The Scavenging Process and Physical Removing Mechanism of Pollutant Aerosols by Different Precipitation Intensities

The aerosol scavenging process of precipitation is an important mechanism for cleaning polluted aerosols in atmosphere. But there are many uncertainties due to complexities of precipitation processes and atmospheric pollutant particulate matter. PM_2.5 scavenging rates by different intensities of precipitation are investigated based on aerosol and precipitation measurements in Beijing from March 2014 to July 2016. Effects of raindrop size distribution, wind speed and rain duration on PM_2.5 scavenging rate are studied. Results show that stronger precipitation is more efficient in removing polluted aerosols in atmosphere. The mean PM_2.5 scavenging rate is 5.1%, 38.5% and 50.6% for light, moderate and heavy rain, respectively. However, PM_2.5 scavenging rate by light rain has large difference. In about 50% light rain cases, PM_2.5 mass concentration decreases, while in the other 50% light rain cases, PM_2.5 mass concentration increases. In all moderate and heavy rain cases, PM_2.5 concentration apparently decreases. Scavenging rates exceed 40% for 10% of light rain cases, 50% of moderate rain cases, and 78% of heavy rain cases. Since light precipitation has generally narrower size distribution and more smaller drops, PM_2.5 scavenging rate by light rain is much lower, while moderate and heavy rain usually have wider size distribution and more larger drops, so that PM_2.5 scavenging rates by these precipitation are much higher. In addition, further investigations indicate that PM_2.5 scavenging rate for light rain is strongly influenced by precipitation duration and wind speed. The longer precipitation duration and higher the wind speed is, the higher the scavenging rate for light rain becomes. In some light rain cases, these factors enhance scavenging rates, but influences of precipitation duration and wind speed on PM_2.5 scavenging rates are relatively smaller for moderate and heavy rain. This is because that the moderate and heavy rain can scavenge most of PM_2.5 in a short time. The size distribution of raindrops is not an important factor to cause the different PM_2.5 scavenging rate for the same rain intensity.