Stable carbon isotopic compositions (δ 13 C) were measured for 23 individual organic species including 9 dicarboxylic acids, 7 oxocarboxylic acids, 1 tricarboxylic acid, 2 α-dicarbonyls, and 4 fatty acids in the aerosols from Gosan background site in East Asia. δ 13 C values of particle phase glyoxal and methylglyoxal are significantly larger than those previously reported for isoprene and other precursors. The values are consistently less negative in oxalic acid (C 2 , average À14.1‰), glyoxylic acid (À13.8‰), pyruvic acid (À19.4‰), glyoxal (À13.5‰), and methylglyoxal (À18.6‰) compared to other organic species (e.g., palmitic acid, À26.3‰), which can be explained by the kinetic isotope effects during atmospheric oxidation of pre-aged precursors (e.g., isoprene) and the subsequent gas-particle partitioning after the evaporation of clouds or wet aerosols. The δ 13 C values of C 2 is positively correlated with C 2 to organic carbon ratio, indicating that photochemical production of C 2 is more pronounced than its degradation during long-range atmospheric transport. The isotopic results also suggest that aqueous phase oxidation of glyoxal and methylglyoxal is a major formation process of oxalic acid via the intermediates such as glyoxylic acid and pyruvic acid. This study provides evidence that organic aerosols are intensively photochemically aged in the western North Pacific rim.Compound-specific stable carbon isotope analysis is a powerful tool to provide important information of the sources and atmospheric processing of organic aerosols. Many studies have revealed that the δ 13 C values of longer-chain dicarboxylic acids are often lower than those of shorter-chain dicarboxylic acids [Wang and Kawamura, 2006]. For example, it has been reported that oxalic acid is more enriched in 13 C than malonic, ZHANG ET AL.

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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>

The secondary aerosol forming potential of ambient air was first measured with the Potential Aerosol Mass (PAM) chamber at Gosan supersite on Jeju island from October 22 to November 5, 2010. PAM chamber is a small flowthrough photo-oxidation chamber with extremely high OH and O 3 levels. The OH exposure in the PAM chamber was (2±0.4)×10 11~( 6±1.2)×10 11 molecules cm -3 s and was similar to 2 to 5 days of aging in the atmosphere. By periodically turning on and off UV lamps in the PAM chamber, ambient aerosol and newly formed aerosol (e.g. called as PAM aerosol) was alternately measured. Aerosol number and mass concentration in the range of 10~487 nm in diameter was measured by SMPS 3034. With UV lamps on, the nucleation mode particles smaller than 50 nm in diameters were formed. Their number concentration was greater than 10 5 cm -3 , leading to increase in aerosol mass by 0~8 μg m -3 . The variations of PAM and ambient aerosols were greatly dependent on characteristics of air masses such as precursor concentrations and degree of aging. This preliminary results suggests that PAM chamber is useful to assess the aerosol formation potential of air mass and its impact on the air quality. The further analysis of data with gaseous and particulate measurements will be done.

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