Abstract
Abstract. The implementation of stringent emission regulations has resulted in the decline of anthropogenic pollutants including sulfur dioxide (SO2), nitrogen oxides (NOx), 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 agriculturally intensive region in the southeastern US 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 was 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 a PM1 pH of 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 ratios 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.
Highlights
Ammonia (NH3) is the most abundant basic gas in the troposphere and plays an important role in many atmospheric processes
The thermodynamic equilibrium model, ISORROPIA II, is used to calculate particle water and pH based on measured inorganic aerosol and gas composition (Nenes et al, 1998; Fountoukis and Nenes, 2007), and these predictions are compared to observed gas–particle partitioning of NH3, HNO3, and organic acids
The diurnal plot does not account for dilution as the boundary layer expanded and only indicates that if emissions were solely from the surface and lower concentrations aloft, these NH3 sources were of significant magnitude
Summary
Ammonia (NH3) is the most abundant basic gas in the troposphere and plays an important role in many atmospheric processes. More wildfires from a changing climate, or from controlled burning for land clearing for agricultural use, may lead to increased NH3 emissions (Reis et al, 2009; Pechony and Shindell, 2010; Warner et al, 2016) These trends suggest that NH3 could play an increasingly important role in atmospheric chemistry. The thermodynamic equilibrium model, ISORROPIA II, is used to calculate particle water and pH based on measured inorganic aerosol and gas composition (Nenes et al, 1998; Fountoukis and Nenes, 2007), and these predictions are compared to observed gas–particle partitioning of NH3, HNO3, and organic acids. These measurements are used to determine how aerosol acidity affects the mass concentration of particle-phase organic acids at this site
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