Role of air stagnation in determining daily average PM 2.5 concentrations in areas with significant impact of long-range transport

In areas where the regional transport of air pollutants exerts a significant impact, ascertaining whether shortterm air stagnation affects PM 2.5 concentrations is crucial for accurate air quality forecasting and effective management planning. However, this research area remains underexplored. In this study, we analyzed the relationship between stagnant atmospheric conditions and daily average PM 2.5 concentrations in areas with a substantial long-range transport impact (LRT). Specifically, we focused on days with elevated PM 2.5 concentrations (daily average >= 35 mu g/m 3 ) from January to March 2019 in South Korea. The analysis was performed using the Weather Research and Forecasting and the Community Multiscale Air Quality Modeling System. Stagnant conditions were quantified using ventilation index (VI), calculated as the product of planetary boundary layer height and 10-m wind speed. The correlation coefficient between daily average VI and PM 2.5 concentration ( r = -0.23) was lower than that between VI and NO 2 concentration ( r = -0.60). This can be attributed to the fact that LRT was 2.9 -fold higher than local emission impact (LEI) during days of elevated PM 2.5 concentrations. Notably, the correlation coefficient between LRT and VI ( r = -0.03) was considerably lower than that of LEI ( r = -0.70). Hence, predicting the daily average PM 2.5 concentration solely based on VI proved challenging in an area characterized by substantial LRT. For future research, in areas where LRT plays a substantial role, accurate prediction of PM 2.5 concentrations requires distinguishing the effects of LRT and LEI on PM 2.5 concentrations.