Even during the heaviest rains (right), rain reduced small particles by just 8.7%. For light to medium rain, the effect on PM 2.5 was close to zero.
With Josh Malina (the nerd behind AQCast) helped us by analyzing 6 years of PM2.5 and weather data for Beijing. The correlation between rain and PM2.5 was far smaller (r = -.04) than for wind (r = -.37). Researchers in Europe have found similar effects for rain.