Reducing particulate NO₃⁻ of PM_2.5 under an ammonia-rich environment: Role of NH₃ and aerosol pH using ISORROPIA-II model

Nitrate pollution is a significant contributor to poor air quality and adverse health effects, especially in NH₃-rich environments. This study investigated the reduction of particulate nitrate (NO₃⁻) in Seosan, South Korea, an NH₃-rich rural area, through two intensive campaigns conducted in winter (December 15, 2020–January 15, 2021) and autumn (November 10, 2022–December 10, 2022). Results showed that NH₃ played a critical role in NO₃⁻ formation, with average NH₃ concentrations of 6.7 ± 3.6 μg m⁻³. NO₃⁻ was the most abundant secondary inorganic aerosol throughout the study, especially during PM_2.5 episodes (hourly PM_2.5 ≥ 50 μg m⁻³), demonstrating a strong correlation between excess-NH₄⁺ and NO₃⁻ concentrations. A sensitivity analysis using the ISORROPIA-II model revealed that reducing the pH of PM_2.5 below 3.0 effectively decreased the gas-particle partitioning ratio of total nitrate, emphasizing the importance of total ammonia control. Specifically, ∼65% reduction in _TNH₃ reduced particulate NO₃⁻ concentrations by ∼20% in winter and ∼34% in autumn, with more pronounced effects during high PM_2.5 alerts. This study indicates that under NH₃-rich conditions, NH₃ reduction policies could be effective for alleviating NO₃⁻ pollution and substantially improving air quality in rural areas.