Seasonal Variation of OC, EC, and WSOC of PM10 and Their CWT Analysis Over the Eastern Himalaya

Abstract

In the present study, seasonal transport trends and potential source regions of carbonaceous species [organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), secondary organic carbon (SOC) and total carbonaceous aerosols (TCAs)] of PM10 was evaluated over Darjeeling, an eastern Himalayas of India during August 2018–June 2019. Backward trajectories, cluster analysis, and concentration-weighted trajectory (CWT) analysis were performed to evaluate the seasonal transport pathway of carbonaceous aerosols over the region. The annual average concentration of PM10 was recorded to be 55 ± 18 μg m−3, which is close to National Ambient Air Quality Standard (NAAQS; 60 μg m−3 for annual PM10) of India. The concentration of PM10 showed maxima in pre-monsoon season (63 ± 21 μg m−3) followed by post-monsoon (56 ± 16 μg m−3), monsoon (51 ± 13 μg m−3) and winter seasons (49 ± 17 μg m−3). The study revealed that the WSOC comprises about 72% of OC concentration with maxima in post-monsoon (81% of OC) followed by winter (74% of OC), pre-monsoon (67% of OC) and monsoon seasons (66% of OC). The concentration of SOC were estimated as 1.4 ± 0.9, 1.7 ± 1.0, 2.4 ± 0.9 and 1.9 ± 0.9 µg m−3 during pre-monsoon, monsoon, post-monsoon and winter, respectively (which are accounted for 27%, 49%, 41% and 35% to the OC mass concentration, respectively). The results indicated that biomass burning could be one of the major sources of carbonaceous aerosols in Darjeeling. Five days backward trajectory analysis (including cluster and CWT analysis) revealed that the air-mass flow of pollutants towards the sampling site of Darjeeling majorly coming from continental sites (Nepal and Indo-Gangetic plain (IGP) region of India) and the Bay of Bengal (BOB).