Absorbing aerosols: contribution of biomass burning and implications for radiative forcing

Abstract

Absorbing aerosols supplements the global warming caused by greenhouse gases. However, unlike greenhouse gases, the effect of absorbing aerosol on climate is not known with certainty owing to paucity of data. Also, uncertainty exists in quantifying the contributing factors whether it is biomass or fossil fuel burning. Based on the observations of absorption coefficient at seven wavelengths and aerosol optical depth (AOD) at five wavelengths carried out at Gadanki (13.5° N, 79.2° E), a remote village in peninsular India, from April to November 2008, as part of the "Study of Atmospheric Forcing and Responses (SAFAR)" pilot campaign we discuss seasonal variation of black carbon (BC) concentration and aerosol optical depth. Also, using spectral information we estimate the fraction of fossil-fuel and nonfossil fuel contributions to absorption coefficient and contributions of soot (Black Carbon), non-soot fine mode aerosols and coarse mode aerosols to AOD. BC concentration is found to be around 1000 ng/m³ during monsoon months (JJAS) and around 4000 ng/m³ during pre and post monsoon months. Non-fossil fuel sources contribute nearly 20% to absorption coefficient at 880 nm, which increases to 40% during morning and evening hours. Average AOD is found to be 0.38±0.15, with high values in May and low in September. Soot contributes nearly 10% to the AOD. This information is further used to estimate the clear sky aerosol direct radiative forcing. Top of the atmosphere aerosol radiative forcing varies between -4 to 0Wm^-2, except for April when the forcing is positive. Surface level radiative forcing is between -10 to -20Wm^-2. The net radiation absorbed within the atmosphere is in the range of 9 to 25Wm^-2, of which soot contributes about 80 to 90%.