Radiative effects of absorbing aerosols over northeastern India: Observations and model simulations

Multi-year measurements of spectral properties of aerosol absorption are examined over four geographically distinct locations of northeastern India. Results indicated significant spatio-temporal variation in aerosol absorption coefficients (σabs) with highest values in winter and lowest in monsoon. The western parts of the region, close to the outflow of Indo-Gangetic Plains, showed higher values of σabs and black carbon (BC) concentration - mostly associated with fossil fuel combustion. But, the eastern parts showed higher contributions from biomass burning aerosols, as much as 20-25% to the total aerosol absorption, conspicuously during pre-monsoon season. This is attributed to a large number of burning activities over the Southeast Asian region, as depicted from MODIS fire count maps, whose spatial extent and magnitude peaks during March/ April. The nearly consistent high values of Aerosol Index (AI) and layer height from OMI indicates the presence of absorbing aerosols in the upper atmosphere. The observed seasonality has been captured fairly well by GOCART as well as WRF-Chem model simulations. The ratio of column integrated optical depths due to particulate organic matter (POM) and BC from GOCART showed good coincidence with satellite based observations, indicating the increased vertical dispersion of absorbing aerosols, probably by the additional local convection due to higher fire radiative power caused by the intense biomass burning activities. The WRF-Chem though underperformed by different magnitude in winter, the values are closer or overestimated near the burnt areas. Atmospheric forcing due to BC was highest (~30 Wm-2) over the western part associated with the fossil fuel combustion.