Dylan Millet D.J. Jacob, D.R. Blake, K. Chance , A. Fried,

1. 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

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

3. HCHO slant columns measured by GOME (K. Chance, T.P. Kurosu et al.) 2.5 2 1.5 1 0.5 LOD 0 -0.5 1016 molecules/cm2 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 HCHO slant columns measured by OMI (K. Chance, T.P. Kurosu et al.)

4. 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 Uncertainty in solar backscatter retrievals of HCHO HCHO Model

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

6. 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? AMF values

7. 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% Model AMF Bias Mean Individual profiles Cloud and aerosol effects on AMF

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

9. 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 isoprene OVOCs NMHCs methane terpenes WHCHO sources & variability Probability Measured column HCHO production rate

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

11. 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

12. 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

13. Chongqing Jakarta

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