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Although meteorological conditions play a significant role in air pollution, research on their effects on the relationship between air pollutants is limited. In this study, trends of six criteria air pollutants were investigated from 15 air quality monitoring stations (AQMSs) in Ulsan, a multi-industrial city in South Korea, during 2015–2019. Unlike CO and O3, SO2, NO2, PM10, and PM2.5 showed statistically significant decreasing trends over the period. The companion relationship between PM2.5 and gaseous pollutants was evaluated by their correlations [R (PM2.5–GPs)]. R (PM2.5–NO2) was relatively high at almost all AQMSs, whereas high R (PM2.5–SO2) was observed near the petrochemical industrial complex, suggesting a great influence of local emissions (vehicles and industries). R (PM2.5–CO) and the standardized regression coefficients of CO obtained from the multiple linear regression model were the highest, indicating that combustion processes may significantly contribute to PM2.5. The effect of temperature (T) was more apparent on R (PM2.5–GPs) than that of relative humidity, with significant values under T > 15 °C. Moreover, R (PM2.5–O3) was positive at the T range of 12–18 °C, suggesting that reducing GPs emitted by industrial facilities during May–June may control PM2.5 and O3 in Ulsan. The methodology demonstrated in this study can be further used for a better understanding of the influences of environmental factors on the secondary PM2.5 formation from gaseous precursors and the R (PM2.5–O3). [ABSTRACT FROM AUTHOR]

Evaluation for the controlling policy's effectiveness to mitigate criteria air pollutants (CAPs) in South Korea during December 1, 2019–March 31, 2020 is difficult because of its coincidence with the COVID-19 social distancing. In this study, we differentiated the influence of three major driving factors (intensive controlling policy by the government, meteorological conditions, and social distancing) to the CAP variation in Ulsan, the largest industrial city in South Korea. In 2013–2019, the concentrations of PM 2.5 (2015–2019), PM 10 , SO 2 , and NO 2 decreased by 6.7, 1.6, 4.2, and 3.3%/year, respectively, whereas the O 3 concentration slightly increased by 0.7%/year. Trend analysis was used to predict the CAP concentrations before (January 1–February 21) and during (February 22–March 31) the social distancing in 2020. The difference between the measured and predicted concentrations was designated as the contribution of the three factors. The controlling policy was the most important driver of the CAP reductions. In particular, its contributions were 94.1% (January 1–February 21) and 87.4% (February 22–March 31) to the PM 2.5 decrease. The change in meteorological conditions considerably affected the CAP reductions, with the highest contributions of 35.2% (January 1–February 21) and 39.2% (February 22–March 31) to the O 3 decrease. On February 22–March 31, the effects of social distancing were 1.6, 0.6, 1.3, and 1.4% to the reduction of SO 2 , NO 2 , PM 10 , and PM 2.5 , respectively. Overall, a decrease in the CAP concentrations was apparent during January–March 2020 in Ulsan primarily due to the intensive controlling policies, not by the COVID-19 social distancing. [Display omitted] • Concentrations of air pollutants, except for O 3 , decreased in Ulsan during 2013–2019. • A controlling policy played a vital role in reducing air pollutants in Jan–Mar 2020. • The contribution of controlling policy to the PM 2.5 and PM 10 decrease was 81–94%. • Meteorological conditions contributed 19–39% of the air pollutant reduction. • The pollutant decrease was affected by the social distancing on Feb 22–Mar 31, 2020. [ABSTRACT FROM AUTHOR]

Atmospheric halogenated polycyclic aromatic hydrocarbons (Halo-PAHs) and parent PAHs were monitored in Ulsan, South Korea for one year (January‒December 2015) to investigate their seasonal patterns, gas/particle partitioning behavior, and the impact of meteorological conditions. The mean concentrations of 24 chlorinated PAHs, 11 brominated PAHs, and 13 parent PAHs in the gaseous and particulate phases were 8.64 and 9.64 pg/m3, 11.6 and 1.62 pg/m3, and 2.17 and 2.40 ng/m3, respectively. Winter had the highest ClPAH and PAH levels, with significant contributions from poly-chlorine groups and high-molecular-weight compounds. However, BrPAHs showed reverse patterns with the highest concentration in summer and the dominant gaseous fraction throughout the year. This finding could be explained by the strong local sources of BrPAHs, related to automobile and petrochemical industries. In contrast, the effects of the temperature inversion layer and atmospheric transport from the outside of Ulsan were more apparent for ClPAHs and PAHs, particularly in winter and spring. Regarding gas/particle partitioning, Halo-PAHs exhibited different seasonal behaviors from those of parent PAHs. The sorption pathway of Halo-PAHs seemed to shift from absorption as the sole dominant mechanism in winter and spring to both adsorption and absorption in summer and fall, while both partitioning mechanisms contributed equally for parent PAHs during the entire year. This study implies that Halo-PAHs and parent PAHs might not share the same atmospheric behavior, possibly due to different characteristics in atmospheric reactions with other chemicals and particle-size distribution. However, there have been limited studies about the formation of Halo-PAHs and their physicochemical properties; hence, further in-depth investigations are of vital importance. Image 1 • Winter had the highest ClPAH and PAH levels with significant contributions of HMWs. • Gaseous BrPAHs reached the maximum concentration in summer due to local sources. • The influence of LRAT on ClPAHs and PAHs was more apparent than on BrPAHs. • TILs were more frequent in winter, leading to higher levels of ClPAHs and PAHs. • The gas/particle partitioning mechanisms for Halo-PAHs were seasonally different. [ABSTRACT FROM AUTHOR]