Parametrization of Acid-Base Dissociation for Atmospheric Nanoparticle Growth Predictions

PAULUS BAUER, Sabrina Chee, Nanna Myllys, James Smith, Kelley Barsanti, University of California, Riverside & Irvine

     Abstract Number: 376
     Working Group: Aerosol Physical Chemistry and Microphysics

Abstract
Measurements and models have shown that growth rates of new particles are often higher than can be explained by the condensation of sulfuric acid (H₂SO₄) alone. In particular, organic compounds are thought to play some role in these higher growth rates. To contribute to the growth of ambient nanoparticles, organic compounds must be of sufficiently low volatility to overcome the Kelvin effect. Organic compounds with very low volatility can be formed through chemistry in the gas phase (e.g., autoxidation) and in/on particles (e.g., acid-base reactions and organic salt formation). The overall growth rate of atmospheric nanoparticles can be calculated as the sum of the growth rates due to condensation of H₂SO₄ and low-volatility organic compounds, reactive uptake, and formation of organic salts. However, due to uncertainties in these processes and the parameterizations used to represent them in models, agreement between measured and modeled growth has been elusive even under controlled conditions.

Here we used computational chemistry and laboratory experiments to test the applicability of bulk phase aqueous acid and base dissociation constants (pK_A and pK_B, respectively) for representing chemical processes in and on atmospherically relevant nanoparticles. Given the dependence of dissociation constants on ionic strength and solvent dielectric constant, there is a range of particle sizes and compositions for which aqueous bulk phase pK_A and pK_B values are likely insufficient to represent relevant acid-base chemistry. For such cases, we apply a parameterization that modifies the dissociation constant based on effects of size and solvent composition. To evaluate the sensitivity of environmental predictions, we implement the newly developed parameterization to model the role of organic salt formation in the growth of atmospheric nanoparticles. The extent to which organic salt formation contributes to nanoparticle growth, and consequently the growth rate, is important for predicting aerosol-climate interactions and health effects.