Dylan MilletD.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

Variability of HCHO over North America: Implications for

satellite retrievals

ICARTT Data Analysis WorkshopUniversity of New Hampshire

August 10, 2005

Space-based measurements of HCHO columns

Can we useHCHO as a proxy for VOC emissions?

OH, h, O3VOC HCHO

HCHO slant columns measured by GOME(K. Chance, T.P. Kurosu et al.)

HCHO slant columns measured by OMI(K. Chance, T.P. Kurosu et al.)

-0.5

0

0.5

1

1.5

2

2.5

LOD

1016 molecules/cm2

Key Questions:

Measurement:1) What is the uncertainty and bias

in HCHO columns measured from satellites?

Interpretation2) 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

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

AMFG: Viewing geometry & SZAw(P): scatteringS(P): HCHO vertical distribution

Use DC-8 vertical profiles

Measured vs. modeled [HCHO], aerosol

“Satellite” clouds

T

s

P

Ps

G dPPSPwP

AMFAMF )()(

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

Aerosol effectsIncrease the AMF (i.e. sensitivity to

HCHO) by ~15%Cloud effects

– Can ↑ or ↓ AMF– Major source of error– Double the uncertainty for a single

sceneRecommend cloud cutoff:

AMF error of 21% @ cloud fraction cutoff of 40%

Model AMF Bias

Mean

Individual profiles

Interpretation of HCHO

• Methane, anthro. & biogenic VOCs

• What drives variability in HCHO?

OH, h, O3VOC HCHO

Methane & OVOCs main HCHO precursors in most of the atmosphere

But variability in column production rate is low

Satellite LOD / HCHO: ~ 5x1011 molec/cm2 /s

isoprene OVOCs

NMHCs methane

terpenes

Measured column HCHO production rate

Pro

bab

ilit

y

HCHO sources & variability

HCHO factors of variability

Isoprene dominant source when HCHO is high

Variability in HCHO over N.America driven by isoprene

NMHCs methane

terpenes

OVOCsisoprene

Measured column HCHO production rate

Co

lum

n H

CH

O

HCHO production yield from isoprene

From measured (HCHO vs. i) &

modeled (ki/kHCHO):

Y = 1.61 ± 0.10

iiHCHO

iHCHO Y

k

k MOD MOD

MOD(flighttrack) OBS

HCHO column mass balance:

ConclusionsAMF

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% (1) for individual scene (less for time averages)

HCHO production & variability

Variability in HCHO over N. America driven by isoprene

Satellite retrievals of HCHO 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%