10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Characterization of Aerosol Composition, Aerosol Acidity and Organic Acid Partitioning at an Agriculture-intensive Rural Southeastern U.S. Site
Theodora Nah, Hongyu Guo, Amy P. Sullivan, Yunle Chen, David Tanner, Athanasios Nenes, Armistead G Russell, Nga Lee Ng, Greg Huey, RODNEY J. WEBER, Georgia Institute of Technology
Abstract Number: 920 Working Group: Aerosol Chemistry
Abstract U.S. air quality regulations have led to decreased emissions in traditional air pollutants such as CO, NOx and SO2. In contrast, NH3 emissions have remained essentially unchanged, and may even increase in the future due to increased agricultural activities from a growing world population. These emission trends point to an increasingly important role that NH3 plays in atmospheric aerosol chemistry. We present real-time aerosol and trace gas measurements from a field study conducted in an agricultural-intensive region in Georgia during the fall of 2016. The goal was to understand how NH3 affects particle acidity and SOA formation via the gas-particle partitioning of semi-volatile inorganic and organic compounds. Particle pH and water 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 at the field site (study average 8.1 ± 5.2 ppb), PM1 are highly acidic with pH values ranging from 0.5 to 3.7 and a study-averaged pH of 2.1 ± 0.6 due to pH buffering by NH3 partitioning between the gas and particle phases. PM1 pH varied by approximately 1.4 units throughout the day. Gas-phase organic acids as large as C5 were detected at the site. Ambient concentrations of gas-phase organic acids ranged from a few parts per trillion by volume to several parts per billion by volume, with the largest concentrations observed for formic and acetic acids. Measured particulate organic acids comprised 6 % of the total organic aerosol mass concentration, with significant contributions from oxalic and succinic acids. The fractions of formic, acetic and oxalic acids present in the particle phase increased with PM1 pH. Measured formic, acetic and oxalic acid gas-particle partitioning ratios were also compared to their corresponding analytical predictions, which were calculated based on the organic acid’s physicochemical properties, ambient temperature, particle water and pH. While the measured oxalic acid gas-particle partitioning ratios (fraction in the particle phase ranged from 39 to 88 % at pH between 1.2 and 3.3) generally agreed with analytical predictions, this was not the case for formic and acetic acids. Possible reasons for these disagreements will be discussed.