Isoprene emissions in Africa inferred from OMI observations of formaldehyde columns

E A Marais; D J Jacob; T P Kurosu; K Chance; J G Murphy; C Reeves; G Mills; S Casadio; D B Millet; M P Barkley; F Paulot; J Mao

doi:10.5194/acp-12-6219-2012

Isoprene emissions in Africa inferred from OMI observations of formaldehyde columns

Atmos Chem Phys. 2012 Jul;12(14):6219-6235. doi: 10.5194/acp-12-6219-2012. Epub 2012 Jul 18.

Authors

E A Marais¹, D J Jacob^{1

2}, T P Kurosu³, K Chance³, J G Murphy⁴, C Reeves⁵, G Mills⁵, S Casadio⁶, D B Millet⁷, M P Barkley⁸, F Paulot², J Mao⁹

Affiliations

¹ Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.
² School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
³ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA.
⁴ Department of Chemistry, University of Toronto, Toronto, Canada.
⁵ School of Environmental Sciences, University of East Anglia, Norwich, UK.
⁶ Instrument Data quality Evaluation and Analysis (IDEAS), Serco Spa Via Sciadonna 24, 00044 Frascati (Roma), Italy.
⁷ Department of Soil, Water and Climate, University of Minnesota, St. Paul, MN, USA.
⁸ Space Research Centre, University of Leicester, Leicester, UK.
⁹ Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA.

Abstract

We use 2005-2009 satellite observations of formaldehyde (HCHO) columns from the OMI instrument to infer biogenic isoprene emissions at monthly 1 × 1° resolution over the African continent. Our work includes new approaches to remove biomass burning influences using OMI absorbing aerosol optical depth data (to account for transport of fire plumes) and anthropogenic influences using AATSR satellite data for persistent small-flame fires (gas flaring). The resulting biogenic HCHO columns (Ω_HCHO) from OMI follow closely the distribution of vegetation patterns in Africa. We infer isoprene emission (E _ISOP) from the local sensitivity S = ΔΩ_HCHO / ΔE _ISOP derived with the GEOS-Chem chemical transport model using two alternate isoprene oxidation mechanisms, and verify the validity of this approach using AMMA aircraft observations over West Africa and a longitudinal transect across central Africa. Displacement error (smearing) is diagnosed by anomalously high values of S and the corresponding data are removed. We find significant sensitivity of S to NO_x under low-NO_x conditions that we fit to a linear function of tropospheric column NO₂. We estimate a 40% error in our inferred isoprene emissions under high-NO_x conditions and 40-90% under low-NO_x conditions. Our results suggest that isoprene emission from the central African rainforest is much lower than estimated by the state-of-the-science MEGAN inventory.

Grants and funding

NNX10AG65G/NASA/United States