The Underappreciated Role of Nonvolatile Cations on Aerosol Ammonium–Sulfate Molar Ratios

Overprediction of fine-particle ammonium-sulfate molar ratios (R) by thermodynamic models is suggested as evidence for an organic film that selectively inhibits the equilibration of gas-phase ammonia with aerosol sulfate and questions the equilibrium assumption between gas and particle phase ammonia (and other species) long thought to apply for submicron aerosol. It is also reported to result in improvements in model-predicted ammonia concentrations. The ubiquity of such organic films would imply significant impacts on many aspects of aerosol chemistry. We test the organic film hypothesis by analyzing ambient observations and find that both R and ammonia partitioning can be accurately reproduced when small amounts of nonvolatile cations (NVC), consistent with observations, are considered in the thermodynamic analysis. Exclusion of NVCs results in predicted R consistently near 2. The error in R is positively correlated with NVC and not organic aerosol mass fraction or concentration. These results strongly challenge the postulated ability of organic films to perturb aerosol acidity or prevent ammonia from achieving gas-particle equilibrium and further confirms that ammonia equilibrium can be accurately predicted with a thermodynamic model that only includes inorganic species, for the conditions considered.