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The widespread presence of organic aerosols (OA) in the atmosphere and their dominant role in climate forcing are increasingly recognized. Measurements from the high‐altitude region provide insights into the natural processes and long‐range transport of aerosols. We studied water‐soluble organic compounds in PM1.1 during January‐March 2020 and February‐April 2021 at Ponmudi (8.8°N and 77.1°E), a tropical hill station (960 m a.s.l.) located on the Western Ghats of India. The 5‐day backward trajectories of air masses presented that continental and marine sources impacted the site during the campaign, and thus we classified samples as mixed and marine air masses. Their molecular fingerprints revealed the prevalence of oxalic acid (C2), followed by terephthalic acid in mixed air mass and malonic (C3) or succinic (C4) acid in maritime air mass aerosol samples. This feature and the phthalic to azelaic acid ratio, as well as the C3 to C4 and fumaric to maleic acid ratios, evidenced that water‐soluble OA were of anthropogenic origin and less photochemically aged in mixed air mass samples, whereas they derived from biogenic sources and more photochemical aging in marine air mass samples. However, we found their comparable contributions in water‐soluble organic carbon under mixed and marine air masses. Linear correlations between C2 relative abundance and its ratios with the precursor compounds implied that the oxidation of precursors to oxalic acid occurs during atmospheric transport. The results revealed that the input of anthropogenic and biogenic precursors followed by atmospheric processing controlled the burden of water‐soluble OA across the Western Ghats. Plain Language Summary: Dicarboxylic acids (DCAs), oxocarboxylic acids and dicarbonyls are ubiquitous water‐soluble components of organic aerosol (OA) that can serve as cloud condensation nuclei and affect the climate of the Earth. We reveal for the first time the molecular fingerprints and sources as well as formation processes of water‐soluble OA in PM1.1 at a tropical hill station in Western Ghats of India. Significant differences in the molecular fingerprints and atmospheric processes of water‐soluble OA were found due to the influence of different air masses. DCAs and related compounds in mixed air mass aerosols originated from anthropogenic sources and produced in the atmosphere by photooxidation during long‐range transport whereas they were of biogenic origin followed by intense photochemical aging of biogenic volatile organic compounds such as isoprene and unsaturated fatty acids during atmospheric transport in marine air mass aerosols. The results revealed that DCAs substantially contribute to water‐soluble OA not only in mixed air mass aerosols but also in marine air mass aerosols at the tropical hill station in the Western Ghats. These findings have implications for assessing climate‐forcing feedback of secondary OA over the Western Ghats that is driven by anthropogenic and biogenic sources followed by photochemical processes. Key Points: Mixed and marine air masses drive the molecular fingerprints of atmospheric water‐soluble organic compounds over the Western GhatsAerosols of mixed air masses comprised dicarboxylic acids (DCAs) of anthropogenic origin and less photochemically processedDCAs were produced from biogenic sources followed by intense photochemical aging in marine air mass aerosols [ABSTRACT FROM AUTHOR]

Abstract The widespread presence of organic aerosols (OA) in the atmosphere and their dominant role in climate forcing are increasingly recognized. Measurements from the high‐altitude region provide insights into the natural processes and long‐range transport of aerosols. We studied water‐soluble organic compounds in PM1.1 during January‐March 2020 and February‐April 2021 at Ponmudi (8.8°N and 77.1°E), a tropical hill station (960 m a.s.l.) located on the Western Ghats of India. The 5‐day backward trajectories of air masses presented that continental and marine sources impacted the site during the campaign, and thus we classified samples as mixed and marine air masses. Their molecular fingerprints revealed the prevalence of oxalic acid (C2), followed by terephthalic acid in mixed air mass and malonic (C3) or succinic (C4) acid in maritime air mass aerosol samples. This feature and the phthalic to azelaic acid ratio, as well as the C3 to C4 and fumaric to maleic acid ratios, evidenced that water‐soluble OA were of anthropogenic origin and less photochemically aged in mixed air mass samples, whereas they derived from biogenic sources and more photochemical aging in marine air mass samples. However, we found their comparable contributions in water‐soluble organic carbon under mixed and marine air masses. Linear correlations between C2 relative abundance and its ratios with the precursor compounds implied that the oxidation of precursors to oxalic acid occurs during atmospheric transport. The results revealed that the input of anthropogenic and biogenic precursors followed by atmospheric processing controlled the burden of water‐soluble OA across the Western Ghats.