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

3. 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.)

4. 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

5. 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

6. 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?

7. 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 cloud fraction cutoff of 40%

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

9. 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

10. 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

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 ( ); lower end of MCM ( ) yields

13. Jakarta
Chongqing

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