Isoprene Emissions in Africa Inferred from OMI Observations of Formaldehyde Columns

We use 2005-2009 satellite observations of formaldehyde (HCHO) columns from the OMI instrument to infer biogenic isoprene emissions at monthly 1 ◊ 1 reso- lution over the African continent. Our work includes new ap- proaches 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 (HCHO) from OMI fol- low closely the distribution of vegetation patterns in Africa. We infer isoprene emission (EISOP) from the local sensi- tivity S = 1HCHO /1EISOP derived with the GEOS-Chem chemical transport model using two alternate isoprene oxi- dation mechanisms, and verify the validity of this approach using AMMA aircraft observations over West Africa and a longitudinal transect across central Africa. Displacement er- ror (smearing) is diagnosed by anomalously high values of S and the corresponding data are removed. We find signifi- cant sensitivity of S to NOx under low-NOx conditions that we fit to a linear function of tropospheric column NO 2. We estimate a 40 % error in our inferred isoprene emissions un- der high-NOx conditions and 40-90 % under low-NOx con- ditions. Our results suggest that isoprene emission from the central African rainforest is much lower than estimated by the state-of-the-science MEGAN inventory.