Multi-year submicron aerosol chemical composition at a high-altitude site in the Western Ghats of India

Abstract

Organic aerosols (OAs) are the key factors influencing air quality and climate change. Studies providing long-term characterizations of the highly time-resolved chemical composition of aerosols and the sources of OAs in the ambient air remain limited due to the challenges associated with continuous observations. Here, we present an analysis of the long-term variability of submicron aerosols using an Aerosol Chemical Speciation Monitor (ACSM). The study presents the interannual and interseasonal measurements of non-refractory submicron particulate matter with aerodynamic diameter ≤1 µm (NR-PM₁) at a high-altitude site in the Western Ghats of India from October 2015 to February 2018. The mean NR-PM₁ concentration observed during the study period was 9.5 ± 8.5 µg m⁻³, dominated by OAs (55 ± 16%) followed by sulphate (SO₄, 27 ± 13%), ammonium (NH₄, 11 ± 6%), nitrate (NO₃, 6 ± 4%), and chloride (Cl, 1 ± 0.5%). The percentage contribution of NR-PM₁ species across different years showed some disparity when compared by season. The diurnal pattern of OAs closely resembled that of NR-PM₁, while NH₄ exhibited a combined diurnal pattern of SO₄ and NO₃. Similar diurnal patterns of NR-PM₁ species were observed across years in each season, though with varying magnitudes. The highest seasonal average concentrations of NR-PM₁, OAs, NO₃ and NH₄ were observed during the winter season, followed by the post-monsoon season. Source apportionment of OAs was carried out using Positive Matrix Factorization (PMF). Secondary factors contributed markedly in the winter, post-monsoon, and summer seasons in 2015 and 2017, whereas in 2016, primary and secondary OAs contributed nearly equally.