American Association for Aerosol Research - Abstract Submission

AAAR 36th Annual Conference
October 16 - October 20, 2017
Raleigh Convention Center
Raleigh, North Carolina, USA

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The Role of Nonvolatile Cations on Aerosol Ammonium-Sulfate Molar Ratios and Aerosol pH

RODNEY J. WEBER, Hongyu Guo, Athanasios Nenes, Georgia Institute of Technology

     Abstract Number: 747
     Working Group: Aerosol Chemistry

Abstract
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 only gas-phase ammonia (but not water or nitric acid) with aerosol sulfate and questions the equilibrium assumption long thought to apply for submicron aerosol. The ubiquity of such organic films would imply significant impacts on aerosol chemistry. We test the organic film hypothesis by analyzing ambient observations and find that 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 for the conditions considered.