Variability of HCHO over North America: Implications for satellite retrievals Dylan Millet D.J. Jacob, D.R. Blake, K. Chance, A. Fried, B.G. Heikes, R.C. Hudman, T.P. Kurosu, H.B. Singh, S. Turquety, S. Wu, and the ICARTT Science Team ICARTT Data Analysis Workshop University of New Hampshire August 10, 2005

Space-based measurements of HCHO columns OH, hu, O3 VOC HCHO Can we use WHCHO as a proxy for VOC emissions?

Key Questions: Measurement: Interpretation HCHO slant columns measured by GOME (K. Chance, T.P. Kurosu et al.) Key Questions: Measurement: 1) What is the uncertainty and bias in HCHO columns measured from satellites? Interpretation 2) What are the main precursors contributing to HCHO columns and variability over North America? 3) What are the implications for retrieving VOC emissions from space?  Address using aircraft measurements -0.5 0.5 1 1.5 2 2.5 LOD 1016 molecules/cm2 HCHO slant columns measured by OMI (K. Chance, T.P. Kurosu et al.)

Uncertainty in solar backscatter retrievals of HCHO Fitting uncertainty (~ 4 x 1015 molec/cm2) Relating slant columns to vertical columns Air mass factor (AMF) AMF depends on HCHO vertical profile Radiative transfer Cloud effects Aerosol effects HCHO Model

Computation of AMF Use DC-8 vertical profiles AMFG: Viewing geometry & SZA w(P): scattering S(P): HCHO vertical distribution Use DC-8 vertical profiles Measured vs. modeled [HCHO], aerosol “Satellite” clouds

AMF values Mean: 1.2 – 1.3 Range: 0.11 - 2.42 Model bias: -5% over continents (-57+70%) +13% over ocean (-14+72%) 25% uncertainty for a single scene What drives the variability?

Cloud and aerosol effects on AMF Model AMF Bias Mean Individual profiles Aerosol effects Increase the AMF (i.e. sensitivity to HCHO) by ~15% Cloud effects Can ↑ or ↓ AMF Major source of error Double the uncertainty for a single scene Recommend cloud cutoff: AMF error of 21% @ cloud fraction cutoff of 40%

Interpretation of WHCHO OH, hu, O3 VOC HCHO Methane, anthro. & biogenic VOCs What drives variability in WHCHO?

WHCHO sources & variability isoprene OVOCs NMHCs methane terpenes WHCHO sources & variability Methane & OVOCs main HCHO precursors in most of the atmosphere But variability in column production rate is low Satellite LOD / tHCHO: ~ 5x1011 molec/cm2 /s Probability Measured column HCHO production rate

WHCHO factors of variability NMHCs methane terpenes OVOCs isoprene Isoprene dominant source when WHCHO is high Variability in WHCHO over N.America driven by isoprene Column HCHO Measured column HCHO production rate

HCHO production yield from isoprene HCHO column mass balance: MOD MOD From measured (WHCHO vs. Wi) & modeled (ki/kHCHO): Y = 1.61 ± 0.10 MOD (flighttrack) OBS

Conclusions AMF Clouds major source of error Increase AMF uncertainty by ~ 2x  Uncertainty in satellite HCHO columns due to the AMF: Mean bias ~ -5% over continents Uncertainty ~ 25% (1s) for individual scene (less for time averages) HCHO production & variability Variability in WHCHO over N. America driven by isoprene  Satellite retrievals of WHCHO can be used as a proxy for isoprene emissions over N. America Estimated average HCHO yield from isoprene oxidation: 1.61±0.10  Upper end of GEOS-Chem (0.9-1.9); lower end of MCM (1.6-2.4) yields

Jakarta Chongqing

Note: bias from assuming constant HCHO & aerosol vertical profiles over land: +5%