Characterization of the seasonal cycle, regional distribution and outflow of South Asian aerosols: A three dimensional chemical transport model analysis

A regional chemical transport model is used to study the aerosol seasonality, regional distribution, and composition over South Asia. Model predictions of sulfate, black carbon, primary organic carbon and total particulate mass are compared with observations from the Atmospheric Brown Cloud Project. The results show anthropogenic aerosols over South Asia peak during the early dry season and remain elevated until the summer monsoon rain scavenges them out from the atmosphere. Modeling results also show a high dust concentration over South Asia with different seasonality than that of anthropogenic aerosols. To improve model prediction skills and better understand aerosol distribution, monthly mean Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) was used in an optimal interpolation assimilation scheme. MODIS AOD and aerosol fine mode fraction information (where available) were used in the assimilation. The optimal interpolation algorithm is able to improve model aerosol total mass prediction skills and better capture temporal variability. Inter annual variability from a four year study shows high coefficient of variation over areas of biomass burning and desert regions reflecting uncertainties in emissions estimates. Outflow of Asian aerosols and trace gases to the Pacific were analyzed using the model constrained by observations from the Intercontinental Chemical Transport Experiment - Phase B (INTEXB) field campaign. Back trajectory and source region tagged tracer modeling tools were used to identify pollutant source areas and possible quasi Lagrangian sampling of the air-mass. INTEXB simulation results show different regional distribution and transport pathways for aerosols and trace gases over the Pacific. The role of aerosols in influencing trace gas concentration via heterogeneous reactions and photolysis rates was shown to be significant over the western Pacific but was found to be minimal over the Central and Eastern Pacific. This study analyzed the role of residential bio-fuel as a sector for carbonaceous aerosol loading over South Asia in the present and near future (2030) based on four different IPCC development pathways. Model results show substantial contribution from bio-fuel to aerosol mass loading with repercussions on human health and implications to radiative forcing over South Asia.