Characteristics of elevated PM2.5 events driven by enhanced organic compound concentrations in a South Korean residential city

Abstract

Organic matter is often the largest contributor to PM_2.5, but its emission sources and formation pathways are very diverse, making it challenging to identify the causes of high concentration episodes. In this study, four groups of organic compounds that contribute to PM_2.5, including sugars, dicarboxylic acids (DA), fatty acids (FA), and pinonic acid (PNA) were measured in a medium-sized residential city in South Korea during three seasons, where high PM_2.5 concentration episodes often occur. The average concentrations of PM_2.5 and total quantified organic matter (∑17 qOM) was 21 ± 12 μg m⁻³ and 391 ± 183 ng m⁻³, respectively. The concentration of ∑sugars was higher during the colder seasons compared to the warm season (32 ± 18 ng m⁻³, 240 ± 109 ng m⁻³, and 231 ± 105 ng m⁻³ in the warm, transition, and cold seasons, respectively). In contrast, ∑DAs exhibited the opposite seasonal trend (234 ± 164 ng m⁻³, 114 ± 103 ng m⁻³, and 140 ± 103 ng m⁻³ in the warm, transition, and cold seasons, respectively). The contribution of ∑FA to qOM was relatively consistent (13.1% in warm season to 15.6% in colder seasons). PNA, a biogenic secondary organic aerosol tracer, had a significantly higher concentration during the warm season (16 ± 13 ng m⁻³ in warm season vs. 3 ± 3 ng m⁻³ in colder seasons). A strong correlation between ∑sugars and ∑FAs (r = 0.72) was observed only in the transition season, when crop residue burning was determined to be important. Unsaturated FAs were likely to be efficiently aged during the cold season since the ratio of C_18:0 to C_18:1, a tracer for the age of aerosol, increased. DAs were generally dominant in the warm season, but also significantly increased during most high PM_2.5 concentration episodes (HCEs; 306 ± 199 ng m⁻³), which primarily occurred in the colder season. During HCEs appearing in colder season, the ratio of malonic acid (C₃) to succinic acid (C₄), a tracer for photochemical aging of air masses, also increased, suggesting that the secondary aerosol formation and aerosol aging significantly enhanced PM_2.5 concentration.