Seasonal Evolution of Atmospheric Water-Soluble Organic Matter in Humid Suburban PM2.5

Water-soluble organic matter (WSOM) is a major component of atmospheric aerosols, yet most constituents remain insufficiently characterized and their seasonal evolution is not well understood. In this study, WSOM extracted from fine particulate matter (PM_2.5) collected across four seasons in suburban Chongqing was analyzed using ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Carboxylic-rich alicyclic and aliphatic compounds emerged as key contributors to the seasonal evolution of WSOM. Seasonal relationships linking the combustion enthalpy of WSOM to the nominal oxidation state of carbon were established, with WSOM in autumn and winter exhibiting broader energy-content distributions. Six distinct molecular evolution pathways were identified during polluted conditions, with methylation/demethylation and hydrogenation/dehydrogenation accounting for about 75% of transformations, whereas sulfation/desulfation emerged as the most thermodynamically active process. These findings provide molecular-level insight into the seasonal evolution of WSOM in humid suburban PM_2.5, offering a scientific basis for suburban air pollution control.