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

The atmospheric visibility in South Korea has not improved despite decreasing concentrations of particulate matter (PM)2.5. Since visibility is influenced by particle size and composition as well as meteorological factors, the light detection and ranging data provided by the National Institute for Environmental Studies in Japan and PM2.5 measurements retrieved from Air Korea 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)·mth-1] and Ulsan [+0.07 (m2·g-1)·mth-1]. 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)·mth-1, respectively. However, both Relative humidity and Absolute humidity 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 particles become smaller each year. 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.