Hydrocarbon Emissions Constrained By Formaldehyde Column Measurements from Gome-2 and OMI

The vertical columns of formaldehyde retrieved from the spaceborne instruments GOME2 and OMI are used to constrain the biogenic, pyrogenic and anthropogenic emissions of formaldehyde precursors NMVOC at the global scale. To this end, those emissions are varied and optimized in the (updated) IMAGESv2 model. The adjoint model technique is used to minimize the bias of the model with observations from either GOME2 or OMI. The optimisation is performed on a monthly basis at the model resolution (2°x2.5°). The a priori biogenic emissions are provided by the MEGAN-ECMWF-v2 inventory for isoprene and from a previous optimization of methanol emissions based on IASI columns. Biomass burning and anthropogenic emissions are taken from GFEDv3 and from a combination of the RETRO global inventory with the regional inventory REASv2.

Given the different local overpass times of GOME2 (9h30 LT) and OMI (13h40 LT), the factors which might affect the diurnal cycle of formaldehyde in the model (chemistry, diurnal profile of emissions, mixing) are examined through sensitivity calculations. The simulated diurnal cycle is evaluated against ground-based measurements obtained by either the MAX-DOAS technique (in Europe, China, and Africa) or by FTIR (in the Indian Ocean). The agreement between simulated and observed normalized columns is found to be generally better in the summer (with a clear afternoon maximum at mid-latitude sites) than in the winter.

Both optimizations infer a reduction of the global biogenic emissions of isoprene (by 10-15%), which is largest (up to a factor of 2) over Eastern U.S. according to GOME2 and over Western Amazonia according to OMI. Those reductions and evaluated by comparisons with arcraft observations for different years (ARCTAS and INTEX-A). Northern Australia, Southern China and Northern Africa are also regions where both sensors indicate an overestimation of MEGAN. Biomass burning emissions appear to be likewise overestimated in Central Canada, Centrafrique and Burma. In contrast with these underestimations, the emissions due with agricultural fires in Northeastern China are strongly increased by the inversion (>50% in June according to OMI). Complex, and conflicting responses are found over Siberia and Alaska, possibly due to larger uncertainties associated to retrievals at those latitudes.