Increased number concentrations of small particles explain perceived stagnation in air quality over Korea

Abstract. The atmospheric visibility in South Korea has not improved despite decreasing mass concentrations of particulate matter (PM)2.5. Since visibility is influenced by particle size and composition as well as meteorological factors, light detection and ranging (lidar) data provided by the National Institute for Environmental Studies in Japan and PM2.5 measurements retrieved from AirKorea are used to determine the trends in PM2.5 mass extinction efficiency (MEE) in Seoul and Ulsan, South Korea, from 2015 to 2020. Moreover, the monthly trends in the Ångström exponent and relative and absolute humidity are determined to identify the factors influencing PM2.5 MEE. The monthly average PM2.5 MEE exhibits an increasing trend in Seoul (+0.04 m2 g−1 per month) and Ulsan (+0.07 m2 g−1 per month). Relative humidity increases by +0.070 % and +0.095 % per month in Seoul and Ulsan, respectively, and absolute humidity increases by +0.029 and +0.010 g m−3 per month, respectively. However, the trends in these variables are not statistically significant. The Ångström exponent increases by +0.005 and +0.011 per month in Seoul and Ulsan, respectively, indicating that the MEE increases as the size of the particles becomes smaller each year. However, due to limitations when obtaining long-term composition data in this study, further research is needed to accurately determine the causes of the increase in PM2.5 MEE. Such an increase in PM2.5 MEE may have limited the improvements in visibility and adversely affected public perception of air quality improvement even though the PM2.5 mass concentration in South Korea is continuously decreasing.