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.
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