Long-term visibility trend representing perceived air quality in the Republic of Korea

This study investigated the trends in visibility and visibility impairment in the Republic of Korea to understand why the public's perceived concentration of fine particles is increasing since 2013 despite their actual decreasing trend. Specifically, the arithmetic and harmonic means in 16 districts in Korea were analyzed. Both the arithmetic and harmonic mean visibility increased from 2003 to 2018 (Period III), despite differences in year-to-year variability and overall rates of change. However, the frequency of haze and mist cases, during which visibility is < 10 km, increased in most districts. Among all districts, Seoul was the only region where the average visibility improved and the frequency of low-visibility cases decreased regardless of the period. Nevertheless, during Period III, visibility in Seoul significantly worsened during winter morning commuting hours on high-humidity days. These factors may have influenced the perception of increased fine particle concentrations. In Seoul, cognitive dissonance between dry-PM_2.5 concentrations and poor visibility—when dry-PM_2.5 concentrations are low but visibility is poor—primarily occurs under conditions of high relative humidity. Therefore, to reduce the gap between actual and perceived fine particle concentrations under high humidity conditions, providing real-time information or forecasts on wet-PM_2.5 concentrations and visibility is recommended. Notably, relative humidity increased in most regions despite rising temperatures during Period III, which appears to have contributed to the increased frequency of mist cases. This necessitates further studies and continuous monitoring.