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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 O 3 , SO 2 , NO 2 , PM 10 , and PM 2.5 showed statistically significant decreasing trends over the period. The companion relationship between PM 2.5 and gaseous pollutants was evaluated by their correlations [R (PM 2.5 -GPs)]. R (PM 2.5 -NO 2 ) was relatively high at almost all AQMSs, whereas high R (PM 2.5 -SO 2 ) was observed near the petrochemical industrial complex, suggesting a great influence of local emissions (vehicles and industries). R (PM 2.5 -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 PM 2.5 . The effect of temperature (T) was more apparent on R (PM 2.5 -GPs) than that of relative humidity, with significant values under T > 15 °C. Moreover, R (PM 2.5 -O 3 ) was positive at the T range of 12-18 °C, suggesting that reducing GPs emitted by industrial facilities during May-June may control PM 2.5 and O 3 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 PM 2.5 formation from gaseous precursors and the R (PM 2.5 -O 3 )., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

The outbreak of COVID-19 in Daegu, South Korea, early in 2020 has led this metropolitan city to become one of the major hotspots in the world. This study investigates the association of meteorology and the new daily COVID-19 confirmed cases and the effects of the city lockdown on the variation in criteria air pollutants (CAPs) in Daegu. Ambient temperature and relative humidity were negatively correlated to the new daily cases and played an important role in the spread of COVID-19. Wind speed could enhance the virus transmission through the inhalation of aerosols and/or droplets and contact with fomites. The lockdown has directly decreased the concentrations of CAPs. In particular, reductions of 3.75% (PM 10 ), 30.9% (PM 2.5 ), 36.7% (NO 2 ), 43.7% (CO), and 21.3% (SO 2 ) between the period before and during the outbreak were observed over the entire city. An increase in O 3 (71.1%) was affected by natural processes and photochemical formation other than the lockdown effects. The three central districts were the areas most affected by the virus and showed the highest reductions in CAPs (except for O 3 ) during the outbreak. Apart from the influence of the lockdown, the decreasing trend in CAPs may be a result of the actions taken by the government to mitigate air pollutants nationwide since 2019. The results of this study can be useful for government and medical organizations to understand the behavior of the virus in the atmosphere. Further studies are necessary to explore the detailed influences of the lockdown on the environment and public life.