Observations of Formaldehyde and Related Atmospheric Species Using Multi-Axis

Spectroscopy

Christopher P. Beekman and Dr. Heather C. Allen

Department of Chemistry, and Environmental Sciences Graduate Program

The Ohio State UniversityJune 19, 2007

Introduction

• Understanding the concentrations and distributions of photochemical species and aerosols in an urban air-shed

• Combination of spectroscopic and meteorological data with photochemical/radiative transfer models

• Important for atmospheric chemistry, health, and climate change modeling

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS)

• Sensitivity to tropospheric absorbers• Vertical profiling of trace gases• NO2, O3, HCHO, HONO, BrO, ClO2

• Ocean Optics USB-2000– 0.7 nm resolution– Coupled to 1” telescope w/

multimode fiber– Low power requirements

• 8 hours with 12 V battery

U. Platt (1994). Air Monitoring by Spectroscopic Techniques. M. W. Sigrist. 27-84.

G. Hönninger, C. Von Friedeburg and U. Platt. Atmospheric Chemistry and Physics 4, 2004.

• Collection of scattered sunlight in the UV-VIS, along different lines of sight

• Analysis of raw atmospheric spectra using modified Beer-Lambert law

• The most basic measured quantity is Slant Column Density (SCD)

• Slant columns at each elevation angle converted to Vertical Column Densities (VCD)

• Conversion factor is Air Mass Factor (AMF), calculated using model of solar radiative transfer

• UVSPEC/DISORT: libRadtran Package

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS)

dssSCD )(

Solar Intensity

0

200

400

600

800

1000

1200

1400

325 350 375 400 425 450 475 500 525

Wavelength (nm)

Inte

nsity

(A.U

.)

VCD

SCDAMF

VCDSCD

B. Mayer and A. Kylling. Atmospheric Chemistry and Physics 5, pp. 1855-1877, 2005.

Slant Column Density (SCD) Retrieval

NO2

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

420 440 460 480 500 520

Wavelength (nm)

Dif

fere

nti

al O

pti

ca

l D

ep

th

Measured

Calculated

Raw Solar Intensity

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

420 440 460 480 500 520

Wavelength (nm)

No

rma

lize

d In

ten

sit

ies

SpectrumMeasured Reference

O4

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010

0.012

420 440 460 480 500 520

Wavelength (nm)

Dif

fere

nti

al O

pti

ca

l D

ep

th

Measured

Calculated

O3

-0.010-0.008

-0.006

-0.004

-0.002

0.000

0.0020.004

0.006

0.008

0.010

0.012

420 440 460 480 500 520

Wavelength (nm)

Dif

fere

nti

al O

pti

ca

l D

ep

th

Measured

Calculated

Radiative Transfer

• Accurate knowledge of vertical structure of atmosphere required

• Aerosol optical depth and vertical distribution are key parameters

• Inclusion improves the model retrieval of Air Mass Factors

Aerosol Optical Depth Profiles

0

0.5

1

1.5

2

2.5

3

3.5

4

0.005 0.025 0.045 0.065

AOD (km-1)

Alti

tud

e (k

m)

Aerosol Profile Retrieval

Greenblatt, G. D., J. B. Burkholder, A. R. Ravishankara. Journal of Geophysical Research, 95, 1990. Wagner, T., B. Dix, C. v.Friedeburg,et al. Journal of Geophysical Research, 109, 2004.

• Aerosol profiling requires a species with known vertical profile

• O4 is most appropriate in UV-Vis region

• Concentration proportional to [O2]2

• 12 typical profiles of aerosols simulated within model

• Comparison of measured and modeled Air Mass Factors of O4 yields best match profiles

O4 Absorbance

0.00

0.05

0.10

0.15

0.20

330 430 530 630

Wavelength (nm)

Ab

so

rba

nc

e

Aerosol Profile Matching

0

0.5

1

1.5

2

2.5

0.000 5.000 10.000 15.000 20.000 25.000Telescope Elevation Angle

dA

MF

Modeled dAMFs

Measured dAMF

Aerosol Optical Depth Profiles

Aerosol Profiles, 5/31/07

0

0.5

1

1.5

2

2.5

3

0.005 0.015 0.025 0.035 0.045 0.055

AOD (km-1)

Alt

itu

de

(k

m)

Aerosol Profiles, 5/31/07

0

0.5

1

1.5

2

2.5

3

0.005 0.015 0.025 0.035 0.045 0.055

AOD (km-1)

Alt

itu

de

(k

m)

Aerosol Profiles, 5/31/07

0

0.5

1

1.5

2

2.5

3

3.5

0.005 0.015 0.025 0.035 0.045 0.055

AOD (km-1)

Alt

itu

de

(k

m)

Volume Mixing Ratios

• Vertical Column Densities can be converted to volume mixing ratios

• Need to define the mixing height h

• Height of lowest layer determined by several methods

– Radiosonde data location of 1st inversion– Height of Aerosol Profile– Box Air Mass Factors Iterative profile variation

202

1VCDVCD

hC

R. Sinreich, U. FrieS, T. Wagner, U. Platt. Faraday Discussions 130, 2005.

HCHO• Primarily an oxidation product of

other VOCs

• Indicator of VOC photochemistry

• 1989: 17% of atmospheric HCHO in Columbus attributed to vehicle emissions

– R. Mukund, T. J. Kelly, C. W. Spicer. Atmospheric Environment 30 (20), 1996.

• 2002: MAX-DOAS measurements of HCHO in Italy

– A. Heckel, A. Richter, et al. Atmospheric Chemistry and Physics 5, 2005.

HCHO + hv H + HCOH + O2 HO2

HO2 + NO NO2 + OHNO2 + hv NO + OO + O2 O3

A. Heckel, A. Richter, et al. , 2005

5-30-2007, HCHO

0

1

2

3

4

5

6

9:36 12:00 14:24 16:48 19:12

Time

VM

R H

CH

O (

pp

b)

HCHO

HCHO, O3 and NO2

5-30-2007, HCHO, O3 and NO2

0

5

10

15

20

25

30

35

9:36 12:00 14:24 16:48 19:12

Time

VM

R H

CH

O, N

O 2 (

pp

b)

0

50

100

150

200

250

VM

R O

3 (pp

b)

HCHO

NO2

O3

5-31-2007, HCHO, O3, NO2

0

5

10

15

20

25

30

35

9:36 12:00 14:24 16:48 19:12

Time

VM

R H

CH

O, N

O 2 (

pp

b)

0

50

100

150

200

250

VM

R O

3 (pp

b)

HCHO

NO2

O3

• Both days: poor air quality

• 5-30-07: Strong spike in AM

• 5-31-07: Elevated NO2 , possible O3 suppression during AM

• Need more information to characterize regimes

Conclusions and Outlook

• MAX-DOAS Effective technique for probing atmospheric photochemistry

• HCHO measurements: – 1989: 4.7 ppb– May and June 2007: 3.0 ppb

• Measurements of O4: enabled the retrieval of the first vertical profiles of aerosols in central Ohio

– Extend to long term, incorporate into photochemical models

Acknowledgements

• Professor Heather C. Allen, Allen Lab Members

• Professor Katherine Calder, Dr. HongFei Li

• Arve Kylling and Bernhard Mayer