The implementation of stringent emission regulations has resulted in the decline of anthropogenic pollutants including sulfur dioxide (SO 2 ), nitrogen oxides (NO x ) and carbon monoxide (CO). In contrast, ammonia (NH 3 ) 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 NH 3 affects particle acidity and 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 HNO 3 -NO 3 − and NH 3 -NH 4 + gas-particle partitioning ratios with measured values. Our results showed that despite the high NH 3 concentrations (study average 8.1 ± 5.2 ppb), PM 1 were highly acidic with pH values ranging from 0.9 to 3.8, and a study-averaged pH of 2.2 ± 0.6. PM 1 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 PM 1 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 PM 1 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 NH 3 concentrations relative to what has been measured in the region in previous studies had minor effects on PM 1 organic acids and their influence on the overall organic aerosol and PM 1 mass concentrations.