1Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
2School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
3Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
4Department of Chemistry, University of Toronto, Toronto, Canada
5School of Environmental Sciences, University of East Anglia, Norwich, UK
6Instrument Data quality Evaluation and Analysis (IDEAS), Serco Spa Via Sciadonna 24, 00044 Frascati (Roma), Italy
7Department of Soil, Water and Climate, University of Minnesota, St. Paul, MN, USA
8Space Research Centre, University of Leicester, Leicester, UK
9Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA
*now at: Earth Atmosphere Science, Jet Propulsion Laboratory, Pasadena, CA, USA
Received: 21 Feb 2012 – Discussion started: 15 Mar 2012
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 (ΩHCHO) from OMI follow closely the distribution of vegetation patterns in Africa. We infer isoprene emission (EISOP) from the local sensitivity S = ΔΩHCHO / ΔEISOP 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 NOx under low-NOx conditions that we fit to a linear function of tropospheric column NO2. We estimate a 40% error in our inferred isoprene emissions under high-NOx conditions and 40–90% under low-NOx 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.
Revised: 12 Jun 2012 – Accepted: 20 Jun 2012 – Published: 18 Jul 2012
Marais, E. A., Jacob, D. J., Kurosu, T. P., Chance, K., Murphy, J. G., Reeves, C., Mills, G., Casadio, S., Millet, D. B., Barkley, M. P., Paulot, F., and Mao, J.: Isoprene emissions in Africa inferred from OMI observations of formaldehyde columns, Atmos. Chem. Phys., 12, 6219-6235, doi:10.5194/acp-12-6219-2012, 2012.