Long term variability of carbonaceous aerosol over Southeast Asia: Association with changes in vegetation cover and biomass burning

This study presents the seasonal variability and long term trends in aerosols distribution over peninsular Southeast Asia(90^°-105^°E, 15^°-28^°N)based on Modern-Era Retrospective Analysis for Research and Application (MERRA-2) reanalysis datasets for a period of 30 years (1985-2015). Variability in the loading and distribution of carbonaceous aerosols in response to changes in biomass burning and vegetation cover over the region has been studied with special emphasis. MERRA-2 reanalysis reveal a distinct seasonality in columnar aerosol optical depth (AOD) distribution, that peaks (0.3-0.45) in the pre-monsoon (March-May) season, with a maximum contribution from carbonaceous aerosols (~60%). The ratio of organic carbon (OC) to black carbon (BC) column mass density indicates a dominant contribution from biomass burning along with biogenic emissions (OC/BC > 10), while the OC (BC) aerosols during pre-monsoon vertically vary between ~6-12 μgm^-3 (~0.6-1.6 μgm^-3) in the lower troposphere (surface to 700hPa) during pre-monsoon. Interestingly, linear regression in monthly variability of the enhanced vegetation index (EVI) over SEA shows a slightly increasing tendency during the recent decade (2004-2015), while the number of fire counts decreased during the same time period. In addition, a slightly positive trend in OC-AOD is likely associated with the increase in vegetation cover that contributes to the rising trend of biogenic OC emissions, while the reduction in biomass burning is linked with the declining trend of BC-AOD. The pre-monsoon average shortwave flux at the top of the atmosphere (TOA) obtained from MERRA-2 over SEA varies from -12 to -6 Wm^-2 and presents a decreasing tendency during the last 30 years due to a decrease in absorbing BC fraction. The temperature anomalies are well related to changes in BC-AOD, and OC-AOD at 850hPa level, revealing important atmospheric heating.