Nitrate-driven high PM2.5 episodes in Seoul during pre-monsoon season

Ammonium nitrate (NH₄NO₃) is the main driver of high PM_2.5 episodes in Seoul, but its formation processes are not fully understood yet. Intensive experiments were conducted at the Korea University campus in Seoul during June ~ August 2018 and April ~ June 2019, when the chemical composition of PM_2.5 including Na⁺, SO₄^2-, NH₃, NO₃^-, Cl^-, Ca²⁺, K⁺, Mg²⁺, OC and EC, and its gaseous precursors including NO_X, HNO₃ and SO₂ were continuously measured. The concentrations of PM_2.5 and its major constituents were noticeably higher in pre-monsoon (June) than summer monsoon (July~August) period. In particular, nitrate concentration was much higher (6.9 μg/m³) during the high PM_2.5 episode (24-hr average PM_2.5 > 35 μg/m³) in June compared to those of non-episode (3.1 μg/m³) and the other two months (0.74 μg/m³). Aerosol liquid water content (ALWC) was calculated using ISORROPIA II model, ALWC was higher during the episode than non-episode and the highest ALWC was found concurrently with the highest NO₃^- concentration (18.2 μg/m³) at night. Concurrent increases of nitrate and ALWC cause aqueous-phase formation and hygroscopic growth of aerosol, which lead to high PM_2.5 concentration. In addition, ALWC was more rapidly increased with the number of accumulation mode particles larger than 100 nm in diameter at higher RH and nitrate concentration. In this study, PM_2.5 mass and nitrate were elevated after the NO_X peak in the morning as well as at dawn. The surface of pre-existing particles was found to be prerequisite for nitrate driven PM_2.5 episode.