Understanding the role of formic acid in fine and coarse particle mode

Abstract

Formic acid partitioning was investigated at two sites in the southeast U.S., Yorkville, GA and Jefferson Street (JST). Previous studies in the southeast have shown that observed formic acid partitioning to fine particles did not agree with predicted partitioning due to very low predicted fine particle pH. This thesis looks to understand the possible causes for this discrepancy. The abundances of nonvolatile cations (NVCs) observed in particulate matter (PM) lead to the investigation of NVC complexation with formic acid to possibly provide additional insight on formic acid partitioning. Bulk particle pH and particle liquid water content were estimated using the ISORROPIA II thermodynamic model. Formic acid partitioning and pH results showed that assuming PM1.0-2.5 was externally mixed from PM1.0 at JST improved the agreement between the measured and predicted formic acid partitioning. This was only for PM1.0-2.5 because the partitioning pH was closer to neutral. However, it could not resolve the disagreement with PM1.0, suggesting that there may be different chemical processes and sources of formic acid that may be contributing to the fine and coarse mode. Increasing particle water concentrations also affected predicted partitioning of formic acid but were not sufficient to agree with observed partitioning. Correlations also indicate that formic acid in the southeast may be associated with mineral dust and biomass burning, suggesting a need to understand the source contributions of formic acid in more detail.