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, PM_2.5, and surface ozone (O₃), 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, PM_2.5, and NO₂ generally decreased, while O₃ 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 PM_2.5 and O₃ concentrations, respectively. However, the relationship between O₃ 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 (MDA8O₃) increased in proportion to the NO₂ level under poor NO₂ conditions (e.g., < 25 ppb in Seoul).