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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. funded by NASA/ACMAP.

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using omi formaldehyde hcho observations to estimate isoprene emissions over africa
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

funded by NASA/ACMAP

OMI HCHO columns

Jan 2006

Jul 2006

and funding from NASA ACMAP

T.P. Kurosu

omi hcho over africa 2005 2009

OMI annual mean

HCHO slant columns

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

1015 molecules cm-2

MODIS leaf area index MODIS fire counts AATSR gas flares

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
Isolating biogenic HCHO in the OMI data

1015 molecules cm-2

converting slant to vertical hcho columns
Converting slant to vertical HCHO columns

vertical

column

slant

column

air mass

factor

HCHO

shape factor

(GEOS-Chem)

scattering weight

= f(albedo, cloud,

aerosol)

OMI HCHO slant columns

biogenic component only

OMI HCHO vertical columns

biogenic component only

MODIS IGBP land cover

1015 molecules cm-2

relating hcho columns to isoprene emission
Relating HCHO columns to isoprene emission

volatile

organic

compound

hn (340 nm), OH

oxidation

VOCi

HCHO

k ~ 0.5 h-1

yield yi

Emission Ei

smearing, displacement

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

Local linear relationship

between HCHO and E

but wind smears this relationship depending on VOC lifetime wrtHCHO production:

Isoprene

WHCHO

a-pinene

propane

detection limit

Distance downwind

100 km

VOC source

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

slide6

OMI HCHO (Jun-Aug 2006)

GEOS-Chem relationship between

HCHO column and isoprene emission

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

OMI-constrained isoprene emission

Model slope (2.4 s) agrees with

INTEX-A vertical profiles (2.3),

PROPHET Michigan site (2.1)

Millet et al. [2008]

new developments in isoprene oxidation mechanism

OH regeneration from isoprene hydroperoxides (Paulot et al., 2009ab)

  • Isomerization of isoprene peroxy radicals (Peeters et al., 2009, 2010)

New developments in isoprene oxidation mechanism

Isoprene peroxy radicals

Isoprene

hydroperoxides

slide8

Flight tracks (Jul-Aug 2006)

and vegetation index (EVI)

Mean vertical profiles (model uses MEGAN isoprene)

isoprene

HCHO

MVK

+MACR

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

HO2

NO

OH

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

Standard

Paulot

Peeters

Model

  • 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?
latitudinal profiles along vegetation gradient in amma

Flight tracks (Jul-Aug 2006)

and vegetation index

OMI HCHO

Latitudinal profiles below 900 hPa

Latitudinal profiles along vegetation gradient in AMMA

HCHO

MVK+MACR

Isoprene

WIND

  • OMI HCHO observations closely track the vegetation gradient
  • AMMA observations indicate no significant lag between isoprene emission and HCHO enhancement
geos chem local relationship between hcho column and isoprene emission
GEOS-Chem local relationship between HCHO column and isoprene emission

Annual mean isoprene emission (2006)

MEGAN inventory

Annual mean column HCHO (2006)

Peeters mechanism

Scatterplot for African continent (2006)

  • 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
omi constrained isoprene emissions from africa comparison to megan inventory

Isoprene emission (annual mean, 2006)

MEGAN

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%