| 07 Oct 2025
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 this climate effect. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer co-located with gas analyzers was deployed during 25 April 2022 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 following by sulfate (20.8 %), BC (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 ageing 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.
The manuscript provides valuable insights into the chemical composition and aging processes of submicron aerosols in the Himalayas, particularly focusing on the impact of biomass burning (BB) aerosols during the pre-monsoon period. The use of high-resolution mass spectrometry along with real-time gas analyzers offers a comprehensive view of aerosol characteristics and their formation mechanisms. Additionally, the combination of positive matrix factorization and air mass trajectory analyses provides a clear understanding of the sources and transport pathways of these aerosols. The manuscript is generally well written, and I recommend it for publication after addressing the following comments:
Comments:
The authors used default RIEs for mass quantification. Did the authors perform any calibration of the AMS? It should be possible to obtain the actual RIE for NH4 from the calibration data.
Regarding the diurnal cycles, could the authors check those during the clean periods? The reason for this is that the sources and processes in the clean and polluted periods are different. The diurnal cycle for the entire study could be primarily influenced by the first polluted period with high mass loadings. Similarly, I suggest the authors examine the contributions of different processes in the two distinct periods (Figure 11).
The SOR is notably low compared to previous studies. It is somewhat surprising that the SO2 mixing ratio is relatively high (~3 ppb on average) in this study. Could this be a real measurement, or might it be influenced by instrument uncertainties? It’s like a baseline there?
For Figure 9, could the authors clarify the labels in the first two plots? Specifically, is it f44 or f(CO2+), and f60 or f(C2H4O2+)? These two have some differences.