Salting Effects Alter the Gas-Aqueous Partitioning and SOA Potential of Formic and Acetic Acid

AMIR BABAEI-GHAREHBAGH, Rose Taylor, Vikram Pratap, Annmarie Carlton, Christopher Hennigan, University of Maryland, Baltimore County

     Abstract Number: 49
     Working Group: Aerosol Chemistry

Abstract
Formic acid (FA) and acetic acid (AA) are abundant atmospheric carboxylic acids that play important roles in wet deposition acidity and secondary organic aerosol (SOA) formation. Despite their significance, observations of FA and AA gas-particle partitioning differ substantially from thermodynamic predictions, contributing to uncertainties in SOA burden estimates. In this study, we systematically investigate the effects of sodium chloride (NaCl) and ammonium sulfate ((NH₄)₂SO₄) on the partitioning behavior of FA and AA using two independent experimental approaches: dual parallel mist chambers and a shared headspace method. Our results reveal a consistent salting-out effect for both compounds in NaCl solutions, with Setschenow constants indicating increased volatilization with rising salt concentrations. In contrast, (NH₄)₂SO₄ was found to have minimal impact on FA and AA partitioning at concentrations up to 2 mol kg-1. Additionally, pH-dependent experiments confirmed that partitioning behavior is strongly modulated by acid dissociation equilibria, especially when the solution pH nears the pKa of the acids. These findings demonstrate that both ionic strength and pH influence gas-aqueous partitioning of small organic acids; - implications for ambient SOA formation and the potential to reconcile observations with thermodynamic predictions will be discussed.