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 flares1015 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 HCHO:
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
HCHOIsoprene
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
HHCHO
88% Organicperoxides
OHOO
OO
OH OHOO
-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 glyoxalOO
O
hn 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
MEGANIsoprene 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%