Evaluating PM_2.5 acidity using thermodynamic models and source contributions in Eastern India

Fine particulate matter (PM_2.5) pollution over India, particularly across the Indo-Gangetic Plain (IGP), is strongly influenced by secondary inorganic aerosols (SIA) formed from abundant alkaline and acidic precursors; however, aerosol acidity and aerosol water content (AWC) remain poorly constrained outside major urban centres. This study presents one of the first thermodynamic model-based, seasonally resolved estimates of fine-mode aerosol pH and AWC from a regional receptor site in eastern India, within the COALESCE (Carbonaceous Aerosols Emission, Source Apportionment and Climate Impacts) framework. Aerosol pH was evaluated using the ISORROPIA-II and E-AIM thermodynamic models in both forward and reverse modes. ISORROPIA forward-mode simulations, which used particle-phase composition alone, yielded physically reasonable, unimodal pH distributions and reduced sensitivity to ion balance uncertainties. Using this framework, aerosols at Mesra were consistently acidic to moderately acidic, with pronounced seasonal variability; annual mean pH ranged from 1.97 ± 0.81 to 3.14 ± 0.71 across sensitivity simulations. AWC exhibited a strong seasonal dependence, with maxima during winter and post-monsoon (~ 36 µg m^-3) and minima during the pre-monsoon period (~ 6 µg m^-3). Aerosol pH covaried positively with AWC and relative humidity and negatively with temperature, reflecting thermodynamic shifts between nitrate- and sulfate-dominated regimes. Principal component analysis identified SIA formation as the dominant contributor to water-soluble inorganic ions, with additional influences from biomass burning, coal combustion, crustal dust, and sea salt. These results provide region-specific constraints on aerosol acidity in eastern India, offering critical insights for air quality management.