Factors controlling the long-term (2009-2015) trend of PM2.5 and black carbon aerosols at eastern Himalaya, India

A first-ever long-term (2009-2015) study on the fine particulate matter (PM2.5) and black carbon (BC) aerosol were conducted over Himalaya in order to investigate the characteristics, temporal variations and the important factors regulating the long-term trend. The study was conducted over a high altitude station, Darjeeling (27 degrees 01'N, 88 degrees 15'E, 2200 m asl) representing a typical high altitude urban atmosphere at eastern Himalaya in India. The average concentrations of PM2.5 and BC over a period of seven years were 25.2 +/- 5.6 mu g m(-3) (ranging between 2.2 and 220.4 mu g m(-3)) and 3.4 +/- 0.7 mu g m(-3) (0.4 to 15.6 mu g m(-3)) respectively. We observed decreasing trends in both PM2.5 (49% at a rate of 170 ng m(-3) month(-1)) and BC (34% at the rate of 20 ng m(-3) month(-1)) mass concentration over this region from 2009 to 2015. We extensively studied the impact of micrometeorological parameters on the long-term trend in PM2.5 and BC through the correlation analysis. The significant changes in boundary layer dynamics over this region played a major role in the decreasing trend of aerosols. The concentration weighted trajectory analysis revealed that the important contributory long-distant source regions for PM2.5 and BC over eastern Himalaya were Indo Gangetic Plane and Nepal. The contributions from these regions were found to be decreased significantly from 2009 to 2015. Investigations on the fire counts associated with the forest fire, and open burning activities through the satellite observations revealed that the decreasing trend in PM2.5 and BC over eastern Himalaya is well correlated to the decreasing trend in the fire counts over IGP and Nepal. We also explored that the changes and up gradation of the domestic fuel at the Indo Gangetic Plane regions in recent years not only improved the regional air quality but also affected the atmospheric environment over the eastern part of Himalaya. (C) 2018 Published by Elsevier B.V.