論文

The quantitative relationship between the observed snowfall and PM2.5 concentration data collected at Niigata Prefecture, Japan, was examined. The PM2.5 concentration decreased as snowfall increased. A composite analysis revealed that the PM2.5 concentration decreased by 27.33% from 1 h before to 1 h after the onset of snowfall, which is statistically significant. The average PM2.5 concentration for all snowfall events decreased over a total of three hours, from 2 h before to 1 h after the onset of snowfall. The average PM2.5 scavenging rate for all snowfall events was 6.15 × 10−6 s−1, which was much higher than that for rainfall events. The scavenging rates were positive at relative humidity (RH) between 50% and 75% and above 95%. It is suggested that the deposition processes of wet scavenging are effective, both in reducing PM2.5 concentration during snowfall and in the RH dependence of scavenging rates.

This study analyzed the observed data of precipitation and PM2.5 concentration collected over two and a half years at Tsujido, Fujisawa City, to examine a quantitative relationship between the two quantities. It was observed that the PM2.5 concentration decreased as precipitation increased. A composite analysis revealed that the PM2.5 concentration on average decreases by 20.99% 1 h after the onset of precipitation as compared to that 1 h before, which is statistically significant based on a paired t-test. The PM2.5 concentrations decreased over 8 h period from 5 h (−5 h) before to 3 h (+3 h) after the onset of precipitation, with a particularly dramatic decrease at the onset of precipitation. Analysis of PM2.5 scavenging rates by precipitation intensity showed that the scavenging rate was 2.03% for light precipitation events, 28.15% for moderate precipitation events, and 26.75% for heavy precipitation events, indicating that the scavenging rate increased with moderate precipitation intensity and above. It is suggested that the deposition processes of rainout and washout are effective in reducing PM2.5 concentration with precipitation.