QUESTIONS

1. If CO emission to the atmosphere were to double, would you expect CO concentrations to (a) double, (b) less than double, (c) more than double?

2. What fraction of NOx emissions are natural?

3. Peroxynitric acid (HNO4) is produced in the atmosphere by addition of HO2 to NO2. In the atmosphere it may either (1) thermolyze or photolyze back to the original reactants, (2) react with OH, or (3) deposit to the Earth’s surface. What are the implications of each of these three sinks for tropospheric NOx and OH?

CHAPTER 12: OZONE AIR POLLUTIONCHAPTER 12: OZONE AIR POLLUTION

SMOG(Sulfurous vs. Photochemical Pollution)

Example: London (also Eastern US)• smoke + fog• SO2

• characterized by inversions, cool weather, coal burning

Example: LA• UV, hydrocarbons, NOx• characterized by hot dry sunny weather, reduced visibility and high oxidant levels

We will be focusing on this kind!

GREAT LONDON SMOG OF 1952

Dec 5-9, 1952

• Cold fog + (high sulfur) coal + diesel buses = “pea souper”• Killed 4000 people (young & elderly), and over 8000 died subsequently • Clean Air Acts of 1956 and 1968 and City of London Act of 1954.

Smog = fog intensified by smoke (Henry Antoine Des Voeux, who first used it in 1905 to describe British urban areas)

http://www.martinfrost.ws/htmlfiles/great_smog.html

Monet painting showing Victorian smog (1900) [Baker and Thornes, 2006]

http://www.feast.org/articles/?ID=331

GOOD AND BAD OZONE

Stratosphere:O2 + UV sunlight = O + OO + O2 O3

Troposphere:CO + NOx + sunlight O3

VOCs

75 ppb (new standard, set in 2008)

OZONE AND PARTICULATE MATTER (PM): THE TOP TWO AIR POLLUTANTS IN THE U.S.

15 g m-3 (annual), 35 (daily)

THE NEW OZONE STANDARDCAUSES MORE U.S. AREAS TO BE OUT OF COMPLIANCE

OZONE POLLUTION IN THE UNITED STATES90th percentiles of summer afternoon (1-5 pm) surface ozone concentrations (ppbv), 1980-1995

Fiore et al. [1998]

OZONE/PM EXPOSURE DEGRADES HEALTH

Unscheduled hospital visits for asthma medication increases with ozone

[Environmental Working Group Report, 2005]

In California alone…

In 1997 EPA estimated that particulate air emissions cause 35,000 premature deaths each year.

OZONE IMPACTS ON VEGETATIONSUM06 Index: ozone dose in excess of 60 ppb during growing season

Documentednegativeeffects

1995 data

TRANSPORT PATTERN ON HIGH-OZONE DAYS

90th percentile O3 concentrations for summers 1991-1995 and mean resultant 850hPa winds on days when O3 > 90th percentile

NON-METHANE VOC EMISSIONS

Vegetation

~ 600 Tg C yr-1

Isoprene, terpenes,oxygenates…

Biomass burning

~ 50 Tg C yr-1

Alkenes, aromatics,oxygenates…

~ 200 Tg C yr-1

Alkanes, alkenes,aromatics…

Industry

Largest global flux is from isoprene (300-500 Tg C yr-1)

OZONE PRODUCTION IN TROPOSPHEREPhotochemical oxidation of CO and volatile organic compounds (VOCs)

catalyzed by hydrogen oxide radicals (HOx) in the presence of nitrogen oxide radicals (NOx)

HOx = H + OH + HO2 + RO + RO2

NOx = NO + NO2

Oxidation of CO:

2

2 2

2 2

2

2 3

2 2 3Net: 2

CO OH CO H

H O M HO M

HO NO OH NO

NO h NO O

O O M O M

CO O CO O

2

2

2 2

2 2

2 3

2 2

2 2

2 3 2

'

Net: 4 ' 2

O

RH OH R H O

R O M RO M

RO NO RO NO

NO h NO O

RO O R CHO HO

HO NO OH NO

RH O R CHO O H O

Oxidation of VOC:

RO can also decompose or isomerize; range of carbonyl products

Carbonyl products can react with OH to produce additional ozone, or photolyze to generate more HOx radicals (branching reaction)

OH can also add to double bonds of unsaturated VOCs

OXIDATION OF HYDROCARBONS CONTRIBUTE TO OZONE FORMATION IN POLLUTED AIR

Alkenes: OH oxidation adds to double bond (does not abstract H as with alkanes). With double bond, alkenes can also be oxidized by ozone

Aromatics (with benzene rings): reactive with OH, via either addition or abstraction source of secondary organic aerosol (SOA)

RH RO2

ROOH

ROR’CHO

OR

R’C(O)R”

NO

HO2

O2

isomdecomphvR O2

alkylradical

alkylperoxyradical

alkoxyradical

OH

aldehyde

ketone

Additional oxidation by NO3 (but only at night!)

> C4:

RONO2

NO < C4:

alkyl nitrates

NO2HO2

Generic Alkane OH Oxidation Scheme (no longer just CO and CH4!)

Can photolyze to produce HOx or react with OH to continue chain…

NO2

RO2NO2

org nitrates

General rules for atmospheric oxidation of hydrocarbons

• Attack by OH is by H abstraction for saturated HCs (alkanes), by addition for unsaturated HCs (alkenes)

• Reactivity increases with number of C-H bonds, number of unsaturated bonds

• Organic radicals other than peroxy react with O2 (if they are small) or decompose (if they are large); O2 addition produces peroxy radicals.

• Organic peroxy radicals (RO2) react with NO and HO2 (dominant), other RO2 (minor); they also react with NO2 but the products decompose rapidly (except in the case of peroxyacyl radicals which produce peroxyacylnitrates or PANs)

• RO2+HO2 produces organic hydroperoxides ROOH, RO2+NO produces carbonyls (aldehydes RCHO and ketones RC(O)R’)

• Carbonyls and hydroperoxides can photolyze (radical source) as well as react with OH• Unsaturated HCs can also react with ozone, producing carbonyls and carboxylic acids

• RO2+R’O2 reactions produce a range of oxygenated organic compounds including carbonyls, carboxylic acids, alcohols, esters…

OZONE PRODUCTION: BASIC CHAIN MECHANISM

O3

OH HO2

h, H2ONO

H2O2

CO, CH4, RH

NO2

h HNO3

OH, M VOC limited

NOx limited

CO, HC, NOx

DEPENDENCE OF OZONE PRODUCTION ON NOx AND HYDROCARBONS

HOxfamily

OH

RO2 RO

HO2

HNO3 H2O2O3

O3

O3

PHOx

4

5

67

89

1/ 23 7

8

( ) 2 ( ) [ ]HOxPP O k NO

k4

39 2

2 [ ]( )

[ ][ ]HOxk P RH

P Ok NO M

“NOx- saturated” or“hydrocarbon-limited” regime

“NOx-limited” regime

RH

NO

O2

NO

NO2, M

NET: RH + 4O2 R’CHO + 2O3 + H2O

OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir pollution model calculation for a typical urban airshed

NOx-saturated(or HC limited)

NOx-limited Ridge