Characterization of Aerosol Composition, Aerosol Acidity and Organic Acid Partitioning at an Agriculture-Intensive Rural Southeastern U.S. Site

The implementation of stringent emission regulations has resulted in the decline of anthropogenic pollutants including sulfur dioxide (SO₂), nitrogen oxides (NO_x) and carbon monoxide (CO). In contrast, ammonia (NH₃) emissions are largely unregulated, with emissions projected to increase in the future. We present real-time aerosol and gas measurements from a field study conducted in an agricultural-intensive region in the southeastern U.S. during the fall of 2016 to investigate how NH₃ affects particle acidity and secondary organic aerosol (SOA) formation via the gas-particle partitioning of semi-volatile organic acids. Particle water and pH were determined using the ISORROPIA-II thermodynamic model and validated by comparing predicted inorganic HNO₃-NO₃^- and NH₃-NH₄⁺ gas-particle partitioning ratios with measured values. Our results showed that despite the high NH₃ concentrations (average 8.1 ± 5.2 ppb), PM₁ were highly acidic with pH values ranging from 0.9 to 3.8, and an average pH of 2.2 ± 0.6. PM₁ pH varied by approximately 1.4 units diurnally. Formic and acetic acids were the most abundant gas-phase organic acids, and oxalate was the most abundant particle-phase water-soluble organic acid anion. Measured particle-phase water-soluble organic acids were on average 6 % of the total non-refractory PM₁ organic aerosol mass. The measured molar fraction of oxalic acid in the particle phase (i.e., particle-phase oxalic acid molar concentration divided by the total oxalic acid molar concentration) ranged between 47 and 90 % for PM₁ pH 1.2 to 3.4. The measured oxalic acid gas-particle partitioning ratios were in good agreement with their corresponding thermodynamic predictions, calculated based on oxalic acid’s physicochemical properties, ambient temperature, particle water and pH. In contrast, gas-particle partitioning of formic and acetic acids were not well predicted for reasons currently unknown. For this study, higher NH₃ concentrations relative to what has been measured in the region in previous studies had minor effects on PM₁ organic acids and their influence on the overall organic aerosol and PM₁ mass concentrations.