Global emissions of non-methane hydrocarbons deduced from SCIAMACHY formaldehyde columns through 2003–2006

摘要

Formaldehyde columns retrieved from the Scanning Imaging Absorption Spectrometerfor Atmospheric Chartography/Chemistry (SCIAMACHY) instrument onboardENVISAT satellite through 2003 to 2006 are used as top-down constraintsto derive updated global biogenic and biomass burning flux estimates for thenon-methane volatile organic compounds (NMVOCs) precursors of formaldehyde.Our interest is centered over regions experiencing strong emissions, and henceexhibiting a high signal-to-noise ratio and lower measurement uncertainties.The formaldehyde dataset used in this study has been recently made availableto the community and complements the long record of formaldehyde measurementsfrom the Global Ozone Monitoring Experiment (GOME). We use the IMAGESv2 globalchemistry-transport model driven by the Global Fire Emissions Database (GFED)version 1 or 2 for biomass burning, and from the newly developed MEGAN-ECMWFisoprene emission database. The adjoint of the model is implemented in agrid-based framework within which emission fluxes are derived at the modelresolution, together with a differentiation of the sources in a grid cell.Two inversion studies are conducted using either the GFEDv1 or GFEDv2 as a priorifor the pyrogenic fluxes. Although on the global scale the inferred emissionsfrom the two categories exhibit only weak deviations from the correspondinga priori estimates, the regional updates often present large departures from theira priori values. The posterior isoprene emissions over North America, amountingto about 34 Tg C/yr, are estimated to be on average by 25% lower than thea priori over 2003–2006, whereas a strong increase (55%) is deduced over thesouth African continent, the optimized emission being estimated at 57 Tg C/yr.Over Indonesia the biogenic emissions appear to be overestimated by 20–30%,whereas over Indochina and the Amazon basin during the wet season the a prioriinventory captures both the seasonality and the magnitude of the observedcolumns. Although neither biomass burning inventory seems to be consistentwith the data over all regions, pyrogenic estimates inferred from the twoinversions are reasonably similar, despite their a priori deviations.A number of sensitivity experiments are conducted in order to assess the impact ofuncertainties related to the inversion setup and the chemical mechanism.Whereas changes in the background error covariance matrix have only alimited impact on the posterior fluxes, the use of an alternative isoprenemechanism characterized by lower HCHO yields (the GEOS-Chem mechanism) increasesthe posterior isoprene source estimate by 11% over northern America,and by up to 40% in tropical regions.