A year-long study on PM2.5 and its carbonaceous components over eastern Himalaya in India: Contributions of local and transported fossil fuel and biomass burning during premonsoon

A year-long (March 2019–February 2020) study on the characterization of fine mode carbonaceous aerosols has been conducted over a high altitude urban atmosphere, Darjeeling (27.01°N, 88.15°E, 2200 m asl) in eastern Himalaya. The fine mode aerosol (PM2.5; 41.7 ± 23.7 μgm−3), total carbonaceous aerosols (TCA; 19.8 ± 7.7 μgm−3), organic carbon (OC; 8.0 ± 3.9 μgm−3) and elemental carbon (EC; 2.0 ± 0.9 μgm−3) exhibited similar seasonal variability with the highest abundance during winter followed by premonsoon, postmonsoon and minimum in monsoon. The OC:EC varied over a range of 2.8–19.4 whereas the secondary organic carbon ranged between 1.9 and 17.1 μgm−3 respectively. Higher PM2.5 associated with higher winds and elevated mixing layer depth suggest a strong influence of regional and long-range transport. In addition to the usual morning and evening rush-hour peaks, the impact of low land plain regions driven by up-slope valley winds was observed for the carbonaceous components. A novel approach has been taken to find out the individual contributions from the local and transported fossil fuel, biomass burning, and biogenic sources to OC and EC during premonsoon. We observed that the local fossil fuel (43%) contributions dominated over the biomass burning (39%) for EC whereas the contributions of local biomass burning and the local fossil fuel were same (46%) for OC. EC exhibited a higher contribution (18%) from the regional/long-range transport compared to OC (8%). IGP and Nepal were found to be the maximum contributing long distant source regions for the carbonaceous aerosol loading over eastern Himalaya. Such individual source apportionment of carbonaceous aerosols over eastern Himalaya makes the study unique and first-ever of its kind and immensely helpful for building robust mitigation action plans.