Investigation of potential source regions of atmospheric Black Carbon in the data deficit region of the western Himalayas and its foothills

In the present study, BC was measured at three western Himalayan sites (Site-A approximate to 650 m, Site-B approximate to 2050m and Site-C approximate to 2600 m a.s.l.) during summer and winter seasons of 2014-2016. The objective of this study is to investigate the variability of BC as a function of meteorological conditions and identify potential combustion sources (biomass vs fossil fuel). Furthermore, boundary layer (ABL) height and wind patterns, MODIS fire maps and Concentrated Weightage Trajectory (CWT) analysis were utilized to study potential BC source regions. The average BC concentration at Site-A, B and C were observed to be 4.98 +/- 2.79, 4.57 +/- 3.54 and 3.01 +/- 2.95 mu gm(-3), respectively. At Site-B, daytime BC was observed to be higher than nighttime, indicating that ABL development and consequent upward transport of pollution from IGP contributed to high BC concentrations in lesser Himalayas. Nevertheless, variable fraction of BC attributable to biomass vs fossil fuel at three stations indicates the effect of transport from IGP overlapped with the effect of mixing with different combustion sources, including local biomass burning practices which modulated BC levels especially at a high-altitude site in winter. On the basis of CWT analysis and MODIS fire counts, emissions from agriculture residue burning in northwestern-IGP impacted the BC aerosols at all monitoring locations, although, based on our in-situ aethalometer measurements, biomass burning fraction of BC remained below 25-30% at Site-A and B. Episodic westerly long-range transport events (from Middle-East, Europe and Central-Asia) can contribute to background BC concentrations especially in wintertime which were, however, in range of sub-mu gm(-3) levels based on our observations at Site-C.