Comparison of Fine Particulate Nitrate Formation in the Presence of Excess Ammonia at Livestock and Crop Agricultural Sites in Summer
JOONWOO KIM, Jiho Jang, Dahye Oh, Haebum Lee, Taewoong Gong, Kihong Park,
Gwangju Institute of Science and Technology Abstract Number: 511
Working Group: Remote and Regional Atmospheric Aerosol
AbstractFine particulate nitrate formation often leads to severe haze pollution in winter when the particle phase is favored in reversible phase equilibrium with precursor gases (e.g., ammonia (NH
3) and nitric acid (HNO
3)) at low temperature. Summertime nitrate formation is relatively unlikely to occur because of high evaporation, but may be altered by excess ammonia. To explore the formation of nitrate in the presence of excess ammonia in summer, we performed intensive field measurements of fine particles and gases at two distinct ammonia-rich agricultural sites (livestock and crop agricultural sites) in Gimje, Korea during the summers of 2020 and 2021. The ISORROPIA II thermodynamic equilibrium model was applied to estimate aerosol pH, aerosol water content, and gas/particle partitioning ratios. The average ammonia concentrations were 81.7±84.3 ppb at the livestock agricultural site and 20.8±14.9 ppb at the crop agricultural site, elevating aerosol pH (3.9±0.3 and 3.4±0.3, respectively). The average mass fraction of nitrate in PM
2.5 was higher at the livestock agricultural site (21.9%) than at the crop agricultural site (19.2%) with higher nitrate partitioning ratios ([NO
3–]/([HNO
3]+[NO
3–]); 0.91±0.11 and 0.69±0.24, respectively). The nitrate oxidation ratios ([NO
3–]/([NO
2]+[NO
3–])) were more correlated with aerosol water content than O
3 concentration, indicating that aqueous reactions at night were important. Secondary inorganic aerosol concentration, aerosol water content, and nitrate partitioning ratios increased collectively, resulting in haze pollution. Our results clearly show that excess ammonia contributes to summertime nitrate formation.