Assessment of carbonaceous aerosols at Mukteshwar: A high-altitude (~2200 m amsl) background site in the foothills of the Central Himalayas

Abstract

The present study examined the equivalent black carbon (eBC) mass concentrations measured over 10.5 years (September 2005–March 2016) using a 7-wavelength Aethalometer (AE-31) at Mukteshwar, a high-altitude and regional background site in the foothills of Indian central Himalayas. The total spectral absorption coefficient (babs) was divided into three categories: black carbon (BC) and brown carbon (BrC); fossil fuels (FF) and wood/biomass burning (WB/BB); and primary and secondary sources. At the wavelength of 370 nm, a significant BrC contribution (25 %) to the total babs is identified, characterized by a pronounced seasonal variation with winter (December–January-February) maxima (31 %) and post-monsoon (October and November) minima (20 %); whereas, at 660 nm, the contribution of BrC is dramatically less (9 %). Climatologically, the estimated BCFF at 880 nm ranges from 0.25 ± 0.19 μg m−3 in July to 1.17 ± 0.80 μg m−3 in May with the annual average of 0.67 ± 0.63 μg m−3, accounting for 79 % of the BC mass. The maximum BCFF/BC fraction reaches its peak value during the monsoon (July and August, 85 %), indicating the dominance of local traffic emissions due to tourism activities. Further, the highest BCWB concentration observed during pre-monsoon (March–May) suggests the influence of local forest fires along with long-range transported aerosols from the low-altitude plains. The increased contribution of BrC (26 % at 370 nm) and WB absorption (61 % at 370 nm) to the total absorption at the shorter wavelengths suggests that wood burning is one of the major sources of BrC emissions. Secondary BrC absorption accounts for 24 % [91 %] of the total absorption [BrC absorption] at 370 nm, implying the dominance of secondary sources in BrC formation. A trend analysis for the measured BC concentration shows a statistically significant increasing trend with a slope of 0.02 μgm−3/year with a total increase of about 22 % over the study period. A back trajectory-based receptor model, potential source contribution function (PSCF), was used to identify the potential regional source region of BC. The main source regions of BC are the northwest states of India in the IGP region and the northeast Pakistan region.