The underappreciated role of nonvolatile cations in aerosol ammonium-sulfate molar ratios
Overprediction of fine-particle ammonium-sulfate molar ratios (inline-formulaR) by thermodynamic models is suggested as evidence for interactions with organic constituents that inhibit the equilibration of gas-phase ammonia with aerosol sulfate and questions the equilibrium assumption long thought to apply for submicron aerosol. This hypothesis is tested through thermodynamic analysis of ambient observations. We find that the deviation between inline-formulaR from a molar ratio of 2 is strongly correlated with the concentration of sodium (inline-formulaNa+), a nonvolatile cation (NVC), but exhibits no correlation to organic aerosol (OA) mass concentration or mass fraction. Thermodynamic predictions of both inline-formulaR and ammonia gas–particle partitioning can accurately reproduce observations when small amounts of NVCs are included in the calculations, whereas exclusion of NVCs results in a predicted inline-formulaR consistently near 2. The sensitivity of inline-formulaR to small amounts of NVCs arises because, when the latter are present but not included in the thermodynamic calculations, the missing cations are replaced with ammonium in the model (inline-formulaNH3–inline-formula
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