and funding from NASA ACMAPOMI HCHO columnsJan 2006 Jul 2006

Using OMI formaldehyde (HCHO) observations to estimate isoprene emissions over Africa

Eloïse Marais, D.Jacob, T. Kurosu, K. Chance, D. Millet, J. Murphy, C. Reeves, M. Barkley, S. Casadio, R. Koster, S. Mahanama, J. Mao, F. Paulot, A. Padmanabhan

T.P. Kurosu

funded by NASA/ACMAP

OMI HCHO over Africa (2005-2009)

• Formaldehyde (HCHO) is produced by atmospheric oxidation of volatile organic compounds (VOCs)

• Observed patterns point to sources from (1) biosphere, (2) open fires, (3) oil and gas industry

• Africa accounts for 20% of global biogenic isoprene emissions in MEGAN inventory

MODIS leaf area index MODIS fire counts AATSR gas flares

1015 molecules cm-2

OMI annual meanHCHO slant columns

1015 molecules cm-2

Isolating biogenic HCHO in the OMI data• Exclude open fire (and dust) influence using MODIS

fire counts, OMI absorbing aerosol optical depth

• Exclude oil/gas industry influence using AATSR gas flare product

MODIS fire count > 0

AATSR fire count > 0

OMI smoke AAOD > regional threshold

BIOMASS BURNING INFLUENCE (exclude)

ANTHROPOGENIC INFLUENCE (exclude)

BIOGENIC (retain)

YES

NO

NO

NO

YES

YES MODIS fire counts

AATSR fire counts

OMI smoke AAOD

OMI Ωs

OMI dust AAOD > 0.1 DUST INFLUENCE

(exclude) YES OMI dust AAOD

NO

Converting slant to vertical HCHO columns

/S AMF 0

( ) ( )AMF w z S z dz

verticalcolumn

slantcolumn

air massfactor scattering weight

= f(albedo, cloud,aerosol)

HCHO shape factor(GEOS-Chem)

OMI HCHO slant columnsbiogenic component only

OMI HCHO vertical columnsbiogenic component only

1015 molecules cm-2

MODIS IGBP land cover

Relating HCHO columns to isoprene emission

VOCi HCHOhn (340 nm), OHoxidation

k ~ 0.5 h-1

Emission Ei

smearing, displacement

In absence of horizontal wind, mass balance for HCHO column WHCHO:

i ii

HCHO

y E

k

yield yi

but wind smears this relationship depending on VOC lifetime wrt HCHO production:

Local linear relationshipbetween HCHO and E

VOC source Distance downwind

WHCHOIsoprene

a-pinenepropane

100 km

detection limit

volatileorganic

compound

…so that HCHO is mainly sensitive to isoprene emission on ~100 km scale

Previous work for N. America showed that HCHO columns provide quantitative constraints on isoprene emission

OMI HCHO (Jun-Aug 2006)

OMI-constrained isoprene emission

GEOS-Chem relationship betweenHCHO column and isoprene emission

Model slope (2.4 s) agrees with INTEX-A vertical profiles (2.3),

PROPHET Michigan site (2.1)

Millet et al. [2008]

NO

OH

OH

OO

34%

O

MVK

Organic nitrates + C5 aldehydes

O

MACRH

O

H

HCHO

88% Organicperoxides

OHOO

OO

OHOH

OO

-hydroxyl (E/Z) -hydroxyl -hydroxyl (E/Z) -hydroxyl

41% 14%15% 23%

HO2 1,6-H shift1,5-H shift

4.7% 7.3%

12%

26% 40%

66% 50%

50% 100%

OOHO

HPALD

MCO3

OO

glyoxal

O

O

methyl glyoxal

OO

O

h OH

Isoprene

New developments in isoprene oxidation mechanism

Isoprene peroxy radicals

• OH regeneration from isoprene hydroperoxides (Paulot et al., 2009ab)• Isomerization of isoprene peroxy radicals (Peeters et al., 2009, 2010)

Isoprenehydroperoxides

Model simulation of AMMA aircraft profiles over W. Africa:sensitivity to isoprene oxidation mechanism

Flight tracks (Jul-Aug 2006)and vegetation index (EVI)

Mean vertical profiles (model uses MEGAN isoprene)

Observations (J. Murphy, C. Reeves, D. Heard)

ModelStandardPaulotPeeters

isoprene MVK+MACR

HCHO

• Simulation of MVK+MACR, lack of OH titration lends confidence in chemistry• Model HCHO is insensitive to choice of mechanism• Discrepancy with boundary layer HCHO: model or measurement error?

OH HO2 NO

Latitudinal profiles along vegetation gradient in AMMA

Flight tracks (Jul-Aug 2006)and vegetation index Latitudinal profiles below 900 hPa

WIND

• OMI HCHO observations closely track the vegetation gradient

• AMMA observations indicate no significant lag between isoprene emission and HCHO enhancement

Isoprene MVK+MACR HCHO

OMI HCHO

GEOS-Chem local relationship between HCHO column and isoprene emission

Annual mean isoprene emission (2006)MEGAN inventory

• Daily model values at 2o x 2.5o resolution show strong linear correlation

• Slope is insensitive to choice of model isoprene oxidation mechanism, local NOx concentrations

• Standard mechanism shows effect of OH titration at high isoprene emissions

Scatterplot for African continent (2006)

Annual mean column HCHO (2006)Peeters mechanism

MEGAN

Isoprene emission (annual mean, 2006)

OMI MODIS IGBP land cover

OMI-constrained isoprene emissions from Africa:comparison to MEGAN inventory

• MEGAN overestimates tropical forest emissions in central Africa, underestimates savanna emissions in southern Africa

• overall African emissions overestimated by 60%