Absorption and radiative characteristics of brown carbon aerosols during crop residue burning in the source region of Indo-Gangetic Plain

Abstract

The north-western Indo-Gangetic Plain (IGP) experiences massive crop-residue burning (viz. paddy and wheat) on an annual and seasonal basis. The long-range transport of the particulates emitted from paddy- and wheat-residue burning (expressed as PRB and WRB, respectively) degrades the air quality, perturb the radiative budget, and alter the atmospheric chemistry of downwind IGP locations. Therefore, chemical, absorption and radiative characteristics of carbonaceous aerosols (total carbon; TC) were explored in this study. The fraction of TC in ambient PM2.5 (particulates with aerodynamic diameter ≤ 2.5 μm) was ~45% during PRB and ~24% during WRB. However, biomass burning emissions were the predominant source of TC during both PRB and WRB. The brown carbon (BrC) aerosols at Beas were ~2–3 times more abundant during PRB than in WRB. However, the absorption properties such as mass absorption efficiency and imaginary component of the refractive index for BrC at 405 nm (expressed as MAEBrC-405, kBrC-405, respectively) and radiative characteristics such as light absorption capacity were similar during both PRB and WRB. The similarity between these absorption and radiative characteristics indicate that BrC aerosols emitted during the burning of different biomass may depend only on their combustion condition. Further, the increased biomass burning emissions were linked with enhancement in the light absorption capacity of BrC during PRB. A similar light absorption capacity was observed (~30 W/g) for water-soluble BrC (WS-BrC) and total BrC during PRB. Moreover, the % contribution of BrC and EC to their total direct radiative forcing (DRFTC) during PRB and WRB (~40% and ~60% for BrC and EC, respectively) were also similar. The WS-BrC constitutes only ~15% of DRFTC during PRB. This difference signifies that non-WS-BrC aerosols were the predominant light-absorbing species during PRB (compared to WS-BrC), which needs to be factored into global climate models to mitigate uncertainties.