Characterization and sources of fine carbonaceous aerosol in winter over a megacity on Indo-Gangetic plain

Abstract

The characteristics and sources of carbonaceous aerosol (CA) was studied during winter in the lower atmosphere of Kolkata, a megacity situated in the lower Indo-Gangetic plain region. The city air was laden with typically high EC (15.4 ± 9.4 μg/m3) and OC (32.2 ± 17.8 μg/m3) in winter. NASA's MERRA-2 reanalysis aerosol products underestimated PM2.5, EC, and OC with a mean bias of 41.0 ± 56.8 μg/m3, 10.1 ± 9.3 μg/m3, and 10.8 ± 18.2 μg/m3, respectively. The EC/BC ratio for the atmosphere of Kolkata was estimated to be 1.03 ± 0.50. Levoglucosan was estimated in PM2.5 as a biomass burning tracer (266.9 ± 189.5 ng/m3). Source apportionment of CA using the tracer method indicated 43% of CA (19% of total PM2.5) was of secondary origin. 49% of total CA (21% of PM2.5) was from fossil fuel combustion, consisting of 23% of EC and 26% OC. Biomass burning contributed to 8% of CA and 3% of PM2.5. The diel trend of EC indicated equivalent primary emission throughout the day except being significantly high in the morning time during rush hours of traffic and increased residential combustion. Despite lower dispersion, OC formation was much higher during sunlight, indicating extensive secondary organic aerosol formation.