Long-range atmospheric transport of PM2.5 and its components in Ulsan, South Korea: chemical characterization, source identification, and nontarget screening

Abstract

Particulate matter with an aerodynamic diameter less than 2.5 µm (PM2.5) is a major air pollutant in Northeast Asia, including South Korea. PM2.5 pollution originates from both local emissions and long-range atmospheric transport (LRAT) from the Asian continent. Although LRAT has been extensively studied, research on PM2.5 components, particularly organic compounds, remains limited. Organic compounds serve as molecular markers, facilitating the identification of specific pollution sources. Advanced receptor models, such as hybrid receptor models and positive matrix factorization (PMF), can significantly enhance source apportionment when integrated with organic markers. This study aims to evaluate the qualitative and quantitative influence of LRAT on PM2.5 and its components in Ulsan, South Korea. Long-term trend analysis, source apportionment using organic markers, source identification of semi-volatile organic compounds, and non-targeted screening to identify key organic markers were conducted to assess transboundary pollution in Northeast Asia. The long-term trends and contributions of LRAT to PM2.5 and its components were analyzed in Ulsan, South Korea, from 2014 to 2023. A 45.6% decrease in the annual mean PM2.5 concentration was observed, primarily due to reductions in local emissions. However, the contribution of LRAT remained substantial, particularly during winter and high-concentration episodes, emphasizing its increasing impact in Ulsan. The Potential Source Contribution Function (PSCF) model identified East and North China as major source regions significantly influencing PM2.5 and its components, such as NO3 -, NH4 +, and EC, during the cold season. Estimations of Contribution of Long-Range Transport (CLRT) revealed that recent PM2.5 levels were related to increasing industrial emissions in eastern and northern China. Notably, NO3 concentrations sharply increased between 2021 and 2023, reflecting enhanced LRAT and secondary formation processes. These findings demonstrate that, while local emission reductions have improved air quality in Ulsan, LRAT remains a critical factor in PM2.5 pollution in Ulsan. The chemical composition and sources of PM2.5 were investigated with a focus on organic species. Intensive PM2.5 sampling was conducted during the spring and fall of 2019, and organic markers, including polycyclic aromatic hydrocarbons (PAHs), n-alkanes, fatty acids (FAs), dicarboxylic acids (DCAs), and sugars, were analyzed. Elevated PM2.5 and OC concentrations were observed in the spring, showing strong correlations. Predominant organic compounds, such as n-alkanes and fatty acids, closely followed PM2.5 trends. Diagnostic ratios indicated significant contributions from fossil fuel combustion and biomass burning, with LRAT playing a greater role in the fall. PMF modeling identified nine primary sources based on PM2.5 components, with secondary inorganic aerosol (SIA) formation accounting for 37.2%. The integration of 31 organic markers further refined source apportionment, effectively differentiating contributions from biomass burning, industrial activities, and fossil fuel-related sources. This study highlights the critical role of organic markers in understanding PM2.5 pollution. The seasonal variations, sources, and LRAT of semi-volatile organochlorine pesticides (OCPs) were investigated. The total concentrations of Σ27 OCPs (42.7–868 pg/m³) were comparable to levels reported by the nationwide monitoring network, with HCB and α-endosulfan identified as dominant compounds. Seasonal analysis revealed higher OCP concentrations in winter due to LRAT effects, while re-emissions from historically contaminated surfaces were dominant in summer. Particulate-bound OCPs, including DDTs, HCHs, and mirex, exhibited notable increases in winter, reflecting enhanced LRAT effects from East and North China. Gas/particle (G/P) partitioning analysis revealed distinct seasonal behaviors of OCPs, with equilibrium states observed in winter and disequilibrium conditions during summer. The concentration- weighted trajectory (CWT) model identified potential source regions, highlighting significant LRAT influences from East, North, and Northeast China, particularly for HCB, PeCB, and mirex. Local emissions from surrounding areas strongly influenced CHLORs and endosulfan concentrations, especially during warmer seasons. This chapter identifies the combined influence of LRAT and local sources on atmospheric OCP levels in Ulsan, South Korea. Finally, to investigate the chemical composition of PM2.5 during high-concentration episodes influenced by LRAT(PLRAT) and local emissions (PLocal), a non-targeted analysis (NTA) was conducted using GC×GC-TOF-MS. A total of 360 compounds were reliably identified and classified into linear hydrocarbons, cyclic hydrocarbons, and aromatic compounds, including nitrated and oxygenated derivatives. Cluster analysis revealed distinct seasonal patterns: winter and fall samples were dominated by high-molecular-weight hydrocarbons and brominated compounds, reflecting industrial and petroleum-related sources. In contrast, samples influenced by local emissions exhibited higher proportions of aromatic and combustion-related compounds, such as nitrogen heterocycles. Among the 65 tentatively identified compounds, significant concentration differences were observed between PLRAT and PLocal episodes, including phthalic acid derivatives. The elevated presence of nitrated and oxygenated compounds in PLRAT samples suggests enhanced atmospheric chemical reactions associated with LRAT. These findings demonstrate the utility of NTA in identifying previously overlooked pollutants and potential indicators of long-range transport.