• • The volume-weighted mean total ion concentration was 246.2 μeq·L–1, with a distinct seasonal pattern: higher in non-monsoon, lower in monsoon, indicating strong dilution by monsoon precipitation and potential impacts on ecosystem acidification and nutrient deposition.
• • PMF source apportionment revealed that during the monsoon, Cl– and Na+ were ~70% from sea salt, while in the non-monsoon, >50% from biomass burning, highlighting the seasonal shift in dominant sources and implications for regional atmospheric chemistry.
• • NO3– was almost entirely from fossil fuel combustion during the monsoon (96%), decreasing to 72% in the non-monsoon, underscoring the persistent influence of anthropogenic emissions even in remote high-mountain areas.
• • Backward trajectory analysis showed that non-monsoon precipitation was dominated by westerly transport (60%), while monsoon precipitation was dominated by southwest monsoon transport (81%), which is critical for understanding long-range transport pathways and source regions.