Understanding of enhanced nitrate in fine particles at agricultural sites in summer with high ammonia level

Nitrate is one of the major constituents of fine particles and has not been effectively alleviated in Northeast Asia. Field measurements of various gases and the chemical composition of fine particles were conducted at two agricultural sites (cropland and livestock) in ammonia-rich environments to understand the effect of ammonia on nitric acid-nitrate partitioning using a thermodynamic model and to suggest a possible strategy to control total nitrate (i.e., nitric acid formation). High nitrate levels were observed at the agricultural sites in summer compared to those at the urban sites. It was found that high level of ammonia in summer led to increased aerosol pH and nitrate fraction. At the cropland site in summer, the daily nitrate fraction was particularly sensitive to aerosol pH, suggesting that ammonia reduction should be effective in decreasing nitrate formation via nitric acid-nitrate partitioning (with a 50% reduction in ammonia, nitrate concentration can decrease by 34%). Aerosol water content also played a significant role in determining nitrate fraction in the aerosol pH range of 2.5-3.0. It was found that the sites were under high NO_x conditions, and that the reduction of OH production (daytime) and O₃ (nighttime) was important for controlling total nitrate, but this is challenging due to the high contributions of background O₃. It was concluded that the reduction of ammonia emissions for the control of the nitrate fraction via gas-to-particle partitioning should be important to mitigate nitrate in fine particles at agricultural sites in summer.