Highly time-resolved chemical characteristics and aging process of submicron aerosols over the central Himalayas

Abstract. Aerosol particles transported from South Asia, especially biomass burning (BB) emission related aerosols during pre-monsoon, have significant climate effect in the Himalayas. The details on complicated physicochemical properties and aging process of aerosols are important for understanding the climate effect. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer co-located with gas analyzers was deployed from 25 April to 25 May 2022 to study the highly time-resolved chemical characteristics and aging process of submicron aerosols (PM1) on the northern slope of the Himalayas. The 10 min resolution mass concentration of PM1 varied from 0.1 to 12.2 µg m−3 during this study, with an average of 1.7 ± 1.6 µg m−3. Organic aerosols (OA) showed a dominant contribution (46.2 %) to PM1 followed by sulfate (20.8 %), black carbon (19.4 %), ammonium (8.5 %), nitrate (4.8 %) and chloride (0.4 %). Evolution of bulk OA in the f44 vs. f60 space showed clear aging process from less aged BB plumes to highly oxidized state in polluted period. Positive matrix factorization (PMF) on the high-resolution organic mass spectra resolved two oxygenated OA (OOA) factors, i.e., a less-oxidized OOA influenced by biomass burning (OOA-BB) and a more-oxidized OOA (MO-OOA). We performed a case study to explore the OOA formation mechanism during long-range transport. The results indicated aqueous-phase process and photochemical reaction together elevated OOA concentrations and aging processing, consistent with secondary inorganic aerosol production. This study underscores the significant occurrence of BB aerosols in Himalayas and provides insights into the oxidative processing in this remote region.