A study of the global cycle of carbonaceous aerosols in the LMDZT general circulation model

Abstract

The global atmospheric cycle of carbonaceous aerosols is simulated in the Laboratoire de Météorologie Dynamique general circulation model, and the subsequent aerosol optical depth is estimated for the period 1997 to 1999. The seasonal and interannual variability in the open biomass burning emissions has been improved by combining existing emission inventories and satellite measured fire counts. The model performance has been thoroughly evaluated against measured aerosol mass concentrations and optical depth in different regions of the globe. At a majority of locations, the modeled mass concentrations of black carbon (BC) at the surface are within a factor of two of observed values. The concentrations of organic carbon (OC) are generally underestimated in comparison to measurements. The discrepancies between model predicted values and measurements are attributable to the difference in time periods between the measurements and model simulations and/or a real underestimation of aerosol emissions in the model. The atmospheric residence times of both BC and OC aerosols are about a week. The hydrophilic fraction of carbonaceous aerosols accounts for about 90% of the total burden. Organic matter (OM) and associated water dominate the optical depth by carbonaceous aerosols with a 86% contribution (global mean of 0.031 at 0.55 μm). Different sensitivity experiments on the transformation time for conversion of hydrophobic to hydrophilic aerosols and emission partitioning show significant changes in the distribution of aerosol burdens and optical depth. The globally averaged burdens change by ±15% and residence times are shorter or longer by about 1 day in the various experiments as compared to the control simulation. In all of these experiments the largest sensitivity in aerosol concentrations is found in the remote regions and in the free troposphere (pressure range of 700–400 hPa). Emissions from biomass burning dominate the burden and optical depth of carbonaceous aerosols in the entire SH and NH tropics, while fossil fuel emissions dominate the NH extratropics. On the global scale biomass burning accounts for 78% of the total carbonaceous aerosol burden (BC + OM) followed by natural secondary organic aerosols (SOA)(14%) and fossil fuels (8%). The contributions to corresponding AOD are similar with the largest contribution from biomass burning (76%) followed by natural SOA (14%), and fossil fuels (10%).