Driving factors to air pollutant reductions during the implementation of intensive controlling policies in 2020 in Ulsan, South Korea

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 PM2.5 (2015–2019), PM10, SO2, and NO2 decreased by 6.7, 1.6, 4.2, and 3.3%/year, respectively, whereas the O3 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 PM2.5 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 O3 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 SO2, NO2, PM10, and PM2.5, 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.