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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