Long-term trends and affecting factors in the concentrations of criteria air pollutants in South Korea

Here, we used a novel approach to analyze the trends in the concentrations of six national criteria air pollutants, carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), PM10, PM2.5, and surface ozone (O-3), between 2001 and 2018, and the factors affecting the concentrations of CO and SO2 at the district level in South Korea (Korea) between 2011 and 2016. We considered four factors affecting the concentrations of CO and SO2: (1) concentration-to-emission ratio, (2) air pollutant-to-CO2 emission ratio, (3) carbon intensity, and (4) area weighted energy supply. We found that the most influencing factors determining the concentrations of CO and SO2 in Korea were the air pollutant-to-CO2 emission ratio and the concentration-to-emission ratio. The annual mean concentrations of CO, SO2, PM10, PM2.5, and NO2 generally decreased, while O-3 levels showed an increasing trend, over the last 18 years in Korea. We also found that NO2 concentration had strong positive and negative correlations with PM2.5 and O-3 concentrations, respectively. However, the relationship between O(3 )and NO2 levels showed an inverted U shape under NO2 super-rich condition (e.g., > 55 ppb in Seoul), and the daily maximum 8-h values (MDA8O(3)) increased in proportion to the NO2 level under poor NO2 conditions (e.g., < 25 ppb in Seoul).