distribution Formaldehyde over North America: Implications for satellite retrievals of formaldehyde columns and isoprene emission
D. B. Millet ([e-mail]) , D. J. Jacob , S. Turquety , R. C. Hudman , S. Wu , A. Fried , J. Walega , B. G. Heikes , D. R. Blake , H. B. Singh , B. E. Anderson , and A. D. Clarke
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7
Harvard University
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NCAR
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University of Rhode Island
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U.C. Irvine
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NASA Ames Research Center
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NASA Langley Research Center
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University of Hawaii at Manoa
1. Distribution of HCHO Over North America
The objective of INTEX-A (July 1 August 15, 2004) was to observe the chemical outflow from North America and infer constraints on chemical sources and export. Here we use HCHO concentrations measured aboard the NASA DC-8 aircraft and simulated using the GEOS-Chem chemical transport model over North America and the adjacent oceans during INTEX-A.
Measurements of formaldehyde (HCHO) columns from space provide constraints on emissions of volatile organic compounds (VOCs). Quantitative interpretation requires characterization of errors associated with HCHO column retrievals and the relationship of these columns to VOC emissions. Here we use aircraft measurements from the summer of 2004 to determine the local relationships between HCHO columns and VOC emissions, quantify the uncertainties in satellite measurements of HCHO columns due to the mass factor computation, and draw conclusions regarding the mapping of VOC emissions from space.
4. Uncertainty in HCHO Column Data from Space
The dominant source of error in HCHO retrievals is the air mass factor (AMF), which defines the relationship between the HCHO abundance along the viewing path and the vertical column amount. The AMF calculation requires external information on atmospheric scattering by air molecules, clouds and aerosols, on the shape of the HCHO vertical distribution, and on the UV albedo of the surface. AMFG: Geometric AMF 0 P: Pressure AMFG AMF = w P S P dP w(P): Scattering weights PS PS S(P): HCHO normalized vertical profile (`shape factor')
( ) ( )
Figure 1. DC-8 flight tracks during INTEX-A. Goal: Employ the extensive mapping of HCHO over North America from the INTEX-A mission to quantify the uncertainties and bias in the AMF calculation. Figure 4. Simulated (G-C) vs. observed HCHO columns (HCHO) during INTEX-A.
Figure 2. Mean simulated (lines) and observed (symbols) HCHO vertical distributions during INTEX-A. Figure 3. HCHO columns over North America. Top left panel: GEOS-Chem simulated columns averaged over the INTEX-A period. Other three panels: HCHO columns computed from simulated and observed [HCHO] during the DC-8 vertical profiles.
2. Relating HCHO Columns to Reactive VOC Emissions
We use the data from the INTEX-A aircraft profiles to determine how column HCHO data from space can be interpreted in terms of the underlying reactive VOC emissions. Figure 5. HCHO column production rates (PHCHO) from different measured VOCs during INTEX-A vertical profiles (OVOCs: oxygenated volatile organic compounds, ANMHCs: anthropogenic nonmethane hydrocarbons). Approximate satellite LOD shown as dashed red line. Figure 6. Relationship between measured HCHO and HCHO production rate from different precursors. Isoprene is the dominant source of HCHO variability.
Approach: Calculate AMFs separately based on measurements model and results for each of the DC-8 vertical profiles. The comparison statistics between the measured and modeled AMFs give a measure of the corresponding error in retrieved satellite HCHO vertical columns.
Figure 8. Measured vs. modeled HCHO AMFs for the DC-8 vertical profiles during INTEX-A.
Major sources of model uncertainty in the AMF calculation are the shape factor, aerosols, clouds, and the surface albedo. We assess the effects of each of these individually using sensitivity calculations. Figure 10. Error in the modeled AMF as a function of the cloud fraction. The solid black line shows the mean bias, and the red line shows the standard deviation in the bias.
Figure 9. Measured and modeled column aerosol optical thickness (OT) and measured cloud OT,
Result: Variability in HCHO over North America in summer is mainly determined by isoprene emission. Satellite retrievals of HCHO can therefore be used reliably as a proxy for isoprene emissions over North America.
3. HCHO Yield from Isoprene
At steady state and in the absence of horizontal transport, the column integral HCHO is related to those of precursors i by Eqn. 1. The slope of a plot of HCHO vs. ISOP, normalized by kISOP/kHCHO, thus represents the molar yield of HCHO production from isoprene oxidation.
Result: Clouds are the primary error source in the AMF calculation; errors in the HCHO vertical profile and aerosols have comparatively little effect. The mean bias and residual 1 uncertainty in the GEOS-Chem AMF calculation increase from <1% and 15% for clear skies to 17% and 24% for half-cloudy scenes. With fitting errors, this gives an overall 1 error in HCHO satellite measurements of 25-31%. We recommend discarding retrieval scenes with greater than 50% cloud cover.
HCHO =
1 k HCHO
k Y
i i i
i
Eqn. 1 kHCHO, ki: Column-average rate constants for chemical loss of HCHO and precursor i Yi: Molar HCHO yield from the oxidation of species i
5. Implications
How accurately we can infer isoprene emissions from HCHO co...