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

In this study, we examine the seasonal variability and long-term trends in aerosol distribution over peninsular Southeast Asia (SEA: 90 degrees-105 degrees E, 15 degrees-28 degrees N) based on MERRA-2 reanalysis datasets for a period of 30 years (1985-2015). Special emphasis has been given on the loading and distribution of carbonaceous aerosols, in response to changes in vegetation cover and biomass burning over SEA. MERRA-2 reanalysis reveals a distinct seasonality in columnar AOD distribution, that peaks (0.3-0.45) in the pre-monsoon (March-May) season, with a maximum contribution from carbonaceous aerosols (similar to 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 similar to 6 and 12 mu gm(-3) (similar to 0.6-1.6 mu gm(-3)) in the lower troposphere (surface to 700 hPa) 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 850 hPa level, revealing important atmospheric heating.