Potential influence of ammonia reduction on particulate nitrate concentration in South Korea

In Northeast Asia, nitrate concentration in fine particles has little decreased even with a significant reduction in nitrogen oxides in recent years. Ammonia reduction can be considered as one of the control strategies for nitrate. This study investigated the role of ammonia in the formation of nitrate in the ambient atmosphere by using various observation data at 10 locations (urban-agricultural, urban-industrial, urban, and remote) in South Korea in 2022 and the E-AIM thermodynamic equilibrium model. In the warm season (May-October), ammonia had a significant effect on particulate nitrate concentration by nitric acid-nitrate partitioning. A sensitivity test suggested that on average across all locations in the warm season, a 50% reduction in ammonia can lead to a 47% decrease in particulate nitrate with a constant total nitrate concentration (= gaseous nitric acid + particulate nitrate). This effect was the weakest at the agricultural site with high ammonia level (19.0 ppb). In the cold season (November-April), the ammonia reduction was not effective in controlling the nitrate by the partitioning, and long-range transport affected nitrate concentration at the remote site. With the consideration of the rapid loss of the gas-phase species in the total nitrate, the partitioning with ammonia reduction was also helpful to control the nitrate. Our findings suggest that ammonia emission reduction via nitric acid-nitrate partitioning should be an effective strategy to decrease particulate nitrate level in the ambient atmosphere particularly in the warm season.