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<p>Ammonium nitrate (NH<sub>4</sub>NO<sub>3</sub>) is the main driver of high PM<sub>2.5</sub> 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<sub>2.5</sub> including Na<sup>+</sup>, SO<sub>4</sub><sup>2-</sup>, NH<sub>3</sub>, NO<sub>3</sub><sup>-</sup>, Cl<sup>-</sup>, Ca<sup>2+</sup>, K<sup>+</sup>, Mg<sup>2+</sup>, OC and EC, and its gaseous precursors including NO<sub>X</sub>, HNO<sub>3</sub> and SO<sub>2</sub> were continuously measured. The concentrations of PM<sub>2.5</sub> 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/m<sup>3</sup>) during the high PM<sub>2.5</sub> episode (24-hr average PM<sub>2.5</sub> > 35 μg/m<sup>3</sup>) in June compared to those of non-episode (3.1 μg/m<sup>3</sup>) and the other two months (0.74 μg/m<sup>3</sup>). 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 NO<sub>3</sub><sup>-</sup> concentration (18.2 μg/m<sup>3</sup>) at night. Concurrent increases of nitrate and ALWC cause aqueous-phase formation and hygroscopic growth of aerosol, which lead to high PM<sub>2.5</sub> 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<sub>2.5</sub> mass and nitrate were elevated after the NO<sub>X</sub> peak in the morning as well as at dawn. The surface of pre-existing particles was found to be prerequisite for nitrate driven PM<sub>2.5</sub> episode.</p>

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