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TEE EFFECT OF WEEKDAYIWEEKEND VARIATIONS IN TROPOSPBERIC OZONE CONCENTRATIONS ON THE AIR TEMPERATURE OF TEE
GREATER TORONTO AREA
Gary Beaney
A thesis submitted in conformity with the requirements for the degree of Master of Science Graduate Department o f Geography
University of Toronto
@ Copyright by Gary Beaney 1998
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THE EFFECT OF WEEKDAk7WEEKEND VARIATIONS IN TROPOSPMERIC OZONE CONCENTRATIONS ON THE AIR TEMPERATURE OF THE GREATER
TORONTO AREA
Gary Beaney
Graduate Deparîment of Geography University of Toronto
Due to the variation of rush-hour traffic, tropospheric ozone concentrations have
been shown to vary between weekdays and weekends. As ozone is a greenhouse gas,
both ozone and temperature data were examined to determine whether weekday/weekend
variations in ozone concentrations resulted in weekdaylweekend temperature variations in
the Greater Toronto Area. Two methodologies were used to isolate the effects of
tropospheric ozone variations on temperature. Weekdaylweekend ozone variations and
associated temperature perturbations in both summer and winter months and in
exceedence and non-exceedence weeks were examined. Exceedence weeks represented
periods in which uncharactenstically high ozone concentrations were observed. Variations
in ozone concentrations encountered in both summer and winter months were not of
sufficient magnitude to result in a noticeable thermal effect. When periods of
uncharactensticaily high ozone concentrations were isolated, the resulting weekdayl
weekend temperature variations were show to be statistically significant at ail three
Greater Toronto Area measurement sites.
1 wish to express my gratitude to those people who assisted in the generation of
this thesis. First and foremost, 1 would like to thank Dr. W. Gough for his instruction,
support, and encouragement throughout the course of this study. In addition, Dr. Gough
and especiaily Peter Jackson deserve thanks for introducing me to the topic of
weekday/weekend ozone variations through Peter's research paper; 'Weekday-Weekend
Variations of Photochemical Air Pollution in the Metropolitan Toronto Region". I would
also like to extend my thanks to Phi1 Kieley of the Ontario Ministry of Environment and
Energy for his contïnued assistance and aid in obtaining my MOEE data, and Juri Werner
for providing the University of Toronto temperature data used in this analysis. Duncan
Fraser and David Yap of the MOEE aiso deserve thanks for their support and
suggestions, especially the analysis of weekday/weekend variations during hi& ozone
events. Appreciation is also due to Bob Rade11 of the MOEE for providing me with
directions to each Greater Toronto Area rneasurernent station. I would also like to thank
my family and Kirsty Duncan for al1 the support and encouragement they have provided
over the last three years. In addition, thanks must be extended to my good friends Bonnie,
Debi and Alex who continudy pufled me back fiom the brink of bum-out and exhaustion.
Finaily, special thanks goes to Emma, whose love, patience and support over the past two
years have proven invaluable in the production of this thesis.
TABLE OF CONTENTS
Abstract ................................................................................................................... Acknowledgments ................................................................................................... Table of Contents .................................................................................................... List of Tables ........................................................................................................... List of Figures .......................................................................................................... List of Appendices ...................................................................................................
.................................. 1 . 0 Urban Induced Variations in Air Temperature
.................................................................... 1.1 The Greenhouse Effect
1.2 Local Vs . Global Scale Perturbations in Greenhouse Gas Emissions .
......................................................................... 2 Ozone as a Greenhouse Gas
2.0 The Absorption of Infiared Radiation by Tropospheric Ozone .........
.................................................. . 2 Z The Vertical Distribution of Ozone
2.2 Review of Literature: Ozone as a Greenhoilse Gas ...........................
.................................... 3 Variations in Tropospheric Ozone Concentrations
3 -0 Factors Muencing Tropospheric Ozone Concentrations ..................
......................................... 3 Local Production of Tropospheric Ozone
3.1.1 Volatile Organic Compounds ............................................
........................... 3.1.2 Removal of Ozone from the Atmosphere
................................................................ 3.1.3 VOC/NO, Ratio
3 -2 The WeekdayiWeekend Ozone Mechanism ......................................
3 -3 Review of Literature: Weekdaymeekend Ozone Variations ............
3-4 Long Range Influence on Local Troposphenc Ozone ................................................................................. Concentrations
.. II
iii iv vi
vii xi
TABLE OF CONTENTS
4 Selection of Study Site ................................................................................. 27
.............................................................. 4.0 The Greater Toronto Area- 27
...................................................................... 4.1 Measurement Stations 29
5.0 Isolating the Radiative Effects of Tropospheric Ozone ..................... 35
5 -0- L Radiative EEects of Summer Vs . Winter Ozone Concentrations .................................................................. 35
5.0.2 Radiative Effects of Exceedence Vs . Non-Exceedence ....................................................... Ozone Concentrations 39
6 Results and Discussion ................................................................................. 43
6.0 Statistical Analysis of WeekdayNeekend Ozone . .
and Temperature Vanations ............................................................. 43
6.1 WeekdayMreekend Variations in Ozone Concentrations (Sumer Vs . Winter) ...................................................................... 44
6.2 Weekdaymeekend Variations in Air Temperature ....................................................................... . (Sumrner Vs Winter) 62
6.3 Weekdaymeekend Variations in Ozone Concentrations .................................................. (Exceedence Vs . Non-Exceedence) 62
6.4 WeekdayAVeekend Variations in Air Temperature .................................................. (Exceedence Vs . Non-Exceedence) 87
7 Conclusions and Recommendations ............................................................ 99
7.0 Conclusions ..................................................................................... 99
................................................ 7.1 Recommendations for Further Study 100
LIST OF TABLES
4.1 Relative VOC to NOx Ratios for Four Major North Amencan Cities ............. .... 27
4.2 Ozone and Temperature Records.. .................................................................. 34
..................................................................... 5.1 S u m e r Exceedence Weeks
6.1 Maximum Difference Between Weekday and Weekend Mean Hourly Ozone Concentrations During Exceedence Vs. Non-Exceedence Weeks (Weekend minus Weekday) ................................................................................ 86
6.2 Maximum Difference Between Weekday and Weekend Mean Hourly Air Temperatures During Exceedence Vs. Non-Exceedence Weeks (Weekend minus Weekday) ................................................................................ 87
LIST OF FIGURES
Bending Within an Ozone MoIecule .................................................. 6
Bond Stretching Within an Ozone Molecule ..................................................... 7
Ozone Isopleth Plot Based on Initial NOs and VOC Concentrations ................... 19
Schematic Example of Variations in the Diurnd Ozone Profile as a Result of Reduced Rush-Hour Traffic ........................................................................... 30
Ozone and Temperature Measurement Stations in the ........................................................................................ Greater Toronto Area 32
Relative Location of the Long Point Measurement Station ................................. 33
Evans/Arnold Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1979 to 1995 (Surnrner Vs . Winter) ............................................ 45
Rathbum/Centennial Park Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1979 to 1995 (Summer Vs . Winter) ..................... 46
PerthlRuskin Mean Hourly Weekday and Weekend Ozone Concentrations for the Penod 198 1 to 1994 (Surnmer Vs . Winter) ..... .................................... 47
Bay/Grosvenor Mean Hourly Weekday and Weekend Ozone Concentrations for the Penod 1990 to 1995 (Surnmer Vs . Witer) ...................................... 48
YongelFinch Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1988 to 2995 ( S u m e r Vs . Winter) ........................................ 49
LawrenceKennedy Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1979 to 2 995 (Summer Vs . Winter) ..................... 50
Long Point Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1984 to 1995 (Surnrner Vs . Winter) ....................................... 51
T-Statistics Representing Hourly WeekdayIWeekend Ozone Variations for the Evans/Arnold Measurement Station (Surnrner Vs . Winter) ...................... 54
T-Statistics Representing Hourly WeekdayIWeekend Ozone Variations for the Rathbudentennial Park Measurement Station (Surnmer Vs . Winter) ......... 55
T-Statistics Representing Hourly Weekdaymeekend Ozone Variations for the PerthRuskin Measurement Station (Sumrner Vs . Winter) .................... ... 56
T-Statistics Representing Hourly Weekdaymeekend Ozone Variations for the Bav/Grosvenor Measurement Station (Sumrner Vs . Winter) ................... 57
vii
LIST OF FLGURES
6.12 T-Statistics Representing Hourly WeekdayNeekend Ozone Variations for the YongeEinch Measurement Station (Surnrner Vs. Winter). ...................... 5 8
6.13 T-Statistics Representing Hourly Weekdaymeekend Ozone Variations for the LawrenceKennedy Measurement Station ( S u m e r Vs. Winter). ............ 59
6.14 T-Statistics Representing Hourly Weekday/Weekend Ozone Variations for the Long Point Measurement Station ( S u m e r Vs. Winter) ......................... 60
6.1 5 Evans/Aniold Mean Hourly Weekday and Weekend Air Temperatures for the Penod 1979 to 1995 (Summer Vs. Winter) ............................................. 63
6.16 UniversityRfoskin Mean Hourly Weekday and Weekend Air Temperatures for the Period 1990 to 1995 (Summer Vs. Winter) ............................................. 64
6.17 LawrenceKemedy Mean Houriy Weekday and Weekend Air Temperatures for the Period 1979 to 1995 (Summer Vs. Winter) ......................................... 65
6.18 Long Point Mean Hourly Weekday and Weekend Air Temperatures for the Period 1984 to 1995 ( S u m e r Vs. Winter) .......................................... 66
6.1 9 T-Statistics Representing Hourly Weekday/Weekend Air Temperatures for the EvandArnoid Measurement Station (Summer Vs. Winter) ...................... 67
6.20 T-Statistics Representing Hourly Weekdaymeekend Air Temperatures for the UniversityRIoskin Measurement Station (Surnmer Vs. Winter). .............. 68
6.2 1 T-Statistics Representing Hourly Weekday/Weekend Air Temperatures ............. for the LawrencehSemedy Measurement Station (Summer Vs. Winter) 69
6.22 T-Statistics Representing Hourly Weekdaymeekend Air Temperatures ......................... for the Long Point Measurernent Station (Summer Vs. wnter) 70
6.23 Evans/Amold Mean Hourly Weekday and Weekend Ozone Concentrations for the Penod 1979 to 1995 (Exceedence Vs. Non-Exceedence).. ...................... 72
6.24 RathburnKentennial Park Mean Hourly Weekday and Weekend Ozone Concentrations for the Penod 1979 to 1995 (Exceedence Vs. Non-Exceedence) ............ ... ..... .... ...-.-..-....... 73
6.25 PertMZuskin Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 198 1 to 1994 (Exceedence Vs. Non-Exceedence). ....................... 74
6.26 Bay/Grosvenor Mean Hourly Weekday and Weekend Ozone Concentrations ....................... for the Period 1990 to 1995 (Exceedence Vs. Non-Exceedence). 75
LIST OF HEURES
6.27 YongelFinch Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1988 to 1995 (Exceedence Vs. Non-Exceedence) ....--. - ---. . . -. - .- - - - - - 76
6.28 LawrenceKennedy Mean Howly Weekday and Weekend Ozone Concentrations for the Period 1979 to 1995 (Exceedence Vs. Non-Exceedence) ...- --.---.----.---------.---.-.-.-.---------. - - - - . - . - - - - - . . . 77
6.29 Long Point Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1984 to 1995 (Exceedence Vs. Non-Exceedence).. ..- -. . . . - - - - -. . . - - - - -. 78
6.30 T-Statistics Representing Hourly WeekdayNeekend Ozone Variations for the Evans/Aniold Measurement Station (Exceedence Vs. Non-Exceedence). -. . -. 79
6.3 1 T-Statistics Representing Hourly Weekday/Weekend Ozone Variations for the RathburnKentennial Park Measurement Station (Exceedence Vs. Non-Exceedence) .... --.. - ... - .-. .- ... .-.. ...-............ .. . . .. . . . . . . 80
6.32 T-Statistics Representing Hourly Weekday/Weekend Ozone Variations for the Perth/Ruskin Measurernent Station (Exceedence Vs. Non-Exceedence).- .. . .. 8 1
6.33 T-Statistics Representing Hourly Weekday/Weekend Ozone Variations for the Bay/Gromenor Measurement Station (Exceedence Vs. Non-Exceedence). . . . 82
6.34 T-Statistics Representing Hourly Weekday/Weekend Ozone Variations for the Yonge/Finch Measurement Station (Exceedence Vs. Non-Exceedence). . . . . . . 83
6.35 T-Statistics Representing Hourly WeekdayAVeekend Ozone Variations for the Lawrence/Kemedy Measurement Station (Exceedence Vs. Non-Exceedence). . . 84
6.36 T-Statistics Representing Hourly Weekday/Weekend Ozone Variations for the Long Point Measurement Station (Exceedence Vs. Non-Exceedence). . . . . -. . - - 85
6.37 Evans/Amold Mean Hourly Weekday and Weekend Air Temperatures for the Period 1979 to 1995 (Exceedence Vs. Non-Exceedence).. ... -. -. -.. -. -. . -. . -. -. 88
6.38 University/Hoskin Mean Hourly Weekday and Weekend Air Temperatures for the Period 1990 to 1995 (Exceedence Vs. Non-Exceedence). .. .-- - -. -. --. - - - --. -. - - 89
6.39 LawrenceKennedy Mean Hourly Weekday and Weekend Air Temperatures for the Penod 1979 to 1995 (Exceedence Vs. Non-Exceedence) ....-. . . . . . . . -. -. . . - - --. 90
6.40 Long Point Mean Hourly Weekday and Weekend Air Temperatures for the Period 1984 to 1995 (Exceedence Vs. Non-Exceedence). .... - .-- - - - - .- -. -. . . - - - 9 1
LIST OF FIGURES
6.4 1 T-Statistics Representing Hourly WeekdayMreekend Air Temperatures for the EvarislArnold Measurement Station (Exceedence Vs. Non-Exceedence) ... .... -... - 92
6.42 T-Statistics Representing Hourly WeekdayMreekend Air Temperatures for the University/Hoskin Measurement Station (Exceedence Vs. Non-Exceedence). . . . . 93
6.43 T-Statistics Representing Hourly WeekdayMreekend Air Temperatures for the Lawrenc&emedy Measurement Station (Exceedence Vs. Non-Exceedence). . . 94
6.44 T-Statistics Representing Hourly Weekday~Weekend Air Temperatures for the Long Point Measurement Station (Exceedence Vs. Non-Exceedence). . ..-.--- --. - - - - 95
LIST OF APPENDICES
Relative RoIes of Volatile Organïc Compounds in Nitric Oxide Scavenging ..... 109
Method Used to Assign Days of the Week to Ozone and Temperature Data Sets ........................................................................................................ 112
............................................ Ozone Summary Statistics (Summer and Winter) 114
Temperature Sumrnary Statistics (Summer and Winter) .................................. 122
.... Ozone Surnmary Statistics (Exceedence and Non-Exceedence) ... ................ 127
Temperature Summary Statisbcs (Exceedence and Non.Exceedence) ............. 135
T-Statistic Values (Summer and W~nter) ......................................................... 140
T-Statistic Values (Exceedence and Non.Exceedence) .................................... 144
Mean HourIy Ozonen-Statistic Cornparison ................................................... 148
CHAPTER I
INTRODUCTION
1.0 Urban Induced Variations in Air Tem~erature
The influence of urban activity on air ternperature has been studied since the early
nineteenth century. Howard (1 833) was the f%st to document urban induced variations in
air temperature in the city of London, England. The temperature variations observed were
terrned the "urban heat island effect". The urban heat island effect is a result of the
physicd nature of urban versus rural areas. Cities are generally found to be warmer than
surroundhg rural sites. The increased warming in urban versus rural areas is attributed
largely to the fate of incoming solar radiation. As compared with cities, a greater
proportion of incoming solar radiation is used to evaporate moisture fiom vegetation or
soi1 in rural areas. Ln cities, the rnajority of incorning solar radiation is absorbed by
buildings and roads, therefore resulting in higher temperatures (Ahrens 1991).
In addition to urban versus rural temperature variations, a number of studies have
discovered weekday/weekend ternperature variations in urban areas. Mitchell (196 1), in
an attempt to attribute temperature increases in urban areas to anthropogenic activity,
observed variations in the urban heat island effect among certain days of the week. Mean
daily temperatures on Sundays were found to be approxirnately 0.5 O C cooler when
compared with those on weekdays. This reduction in air temperature was attributed to
reduced anthropogenic activity on weekends versus weekdays.
Lawrence (1 97 1) obsewed weekday/weekend temperature variations of sirnilar
magnitude but opposite in sign in the city of London, England. Of signifïcant importance
in this particular study was the separation of ternperature data into summer and winter
seasons. While no weekday/weekend variations in temperature were observed in winter
months, during summer rnonths, mean weekend temperatures were shown to rise as much
as 0.5 O C when cornpared with mean weekday temperatures. Although this
weekday/weekend temperature variation was attributed to the 'keekly pattern of air
pollution7', no particular poilutants were identified.
A recent study by Gordon (1994) discovered mean temperature anomalies
(deviations £kom the long terni mean) in the northern hemisphere to be greatest on
weekdays as opposed to weekends. Gordon attnâuted this variation to reduced industrial
activity and therefore energy consumption on weekends. However, Lenschow (1 994)
demonstrated that variations in human energy consumption were not large enough to
account for the weekday/weekend temperature variations observed by Gordon. Lenschow
therefore attnbuted these variations in weekday/weekend temperature anomalies to higher
production of aerosols and therefore aerosol cooling during the week.
While in ali three studies significant weekday/weekend ternperature variations were
observed, no specific causai factor was identified. The presence of weekday/weekend
temperature variations in summer months and the lack thereof in winter observed by
Lawrence (1 97 1) suggests a possible Link with intensity of solar radiation. A number of
anthropogenic pollutants are known to interfere with the release of radiation absorbed by
the Earth' s surface. Such polIutants are termed 'greenhouse gases' and t heir resulting
thermal effect is termed 'the greenhouse eKect7.
1.1 The Greenhouse Effect
The effect of greenhouse gases on air temperature is a welf established
phenornenon. On average, f3ly per cent of the incoming solar radiation is absorbed by the
Earth. Most of the incoming solar radiation f d s into the visible portion of the
electromagnetic spectrum (0.4 - 0.75 pm) and is therefore considered short wave
radiation. For the temperature of the Earth to remain constant, the amount of short
wave solar radiation absorbed by the Earth must equal the amount of long wave terrestrial
radiation released to space. The radiation released fiom the earth has wavelengths ranging
fkom 4 to 50 Pm. Energy released at these wavelengths is commonly referred to as
infiared radiation.
Ifall solar energy absorbed by the Earth were released to space, an average global
temperature of - 1 5 O C would persist (Baird 1995). Naturally occumng greenhouse gases
absorb a portion of this outgoing infiared radiation. According to Kirchhoff s Law, for a
gïven temperature, the absorptivity and emissivity of a substance for a certain wavelength
of radiation are equal. The infked radiation absorbed by greenhouse gases is therefore
re-emitted. While a portion of this re-emitted infiared radiation is lost to space, an equal
amount is radiated back to the Earth's surface. The portion of the re-emitted radiation
that ends up back at the Earth's surface is commody referred to as counter-radiation and
results in an increase in surface temperature (Oke 1990). The presence of naturally
occurring greenhouse gases results in an average global temperature of approximately
+15 O C (Baird 1995).
If the concentrations of these greenhouse gases increase, a greater proportion of
the Earth's outgoing infiareci radiation will be redirected, leading to a fùnher increase in
surface temperatures. In addition, as global temperatures increase, more water vapour is
produced through evaporation. As water vapour is a highly effective greenhouse gas,
increased concentrations result in a fùrther temperature increase. A number of greenhouse
gases, particularly carbon dioxide (CO2), methane (Cm), nitrous oxide (NzO),
chlorofluorocarbons (CFCs), and troposphenc ozone (O3) are released into the
atmosphere as a result of anthropogenic activity.
1.2 Local Vs. Globai Scale Perturbations in Greenhouse Gas Emissions
Most greenhouse gases produced through anthropogenic activity influence climate
over large scale areas. Assessing the impact of increases in greenhouse gases on global
climate has been studied extensively. The Intergovemmental Panel on Climate Change
(PCC) was established by the World Meteorological Organization in conjunction with the
United Nations in 1988 to assess the impact of such pollution related problems on global
climate (Houghton et al. 1996). Recently, however, variations of a known greenhouse
gas, ozone, have been observed on a relatively small scale - that of major metropditan
centres in industrialized nations.
Troposphenc ozone concentrations have been shown to increase on weekends
versus weekdays in major urban centers throughout the northern hernisphere (Bower et al.
1989; Colbeck 1990; Summers 1996). These weekday/weekend ozone variations have
been linked to variations in automobile ernissions, particularly rush-hour traffic. As ozone
is a photochernical oxidant, a strong positive relationship is known to exist between ozone
and temperature. As air temperatures increase so do ozone concentrations, due to their
reliance on intensity of solar radiation. Therefore, while under normal circurnstances, the
effect of temperature on ozone is easily observed, the effect of ozone on temperature is
dficult to discem. These weekday/weekend variations in tropospheric ozone
concentrations present an oppominity to examine the effects of variations in tropospheric
ozone concentrations on air temperature.
1.3 Obiectives
Although the overd climate of a city is determined by macro and regional cfimatic
conditions, a minor change at the local level c m contribute to these larger scale
conditions. For example, the majority of data used to develop the measured gfobal
temperature record (1 861 - 1994) originate f?om measurement stations in, or adjacent to,
urban centers or airports It has been suggested that this global temperature record may
have been cccontaminated" by misrepresentation due to urban-scale thermal effects (Oke
1997). In other words, the air temperatures expenenced in large urban centres may not be
representative of true air temperatures removed From urban influence.
In order to determine the extent of these misrepresentations, the precise thermal
nature of large urban settings must be determined. The study of variations of tropospheric
ozone concentrations within an urban setting will help clarifL, ifonly in part, the effect of
concentrated anthropogenic activity on ambient air temperature.
The objective of this thesis is to determine whether weekday/weekend variations in
tropospheric ozone concentrations could directly influence ambient air temperatures
within an urban setting. For reasons outlined in section 4.0, the city of Toronto, Ontario
was selected as the study site. The geographical hypotheses tested in this thesis are as
follows:
Hypothesis 1 - Tropospheric ozone concentrations v q between weekdays and
weekends within the city of Toronto;
Hjrpothesis 2 - Air temperatures vaty between weekdays and weekends within the
City of Toronto;
Hypothesis 3 - Weekday/weekend variations ui tropospheric ozone concentrations
remit in weekday/weekend variations in air temperature within the
city of Toronto.
1.1 Owanuation
This chapter has provided a bief introduction to the greenhouse eEect and the
possible influence of variations of anthropogenic pollutants on local surface temperatures.
Further detail of the chernical and physical processes involved will be added in subsequent
chapters.
The role of tropospheic ozone as a greenhouse gas, in addition to a bnef Iiterature
review thereof, is presented in chapter two. The factors infiuencing local concentrations of
tropospheric ozone including both geographical factors and chernical production are
presented in chapter three. In addition, the mechanisms involved in weekday/weekend
ozone variations, a brief literature review thereof, and the influence of longrange
pollutants on local ozone concentrations is presented and discussed.
Chapter four outlines the selection of the study site and provides a description of
each rneasurement station used in the analysis. The rnethods of analysis used to determine
the presence of weekday/weekend tropospheric ozone and air temperature variations are
outlined in chapter five. The results of these analyses and discussions thereof are
presented in chapter six. Conclusions and recornmendations for further shidy are
summarized in chapter seven.
C W T E R 2
OZONE AS A GREENHOUSE GAS
2.0 The Absorption of Infrared Radiation bv Tropos~heric Ozone
Due to their chemical natures, dïïerent molecules (O3, CO2, H20) absorb different
wavelengths of radiation throughout the electromagnetic spectrurn. It is the specific
chernicai nature of ozone that alIows it to tùnction as an effective greenhouse gas.
Molecules absorb radiation through a process cdled excitation. Excitation
involves the transformation of a molecuIe from its ground state (the lowea possible
energy levei for a particular molecule) to an excited state. As there are no permissible
energy levels between a ground state and an excited state, a molecule can only absorb
radiant energy with a wavelength that corresponds precisely to the dinerence between
these two energy levels (Seinfeld and Pandis 1998).
The specific wavelength required to transform a molecule fiom its ground state to
an excited state is determined by the Mbrationai motion within the molecule. Al1
molecules containing three or more atoms, such as ozone, possess two types of vibrational
motion; bond stretching and bending vibrations. Bending vibrations involve the oscillation
of distance between two atoms bonded to a cornmon atom but not each other (Figure 2.1).
F e 2 1 : Bending Vibrations Within an Ozone Molecule
The vibration that results £kom this departure f?om a collinear geometry dows the
molecule to absorb infrared radiation. In addition to bending vibrations, molecules cm
undergo bond stretching. Bond stretching involves the oscillatory motion of two bonded
atoms relative to each other (Figure 2.2).
During the expansion and contraction in both bond stretching and bending
vibrations the centres of positive and negative charge within the molecule no longer
coincide. This separation of charge within a molecule is called a dipole moment.
Figure 2.2: Bond Stretching Within an Ozone Molecule
Exposure to Mared radiation of the appropriate wavelength enhances these oppositely
directed forces, causing accelerations on nuclei and electrons at one end of the molecule as
opposed to the other end of the rnolecule (Baird 1995). These accelerations lead to the
excitation of the molecule.
The absorption of infiared radiation by a molecule c m oniy occur when the
Eequencies of light and one of these vibrations match. The bending vibrations of ozone
occur near the sarne fiequency as those of COz. Absorption of infiared radiation in this
range by ozone therefore has little effect on the overall greenhouse effect. The bond
stretching vibrations within an ozone molecule however, occur at 9.6 pm (AUoway and
Ayres 1993). Ozone can therefore absorb radiation at this wavelength. Although the
fiequency of vibration within an ozone molecule determines to a large extent the
wavelengths of radiation absorbed, a slightly wider range of radiation (9 - 10 pm) can be
absorbed by ozone. The reason ozone can absorb infiared radiation within a range (9 - 10
pm) around a precise wavelength (9.6 pm) involves the rotational energy of the molecule.
The energy associated with the rotation of a molecule about its intemal axis is
either slightly increased or decreased when infiared light is absorbed. This change in
rotation and therefore energy enables the absorption of infiared radiation at sIightly higher
or lower fiequencies than the exact frequency of vibration (Baird 1995). It is the
absorption of radiation within this particular range (9 - 10 pm) that makes ozone such an
effective greenhouse gas.
The Earth radiates most of its energy at wavelengths between 4 and 25 Pm. The
combination of water vapour and carbon dioxide in the atmosphere absorbs the rnajority of
infiared radiation emitted from the Earth in two large portions of the electromagnetic
spectrum; Eom 1 to 8 pm and at wavelengths longer than 12 (Ahrens 1991).
Radiation emiited in the range fiom 8 to 12 w, however, is not absorbed by water
vapour or carbon dioxide. This portion of the electromagnetic spectrum acts as a
window, allowing nearly eighty percent of the radiation emitted in this region to escape to
space (Seinfeld and Pandis 1998).
As the specific wavelengths of radiation absorbed by ozone (9 - 10 prn) lie within
this window, relatively smail changes in ozone concentrations can significantly enhance the
greenhouse effect. Due to the appreciable absorption of radiation within this window, on
a per molecule basis, ozone can absorb 33 tirnes more radiation than COz (Aüoway and
Ayres 1993).
2.1 The Vertical Distribution of Ozone
The relative radiative effects of ozone molecuIes depend heavily on their vertical
distribution within the atmosphere. Within the troposphere (part of the atrnosphere Eorn
the surface to 1 1 km), an increase in altitude is accompanied by a decrease in air
temperature. The rate of heating or cooling of air temperature in unsaturated air, cailed
the dry adiabatic lapse rate, is approxirnately 9.8"C for evety 1000 m change in elevation
(Oke 1990).
As ozone molecules increase in altitude through the troposphere, surrounding
temperatures gradually decline. Wien's law states that a decrease in the temperature of a
body not only decreases the total radiant output, but also decreases the proportion of
shorter wavelengths of which it is composed (Oke 1990). Since ozone molecules are at
temperatures lower than at the Earth's surface, they emit infiared radiation at a lower
intensity than if they were at the temperature of the Earth's surface. Therefore, as
atmospheric temperatures decrease, ozone molecules become net absorbers of radiation
(Seinfeld and Pandis 1998). The absorption of radiation by tropospheric ozone tends to
warm the atmosphere around it. The greater the elevation of tropospheric ozone
molecules, the greater the temperature ciifFerence between molecules and the Earth7s
surface. This temperature difference lads to a decrease in emission and therefore an
increase in retention of infiared radiation, therefore resulting in increased atmospheric
temperatures.
Within the region just above the troposphere, there is no accompanying chanje in
temperature with altitude. The absorption of infiared radiation by ozone throughout this
portion of the atmosphere, cded the tropopause (the altitude of which c m vary) therefore
remains constant-
Above the tropopause, an increase in dtitude is accornpanied by an increase in
temperature. This portion of the atmosphere, cded the stratosphere, extends fiom -20
km to 50 km. Increases in altitude through this portion of the atmosphere are therefore
associated with increases in the temperature of ozone molecules. As the temperature of
ozone molecules approaches that of the Earth's surtàce, the net absorption of infiared
radiation is reduced. Once the temperature of ozone molecules surpasses that of the
Earth's suface, ozone becornes less effective at absorbing terrestrial radiation. At such
high altitudes ozone therefore tends to intercept incoming solar radiation more efficiently
than outgoing infkared radiation. Therefore, while increases in lower stratospheric ozone
cm continue to increase atmospheric temperature through re-emittence of both terrestrial
and soiar radiation in the form of infiared radiation, increases in upper stratospheric ozone
result in reduced surface temperatures as the arnount of solar energy reaching the Earth's
surface is reduced (Lacis et al. 1990).
2.2 Review of Literature: Ozone as a Greenhouse Gas
The effect of variations in the vertical distribution of ozone was examined by Lacis
er ai. (1990). A one-dimensional radiative-convective mode1 was used to successively
move a small increase in ozone concentration &om layer to layer up through the
atmosphere. Both decreases in stratosphenc ozone and increases in tropospheric ozone
were shown to result in warmer global surface temperatures. Variation in surface
temperature was attnbuted mainly to variations in ozone concentrations in the upper
troposphere and lower stratosphere. Variations in surface temperature due to increasing
ozone concentrations near ground level was determined to be negligible.
Wang et al. (1 993) examined the climatic implications of ozone perturbations
using variations in radiative forcing as opposed to changes in actual surface temperature.
Radiative forcing, expressed in Watts per square meter (Wmm2), is defined as the
periurbation in the net radiative flux at the tropopause due to changes in concentrations of
trace gases (Schimel et al. 1996). From a radiative standpoint, the Earth's surface and the
troposphere are a tightly coupled system. This is due, in large part, to non-radiative heat
exchanges ( E g the hydrological cycle). The troposphere and stratosphere however, are
weakiy coupled. Due to this weak coupling, the troposphere-surface system and
stratosphere c m respond independently to radiation perturbations, either solar or
terrestrial. Changes in surface temperature are therefore dnven by changes in the net
radiation at the tropopause (radiative forcing) and not those observed at the Earth's
surface or the top of the atmosphere (Harvey 1998).
Using ozone sonde data for tropospheric ozone and satellite data for stratospheric
ozone, Wang et al. (1993) studied variations in ozone vertical distribution f?om the late
1960's to the early 1990's throughout the northem hemisphere. A decrease in
stratosphenc ozone concentration and an increase in tropospheric ozone concentration
were observed at al1 stations. These decreases in stratospheric ozone and increases in
troposphenc ozone were both shown to result in a net warming of the troposphere-surface
system.
In order to determine the significance of ozone perturbations in the overall
greenhouse effect, changes in ozone concentrations were compared to changes in total
greenhouse gases including CO2 and C& at Hohenpeissenberg (located near Munich,
Germany). The total radiative forcing caused by changes in CO2, CI&, N20,
chiorofluorocarbons (CFCs), and O3 h m 1971 to 1990 was calculated to be 0.79 and
1.05 ~ r n - ~ for January and July respectively. The radiative forcing associated with O;
changes alone over this same period was 0.41 ~ m ' ~ for January and 0.57 ~ r n - ~ for July.
Approximately fifty per cent of the greenhouse wanning at Hohenpeissenberg was
therefore attributed to ozone.
The relative importance of pemirbations in troposphenc versus stratospheric
ozone concentrations was discussed by Schwarzkopf and Rarnaswamy (1 993). Ushg a
radiative transfer rnodel, the variations in radiative forcing produced by changes in altitude
of both stratospheric and troposphenc ozone were examined. An almost linear variation
in radiative forcing was shown to occur with altitudina1 variations in both stratospheric
and tropospheric ozone. Based on these near-Iinear relationships, a stratospheric radiative
forcing gradient and a troposphenc radiative forcing gradient were established.
At rnid-latitudes, the stratospheric radiative forcing gradient ranged fiom -0.003
to 0.007 w ~ * ~ / D u (a Dobson Unit (DU) is a measure of the integrated ozone in a layer, 1
DU = 2.69 x 1016 molecules (Seùifeld and Pandis 1998)) while the tropospheric
radiative forcing gradient was approxhately 0.28 w ~ - ~ / D u . These results indicate an
increased sensitivity of the sufiace-troposphere system to pemirbations in tropospheric
ozone versus stratospheric ozone concentrations.
Hauglustaine et of. ( 1994a) attempted to detemine the radiative forcing resulting
from changes in atmospheric composition since pre-industrial times. The effects of
perturbations of a nurnber of greenhouse gases including ozone, were examined using an
interactive chernical-dynamical-radiative two-dimensional model.
The radiative perturbation produced by increases in tropospheric ozone
concentrations was shown to be substantial, leading to an average global forcing of 0.55
wnf2. A strong meridional variation in the effects of tropospheric ozone perturbations
was shown to exist with a maximum forcing of 0.8 ~ r n - ~ between 20"N and 40%
latitude. Approlcimately seventeen per cent of the total perturbation of radiatve forcing by
greenhouse gases at mid-Iatitudes was attributed to the steady increase in anthropogenic
ozone precursors and thus tropospheric ozone concentrations in the mid-latitudes of the
northern hemisp here.
Hauglustaine et al. (1994b) further examined the effects of ozone pemirbations on
climate by attempting to determine the influence of aircraft emissions. High altitude
aircrafl emissions of NOx were modeled using the sarne coupled chernical dynarnical
radiative 2-D model as in the previous study.
High altitude NOx emissions were shown to peak between 10 and 2 1 km altitude in
the 30% - 50% latitudind range. Due to higher residence time in the upper troposphere,
the effect of aircraft NOx eMssions on ozone perturbations was found to be nine times
more effective than ground based emissions. Mid-latitude summer increases in ozone of
approximately seven per cent, resulting in a positive radiative forcing of 0.08 wrne2, was
attributed to increases in aircrafl NO, emissions.
Although the observed ozone perturbation was attributed to increases in aircrafl
NO, emissions, the presence of nitrogen oxides alone does not result in a net ozone
increase. Adequate concentrations of non-methane hydrocarbons (NMHCs) must be
present in order for ozone concentrations to increase. The model used by Hauglustaine et
al. fails to incorporate NMHCs in the upper atmosphere. As the presence or absence of
NMHCs would have a profound effect on variations in ozone concentrations, their
incorporation into ozone modeling studies is essential. Therefore, although the radiative
forcing associated with a given increase in tropospheric ozone concentrations is valid, the
magnitude of the estimated ozone increase remains questionable.
Variations in non-methane hydrocarbon concentrations were included in an ozone
modeling study by Bernsten et al. (1996). A three dimensional tropospheric chernical
tracer model was used to study the impact of variations in anthropogenic emissions from
1980 to 1987 on ozone levels in Asia. Based on the results of this model, the most
significant increases in averaged ozone concentrations were obsenred over the heavily
industrialized parts of Japan.
A radiative transfer model was used to calculate the radiative forcing caused by
these variations in ozone concentrations. The average northem hemisphere radiative
forcing was calculated to be 0.13 ~ r n - ~ . When regions with large ozone perturbations,
such as Japan, were isolated, a radiative forcing of 0.5 ~ r n - ~ was observed. The radiative
forcing by ozone over heavily industrialized areas was shown to be of sufficient magnitude
to offset sulphur dioxide (SOÎ) cooling by as rnuch as fi& per cent. Increases in SOz
concentrations have been shown to result in decreases in air temperature due to reflection
of incorning solar radiation (Charlson et al. 1992; Kiehl and Briegleb 1993; Taylor and
Pemer 1994). As global S 0 2 concentrations decline due to efforts to eliminate acid rain,
the thermal effects of increasing tropospheric ozone concentrations may become more
pronounced.
The majonty of researchers examining the effects of ozone perturbations on
ciirnate work under the assumption that c h a t e change is proportional to radiative
forcing. Hansen et al. (1997) used a three dimensional general circulation mode1 to
determine how effective changes in radiative forcing are in predicting climatic change.
The observed effects of the theoreticai removal of ail ozone from the atmosphere on both
the instantaneous radiative flux at the tropopause and air temperatures were examined.
While the theoretical removal of all ozone eorn the atmosphere resulted in a
drarnatic increase in the radiative flux at the tropopause (3.44 ~ r n - ~ ) , decreases in both
stratosphenc and tropospheric temperatures were observed. Stratospheric temperatures
were shown to cool by as much as 80°C, resulting Eom the elimination of absorption of
solar radiation by stratospheric ozone. While tropospheric temperatures were also shown
to decrease, a change in surface temperature of ody - 1 O C was observed. The relatively
minor change in sufiace temperature as opposed to that in the stratosphere is annbuted to
the near canceling effects of increased solar heating and reduced greenhouse heating.
The results of this study therefore illustrate the importance of the identification of
the thermal eEects of ozone variations using temperature data as opposed to radiative
forcing estimates. If variations in surface temperatures can be successfùlly linked with
ozone perturbations, an accurate assessrnent of the climatic effets of future increases in
tropospheric ozone concentrations cm be made.
CBAPTER 3
VARIATIONS IN TROPOSPEERIC OZONE CONCENTRATIONS
3.0 Factors Influencine Tro~ospheric Ozone Concentrations
Several geographicai phenornena can influence the production and persistence of
high concentrations of tropospheric ozone within an urban setting. Metropolitan areas
situated in valieys or surrounded by mountains are more prone to thermal inversions.
Thennal inversions are perbds in which air temperature increases with increasing altitude
within a certain altitude range, resulting in a layer of warrner air above cooler air.
Temperature inversions are ofken a result of the advection of a high pressure
weather system into a region. The anticyclonic movement of air which accompanies high
pressure systems results in the descent of air which warms as it subsides and compresses
(Ahrens 199 1). This w m parce1 of air Iying above the cooler surface air suppresses the
vertical movement of air contaminants and thus allows the* concentrations to increase.
The presence of mountainous terrain can inhibit the movement of this stagnant air mass
out of the region therefore allowing concentrations of pollutants to increase over
prolonged periods of time (Baird 1995).
Proximity to large bodies of water and associated sea- or lake-breezes cm also
influence pollution leveis within a city. The land-breezellake-breeze cycle involves the
movement of an air mass to and from a body of water due to variations in the heating and
cooling rates of various terrestriai and aquatic surfaces. Due to the rapid cooling of the
land after sunset, air tends to rise above the warmer lake, resulting in a land-breeze (land
to lake). This breeze cm carry a large portion of air pollutants, produced over the city
during the day, out over the lake.
Upon sunrise the next day, the thermal r ising of air above the land, which w m s
more rapidly than the water, leads to a lake-breeze (lake to land). The result of this lake-
breeze is ofien the advection of the pollution Iaden air mass removed the night before,
back over the city (Nriagu and Simmons 1994).
Variations in troposphenc ozone concentrations due to lake- and sea-breezes have
been observed in a number of coastal cities (Blumenthal et al 1978; Westberg et al. 198 1;
Wakamatsu et al. 1983; Gusten et al. 1988). Lyons and Cole (1976) observed an increase
in ozone concentrations of 100 ppb upon advection of a land-breeze dong the western
shore of Lake Michigan.
3.1 Locd Production of Tro~ospheric Ozone
n i e concentrations of tropospheric ozone rneasured at any location, in conjunction
with the geographical nature of the particdar city, is prirnarily influenced by the local
emissions of ozone precursors through anthropogenic activity. While the photochernical
production of ozone is a complicated and detailed process, examination in its entirety is
necessaty to fully understand weekday/weekend variations in ozone concentrations.
Ozone is produced through the combination of molecular oxygen (02) with atomic
oxygen (O), in the presence of a third inert entity (M) such as N2 or 0 2 (Eq. 3.1). This
third entity absorbs the excess energy produced by the reaction and therefore stabilizes the
ozone molecule produced.
The majority of atornic oxygen in the atmosphere results fiom the
photodissociation of nitrogen dioxide (NO& When the energy of incoming photons (solar
radiation) exceeds the binding energy of the chernical bonds involved, photodissociation
occurs. Nitrogen dioxide photodissociates when exposed to light with wavelengths of
approximately 400 nm (Seinfeld and Pandis 1998) as indicated in Equation 3 -2.
Once the ozone molecule has been formed (3.l), it quickly reacts with the nitric oxide
(NO) produced in reaction (3.2), reforming NO2 and 0 2 (Eq- 3 3).
Therefore based on these reactions aione, ozone would remain in a state of equilibrium
and sigdicant increases in concentration would not be possible.
The presence of hydroxyl radicals (OH) in the atmosphere is indirectly responsible
for breaking this ozone-equilibrium cycle, therefore allowing ozone concentrations to
increase. The three pnmary sources of OH radicals in the atmosphere are: the
photodissociation of nitrous acid (HONO), the photodissociation of ozone, and the
reaction of HOz radicals with NO. Nitrous acid, which is often found in nighttime urbm
atmospheres as a result of the combination of NO2 with water vapour, c m
photodissociate to produce OH radicals as indicated in Equation 3.4.
HONO + hv + OH- + NO
Irradiation with li&t in the range of 200-300 nm (Alloway and Ayres 1993) cm
photodissociate ozone to forrn molecular oxygen and an electronicdly excited oxygen
atom (o('D)).
While the majority of o('D) atoms produced react with N2 or O2 and revert back to
ground state oxygen atoms (O),
a small fraction of o('D) reacts with water vapour to produce OH radicals.
The OH radicais produced can then react with carbon monoxide, a combustion emission,
and molecular oxygen to produce carbon dioxide and an HOz radicd.
*- CO + OH- + CO* + HOz-
This H a radical can then react with NO to reproduce NO2 and an OH radical.
This reaction is extremely significant in ozone chemistry for three reasons: 1) NO is
converted to NO2 therefore removing its abïiity to scavenge ozone; 2 ) NOz is once again
available to photodissociate and lead to the formation of an additional ozone molecule;
and 3) an OH radical is produced which, through reaction with Volatile Organic
Compounds, enables ozone concentrations to increase.
3.1.1 Volatile Omanic Compounds
Through reaction with OH radicals, volatile organic compounds (VOCs) such as
alkanes, alkenes, or aldehydes are able to react with NO and therefore negate its role as an
ozone scavenger. The majority of Volatile Organic Compounds involved in the
production of ozone are carbon-hydrogen compounds (hydrocarbons). The term 'Non-
Methane Hydrocarbons (NMHC)' is therefore also used to denote this particular farnily of
atmospheric pollutants. Methane is excluded due to its poor reactive capabilities and
therefore insignificant impact on ozone formation.
While sources of VOCs are varied, a large proportion of atmospheric
concentrations results fkom the incomplete combustion of fossii fùels fiom both
automobiles and power generation facilities. The relative roles of aikenes, alkanes,
aldehydes, and aromatics in NO scavenging are discussed in Appendix A.
3.1.2 Removal of Ozone from the Atmosphere
M e r sunset the photolytic production of OH radicals and atomic oxygen
decreases rapidly, therefore halting the production of ozone. M e r photochernical
production has ceased, ozone concentrations gradually decline via a number of chemicai
and physical pathways.
The primary ozone-removai mechanisrn after sunset is reaction with nitnc oxide
through reaction (3 -3). As NO concentrations decline, reaction with other atmospheric
constituents such as NO2 (Eq. 3.10) or HO2- (Eq. 3 - 1 1) become more significant.
While such reactions occur during the day as well as at ni&, the rates of reaction are so
slow that any daytime infiuence is negligible (Nriagu and SUnmons 1994).
In addition to removai through chernicd reaction with airbome constituents, ozone
concentrations can be significantly reduced through both wet and dry deposition. The
occurrence of precipitation events, day or night, can remove ozone fkorn the atmosphere
(wet deposition). While ozone concentrations rebound quickly after precipitation events
during daylight hours (Kieley 1998), the removal o f ozone by precipitation d e r sunset
will result in depressed concentrations until s u ~ s e , therefore dramatically reducing
ambient ozone concentrations.
Dry deposition of ozone can result through interaction with both flora and fauna.
Reductions in annual growth of severai tree species, including loblolly pine and red spruce,
have been observed due to reaction with ozone (Lovett and Hubbell 199 1; Tjoelker el al.
1993; Duckmanton and Widden 1994; McLaughlin and Downing 1995). In addition
ozone has been shown to have adverse affects on human health through reaction with the
mucous membranes of the nose and throat (Baird 1995). People with respiratory
problems have been shown to expenence adverse health affects when exposed to high
concentrations of ozone (Burke 1987; Bown 1994).
3.1.3 VOC/NO. - Ratio
The concentration of tropospheric ozone present in large urban areas is highly
dependent on the ratio of VOCs to NO, in the atmosphere. As both VOCs and NO, react
with hydroql radicals, the relative proportion of these pollutants determines whether
ozone concentrations are kept in check or are dowed to increase. The rates of reaction
of VOCs and NOz with OH radicals are equal when the VOC to NOz ratio is
approxhately 5.5: 1. When the VOC to NO2 ratio is less than 5 -5: 1, OH reacts
predominantly with NO2, removing OH radicals fiom the VOC oxidation cycle and
inhibitïng the production of ozone (Seinfeld and Pandis 1998). Under theses
circumstances, a decrease in NO, concentrations wilI result in an increase in ozone
concentrations.
It is important to note however, that NO2 is an important ozone precursor.
Therefore, once NO1 concentrations becorne too low, ozone production can be inhibited.
The non-hear nature of the L'OC-NOx relationship is illustrated in figure 3.1.
VOC, ppm carbon
Figure 3.1: Ozone isopleth plot based on initial NOs and VOC concentrations. For a fixed VOC concentration of 0.6 ppm, a decrease in NOs concentration fi-om 0.20 to O. I6 ppm results in an increase in ozone concentration frorn O. 16 to 0.20 ppm. A decrease in NOx concentration fiom 0.08 to 0.04 ppm however is shown to result in a decrease in ozone concentration fi-om 0.2 1 to 0.16 pprn.
Source: Colbeck and Mackenzie 1994
3.2 The Weekdavweekend Ozone Mechanism
Weekday/weekend variations in ozone concentrations result from the absence of
early moming rush-hour trafic on weekends. Reduced tra£Ec flow on weekends lowers
atmospheric NOx concentrations and therefore dows ozone concentrations to increase.
In addition to enhancing the VOC oxidation cycle, early moming NO, emissions actually
breakdown ozone before any photochernical production can take place.
On weekdays, moming rush-hour trafic begins well before sunrise. As ozone
concentrations cannot begin to increase until sunlight breaks down NO*, NO released from
automobiles scavenges ozone (Eq. 3 -3) and lowers early moming concentrations.
Therefore, when the Sun nses and ozone concentrations begin to increase, they are
beginning from a lower level than on weekends, when this early moming ozone
scavenging by rush hour trafic does not occur (Figure 3-2). The absence of this early
moming ozone reduction on weekends leads to higher weekend ozone concentrations.
Hour of Day
Figure 3.2: Schematic example of variations in the diunial ozone profile as a result of reduced rush-hour traffic.
3.3 Review of Literature: WeekdavAWeekend Ozone Variations
The weekday-weekend variation in atmosphenc pollution was first observed by
Haagen-Smit and Brunelle (1958) in Los Angeles, California. The oxidation of
phenolphthalin to phenolphthalein was used to test the oxidizing effectiveness of the
atmosphere. Sundays were show to exhibit the lowest oxidant values of any day of the
week. Oxidant values were shown to rise until Thursday and then gradually decrease back
to Sunday levels. This variation was amibuted to anthropogenic activity, particularly the
reduced intensity of t r a c flow and industrial activity on weekends.
In studying the day-to-day variation of polIutants in downtown Los Angeles,
Schuck et al. (1966) found hydrocarbon and NO, concentrations to be 20 and 40% higher
respectively on weekends. In addition, ozone values were s h o w to increase 10 to 20
percent on weekends.
Schuck et aL attempted to determine the spatial extent of the weekday-weekend
oxidant effect by examining daily maximum ozone values for several additionai stations.
Three distinct weekday/weekend ozone patterns were discovered over the Los Angeles
basin. The southern stations (Inglewood, Long Beach) showed a considerable increase in
ozone concentrations on weekends. The northern stations (Burbank, Pasadena, Azusa)
exhibited a tendency toward a decrease in ozone on weekends and the central stations
(downtown Los Angeles, USC Medical Center, Hollywood Freeway) showed variable
results.
While Schuck el al. (1 966) helped illuarate the geographical variability of day-to-
day oxidant concentrations, no conclusions as to the cause of these variations were
presented. The data for the Los Angeles Basin study were divided into the two si.- month
penods of January to June and July to December, therefore cutting the most intense
photochemical season in haK This division resulted in anomalous readings for the central
stations (e-g. Hollywood Freeway showed a weekend oxidant increase fiom Ianuary to
June and a weekend oxidant decrease fiom July to December).
As ozone is a photochemical pollutant, ozone concentrations, and variations
thereof, are most pronounced during times of maximum sunlight intensity. Bruntz et a[
(1 974) examined hourly mean ozone concentrations in New Jersey and New York.
Average diumal curves were produced for three time periods: weekdays (Mondays
to FrÏdays), Saturdays, and Sundays. Mernoon ozone peaks for all three time penods
were relatively the same. However, 6: 00 am. to 10:OO a.m. ozone concentrations were
shown to be higher on weekends versus weekdays, with the highest concentrations
occuning on Sundays. This early moniing ozone increase was attributed to less vehicular
t r a c and therefore less NOx scavenging of ozone on weekends versus weekdays.
Cleveland et al. (1974) used the statistical method of quantile-quantile (Q-Q) plots
to help detennine the magnitude of weekday-weekend ozone variations in New Jersey and
New York. This statistical method allows one to compare the entire range of ozone
values. Jus as a quartde divides a data set into four equd sets, a quantile divides a data
set into q equal subsets (=th and Amrhein 1991). Therefore, the qth percentile of a
data set infers that q percent of the data is less than or equd to the gven value (Cleveland
and McRae 1978). Sunday quantiles fiom the period iMay through September of 1972
and 1973 were plotted against corresponding weekday quantiles. While a few extreme
ozone maxima were s h o w to occur on weekdays, the majority of ozone maxima and ail
average ozone quantiles where shown to be higher on weekends.
Lebron (1975) developed a smog index to determine whether significant smog
variations occurred between weekdays and weekends in the Baltimore-Washington
metropolitan area. Hourly ozone readings fiom 1 1:00 am. to 7:00 p.m. were recorded
£kom June to September 1972 and 1973. The index values were obtained by setting any
value less than 0.04 ppm to zero. The results of the Kruskd-Wallis non-pararnetnc
analysis of variance test indicated that no day of the week had sigificantly higher ozone
indices than any other. However, Bruntz et al. (1 974) observed weekday-weekend ozone
variations in the early hours, and therefore at low concentrations, in New Jersey and New
York. Lebron's study illustrates the importance of examining the entire range of ozone
concentrations when studying weekday-weekend vuiations.
Elkus and Wdson (1976) examined the possibility of traftic atîenuation as the
cause of weekday-weekend ozone variations in Los Angeles. Hourly average NO,
hydrocarbons, and O, (total oxidant - a measure of several oxidizing molecules, largely
ozone) were examined over the period 1965 to 1972. In addition, Los Angeles county
traffic count data were used to deterrnine weekday-weekend t r a c patterns. Daily
average trafic was shown to decrease by 20 per cent on weekends as opposed to
weekdays. Both NO, and hydrocarbon concentrations were found to decrease on
weekends. Weekend oxidant concentrations were found to increase by an average of 8
per cent.
In Hudson County, New Jersey, Graedel et a(. (1977) examined variations in
ernissions fiom both t r a c and power generation facilities and their effect on weekday-
weekend ozone variations. Sunday ozone concentrations were shown to increase while
concentrations of NO and NO2 were shown to decrease. The early moming excess in
ozone on Sunday mornings was attributed to decreased ozone scavenging by NO,.
The spatial extent of the weekday-weekend ozone variations was further illustrated
by KarI (1978). Air poilution data were collected from 25 air monitoring sites in the
greater St. Louis area. Average daily ozone concentrations were calculateci for both
Sundays and weekdays and plotted on Q-Q plots. Both uuier-city and transitional sites
showed a Sunday increase in ozone concentrations. Outer sites showed an ozone decrease
on Sundays venus weekdays, however the magnitude of this dserence was much reduced
compared to inner and transitional sites.
Decreases in NO2 averages were observed on Sundays at d l stations for a11
quantiles. The largest decrease was observed at the inner sites. Sunday NO
concentrations were also shown to decrease at the imer and transitional sites, however
concentrations at the outer sites were too low to obtain any statisticdy significant
reading .
It was suggested that high NO concentrations during the week at inner and
transitional sites led to the reduction of weekday ozone levels. When these weekday NO
concentrations were negligible (outer sites), weekday ozone concentrations remained
relatively sirnilar to weekend levels.
Bower er al. (1 989) studied urban versus rural weekday-weekend ozone variations
throughout Great Britain. Weekday to Sunday ozone variations fiom two areas
infiuenced by heavy trafic fIow (Centrai London and Stevenage) and seven rural stations
(Sibton, Aston W, Lullington Heath, Strath Vaich, Hi& Mufles7 Lough Navar, and
Yarner Wood) were examined.
Both Stevenage and Central London showed higher Sunday ozone concentrations
as compared to weekday levels. In addition, the morning ozone minimum observed in
most urban centers was shown to be present on weekdays and absent on Sundays. The
seven rural stations showed relatively little variation between weekday and Sunday ozone
concentrations. The lack of Sunday to weekday variation of rural ozone was attributed to
lack of auto emissions and resultant NOx scavenging of ozone.
Colbeck (1990) studied weekday-weekend variations in ozone at two mral sites in
north-west England. One rural site (Hazelrigg) was adjacent to a major highway. The
other rural site (Stodday) was relatively isolated fiorn large sources of automobile
emissions. Using Q-Q plots, Colbeck found that Sunday hourly ozone levels were
approximately 2 ppb higher than weekday concentrations at HazeIrigg. Weekday diumal
ozone variations dso exhibited the rush-hour minimum as observed in previous studies.
No weekday-weekend variations in ozone concentrations were observed at Stodday, once
again illustrating the importance of auto-emissions on weekday-weekend ozone variations.
Pryor and Steyn (1994) examined the magnitude of the weekend effect between
the 1980's and 1990's in the Lower Fraser Valley of British Columbia. The average of the
highest twenty five per cent of daily ozone values were presented on Q-Q plots. The
magnitude of the weekend effect was shown to increase when comparing the period 1984-
1986 with the period 1989-1991.
Altshuler et al. (1995) examined the number of ozone guideline exceedence days
by day of the week in Northern California. A significant increase in nurnber of exceedence
days was observed on weekends versus weekdays. In addition to reduced automobile use,
NOs emissions frorn fossil-fuel power generation facilities and diesel-powered trucks and
buses were shown to decrease 20% on weekends, therefore reducing the rate of weekend
ozone scavenging by NO,.
Surnmers (1 996) examined weekday-weekend ozone variations at various sites
throughout Canada. Average daily maximum values were deterrnined for each day of the
week fiom 1980 to 1993. The urban centers of Montreal, Vancouver, and Toronto
showed weekend ozone increases in the range of 10 to 35 per cent. Rural stations
throughout southern Ontario downwind of urban centers showed weekend increases from
4 to 8 per cent.
The relationship between the morning increase in NO and reduction in ozone was
tested for aii sites on weekdays, Saturdays, and Sundays. An ozone decrease of 1 ppb was
observed for every 1 ppb increase in NO for the first 5 ppb. As NO increased beyond 5
ppb, decreases in ozone concentrations slowed as little ozone remained. However, as the
statistical signifïcance of this relationship was not provided the validity of these results is
questionable.
Four of the thirteen weekday-weekend ozone studies discussed involved the city of
Los Angeles, California. In addition to meeting al1 of the geographical requirements for
the formation and proliferation of tropospheric ozone, Los Angeles has a poorly
developed public transportation system and therefore experiences extremely high tr&c
volumes (Baird 1995). J3gh ozone concentrations experienced in the city of Los Angeles
are therefore the result of local ozone production.
Areas such as New York, Baltimore, St. Louis, and Toronto, while producing
much less ozone on a local scale, still however incur periods of unusually high ozone
concentrations. These high concentration events are usually the result of the intrusion of
long-range pollutants.
3.4 Long Range Influence on Local Tropospheric Ozone Concentrations
The long range transport of tropospheric ozone and its precursors is best
illustrated by Wolfand Lioy (1980). They chronicled the development, progress, and
effects of an "ozone river" formed in the southern United States in the late 1970's.
Wolf and Lioy's observations began with the development of a high pressure
system of predominantly maritime tropical air over the Gulf of Mexico on JuIy 12, 1977.
The coastai area between Corpus Christi, Texas and New Orleans, Louisiana was known
to experience extrernely high hydrocarbon emission densities as well as abundant sunshine.
The "'ozone river" was established by the advection of this tropical high-pressure system
from the highly polluted area of the Texas-Louisiana GulfCoast to the northeastem
Atlantic Coast. Average ozone concentrations throujhout this ozone river (120-130 ppb)
were approxirnately 100 ppb above usuai levels.
Ozone concentrations of 150 ppb and 200 ppb were recorded in the Washington
D.C. - Baltimore area between July 15 and 19 with the highest concentrations being in
Connecticut on July 19 at 328 ppb. This air mass eventudly reached New York City on
Jdy 23 at which t h e ozone concentrations were 90-100 ppb. The reduced ozone
concentrations encountered at New York were attribgted to increased wind speeds
experienced during the latter part of the joumey, which would lead to increased dispersion
of the plume. In addition to the ozone carried dong by this hi&-pressure system, ozone
concentrations may have been supplemented by emissions encountered dong the way.
Even though cities such as New York and Toronto have well developed systems of
public transportation and are situated in well ventilated areas, they still incur periods of
high ozone concentrations. These unusually high ozone events are the result of the
intrusion of pollution laden weather systems such as that outlined by Wolf and Lioy
(1980).
CHAPTER 4
SELECTION OF STUDY SITE
4.0 The Greater Toronto Area
Compared to other major rnetropolitan areas such as Detroit, Chicago and New
York, the VOC to NOs ratio encountered in the city of Toronto is relatively low (Table
4.1). NOx emissions from the Lakeview coal-fired power plant are thought to play a
Tabie 4.1 : Relative VOC to NOs Ratios for Four Major North American Cities
Total VOCs NO, VOCd NO,
City (tome m2yr-') (tonne m 2 - ' ) (ppbc/ppbv)
Toronto 0.99 x lo4 0.77 x 104 Detroit 1.03 x loJ 0.34 x lo4 Chicago 1.00 lo4 0.33 x lo4 New York 1.98 x 104 0.89 x lo4
Source: Lin et al. 1995
significant role in the low VOC to NOs ratio in the city of Toronto. The Lakeview station
is located approximately 25 km to the southwest of downtown Toronto and accounts for
fifty per cent of the NOs emissions in the Toronto urban area (Lin et al. 1995). NOs
emissions from a sirnilar power plant 6 km southwest of Melbourne, Australia were shown
to decrease ozone concentrations by as much as 20 ppb (Hess 1989).
In addition, the presence of major transport routes in the downtown core, which
are heavy sources of NO, emissions, can result in a reduced VOC to NOs ratio and
therefore reduced ozone concentrations. Proximity to major transport routes in Montreal
has been shown to reduce maximum daily ozone concentrations in the downtown core by
as much as fifly per cent (McKendry 1993).
Due to this low VOC to NOs ratio, a decrease in NO, concentration will result in
an increase in ozone formation (as long as NOx concentrations remain high enough for
ozone to be produced) (see section 3.1.3). As rush-hour induced NOx concentrations are
known to fluctuate between weekdays and weekends within most urban centers, this low
VOC to NO, ratio rnakes ozone concentrations within the city of Toronto highly
susceptible to weekday/weekend variations.
Although the low VOC to NOx ratio dramatically inhibits the production of ozone
within the city of Toronto, unusudy high ozone concentrations are still encountered.
These high ozone concentrations are predominantly the result of the long-range transport
of pollutants into the city.
Fifty to sixty percent of the ozone encountered in the city of Toronto is the result
of long-range transport (Yap et al. 1988). Above average ozone concentrations in
southem Ontario have been Linked to a southerly to southwesterly flow on a number of
occasions (Chung 1977; Mukammal et al. 1982; Heidom and Yap 1986; Lin et uZ. 1996).
During the summer the heavily industrialized areas of the U. S. to the south and
southwest of Ontario are frequently under the influence of a large amplitude ridge
extending korn the Bermuda High, which leads to very Little circulation in the area
(Mukammal et al. 1982). In the presence of a southwesterly flow, this moist stagnant air
is advected into southern Ontario carrying pollutants accumulated fiom areas such as
Cleveland, Detroit, or Chicago (Chung 1977). In Ontario, 95% of episode days (days on
which ozone concentrations exceed 80 ppb at a number of stations) between 1979 and
1988 occurred when iduenced by southerly flows (Yap et al. 1988).
The incursion of weather systems containing either ozone or its precursors cm
result in elevated local ozone concentrations. As the VOC/NOx ratio in the city of
Toronto is relatively low, a large influx of VOCs into the area would result in increased
local ozone production. A similar situation was observed by Hess (1989). Advection of
pollution laden weather systems £?om a region where hydrocarbon emissions were
relatively high was shown to increase local ozone concentrations between 10 and 40 ppb.
Such long-range pollutants are most ofien encountered at the receding edge of a
southwesterly high-pressure system, which allows an enhanced residence time over areas
of high precursor ernissions (Chung 1977; Wolf and Lioy 1980; Kelly et aL 1986). The
intrusion of such pollution laden high-pressure systems into the Greater Toronto Area can
lead to the occurrence of high local concentrations even when local atmosphenc
conditions (temperature, sunlight intensity, etc.) are not conducive to ozone production.
4.1 Measurement Stations
Ozone concentrations at various locations throughout an urban center can Vary
considerably. Increased traf£ïc congestion w i t h the downtown core leads to higher
concentrations of nitric oxide (NO). In addition, the presence of t d buildings inhibits the
dispersal of downtown pollutants by wind, allowing concentrations to increase (Katsoulis
1995). As a result of increased scavenging by higher NO concentrations, downtown
ozone concentrations are usually lower than those of better ventilated, less congested
areas of the city. When compared to suburban rneasurement sites, increased ozone
scavenging by NO has been shown to reduce downtown ozone concentrations in the city
of Toronto by as much as f3ty percent (Liu and Rossini 1996).
In order to account for the variable nature of tropospheric ozone concentrations,
ozone data from six measurement sites throughout the Greater Toronto Area were
obtained fi-om the Ontario Ministry of Environment and Energy (MOEE). HourIy ozone
concentrations had been measured with a chemiIuminescent detector and recorded in
increments of parts per biliion (ppb). Cherniluminescence involves the measurement of the
wavelength of light emitted when a gaseous compound is introduced to a sample of
ambient air. In the case of ozone, the wavelength of visible light emitted when ethylene is
introduced to ambient air is directly proportional to the ozone concentration.
For the purpose of this study it is important to note that the Greater Toronto Area
was divided into four distinct regions: Etobicoke (west); North York (north); Toronto
(central); and Scarborough (east). Ozone data were obtained fiom measurernent stations
in each of these regions in an attempt to represent, as well as possible, tropospheric ozone
concentrations throughout the Greater Toronto Area.
The first of the two Etobicoke sites (Station 3 5033) is located approximately 50
meters north-east of the intersection of Evans and Arnold Avenue in a large open field.
The measurernent station was approximately 100 meters south of the Gardiner
Expressway. This highway acts as a major cornmuting artery for the city of Toronto and
is therefore highly susceptible to variations in traftic flow.
The second Etobicoke site (Station 35003) is located approximately 500 meters
north of the intersection of Rathburn and Centennial Park Road. The measurement station
was situated in a large parking lot adjacent to the Centennial Park arena. While location
within a parking lot could result in biased ozone readings (due to a concentrated release of
ozone precursors) heavy tr&c volumes within this parking lot were rarely encountered
(Rade11 1 998).
The first of two Toronto sites (Station 3 1 120) is Iocated at the intersection of
Perth and Ruskin Avenue. While dl other measurement stations used in this snidy were
located at ground level, this station was located on the second floor roof of St. Luigi
Elementary School. The elevation of this measurement site could Iead to increased wind
dispersal of pouutants compared to a ground based station. The measurement station was
located within a quiet residentid area approximately 350 meters east of Dundas Street. As
Dundas is a relatively high volume road, ozone measurements fiom this site could be
susceptible to rush-hour precursor emission variations.
The second Toronto site is located in the downtown core of the city. This
measurement station was orïguiaily located at 26 Breadalbane St. (Station 3 1104), but was
relocated one City block north to the corner of Bay and Grosvenor St. (Station 3 1103) in
October of 1990. Due to the short distance between sites and relatively similar urban
surroundings, the change in location of this site should have had little affect on local ozone
readings. The new location of the downtown station is surrounded by tail buildings,
resulting in reduced wind ventilation, as is the previous location. In addition, traffic
volumes adjacent to each site were virnially identical.
The North York (Station 34020) site is located approxirnately 20 meters west of
the intersection of Yonge Street and Finch Avenue in a large cornmuter parking lot.
While the general area was open and well ventiiated, the proxïmity of this measurement
station to such a highly concentrated source of automobile emissions should result in an
enhanced sensitivity to variations in rush-hour tratfic.
The Scarborough measurement site (Station 33003) is located approximately 20
meters south of the intersection of Lawrence Avenue and Kennedy Road. The
measurement station was situated in a large open field, therefore being highly susceptible
to wind dispersion. In addition, this measurement site is located approxirnately 500 meters
wea of a large cornmuter parking lot and should therefore be highly susceptible to
variations in rush-hour traEc.
While all six of these rneasurement sites provided hourly ozone data, only two of
them, EvandAmold (Etobicoke) and Lawrence/Kemedy (Scarborough), had
corresponding temperature records. Downtown temperature data were obtained fkom the
St. George Campus of the University of Toronto to be used with the Bay/Grosvenor
ozone data. The University of Toronto measurement site was located approxirnately 20
meters north-west of the intersection of University and Hoskin Avenue. While this
temperature station was located approximately 760 meters to the north-west of the
corresponding downtown ozone site (Bay/Grosvenor), both sites were subject to
comparable t r a c volumes and restricted pollution dispersion due to adjacent urban
topography. AU hourly temperature values were measured in Celsius degrees and
recorded with a dry-bulb thennometer. Figure 4.1 displays the relative location of
measurernent stations, both ozone and temperature, within the Greater Toronto Area.
In addition to the six Greater Toronto Area measurement sites, a remote MOEE
rneasurement station, Long Point Provincial Park, was used as a control site. Long Point
Provincial Park is located approximately 150 kilometers south-south west of the city of
Toronto (Figure 4.2). Both hourly ozone and temperature data were obtained from this
site to examine weekday/weekend variations in ozone concentrations and air t emperature
in the absence of urban induced rush-hour t r a c .
The Long Point measurement site (Station 22901) is located approximately 50
meters north east of a large parking lot and 100 meters south of the shore of Long Point
Bay. While such a park would not be influenced by rush-hour t r a c , the possibility of
unusual traffic patterns on statutory hoiidays is possible. However, visual observations
made at this site on Iuly 1, 1998 (Canada Day) revealed no unusualiy high t r a c volumes.
Even though ozone concentrations at Long Point are regularly the highest in Ontario, due
irniCentennlal Park
Figure 4.1 : Ozone and Teinperatiire Measiireineiit Stations witliiii the Greater Toronto Area
o - Ozone and Temperature
- Ozone F - . A
Ratht~r T
b N P
.- 3:
Gardiner Expwy
0.0 20 4.0 bn h
1 cm rspresenh 1 .S km
4
Source: Ontario Ministry of Eiivironiiieiii and Eiiergy 1997
to the incursion of pollution laden weather systems fiom the United States, the absence of
local rush-hour traffic rnakes Long Point an ideal control site.
As the maximum amount of data available from each site was used in an attempt to
best represent average ozone and temperature variations, the Iength of ozone and
temperature records do not necessarily correspond between sites. The maximum length of
data sets for each partïcular station was determined by the maximum corresponding length
of both ozone and temperature records. The length of both ozone and temperature
records for al1 sites used in this study are presented in Table 4.2.
Table 4.2: Ozone and Temperature Records
Measurement Station Ozone Temperature
Evans/ArnoId 79/0 1/0 1 - 95/12/3 1 79/0 RathbudCent. Pk. 79/0 1/0 1 - 95/12/3 1 Perth/Ruskin 8 1/03/20 - 94/11/03 Bay/Grosvenor (Univ./Hos.) 90/0 1 /O 1 - 95/ 12/3 1 90/0 YongeEinch 88/06/0 t - 95/12/3 1 Lawrence/Kemedy 79/01/01 - 95/12/31 79/01/01 - 95/12/3 1 Long Point 84/05/0 1 - 95/1 2/3 1 84/05/0 1 - 95/ 12/3 1
5.0 Isolatine the Radiative Effects of Tropos~heric Ozone
Tropospheric ozone is not the only atmospheric pollutant known to Vary in
concentration between weekdays and weekends. Nitrogen dioxide, a known ozone
precursor, has also been show to Vary in accordance with the attenuation of rush-hour
t r a c . The oxidation of NOt resuits in the formation of nitrate aerosols. Nitrate aerosols
are capable of reflecting incoming solar radiation, reducing the amount of solar radiation
reaching the Earth's surface, therefore resulting in cooler air temperatures. As the
radiative effects of nitrate aerosols are dependent on the presence of solar radiation,
aerosol cooling is isolated to daylight hours (Preining 199 1; Chadson et al. 1992; Pemer
et al. 1994). Reductions in NOz on weekends, as a result of reduced trafic flow, colilci
therefore result in an increase in incoming solar radiation and therefore increased surface
temperatures.
Two separate methodologies have been undertaken in this thesis to attempt to
isolate the effects of variations in tropospheric ozone concentrations on air temperature.
These methodologies involve: 1) the cornparison of the themal effects of surnmer versus
winter ozone concentrations; and 2) the examination of periods of uncharactenstically high
ozone concentrations and their resultant thermal influence.
5.0-1 faadiative Effects of Summer Vs. Winter Ozone Concentrations
As tropospheric ozone is a photochexnical oxidant, concentrations are highly
dependent on relative sunlight intensity. During winter months, due to reduced sunlight
intensity, ozone concentrations are drarnaticaily reduccd. As the radiative effects of ozone
depend highly on the concentrations thereof within the atmosphere, reduced
concentrations durhg winter months should result in a reduced thermal effect.
Nitrogen dioxide however is not photochernical in nature. As this pollutant is not
afSected by reductions in solar intensity, its concentrations and resultant radiative effects
should remain relatively constant through both summer and winter months. Any
temperature perturbations resulting from weekday/weekend variations in ozone
concentrations should therefore be of p a t e r magnitude in summer months when
concentrations are at their highest.
The first step in the analysis of both ozone and temperature data involved assigning
the appropriate days of the week (Monday - Sunday) to each date within aich data set
(see Appendix B for method used to assign days of the week). Second, each data set was
separated into s u m e r and winter months. As the purpose of this study was to examine
the effects of ozone concentrations when at their highest and lowest, the penods of May 1
to September 30 and November 15 to March 15 were used to represent summer and
winter months respectively. These specific stratifications were used by Surnmers (1 996),
who successtùily identifled variations in sumrner vernis winter ozone concentrations in the
city of Toronto.
The summer and winter data sets were further subdivided by separating weekday
and weekend values. This stratification of data made it possible to compare weekday
versus weekend ozone concentrations and temperature values in both summer and winter
months. As al1 data sets consisted of hourly values, mean hourly ozone concentrations
and mean hourly temperature values were calculated for each hour (1 to 24) in each data
set. The mean hourly weekday (Monday to Fnday) values were then compareci with the
associated mean hourly weekend (Saturday and Sunday) values to determine whether any
weekdaylweekend variations were present .
A two-tailed independent sample t-test was used to determine the significance and
direction of the difference between each mean hourly weekday and rnean hourly weekend
value. In the case of ozone, the nuIl hypothesis tested was:
H, - No statistically significant difference exists between mean weekday and mean
weekend ozone concentrations.
Rejection of this null hypothesis would therefore lead to the acceptance of the alternative
hypothesis:
HI - A statistically significant daerence exists between mean weekday and mean
weekend ozone concentrations.
The level of sigdicance chosen for ali t-tests was 0.05, thus reducing the probability of
wrongly rejecting the nul1 hypothesis to 5 per cent.
Before the t-test could be applied to compare the mean hourly weekday and mean
hourly weekend ozone values, two requirements had to be met. First, the data set
represented by each mean value had to be normally distnbuted. Surnrnary statistics (mean,
standard deviation, etc.) were caicuiated for each hour of each data set (Appendix C).
Upon examination of these statistics al1 hourly ozone data sets were found to be positively
skewed.
The hi& degree of data skewness observed was the result of unusually Iow ozone
concentrations found throughout the ozone records. These periods of unusually low
ozone concentrations were likely the result of precipitation events. In the presence of
precipitation, ozone can be quickly washed out of the atmosphere, dramatically reducing
arnbient ozone concentrations (MOEE 1998). The inclusion of these uncharacteristically
low ozone concentrations resulted in a positively skewed distribution. While precipitation
records were not available for cornparison, these periods of dampened ozone
concentrations were present simuItaneously at al1 sites throughout the Greater Toronto
Area. The simultaneous occurrence of these periods of unusually Iow ozone
concentrations at more than one monitoring site ruled out instrument error as their cause.
In order to approximate a normal distribution, each data value was replaced by its
common log. As zeros could not be logged, al1 zero values were replaced with a
representative small value (O. 1) before logging commenced. The Iogging of each hourly
ozone data set resulted in a log-normal distribution, thereby meeting the first requirement
of the t-test.
The second requirement of the t-test involved the variances of the two data sets
under study. While equality of variances is not required, the magnitude of any difference
is important as it wiii determine which of two forrnulae will be applied to calculate the test
statistic. The relative equality of sample variances is determined by estimating the ratio of
the greater variance to that of the lesser (Shaw and Wheeler 1985). This provides a
variance ratio which is tenned the F-statistic:
where V' is the greater and Vz is the fesser of the two variances. The appropriate critical
value (Fc) to which this F-statistic is compared is obtained fiom an F-table using the
degrees of fieedom of each sample (degrees of freedom = rz-1, where n represents sample
size) .
The Fc value for large sarnples in excess of 120 values is 1.0. As the ratio of
greater to lesser variance will aiways equal or exceed 1 .O, the assumption of equal
variances for large sarnples must always be rejected. As dl ozone data sets, both sumrner
and winter, had sample sizes in excess of 120 values, the test statistic which does not rely
on equd variances was employed in al1 cases. The formula for this test statistic was:
where: X = mean of variable X r,, = sample size of variable X - Y = mean of variable Y 4 = sample size of variable Y V', = variance of X ~d = test statistic (mering variances) Pi. = variance of Y
The t-test was used in a similar manner to compare al1 mean hourly weekday and
weekend temperature values. The following hypotheses were tested for aII hourly
temperature data sets, both summer and winter:
IfL, - No statistically significant difference exists between mean weekday and mean
weekend air temperatures;
Hi - A statisticaily sigdicant difTerence exists between mean weekday and m m
weekend air temperatures.
Upon examination of temperature sumrnary statistics (Appendix D) aU hourly temperature
data sets were shown to be negatively skewed. In order to meet the t-test requirement of
a normal distribution aII temperature values were subtracted fiom a number greater than
the highest value (50), therefore changing the skewness fiom negative to positive. Each
resdting value was then replaced by its cornmon log, resulting in a log-normal
distribution.
As all sample sizes exceeded 120, the assumption of equai variances was once
again rejected for aIi data sets. The same test statistic formula (5.1) used with the ozone
data was therefore applied to determine whether a statistically significant difference
existed between rnean hourly weekday and mean hourly weekend temperatures.
The presence of weekday/weekend temperature perturbations in summer months
and the absence thereof in winter months will support the inference that weekday/weekend
variations in ozone concentrations affect air temperature. If weekdajdweekend
temperature perturbations are observed of similar magnitude in both summer and winter
seasons, variations in tropospheric ozone cm be ruled out as a cause.
5.0.2 hdiative Effects of Exceedence Vs. Non-Exceedence Ozone Concentrations
During summer months the Greater Toronto h e a is fkequently subjected to
periods of unusually high ozone concentrations. These high ozone events are often the
result of the incursion of a pollution laden high pressure system. In a fûrther attempt to
isolate the thermal effect of tropospheric ozone, periods of unusually high ozone
concentrations and associated weekday/weekend temperature variations were observed.
Weekday/weekend temperature perturbations during these high ozone events were
compared with periods where average ozone values prevaiied.
As rnentioned earlier, the a e c t of ozone on air temperature is highly dependent on
the concentrations thereof in the atmosphere. Periods of uncharacteristicaily high ozone
concentrations should therefore result in an enhanced greenhouse effect. Any temperature
perturbations resultuig from weekdaylweekend variations in ozone concentrations should
therefore be more pronounced when concentrations are at their highest.
The curent ambient air quality cnterion for ozone used by the Ontario Ministry of
Environment and Energy is 80 ppb. Days in which ozone concentrations equal or exceed
80 ppb at a number of measurement stations are considered 'episode days' (MOEE 1995).
This cnterion was used to isolate periods of unusudly high ozone concentrations within
the Greater Toronto Area-
Maximum ozone concentrations for each day of each ozone record were identified.
As episode days rarely occur in winter months, only summer months were examined.
Weeks in which the ozone criterion of 80 ppb was equaled or exceeded on at least one day
were separated fkom each ozone data set. These weeks were termed "exceedence weeks"
and represented periods of unusudly high ozone concentrations within the Greater
Toronto Area. The remaining weeks in each data set were termed "non-exceedence
weeksyy and represented periods where no unusually high ozone concentrations occurred.
Due to the varying nature of sources and sinks of ozone and its precursors within
the Greater Toronto Area, the number of exceedence weeks did not correspond between
each measurement site (Table 5.1). The identification of exceedence versus non-
Table 5.1 : Summer Exceedence Weeks
Measurement Site Number of Weeks Percentage of Weeks
Evans/ArnoId 80 RathbudCenterinid Pk- 123 PerthIRuskin 1 07 BayIGrosvenor (Univ./Hoskin) 14 Y onge/Finc h 44 LawrencdKemedy 110 Long Point 224
exceedence weeks was therefore undertaken separately for each measurernent station. The
corresponding exceedence weeks were extracted fiom each temperature data set, allowing
one to examine the effects of unusudy high ozone concentrations on air temperature.
Mean hourly ozone concentrations and mean hourly temperature values were
calculated for both exceedence and non-exceedence data sets, for ad measurement
stations. The mean hourly weekday values were once again compared with their
associated mean hourly weekend values using the two-tailed independent sample t-test.
The following hypotheses were tested:
H, - No Statisticaüy sigruficant dierence exists between mean weekday and mean
weekend ozone concentrations;
Hi - A statistically significant difference exists between mean weekday and mean
weekend ozone concentrations.
These hypotheses were applied to both exceedence and non-exceedence data sets. Upon
examination of exceedence and non-exceedence ozone summary statistics (Appendix E) al1
hourly ozone values were shown to be positively skewed. Each value was therefore
replaced by its cornmon log in order to approxhate a normal distribution.
As the number of exceedence weeks examined in the Bay/Grosvenor and
Yonge/Finch data sets was relatively low, the sample sizes for these data sets did not
exceed 120. Therefore, based on the cornparison of the F-statistic with the associated Fc
value, the assumption of equal variances was accepted for a number of mean hourly values
(Bay/Grosvenor: Hrs 1-7, 9- 1 1, 15, 18, and 24, Yonge/Finch: 2, 1 1, 12, 15, 16, 18, and
20-24). These hourly data sets (weekday vs. weekend) were therefore compared using the
t. test statistic formula which requires equal variances:
where: = mean of variable X rz, = sarnple size of variable X - Y = mean of variable Y n, = sample size of variable Y Vx = variance ofX & = test statistic (equal variances) V,, = variance of Y
The remaining hourly data sets which fded to meet the assumption of equal variances
were compared using the td test statistic formula (5.2) discussed in section 5 -0.1.
Fhally, the following hypotheses were tested for all hourly temperature data sets,
both exceedence and non-exceedence:
H,, - No Statistically significant dserence exists between mean weekday and mean
weekend air temperatures;
Hl - A statistically significant difference exists between mean weekday and mean
weekend air temperatures.
Upon examination of exceedence and non-exceedence temperature summary statistics
(Appendix F), all hourly temperature values were show to be negatively skewed. Al1
temperature values were therefore subtracted from a high value (50) and replaced by their
common logs, therefore approximating a normal distribution.
As no corresponding temperature site was available for the YongeEinch ozone
site, onl y hourly t emperature data sets from the University/Hoskin (Bay/Grosvenor)
measurement station had sarnple sizes under 120 and therefore met the assumption of
equal variances. The te test statistic formula was used to compare al1 twenty four mean
hourly weekday/weekend values for the University/Hoskin exceedence data set. The
remainder of mean hourly temperature values for ail other exceedence and non-
exceedence sites, which failed to meet the assumption of equal variances, were compared
using the ld test statistic formula (5.2).
The presence of weekday/weekend temperature perturbations in exceedence weeks
and the absence thereof in non-exceedence weeks wiU fùrther support the inference that
weekday/weekend variations in ozone concentrations affect air temperature. The presence
of weekday/weekend temperature perturbations of similar magnitude in both exceedence
and non-exceedence weeks will rule out the effect that tropospheric ozone concentrations
have on air temperature.
CaAPTER 6
RESULTS AND DISCUSSION
6.0 Statistical Analvsis of Weekdavmeekend Ozone and Temperature Variations
Before the results of the analyses of both ozone and temperature data c m be
discussed, the methods used to present mean hourly values and their differences should be
explained. Mean hourly ozone concentrations and temperature values were determined
for weekdays and weekends when comparing both summer vs. winter and exceedence vs.
non-exceedence penods. The hourly values, both ozone and temperature, recorded by the
Ontario Muiistry of Environment and Energy, represent mean values within each pmicular
hour. Each hourly value represents the average value recorded during a particular hour
(e.g. hour 1 = mean value recorded nom niidnight to 1:00 am).
The student's t-test was used to compare al1 mean hourly weekday to weekend
values, both ozone and temperature. The critical t-statistic of 1.96 was determined to
represent the 0.05 significance level. Any t-statistic value which equaled or exceeded this
critical t-statistic represented a statisticdy sign5cant diierence between mean hourly
weekday and mean hourly weekend values. The direction of the weekday/weekend
variations observed was determined by the sign of the t-statistic. A positive t-statistic lead
to the inference that weekend values were higher than associated weekday values. A
negative t-statistic lead to the inference that weekend values were lower than associated
weekday values.
As a t-statistic value of 0.0 would represent no weekdaylweekend variation and a
value of t1.96 would represent a significant weekdaylweekend variation, cornparison of
the t-statistic values was used to determine the relative magnitude of hourly
weekday/weekend variations of both ozone and temperature within each data set. It is
important to note, however, that cornparison of t-statistic values between data sets ( e g
S u m e r Vs. Wmter) was not permitted due to varying sample sizes. The actual t-statistic
values calculated for surnrner vs. winter and exceedence vs. non-exceedence periods are
presented in appendices G and H respectively.
6.1 Weekdav/Weekend Variations in Ozone Concentrations (Summer Vs. Winter)
Due to the photochernical nature of tropospheric ozone it had been suggested that
concentrations thereof will be higher in summer than in winter months. The identification
of this summer/winter stratification, in addition to weekday/weekend variations, should
help isolate the thermal effects of troposphenc ozone.
To provide a preliminary examination of variations in summer versus winter ozone
concentrations in the Greater Toronto Area, mean hourly weekday and mean hourly
weekend ozone concentrations were compared for both summer and winter months
(Figures 6.1 - 6.7). Actual mean hourly values are presented in Appendix C . As
expected, due to reduced intensity of solar radiation d u ~ g winter months, winter ozone
concentrations were depressed when compared with summer concentrations at al1
rneasurement stations.
Maximum ozone concentrations were observed between hours 14 and 15 on both
weekdays and weekends at al1 Greater Toronto Area rneasurement stations. The timing of
minimum ozone concentrations, however, varied between weekdays and weekends.
Minimum weekday ozone concentrations occurred in hour 7 and hours 8-9 during summer
and winter months respectively. Due to their strong dependence on the timing of sunrise,
minimum ozone concentrations were likely observed an hour later during winter as
opposed to summer months due to the adjustrnent of daylight savings time and resulting
later sunrise. The Iatest minimum ozone concentration was obsei?red at the
Bay/Grosvenor measurement station at hour 9. This minimum ozone lag could be the
result of t d buildings in the downtown core inhibiting sunlight fiorn reaching the
Bay/Grosvenor measurement site. In addition, of al1 the Greater Toronto Area
measurement stations, this site was likely the last to encounter rush-hour traEc.
Minimum weekend ozone concentrations, both summer and winter, were observed
during hour 1 at al1 but one rneasurement site. The differing times of minimum ozone
concentrations between weekdays and weekends is Iikely a product of early momuig
ozone scavenging by rush-hour tra£Ec. On weekdays, not only have ozone concentrations
been gradually declining throughout the night, but NO emissions from rush-hour traffic
drarnaticaffy reduce early morning concentrations. Ozone concentrations do not begh to
Figure 6.1: Evans/Arnold Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1979 to 1995 (Surnmer Vs. Winter)
- 1
j Summer 40.0 -f
i
Hour
t -
t Weekday - - - - - - Weekend ,
Source: Ontario Ministry o f Environment and Energy 1997
Figure 6.2: RathbudCentenniai Park Mean Hourly Weekday and Weekend Ozone Concentrations for the Penod 1979 to 1995 (Summer Vs. Winter)
Eour
Winter 40.0
Hour
I Weekchy - - - - - - Weekend ,
Source: Ontario Ministry o f Environment and Energy 1997
Figure 6.3: PerthlRuskin Mean Hourly Weekday and Weekend Ozone Concentrations for the Penod 198 1 to 1994 ( S u m e r Vs. Witer)
Surnrner
I
Eour
1
0.0 ' I
t 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
, - Weekday - - - - - - Weekend l
Source: Ontario Ministry of Environment and Energy 1997
Figre 6.4: BayIGrosvenor Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1990 to 1995 (Summer Vs. Winter)
i Winter 40.0 +
Hour
Weekdaj- - - - - - - Weekend
Source: Ontario Ministry o f Environment and Energy 1997
Figure 6.5: Yonge/FUich Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1988 to 1995 (Sumer Vs. Winter)
1 2 3 4 5 6 7 8 9 I O 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
Winter
0.0 1
1 2 3 4 5 6 7 8 9 I O 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
Source: Ontario Ministry of Environment and lnergy 1997
Figure 6.6: Lawrence/Kemedy Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1979 to 1995 (Summer Vs. Winter)
? Summer
Hour
i T
Winter 30.0 7
Hour
, - Weekday - - - - - - Weekend
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.7: Long Point Mean HourIy Weekday and Weekend Ozone Concentrations for the Penod 1984 to 1995 (Summer Vs. Wmter)
Summer
Hou r
Hour
I - Weekday - - - - - Weekend 1
Source: Ontario Ministry of Environment and Energy 1997
rise until sunrise occurs and photochernical reactions begin to take place. On weekends
however, no early moming rush-hour scavenging takes place. Minimum weekend
concentrations are therefore restricted to those at night as ozone concentrations gradually
decline.
Minimum weekend ozone concentrations were shown to occur at hour 6 at
Evans/Aniold and not hour 1 like the rest of the Greater Toronto Area sites. The
proximity of this measurement site to the Gardiner Expressway is likely the cause of this
anomalous result. As the Gardiner Expressway is a major transportation route within the
Greater Toronto Area, early moming traffic may occur on both weekdays and weekends
adjacent to this rneasurement station. It should be noted, however, that the weekday
ozone minimum of 4.3 ppb was considerably less than the weekend ozone minimum of
10.4 ppb, therefore suggesting heavier weekday traffic.
As sunlight is required for ozone concentrations to increase, one rnight expect
ozone concentrations to continually decline up until sunrise and minimum weekend ozone
concentrations to coincide in t h e , if not in magnitude with weekday minimums. Figures
6.1 to 6.6, however, show a gradud increase in ozone concentrations just &er midnight.
This gradual increase in ozone concentrations c m be attributed to the urban heat island
effect of major metropolitan areas. As rural areas cool faster than cities, air tends to rise
above the warmer urban area, resulting in a rural to urban breeze. This rural to urban
breeze, referred to as a 'country breeze', results in the turbulent mWng of the atmosphere
(Ahrens 1991). This turbulent mWng can result in the downward transport of upper level
ozone, replenishing ozone scavenged by nitric oxide. As nighttime progresses and urban
temperatures fan, the turbulent mixing and resulting downward flow of ozone dirninishes
and ozone removal once again begins to take precedence (Mukarnmal et. al. 1985).
The maximum difference between weekday and weekend ozone concentrations
during summer months occurred at hour 8 at all Greater Toronto Area sites. The
maximum difference between weekday and weekend concentrations during winter months
occurred at hour 9. The timing of the maximum ciifference between weekday and
weekend ozone concentrations was likely a resdt of early moming rush-hour traffic.
Before sumise, auto emissions break down ozone present in the atmosphere, resulting in
depressed weekday concentrations. Mer s u ~ s e , however, the presence of NO2 in the
atmosphere resdts in ozone production. The maximum difference between weekday and
weekend ozone concentrations occurred during the transition period from ozone
scavenging to ozone production. The largest diffierence between mean hourly weekday
and weekend ozone concentrations (1 0.1 ppb) was observed at the YongdFinch
measurement site (Figure 6.5). This measurement site may be highly athrned to variations
in rush-hour traific due to its proximity to a large commuter parking lot (see section 4.1).
Due to the lack of early rnorning rush-hour tr&c at the control site of Long Point,
weekday and weekend ozone concentrations appeared to be relatively similar in both
summer and winter months. NI minimum ozone concentrations at Long Point occurred
between hours 5 and 6. Unlike Toronto, Long Point ozone concentrations, both weekday
and weekend, were able to gradually diminish throughout the night until sunrise.
Surnmer weekday/weekend variations in ozone concentrations were shown to be
statisîicaliy significant at al1 Greater Toronto Area sites from hours 6 to 23 (Figures 6.8 -
6.14) (Appendix 1 shows the relationship between mean hourly ozone and t-statistic
graphs). Statistically significant weekday/weekend ozone variations occurred at hour 5 at
the Evans/ArnoId (Figure 6.8), RathbudCentennial Pk. (Figure 6.9), YongdFinch
(Figure 6-12), and Lawrence/Kemedy (Figure 6.13) measurement sites. These sites, being
located on the outskirts of the city of Toronto, would be the first to encounter rush-hour
trafic heading into the city centre and would therefore be susceptible to ozone scavenging
earlier in the morning than at central city sites. A statistically significant
weekday/weekend variation in ozone concentrations was not observed until hour 6 at both
the PerthRuskin (Figure 6.10) and Bay/Grosvenor (Figure 6. L 1) measurement sites.
Statistically sigmfïcant weekday/weekend ozone variations were shown to occur in
winter as well as summer months. Significant weekday/weekend variations appeared to
occur one hour behind (hr 7 vs. hr 6) summer variations due to later s u ~ s e dunng winter
mont hs.
Weekday ozone concentrations were show to be signifïcantly higher than
weekend concentrations during hours 1 to 3 at the Pertb/Ruskin, Bay/Grosvenor, and
Lawrence/Kemedy measurement sites. These low nighttime weekend ozone values could
Figure 6.8: T-Statistics Representing Hourly Weekday/Weekend Ozone Variations for the EvandArnold Measurement Station (Summer Vs. Winter)
25.00 , Summer
Winter
Figure 6.9: T-Statistics Representing Hourly Weekdaymeekend Ozone Variations for the RathburniCentennial Park Measurement Station ( S u m e r Vs. Winter)
-y---- ---- - - -
Summer
Winter
Figure 6.1 1 : T-Statistics Representùig Hourly Weekday/Weekend Ozone Variations for the Bay/Growenor Measurernent Station ( S u m e r Vs. Wmter)
Hour
Winter
L . . . - - - . - - - . .
1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 IO Il 22 23 24
Hour
Figure 6.12: T-Statistics Representing HourIy Weekday/Weekend Ozone Variations for the YongelFinch Measurement Station (Surnrner Vs. Whter)
-5.00 --- --
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 12 U 21
Hour
Winter
1 2 3 4 5 6 7 8 9 10 I l 12 13 14 15 16 17 18 19 20 21 2 23 24
Hour
Figure 6.13 : T-Statistics Representing Hourly WeekdayMreekend Ozone Variations for the LawrenceKemedy Measurement Station (Summer Vs. W-îter)
be the result of increased evening and nightîirne tr&c flow on weekends as opposed to
weekdays. As ozone cannot be produced after sunset, any auto emissions released in the
evening or at night would act solely as ozone scavengers. Ifeveninç and nighttime t r a c
volumes were higher on weekends as opposed to weekdays, this increased nighttime
scavenging would result in lower weekend ozone concentrations. These occurrences were
most significant at inner city sites (Bay/Grosvenor, PertMZuskin) where one would expect
evening and nighttime trafic flow to be highest.
The magnitude of weekday/weekend variations in ozone concentrations durin3
surnmer months appeared greatest between hours 7 and 8 at ail Greater Toronto Area
sites. The magnitude of weekday/weekend variations gradually declined until hour 15. At
this time, aflemoon rush-hour began, depressing weekday ozone concentrations and
therefore increasing the dserence between weekday and weekend ozone values. The
winter weekday/weekend ozone variations were shown to follow a sirnilar pattern to those
during sumrner months.
During sumrner months at Long Point, no statistically significant
weekdaylweekend ozone variations were observed (Figure 6.14). During winter months,
however, weekend ozone concentrations were s hown to be significantly higher than
weekday concentrations during hours 1 1 to 24. As Long Point is relatively isolated fiom
any local urban influence, the rnajorïty of pollution recorded at this measurement station is
the result of long-range transport. Ozone concentrations at Long Point are regularly the
highest in Ontario due to the incursion of pollution laden weather systerns frorn cities such
as Chicago, Detroit, and Cleveland (MOEE 1995).
Due to the lack of any local ozone precursor emissions at Long Point, local ozone
concentrations should be highly susceptible to variations in concentrations of long-range
poliutants. Weekdajrlweekend variations in ozone concentrations observed at Long Point
were therefore Wtely the result of the incursion of poliutants from a source where urban-
induced weekday/weekend variations occur. The isolation of these ozone variations to
winter months could be the result of increased winter use of coai-fired power plants, and
resulting NO, emissions, from large metropolitan areas to the south and southwest.
6.2 WeekdayMreekend Variations in Air Temperature (Suminer Vs. Winterl
To provide a preliminary examination of weekday/weekend temperature variations
in summer versus winter months, mean hourly weekday and mean hourly weekend
temperature values for both seasons were compared (Figures 6.15 - 6.18). Actual mean
hourly temperature values are presented in Appendix D.
Weekday/weekend mean hourly temperature values were shown to undergo
expected diumal variations in both surnmer and winter months. Weekend mean hourly
temperatures appeared to be slightly lower than associated weekday temperatures at the
Evan JAmold, Universityhioskin, and LawrenceKennedy measurement sites during
winter months. During summer monthq weekend mean hourly temperatures appeared
slightly iower than weekday temperatures at the Evans/Amold and University/Hoskin
measurement sites. At the Long Point measurement site no weekday/weekend variation in
temperature was observed.
The statistical significance of these weekday/weekend variations in air temperature
is displayed in Figures 6.19 to 6.22. No weekday/weekend temperature variations were
signifïcant at the 95 per cent confidence level. While not statistically significant, mean
hourly weekday temperatures appeared to be higher than associated weekend
temperatures at the Evans/Aniold and University/Hoskin sites during both summer and
winter months. However, while a srna11 weekday/weekend effect appeared to be present at
the LawrenceKennedy measurement site during winter months, no observable
weekday/weekend trends occurred during summer months. As ozone concentrations have
been shown to increase on weekends versus weekdays at al1 three Greater Toronto
Measurement sites (Figures 6.1, 6.4 and 6.6), the observed weekday/weekend temperature
variations suggest the possible presence of an additional temperature mechanism
independent of ozone, acting on a weekly cycle.
6.3 WeekdaylWeekend Variations in Ozone Concentrations (Exceedence Vs. Non- Exceedence)
Weekday/weekend variations in ozone concentrations were examined for both
exceedence and non-exceedence weeks to determine whether penods of
Figure 6.15: EvandArnold Mean Hourly Weekday and Weekend Air Temperatures for the Period 1979 to 1995 ( S u m e r Vs. Winter)
22.0 1 Summer
Hour
I Winter 0.0 ;
Hour
. - Weekday - - - - - - Weekend j 1
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.16: University/Hoskin Mean Hourly Weekday and Weekend Air Temperatures for the Period 1990 to 1995 (Summer Vs. Winter)
Summer
Hour
i Winter
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.17: Lawrence/Kemedy Mean Hourly Weekday and Weekend Air Temperatures for the Period 1979 to 1995 (Summer Vs. Winter)
Summer
--- -
1 1 i l Winter
Eour
, - ! Weekday - - - - - - Weekend
- - - - - -- - - --
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.18: Long Point Mean Hourly Weekday and Weekend Air Temperatures for the Period 1984 to 1995 (Summer Vs. Winter)
Summer
1 2 3 4 5 6 7 8 9 IO 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
Winter - -
Eour
1 - Week&y - - - - - - Weekend i
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.19: T-Statistics Representing Hourly Weekday/Weekend Air Temperature Variations for the Evans/Arnold Measurement Station (Surnmer Vs. Winter)
Summer - - * - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - * - - - * - *
3.00 7 ...
, Winter 1
I z . o o + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
!
Fi,gre 66.1: T-Statistics Representing Hourly Weekday/Weekend Air Temperature Variations for the LawrenrdKennedy Meastirernent Station (Surnmer Vs. Wmter)
Hoar
uncharactensticaily high ozone concentrations result in an enhanced thermal effect. Any
weekday/weekend temperature variations resulting fiom variations in ozone
concentrations should be most pronounced when ozone concentrations are at their
highest .
Weekday/weekend variations in hourly ozone concentrations in exceedence versus
non-exceedence weeks are presented graphically in Figures 6.23 to 6.29. Actual mean
hourly ozone concentrations are presented in Appendix E. Ozone concentrations were
shown to be considerably higher on both weekdays and weekends dunnj exceedence vs.
non-exceedence weeks. The weekday rush-hour minùnum was observed at all Greater
Toronto Area sites. The lack of an associated weekday rush-hour minimum at the Long
Point control site once again illustrates the effect of anthropogenic activity on the diurnal
variation of ozone concentrations.
During exceedence weeks, more ozone is scavenged by early momùig rush-hour
than in non-exceedence weeks. While the time available for early morning rush-hour
tr&c to scavenge ozone is the sarne for both exceedence and non-exceedence weeks
(hour 4 to sumise), the amount of ozone available to be scavenged is greater. The
increased early morning ozone scavenging by rush-hour traffic during exceedence weeks
leads to an increase in the magnitude of the difFerence between weekday and weekend
concentrations (Table 6.1).
The statistical significance of weekday and weekend variations in mean hourly
ozone concentrations in both exceedence and non-exceedence weeks is displayed in
Figures 6.3 0 to 6.36. Statistically significant weekday/weekend ozone variations were
observed at al1 Greater Toronto Area measurement sites.
During exceedence events, no statistically significant weekday/weekend ozone
variations were observed during hours 1 to 3 at any Greater Toronto Area site.
Statistically significant weekday/weekend ozone variations were observed at hours 1 and 2
at al1 Greater Toronto Area measurement stations dunng non-exceedence events. When
mean surnmer ozone concentrations were examined for al1 weeks (section 6.1), weekday
concentrations were shown to be significantly higher than weekend concentrations during
hours 1 to 3 at several Greater Toronto Area measurement sites. This result lends support
Figure 6.23: Evans/Arnold Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1979 to 1995 (Exceedence Vs. Non-Exceedence)
i Exceedence Weeks
r 1 - -
C - \ - -
i
Hour
+ 1 Non-Esceedence Weeks 50.0 -
I
Hour
1 - j Weekday - - - - - - Weekend
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.24: Rathburn/Cente~iai Park Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1979 to L 995 (Exceedence Vs. Non-Exceedence)
Hour
Non-Esceedence Weeks 50.0
i
Eour
; - Weekday - - - - - - Weekend i
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.25: Perth/Ruskin Mean Hourly Weekday and Weekend Ozone Concentrations for the Penod 2 98 1 to 1994 (Exceedence Vs. Non-Exceedence)
I
Non-Esceedence Weeks 50.0 i
0.0 T . . . I
1 2 3 -1 5 6 7 8 9 10 I I 22 13 1 4 1 5 16 17 18 1 9 2 0 2 1 2 2 2 3 23
Hour
1- Weekday - - - - - - Weekend ! I
Source: Ontario Ministq of Environment and Energy 1997
Figure 6.26: Bay/Grosvenor Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1990 to 1995 (Exceedence Vs. Non-Exceedence)
l - l Weekdq - - - - - - Weekend 1
- -- - - - -
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.27: YongGinch Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1988 to 1995 (Exceedence Vs. Non-Exceedence)
1 2 3 4 5 6 7 8 9 IO 1 1 12 13 1 4 1 5 16 17 18 1 9 2 0 2 1 22 2 3 2 4
Hou r
j Non-Esceedence Weeks 50.0 -
1
Hour
1 - I Weekday - - - - - - Weekend !
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.28: Lawrence/Ke~edy Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1979 to 1995 (Exceedence Vs. Non-Exceedence)
*
i Non-Exceedence Weeks
50-0 i
Hour
! - Weekday - - - - - - Weekend ;
Source: Ontario Muiistry of Environment and Energy 1997
Figure 6.29: Long Point Mean Hourly Weekday and Weekend Ozone Concentrations for the Period 1984 to 1995 (Exceedence Vs. Non-Exceedence)
Exceedence 1
Hour
60.0
1 i Non-Esceedence Weeks
50.0
Hour
I 1 - Weekday - - - - - - Weekend /
1
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.20: T-Statistics Representing Hourly Weekdaymeekend Ozone Variations for the EvdArnold Measurement Station (Exceedence Vs. Non-Exceedence)
--
Esceecience Weeks
. .
Non-Esceedence Weeks
Figure 6.3 1 : T-Statistics Representing Hourly Weekday/Weekend Ozone Variations for the RathburnICentenriial Park Measurement Station (Exceedence Vs, Non-Exceedence)
Esceedence Weeks
Rour
- -
Non-Esceedence Weeks
Figure 6.32: T-Statistics Representing Hourly WeekdaylWeekend Ozone Variations for the PerWRuskin Measurement Station (Exceedence Vs. Non-Exceedence)
Exceedence Weeks
Hour
. . -
Non-Esceedence Weeks
Hour
Figure 6.33: T-Statiçtics Representing Hourly WeekdaylWeekend Ozone Variations for the Bay/Grosvenor Measurement Station (Exceedence Vs. Non-Exceedence)
--
Esceedence Weeks
1.- . - ---- - -- I-____I _ .. .
1 1 3 4 5 6 7 g 9 10 11 12 1; 14 15 16 17 18 19 20 21 3 14
Honr
T Non-Esceedence Weeks
Figure 6.34: T-Statistics Representing Hourly WeekdayNeekend Ozone Variations for the YongelFinch Measurement Station (Exceedence Vs. Non-Exceedence)
. . . . . . . . . - . . - . . .. . . . . . . . . . .-- - . - . -. -. . . .. . - . - - . .. * *. ---.- -.. .--.- ...-.. . . -. .
Exceedence Weeks
Hour
T --A- - - -
Non-Esceedence Weeks
1 --
1 2 5 4 5 6 7 8 9 10 I I 12 13 14 15 16 17 18 19 10 LI 22 Li 24
Hour
Figure 6.35: T-Statistics Representing Hourly WeekdayAVeekend Ozone Variations for the LamenceKennedy Measurement Station (Exceedence Vs. Non-Exceedence)
1 Esceedence Weeks
T- ._ _ _ _ _ _ - - _ _- - __ - _ - . _ _. _ I__ __ _ __ -_ _. -- --
Non-Esceedence Weeks
Figure 6.36: T-StatistÏcs Representing Hourly WeekdayIWeekend Ozone Variations for the Long Point Measurement Station (Exceedence Vs. Non-Exceedence)
...................................................................... ..-.. - ....................................
Esceedence Weeks
to the inference that nighmme urban tr&c can reduce ambient ozone concentrations.
During exceedence events, when ozone concentrations are uncharacteristically hi& the
ozone scavenging effect of increased weekend nighttime traffic is not enough to
significantly reduce weekend ozone concentrations. However, during non-exceedence
events, a comparable release of auto emissions can result in a larger percentage reduction
in ozone concentrations,
Table 6.1 : Maximum Difference Between Weekday and Weekend Mean Hourly Ozone Concentrations During Exceedence Vs. Non-Exceedence Weeks (Weekend minus Weekday)
Maximum Difference (ppb) Maximum Difference (ppb) Measurement Site Exceedence Weeks Non-Exceedence Weeks
At the Long Point control site, statisticdy significant weekday/weekend variations
in ozone concentrations were observed at hours 1 and 3 (Figure 6.36). Unlike associated
urban variations, weekend concentrations at Long Point were shown to be significantly
higher than weekday concentrations during these two hours. While not statistically
sipifkant at the 95 per cent level, t-statistic values representing weekday/weekend
variations dunng hours 1 to 3 for the Evans/Arnold (Figure 6-30), RathbumKentennial
Park (Figure 6.3 1 ), Yongfinch (6.34), and LawrencdKemedy (Figure 6.3 5)
rneasurement stations changed fiom negative (weekday higher than weekend) to positive
(weekend higher than weekday) when exceedence weeks were examined. As ozone
concentrations at Long Point were consistently higher than those of the Greater Toronto
Area, the change in eady hour t-statistic values fiom negative to positive at this station
was of greater magnitude and likely resulted in t-statistic values reaching the 1.96
signxcance level. The statisticdy significant weekday/weekend ozone variations
observed at the Long Point measurement station were likely a result of the long-range
transport of pollutants from upwind metropolitan areas.
6.4 WeekdavAVeekend Variations in Air Temperature (Exceedence Vs. Non- Exceedence)
Mean hourly weekday and weekend temperature values for both exceedence and
non-exceedence weeks are presented in Figures 6.37 to 6.40. Actual mean hourly
temperature values are presented in Appendix F. At al1 three Greater Toronto Area sites,
weekend temperatures appeared higher than weekday ternperatures during exceedence
weeks. During non-exceedence weeks, mean hourly weekend ternperatures appeared
lower than weekday temperatures at ail three Greater Toronto Area sites (Table 6.2).
Table 6.2: Maximum Ditference Between Weekday and Weekend Mean Hourly Air Temperatures During Exceedence Vs. Non-Exceedence Weeks (Weekend minus Weekday)
Maximum Difference ( O C ) Maximum DiEerence (OC) Measurement Site Exceedence Weeks Non-Exceedence Weeks
The statisticai significance of these weekday/weekend variations in air temperature
are displayed in Fi y e s 6.41 to 6.44. Statistically signdicant weekday/weekend variations
in air temperature were observed at ail three Greater Toronto Area measurement sites.
No statisticaily significant weekday/weekend ternperature variations were observed at the
Long Point control site. At the University/Hoskin and LawrenceKennedy sites,
significant temperature variations were obsenred during the early hours of 1 to 5 and 1 to
Figure 6.37: EvandArnold Mean Hourly Weekday and Weekend Air Temperatures for the Period 1979 to 1995 (Exceedence Vs. Non-Exceedence)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
Hour
f < - l Weekday - - - - - Weekend i
--
Source: Ontario Miniary of Environment and Energy 1997
Figure 6.3 8: University/Hoskin Mean Hourly Weekday and Weekend Air Temperatures for the Period 1990 to 1995 (Exceedence Vs. Non-Exceedence)
Hour
, - ! Weekday - - - - - - Weekend i
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.39: Lawrence/Kennedy Mean Hourly Weekday and Weekend Air Temperatures for the Period 1979 to 1995 (Exceedence Vs. Non-Exceedence)
Eour l
1 2 3 4 5 6 7 8 9 IO 11 12 13 13 15 16 17 18 19 20 21 22 23 24 1
I Hour I
i - Weekday - - - - - - Weekend ; I
i I
Source: Ontario Ministry of Environment and Energy 1997
Figure 6.40: Long Point Mean Hourly Weekday and Weekend Air Temperatures for the Period 1984 to 1995 (Exceedence Vs. Non-Exceedence)
1 2 3 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Eour
Non-Esceedence Weeks 20.0 1
Hou r
r ! - Weekday - - - - - - Weekend i
Source: Ontario Miniary of Environment and Energy 1997
Figure 6.4 1 : T-Statistics Representing Hourly Weekday/Weekend Air Temperature Variations for the EvandAmold Measurement Stn. (Exceedence Vs. Non-Exceedence)
4-00
Exceedence Weeks 3.00
2.00
1.00
0.00
1 Non-Escedence Weeks 1 3.00 T
Figure 6.42: T-Statistics Representing Hourly WeekdayANeekend Air Temperature Variations for the UniversitylHoskin Station (Exceedence Vs. Non-Exceedence)
4.00 --
Esceedence Weeks 3.00 T
. .
Non-Esceedence Weeks
Figure 6.43: T-Statistics Representing Hourly Weekday/Weekend Air Temperature Variations for the Lawrence/Kemedy Station (Exceedence Vs. Non-Exceedence)
I I Escdence Weeks
I
4.00 L 1 1 3 4 5 6 7 8 9 10 11 11 13 I4 15 16 I7 18 19 1 0 1 1 2 2 2 3 24
Hour
Figure 6 -44: T-S tatistics Representing Hourly Weekdaymeekend Air Temperature Variations for the Long Point Measurement Stn. (Exceedence Vs. Non-Exceedence)
l 4-00 ,- - - . -
1
I Esceedence Weeks 3.00 -,-
I
-4.00 L - - - - . . . 1 2 3 4 5 6 7 8 9 10 11 1 1 1 3 14 15 16 1 7 18 19 10 1 1 22 23 24
Honr
6, respectively. Significant temperature variations were observed at hours 1 to 10 and 2 1
to 24 at the EvandAmold site. The duration ofweekday/weekend variations in air
temperature was more pronounced at the outer measurement sites (Evans/Arnold,
Lawrence/Kemedy) than of the downtown measurement site (University/Hoskin). In
accordance, the duration of weekday/weekend variations in ozone concentrations was
most pronounced at the two outlying sites. Significant weekday/weekend variations in
ozone concentration occurred between hours 3 and 24 at the EvandArnold (Figure 6.4 1)
and Lawrence/Kennedy (Figure 6.43) measurement sites and hours 5 and 24 at the
University/Hoskin (Figure 6.42) site during exceedence weeks. The reduced duration of
significant weekday/weekend temperature variations at the downtown site was likely the
result of the reduced duration of significant weekday/weekend ozone variations.
At al1 three measurement stations the most significant weekday/weekend
temperature variations were observed between hours 1 and 3 . The isolation of significant
temperature variations to early and late hours and therefore non-photochemicdly active
penods was Likely the result of a lagged radiation release by local tropospheric ozone.
During the day when temperatures are high, ozone absorbs and re-emits radiation at
relatively the same energy ievel. As air temperatures decrease, while ozone absorbs
radiation of the same intensity, the re-emitted radiation is of a lower intensity (see section
2.1) As the radiation ernitted is of a lower intensity than that absorbed, it takes longer to
re-emit the absorbed energy, thus producing a lagged temperature response. In addition,
the absorption and retention of radiation by tropospheric ozone tends to warm the
surrounding atmosphere. Therefore, as air temperatures begin to cool after sunset, the
radiative abilities of ozone become more pronounced.
It is interesting to note that when al1 periods of uncharacteristicaily high ozone
were removed, the resuiting data sets (non-exceedence weeks) exhibited
weekday/weekend temperature variations opposite in sign (weekday higher than weekend)
to those observed during exceedence events. While only significant at hours 7 and 8 at the
Evans/Arnold site, weekday temperatures appeared to be higher than weekend
temperatures during non-exceedence weeks. Similar weekdaylweekend variations in
temperature have been attributed to reduced anthropogenic activity on weekends (Mitchell
196 1; Gordon 1991; Lenschow 1994).
The weekdaylweekend variations during non-exceedence weeks observed in the
Greater Toronto Area appeared to increase in magnitude £iom hours 3 to 7 at the
Evans/Arnold and LawrenceKemedy sites. At the downtown measurement site
(University/Hoskin), where the greatest difference between weekday and weekend activity
would be expected, non-exceedence weekday/weekend variations increased in magnitude
fiom hours 3 to 12- While these non-exceedence weekday/weekend variations were not
statistically significant, they do suggest the presence of a rnechanisrn that depresses
weekend temperatures. The inclusion of this effect when examining exceedence weeks
was likely the result of the reduced magnitude of weekdaylweekend variations from the
hours 3 onward observed at ail Greater Toronto Area sites.
6.5 Linking Observed Ozone and Air Temperature Variations
The radiative forcing associated with specific variations in tropospheric ozone
concentrations has been studied on a number of occasions. The majonîy of these studies
concentrate on variations in ozone concentrations on global or hemispheric scales. Forster
et al. (1996) estimated the northern hemispheric radiative forcing associated with
increases in tropospheric ozone concentrations since pre-industrial tirnes to be 0 -6 1 ~ r n - ~ .
Hansen et al. (1997) suggested a doubhg of present day tropospheric ozone
concentrations would result in an increase in surface temperature of O S OC.
While such large-scde studies are significant, it is dficult to apply their results to
small scale ozone variations, such as those observed in the Greater Toronto Area. The
high ozone concentrations encountsred, and short time scales over which they c m Vary,
over large metropolitan areas are considerably different fiom the conditions under which
large-scale radiative-transfer models are implemented.
In addition, the majority of models used to determine the thermal effect of
variations in ozone concentrations consider only changes in total column ozone. As was
mentioned in section 2.1, the radiative forcing associated with variations in ozone
concentrations is highly dependent on altitude and therefore surrounding air temperature.
The radiative effects of troposphenc ozone are most pronounced at the tropopause, where
atmosphenc temperatures are at their coolest. As was observed in Figures 6.41 to 6.43,
the weekend versus weekday temperature increases were most pronounced during eariy
hours of the moming, when ambient air temperatures were at their lowest. Failure to
incorporate such air temperature variations when attempting to determine the thermal
effect of variations in ozone concentrations would likeIy result in an underestirnation of
their actual thermal influence.
Therefore, although weekday/weekend air temperature variations were isolated to
periods of uncharacteristicaily high ozone in the Greater Toronto Area, due to the
inapplicability of existing radiative-transfer mode1 results to small scale ozone variations, a
direct cause-and-effect relationship between obsewed ozone and temperature variations
could not be made. Further study is required to determine whether observed
weekday/weekend temperature variations were solely the result of variations in
tropospheric ozone concentrations or whether additional atmospheric constituents were
involved. The lack of any significant weekday/weekend air temperature variations at the
Long Point control site does however support the inference that the observed thermal
variations were a product of anthropogenic activity.
CfIAPTER 7
CONCLUSIONS AND RECOMMENDATIONS
7.0 Conclusions
The pnrnary airn of this thesis was to determine whether weekday/weekend ozone
variations result in weekday/weekend variations in air temperature. Both ozone and
temperature data were obtained fiom several Greater Toronto Area measurement stations.
These data were used to determine whether weekday/weekend variations in troposphenc
ozone concentrations result in weekday/weekend temperature variations. As a number of
atmospheric pollutants are known to Vary in concentration between weekdays and
weekends, two separate methodologies were used in an attempt to isolate the effects of
tropospheric ozone variations on temperature.
The first method employed involved the c o m p ~ s o n of weekday/weekend ozone
variations and associated temperature perturbations in both summer and winter months.
As ozone concentrations have been shown to be highest dunng summer months, any
associated weekday/weekend temperature variations should be most pronounced during
this period. The second method employed involved the cornparison of weekdayiweekend
variations in ozone concentrations in exceedence and non-exceedence weeks. Exceedence
weeks represented periods in which uncharacteristicdy high ozone concentrations were
observed. As the thermal effects of ozone depend hiJhly on the concentration thereof in
the atmosphere, weekday/weekend temperature variations resulting fiom ozone
perturbations should be most pronounced during exceedence weeks.
The timing of auto emission release was shown to be of significant importance in
producing weekdayheekend ozone variations. Auto emissions released f i e r sunrise led
to increased ozone concentrations. Auto emissions released before sunnse however,
reduced ozone concentrations. The attenuation of before-sunrise rush-hour traEc on
weekends therefore resulted in reduced early moming ozone scavenging and increased
weekend ozone concentrations.
The statistical significance of differences between each mean hourly weekday and
mean hourly weekend ozone concentration and temperature value were established using
the two-tailed independent sample t-test. Statistically significant weekday/weekend
variations in ozone concentrations were obsetved at all Greater Toronto Area
measurement stations during Nmmer, winter, exceedence, and non-exceedence periods.
Weekday/weekend variations during summer and exceedence periods were of greater
magnitude than those observed during winter and non-exceedence periods respectfully.
Variations in mean hourly ozone concentrations encountered in both sumrner and
winter months did not appear to be of sufficient magnitude to result in a noticeabte
thermal effect. When exceedence and non-exceedence weeks were separated however,
two distinct weekday/weekend patterns were observed. During exceedence weeks,
weekend temperatures were higher than weekday temperatures. During non-exceedence
weeks however, weekend temperatures were lower than weekday temperatures. When
these data sets were not separated, but examined as a whole (Sumer), these opposite
weekdaylweekend patterns may have been of sufficient magnitude to counteract one
another resulting in no statistically significant weekday/weekend temperature variation
during summer months.
When periods of uncharacteristically high ozone concentrations (exceedence
weeks) were isolated, the resulting weekday/weekend temperature variations were shown
to be statistically significant at al1 three Greater Toronto Area measurement sites.
Statistically significant temperature variations were observed during the non-
photochemically active hours of the day. The restriction of this effect to exceedence
weeks however, supports the assumption that the observed weekday/weekend
temperature variations may have been the result of weekday/weekend variations in ozone
concentrations. The assumption that the observed weekday/weekend air temperature
variations were the result of anthropogenic activity was however supported by the relative
lack of statistically significant weekday/weekend temperature variations at the Long Point
control site.
7.1 Recommendations for Further Studv
This thesis has demonstrated that weekdaylweekend variations in tropospheric
ozone concentrations may result in weekday/weekend air temperature variations.
Weekday/weekend temperature variations were restricted to periods of
uncharacteristicaiiy hi& ozone concentrations. The following recommendations are based
on the findings of this thesis.
1. Due to high standard deviations observed in hourly average ozone data sets, no
specific part per billion weekday/weekend variation could be inferred. The high
standard deviations observed were the result of periods of unusually low ozone
concentrations throughout each ozone record. As these periods were shown to
correspond simultaneously between dl ozone stations, they codd not be attributed to
measurement error. These periods of unusually low ozone were iikely the result of
precipitation events. Precipitation cm 'wash' ozone out of the atmosphere
drarnatically reducing arnbient concentrations. Through cornparison with precipitation
records, penods of low ozone resulting £iom precipitation events could be identified
and removed. This would increase the validity of part per billion estimates of
weekday/weekend ozone variations by reducing the standard deviation.
2. While not statistically significant at the 95 per cent level, weekday mean hourly
temperatures were shown to be higher than associated weekend temperatures during
non-exceedence weeks. This reversed weekdaylweekend temperature effect seemed
most pronounced when ozone concentrations were at their lowest. The separation and
examination of periods of uncharacteristically iow ozone concentrations ( e g O to 5
ppb) may enable one to observe the true magnitude of this reversed weekday/weekend
temperature effect. If the magnitude of this effect could be detemùned, its
counteracting influence could be removed when exarnining temperature variations
during exceedence events, further isolating the effects of ozone variations on air
temperature. The removal of this counteracting effect could therefore make it possible
to attribute specific degree Celsius variations in temperature to specific part per billion
variations in ozone.
3. In this midy, 'exceedence weeks' represented weeks in which ozone concentrations
on at least one day reached or exceeded 80 ppb. The separation and analysis of these
exceedence weeks on an individual basis would provide a more detailed analysis of
weekday/weekend ozone variations and associated temperature perturbations. The
inclusion of wind direction data in each of these analyses would make it possible to
determine whether peiiods of unusually high ozone were the result of the incursicn of
a pollution laden weather system into the area.
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APPENDM A
Relative Roles of Volatile Organic Compounds in Nitric Oxide Scavenging
The cornparison of the reaction sequences of the following three families of VOCs
illustrates the complexity and importance of understanding VOC-NO interactions.
A.l Alkanes
Alkanes react with OH radicais via H-atom abstraction. That is, the OH radical
absîracts an H atom from the alkane, producing an akyl radical and water vapour (the
notation 'R' is used to represent ail organic radicals).
RH + OH* + R- + H20 (A- 1
The resulting atkyl radical is then fiee to react with rnolecular oxygen to form an alkyl
peroxy radical @O2).
This alkyl peroxy radical is responsible for the removal of NO from the atrnosphere via the
following reaction.
The alkoxy radical produced in this reaction can then react with molecular oxygen to form
a carbonyi species and an HOz radical (R' represents a hydrocarbon group which may
dBer fiom R).
RO* + O2 + R'CHO + HOy (A-4)
As rnentioned earlier, this HOa radical can convert an additional NO molecule to NO2 and
produce an additionai OH radical (reaction 3 -8) therefore leadhg to further alkane
oxidation and nitnc oxide removd.
A.2 Aldehydes
Aldehydes react in a sirnilar manner to alkanes, once again through H-atom
abstraction, this t h e forming an acyl radical (RCO).
RCHO + OH- --+ RCHO + H20
The acyl radical reacts with molecular oxygen, forming an acyl peroxy radical.
This acyl peroxy radical can then react with NO to form NOz.
While this reaction sequence is simila. to that of an alkane, in this case, only one molecule
of NO is converted to NOa and no OH radicals are produced in the process.
A.3 Alkenes and Aromatic Hvdrocarbons
Both alkenes and aromatics proceed via OH radical addition rather than H-atom
abstraction. To fùrther illustrate the process of OH radical addition, the reaction of an OH
radical with the simplest alkene (ethene) is shown.
C2& + OH- + HOCH2CH2* (A-8)
HOCH2CH2* + O2 + H0CH2CH2O2* (A-9)
HOCH2CH202. + NO + NO2 + HOCH2CH20. (A. 10)
I l l
The decomposition of the HOC&CH20 radical eveniudy leads to the production of an
HOz radical, therefore providing an additional NO to NOz conversion and OH radical.
Alkanes, alkenes, and aromatic hydrocarbons scavense more NO per OH radical
than aldehydes. In addition, the oxidation of these particular pollutants leads to the
production of additional OH radicals, therefore resulting in the proMeration of the NO
scavenging process.
Alkanes, aikenes, and aromatics are the primary hydrocarbons found in automobile
emissions while aldehydes are rnainly formed from the degradation of other organic
compounds, such as industrial solvents, in the atrnosphere. Therefore, the VOCs moa
effective at NO scavenging and indirectly increasing ozone concentrations are those
produced by automobiles.
Method Used to Assign Days of the Week to Ozone and Temperature Data Sets
The fkst s e p in data preparation invoived assigning the appropriate day of the week
(Monday - Sunday) to each date within the data set. This was accomplished by first
detemiinùig the appropriate day of the week for the fïrt date in each data set (E-g.
January 1, 1979 = Monday). Once the initial day of the week in the data set had been
detennined, these days of the week (Monday = 1, Tuesday = 2. ..Sunday = 7) were simply
extended down the remainder of the data set. The initial days of the week were
determïned using both DOS and UNTX systems to ensure accuracy.
Upon visual examination of the data in each file, several daily or rnonthly gaps
were discovered. As the accurate identification of the day of the week (Monday - Sunday)
was crucial to the anaiysis of data, these gaps had to be identsed and filled-in
appropriately before days of the week could be assigned (Figure 4.1).
Year Month Day Dav of Week - - 1979 1 10 3 (Wed.) 1979 1 II 4 (Thurs.) 1979 1 14 5 (Fn.) 1979 1 15 6 (Sat.)
Year Month Day Dav of Week - - 1979 1 10 3 (Wed.) 1979 1 II 4 (Thurs.) 2979 1 12 S (Fri. ) 1979 1 13 6 (Sat.) 1979 1 14 7 (Sun.) 1979 1 15 1 (Mon.)
Figure A. 1 : Identification of gaps in data sets. Failure to identify the gap between Ianuary 1 1 and Januaiy 14 would have resulted in the improper designation of days of the week to the remainder of the data set.
In order to ident* these gaps, the numbers h m 1 to 3 1, 1 to 28, 1 to 30, etc. within each
month of each year were summed to provide the value of 5738. Ifthe sum of these values
did not equal5738 (5767 for leap years), the year in question would be visually checked,
day by day, for missing values. Once the gaps had been identified, appropriately flled, and
the mm provided the correct value, the appropriate days of the week would be assigned.
Several checks throughout each data set were made to ensure the day of the week
corresponded correctly with that specific date. This process was repeated for each year of
each data set used.
APPENDM C
Ozone S m q Statistics (Summer and Winter)
* C4
CC c.
CI CI
- CI
- - CI
3 - 5
CI - 2
13
2
r: - -1 -i.
- CiI
- Y CI
5
X
CI
\S
Wh
-?
CC.
S s = i r: N S C u -
G- - - ; S f =
2712 - Y Li( c.
= W! b- x z = - *. ? - Y. N - - c- G- 3 = N
3- s* 5 S l C 3 N - - - v, cc: 2- CP.
*. rl . 'c G 3 CC.
\s 3- b z = CL.
Y i . ? ? ,.f:= '?.Y X C I ^ , CC. - + - L / : g C " q - U! Y. W. = TC, - 5 -. 1 3 e. C J - - 3 'c . - . W, - = N - 'f: 9- Tt - A - 4 -
W - rc: -. e- x = = - M
G* \s s s = Q- =- fi- 2 - c - -. =. ='? e F. = - - + - % W. cc. = c. - -- ? 9- -t CL. = - - ' = r v ; A , 2- 2- -- . -- s 2 2 -
7
r' r
r c-
- e
C - P
- " - X - r- - '"
iC - -t - r - N - - - - - 3
X;
P
G
V?
t
CC.
r.4
-a 0 - g 2 Li L
= 3 -- 3 2
- C
P C
C C
- C
- b
C
C - 3 - P
i
V - 2
CC -
CI -
C - - =
x
Is
\f
lr
*
rT:
J* g - C C - E 4
Li-
5 2
1 C
.? C
(4 c.
C
C4
- - l?
3 - X - C4 - 5
rC; - 2
CC: - 2
d
d
- Y I
3
X
P
Q
iA
-t
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6
T
C
P C
C C
- c
A
i:
C - - 3i - c' - \L - cc -
2
CC - C - - - - - - 3
X
CI
\<-
'c
*
rc:
Li-
2 4 L
s 3 3 0 V3z 1:
S W. cr. w; =-
CC. -
- - 5 35 CU- s *el
- C
F C
C C
- C
- b
C
- " - 31 - l- -
5
iC - 2
PC - CI - - - Li - - 9
X
t-
a
V,
d
Y.
= V! t V; CC: s u - r N
0
O s O C
+ =- T Cr. ch. = VI CU
2" -
Li-
";. r rc. - S CV -
Temperature Summary Statistics (Summer and Winter)
- cc. CC.
A - =- =- ('cl-?
d C4
r CI
h CI
- fl
C - ci
3 - X - c
2
VI - 3
CI - C4l - CI - C5 I i(
D
X
e
\S
'n
ct
Cn.
Li- O C = E 2 2 1: 2
G y - - ic, = c.l
't P.
r+ m
fi n
- c.
C - C I
9 - T
2
c - lr: - C
rC - Si
d
d
- - d
=r
x
fi
a
W.
l-
CC.
N
P {- O - b, O 4
s S -4
3 2
- X; cc. y- \s c j
Ozone Summary Statistics (Exceedence and Non-Exceedence)
r r 9 a m - --
u-! =* w, S \ S Î CL, -
' ? + Y P Z -
- G- 7 'fa 2 = CC.
'f! ? ? N cc. = C. -
x; 5 =- G E - -
F: (U- 'e- 3 5 1 = CC. -
-. ? =- \S 'P. - - -
- d - ? v. =:
cc. -
CT?'? f i e A l -+ crl
l." \s e m = Pi-
'-7 t t FI = CC.
='? - + G W. = CC. -
CC. - 'l \ s & - - - Y
? ? ? CC. CU Y - -
N t " . 7 W. y. -l- -
N.? - s = F.
2 2 9 CC. -
Temperature Summary Statistics (Exceedence and Non-Exceedence)
z - X Oc - T s
w! x- e b * " - = *-Se - *- =!
3 5
e S cc: lr; = - - yc z,=. -
c.
î cc, y Cs(
\se- = v, = N
Li w- 5 C". - v. = N
N - ? - * Y N - *- . - W. N - -- P. rc. - . - W C N
Q- " . - - ' P a = N
= v. = N
'". 3- 3 T Z 3
?Fr;. E t =
"?". b -f = - G- y cc: 2 3 2
3- q 5 z * = = ? ? Y z-F= = ri rc:
5 x 5 ' P a ? = -iI
? b. - 2 -r = -- ? l-: 2 - s
- ". a * = CI
CC. cc F.
- s i * 2- In s
s L s - ". v- * i r . G S C1
X ? ? W. W. S CI
-- 3- ? 2 ln s
T F ? G ' r 0 C O
3- "I * ir; S -
r? v r: fi- e- --.
'? -. . b a s - - i". Z b S .
I
GZ2
C! -- -? b \ S S - - œ + 6 -- A - 4
??t \Sec - ? = ? Y G W. = - ? ? T ic, \S S I
- - -? 2- v2 s
W b - t ? \ s =
"i. N ? C S
'5 m- t. d
ic: y ? Z G z ;
222 "' "1 T- t a s œ
".? ~r- ic, s .-.
=- =? 3 ic. W. C -. - x t- w; v; e -.
7 W. ic* = L
v 3- * v; ir. S L
zq =- ? ic, \S Î iI
T F ? G i r r S - ". 3- -t '?_ Ir, s
3- 3- +- 5 Ir, s
? N ? z \ s s - - . ? & C S - ==. m. ? \ 5 \ S S - 3- -- 4'. \ S e s - \Sn? zvzcz
Y - ? c e s - -. = ** W, c s œ . ? W. W. s d
S N * 3\55
= * W! f3"
a q \ s . cc. \î S -.
c. w! C G = - - 5 'C:
+ a 5 O
7 5 1 2 \ 5 5
2 2 2 CI
? Y 1 -
APPENDM G
T-Statistic Values (Summer and Winter)
Table G. 1 : Ozone 'r-Statistics (Suinnier and Winter)
BriyIG ros\wor Hour 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
RrithbiiriilCciil. Pk. Hour 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 I X 19 20 21 22 23 24
Y o~i~clFiiicli Hour 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24
5 8 O O
5 üj 0 0
g? $ 8 8 4 ' ~
£3 9 0
8 I z 9-
CU Q Q. ? Q m O lc k u? 4 '=
8 % Q N
8 8 Q @ J
8 8 ci cu
5 O Cu
8 I8 CU-
&? z N
E 5 O - 8 c O - $ g O O
!e Q - Z 8 0 Q
g 9 O 4
q O 9
q 8 09' CV b 'Y 9 9 0 * 2 2 Li
,O Li
c - 2 z
APPENDM H
T-Statistic Values (Exceedence and Non-Exceedence)
Table H. 1 : Ozone T-Statistics (Exceedence and Non-Exceedence)
R:ithburii/Cciil. Pk.
BayIGros\~ciior Hour 1 1 2 3 4 5 G 7 8 9 I O 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24
I
Ilour
Pcrtli/Ruskin
1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 23 21
Hour 1 2 3 4 S G 7 8 9 O 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24
Y oii~c/Fincli Hoiir 1 2 3 4 5 6 7 8 9 IO 1 1 12 13 11 15 16 17 18 19 20 21 22 23 21
-i- N
CC. C 1
Cu N
- c.l
- V
N
3 - X) - IZ
2
'C - t
r: - N - - - - Y - 3
OC
t-
c
'c
v
r:
CU
U
O 5 0 2 k 3- s P
C
C C
C C
M
C
- * C
L -
Y - r -
- ir -
1 - c-
CI -
- - 3
X
b
c
'O
t
CC.
N i; .- 2 L
3 3
APPENDM I
Mean Hourly OzoneIT-Statistic Cornpanson
Figure 1.1 : Comparison of weekday/weekend ozone concentrations with the statistical significance of weekday/weekend ozone variations at the Evans/Arnold Measurement Site Error bars represent the 95 per cent confidence Iimit.
Hour
- Weekday - - - Error Bar - Weekend - - - - - - Error Bar