Characteristics, source apportionment and long-range transport of black carbon at a high-altitude urban centre in the Kashmir valley, North-western Himalaya

Abstract

Six years of data (2012–2017) at an urban site-Srinagar in the Northwest Himalaya were used to investigate temporal variability, meteorological influences, source apportionment and potential source regions of BC. The daily BC concentration varies from 0.56 to 40.16 μg/m3 with an inter-annual variation of 4.20–7.04 μg/m3 and is higher than majority of the Himalayan urban locations. High mean annual BC concentration (6.06 μg/m3) is attributed to the high BC observations during winter (8.60 μg/m3) and autumn (8.31 μg/m3) with a major contribution from Nov (13.88 μg/m3) to Dec (13.4 μg/m3). A considerable inter-month and inter-seasonal BC variability was observed owing to the large changes in synoptic meteorology. Low BC concentrations were observed in spring and summer (3.14 μg/m3 and 3.21 μg/m3), corresponding to high minimum temperatures (6.6 °C and 15.7 °C), wind speed (2.4 and 1.6 m/s), ventilation coefficient (2262 and 2616 m2/s), precipitation (316.7 mm and 173.3 mm) and low relative humidity (68% and 62%). However, during late autumn and winter, frequent temperature inversions, shallow PBL (173–1042 m), stagnant and dry weather conditions cause BC to accumulate in the valley. Through the observation period, two predominant diurnal BC peaks were observed at ⁓9:00 h (7.75 μg/m3) and ⁓21:00 h (6.67 μg/m3). Morning peak concentration in autumn (11.28 μg/m3) is ⁓2–2.5 times greater than spring (4.32 μg/m3) and summer (5.23 μg/m3), owing to the emission source peaks and diurnal boundary layer height. Diurnal BC concentration during autumn and winter is 65% and 60% higher than spring and summer respectively. During autumn and winter, biomass burning contributes approximately 50% of the BC concentration compared to only 10% during the summer. Air masses transport considerable BC from the Middle East and northern portions of South Asia, especially the Indo-Gangetic Plains, to Srinagar, with serious consequences for climate, human health, and the environment.