Thermodynamic insights into the molecular evolution of water-soluble organic matter in suburban PM2.5

Water-soluble organic matter (WSOM) is a major aerosol component, yet its seasonal molecular variability and thermodynamic evolution remain poorly understood. Here, WSOM extracted from fine particulate matter (PM 2.5 ) collected across four seasons in humid suburban Chongqing was analyzed using ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. Carboxyl-rich alicyclic molecules and aliphatic compounds were key contributors to seasonal differences in molecular composition, with autumn and winter WSOM exhibiting broader combustion-enthalpy distributions than spring and summer WSOM. Biomass burning and fatty-acid-rich primary emissions contributed more in autumn and winter, consistent with enhanced accumulation during cold-humid pollution episodes. Furthermore, six distinct molecular transformation pathways were identified as pollution levels increased. Among these, methylation/demethylation, hydrogenation/dehydrogenation, and hydroxylation/dehydroxylation accounted for approximately 86.8% of transformations, whereas sulfation/desulfation showed the strongest thermodynamic sensitivity. These findings provide molecular-level insights into the seasonal evolution of WSOM in suburban PM 2.5 , offering a scientific basis for suburban air pollution control.