Impact of Biomass Burning Plumes on the Size-Segregated Aerosol Chemistry over an Urban Atmosphere at Indo-Gangetic Plain

A study was conducted on the impact of biomass burning plumes transported from the Eastern Ghats region on the chemical characteristics of size-segregated aerosols over Kolkata, a tropical megacity situated in the eastern part of the Indo-Gangetic Plain. A Micro Orifice Uniform Deposit Impactor (MOUDI) was used to collect size-segregated aerosols affected and unaffected by the influence of transported biomass burning plumes during the pre-monsoon season (March-May) in 2016. Satellite observations revealed the presence of a thick polluted aerosol layer over Kolkata during the fire episodes, whereas a clean marine aerosol layer was observed during the normal episodes. Transported plumes enhanced the fine-mode aerosols, whereas the coarse-mode aerosols remained unaffected. PM1.0-0.1 and PM0.1 were greatly affected by the transported plumes, with a threefold increase during the fire episodes. K+, NH4+ and SO42- showed significant increases in the accumulation-mode (PM1.0-0.1) under the influence of biomass burning plumes, whereas NO3- showed an increase in the coarse-mode aerosols. The transported plumes did not change the mass-size distribution patterns of K+, NH4+ and SO42- (in which the unimodal fine-mode was dominant), but NO3- showed a change from a bimodal to a unimodal coarse-mode distribution under the influence of the plumes. It was observed that the biomass burning plumes interacted strongly with the accumulation- and superfine-mode (PM0.1) sea-salt particles, leading to the depletion of chloride. The maximum chloride depletion (similar to 80%) was observed in the size range of 0.560.32 mu m. It was observed that non-sea-SO42- was the key constituent responsible for the chloride loss from sea-salt particles. The biomass burning plume was found to have the potential to shift the dominance from coarse- to fine-mode particles in the mass-size distribution pattern of the total aerosols.