Effect of Elevated Ammonia Concentrations on Secondary Organic Aerosol Formation during the Georgia Tech Ambient Field Study in Yorkville, Georgia

Secondary Organic Aerosols (SOA) are suspended particles formed in the atmosphere through the oxidation of emitted Volatile Organic Compounds (VOC). SOA make up a significant portion of ambient aerosol particles and therefore have an important impact on climate and human health. The formation of SOA is strongly influenced by inorganic species such as ammonia, however this influence is not well understood. Since ammonia emissions are expected to increase due to growing population and changing climate, we conducted an ambient field study to better understand the formation of organic aerosols in ammonia-rich environments. The study was conducted at a rural site in Yorkville, Georgia from August – October 2016. The Yorkville site is characterized by elevated concentrations of ammonia (10-20 ppb) due to a nearby cow pasture and high biogenic VOC emissions typical of the southeastern United States. The site is also part of the SouthEastern Aerosol Research and Characterization (SEARCH) network.

A suite of instruments was deployed to characterize gas and particle phase compositions over the period of the study. A Scanning Mobility Particle Sizer (SMPS) spectrometer consisting of a Differential Mobility Analyzer (DMA) and Condensation Particle Counter (CPC) was used to determine the concentration and particle size distribution of ambient aerosol particles in the size range of 10-300 nm. Additionally, a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) was used to determine the chemical composition of sub-micron non-refractory aerosol. Aerosol mass concentration was found to be relatively constant with slight diurnal variation. The particle size distribution for number concentration showed significant variation between day and night and revealed numerous nighttime growth events. The SMPS data were compared to AMS data to determine the chemical species present during episodes of high aerosol loading and growth events. Meteorological measurements from nearby SEARCH instruments were used to determine potential factors affecting mass concentration. From these and future results, the formation of SOA in the presence of elevated ammonia concentration will be evaluated.