Atmospheric polychlorinated naphthalenes in Ulsan, South Korea: Seasonal variations, source apportionment, and meteorological effects

Abstract

Polychlorinated naphthalenes (PCNs) are toxic, persistent organic pollutants primarily emitted unintentionally through industrial combustion. Despite being phased out, they remain in the environment due to their persistence and continued emissions. PCN levels vary by location and season, often higher in urban, industrial areas like Ulsan, South Korea. Prior studies found PCNs in various media, with combustion processes as key sources. However, PCN air monitoring in Ulsan has been limited. This study fills that gap by examining atmospheric PCN concentrations, seasonal trends, and gas-particle partitioning over a year to better understand their behavior and inform control strategies. Air samples were collected weekly in 2016 at a suburban site in Ulsan, South Korea, using high-volume samplers that captured both gas and particulate phases. Samples were seasonally categorized and stored at –4°C until analysis. Pre-treatment involved cleaning and preparing filters and PUFs, followed by Soxhlet extraction, purification, and concentration of the extracts. PCNs were analyzed using GC- HRMS, with surrogate and internal standards added for quality control. A total of 55 congeners were quantified, excluding mono- and di-CNs due to low recovery. Meteorological and air pollutant data were sourced from local and national monitoring systems. Gas/particle (G/P) partitioning coefficients were calculated to understand phase distribution, using total suspended particles and sub-cooled vapor pressure for regression analysis. The annual mean concentration of Σ62 PCNs in Ulsan were 5.30 pg/m³, predominantly in the gas phase, with tri- and tetra-CNs as the dominant homologues. PCN levels were on a middle ground between urban areas and background areas worldwide. High concentration events were found during summer and fall. Seasonal trends followed Summer > Fall > Spring > Winter. Meteorological conditions influenced PCN concentrations in the atmosphere, and correlations with CO, PM10, PM2.5 and SO2 indicated an industrial source contribution. The increased presence of highly chlorinated PCNs during summer further suggested local industrial impacts. These findings were consistent with source identification analyses, which showed higher PCN concentrations associated with prevailing winds from the eastern and southeastern regions of Ulsan. Over 90% of PCNs were in the gaseous phase, with log–log regression of partitioning coefficients indicating adsorption dominated in colder months and absorption in warmer ones. Toxic equivalency analysis showed congeners PCN-66/67 and PCN-73. As these congeners are associated with combustion sources, the results suggest that local industrial emissions not only increase PCN concentrations but also elevate their toxicological risk. This is the first study in South Korea to assess atmospheric PCNs with both gas-particle partitioning and long-range transport, offering a model for future monitoring efforts.