Deciphering aerosol carbon fractions in PM2.5 & PM10 and their possible sources in Eastern Himalaya region

Abstract

This study provides a comprehensive assessment of particulate matter (PM10 and PM2.5) and their carbonaceous fractions-organic carbon (OC) and elemental carbon (EC), across the Sikkim Himalaya region during the summer and winter of 2022. Sampling was conducted at four strategically selected locations: Gangtok (1151m), Pelling (1962m), Mangan (1353m), and Lachen (2740m). Due to harsh winter condition, Lachen was inaccessible, limiting winter sampling to the remaining three sites. Quartz filter-based low volume air sampling was employed to ensure high precision data collection. Annual average concentrations of PM2.5 exceeded the National Ambient Air Quality Standards (NAAQs) of 40 μg/m³, with the values of 66.48 ± 10.81 μg/m³ (Gangtok), 42.05 ± 17.48 μg/m³ (Pelling), and 61.39 ± 13.24 μg/m³ (Mangan). PM10 levels followed a similar trend: 89.99 ± 12.04 μg/m³ (Gangtok), 58.64 ± 20.17 μg/m³ (Pelling), 82.71 ± 14.94 μg/m³ (Mangan). Winter exhibited significantly higher OC and EC concentrations, especially in Gangtok, attributed to intensified vehicular emissions and biomass burning. The OC/EC ratio exceeding 2 in winter suggests enhanced secondary organic carbon formation, predominately from biomass and coal combustion. Gangtok exhibits the highest SOC/OC ratio, indicating enhanced secondary organic aerosol formation, likely due to higher anthropogenic emissions or favourable atmospheric conditions. In contrast, lower SOC/OC ratios in Pelling and Mangan suggest weaker atmospheric processing or lower VOC emissions. Correlation analysis indicates primary emissions as the dominant source of OC and EC, with transboundary pollution playing a critical role. Air mass Trajectory analysis highlights substantial pollution transport from Afghanistan, and the Indo-Gangetic plain, emphasizing the regional impact on Himalayan air quality. This study underscores, the urgent need for integrated air quality management strategies to mitigate both local and cross-border pollution, safeguarding the fragile Himalayan ecosystem and public health. A combination of emission control policies, real-time monitoring, and predictive modelling is essential to address the escalating air pollution crisis in this ecologically sensitive region.