Low-Molecular-Weight Carboxylic Acids in the Southeastern US: Formation, Partitioning, and Implications for Organic Aerosol Aging

While carboxylic acids are important components in both particle and gas phases in the atmosphere, their sources and partitioning are not fully understood. In this study, we present real-time measurements of both particle- and gas-phase concentrations for five of the most common and abundant low-molecular-weight carboxylic acids (LMWCA) in a rural region in the southeastern U.S. in Fall 2016. Through comparison with secondary organic aerosol (SOA) tracers, we find that isoprene was the most important local precursor for all five LMWCA but via different pathways. We propose that monocarboxylic acids (formic and acetic acids) were mainly formed through gas-phase photochemical reactions, while dicarboxylic acids (oxalic, malonic, and succinic acids) were predominantly from aqueous processing. Unexpectedly high concentrations of particle-phase formic and acetic acids (in the form of formate and acetate, respectively) were observed and likely the components of long-range transport organic aerosol (OA), decoupled from their gas-phase counterparts. In addition, an extraordinarily strong correlation (R-2 = 0.90) was observed between a particulate LMWCA and aged SOA, which we tentatively attribute to boundary layer dynamics.

Automated summary

Carboxylic acids play important roles in both particle and gas phases in the atmosphere, with their sources and partitioning not entirely understood. Isoprene is identified as a key precursor for LMWCA, with monocarboxylic acids likely formed through gas-phase photochemical reactions and dicarboxylic acids predominantly from aqueous processing. Unexpectedly high concentrations of particle-phase formic and acetic acids, possibly components of long-range transport OA, were observed. Additionally, a strong correlation between particulate LMWCA and aged SOA suggests a potential link to boundary layer dynamics.