Assessing pH- and temperature-dependence in the aqueous phase partitioning of organic acids and bases in the atmosphere

Abstract

The gas-particle partitioning of low-volatility and semi-volatile organic compounds (L/S-VOCs) plays a dominant role in the formation of secondary organic aerosol, carrying implications for the health and climate effects of atmospheric particulate matter. Partitioning into aqueous particles and cloud droplets can also impact the fates of L/S-VOCs in the atmosphere. As the NH₃/NH₄⁺ conjugate pair begins to dominate the buffering capacity of the atmospheric aqueous phase, there is a growing need to consider how changing particle acidity may impact the phase distribution of different ionizable compounds. In this work, we use a partitioning space framework and graphical assessment method to predict the effects of varied pH and temperature on the partitioning behavior of 24 ionizable organic compounds, including carboxylic acids and amines. As pH increases from 2 to 6, amines exhibit significantly increased affinity for the gas phase, whereas a preference for the aqueous phase is generated among several weak acids that would otherwise have remained vapors. We find that temperature can have a strong influence on the partitioning of some compounds. However, temperature-dependence can vary widely between compounds, and our analysis was limited by a lack of enthalpy values, necessitating reliable thermodynamic data for a larger number of L/S-VOCs. We implement a new visualization to investigate the partitioning behavior of lesser-studied compounds under varied conditions, and through this approach we see that aerosol liquid water content can greatly impact pH-sensitivity in partitioning.