不同降水强度对PM_(2.5)的清除作用及影响因素

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

云和降水过程是大气污染物的重要清除途径,但由于降水过程和大气污染颗粒物本身的复杂性,目前降水过程对大气污染物的清除机制及影响因素有待深入研究。该文利用2014年3月—2016年7月在北京地区连续观测的PM_(2.5)和降水数据,研究了不同降水强度对PM_(2.5)的清除率,以及雨滴谱、风速和降水持续时间对PM_(2.5)清除率的影响。研究表明:降水强度越大,对PM_(2.5)清除效率越高。小雨、中雨和大雨对PM_(2.5)清除率平均值分别为5.1%,38. 5%和50.6%。小雨不但对PM_(2.5)的清除率最低,而且对PM_(2.5)的清除效果也存在很大差异,约50%的小雨个例中PM_(2.5)质量浓度出现减小情况,而另外50%的小雨个例中,PM_(2.5)质量浓度出现增加情况。在持续时间长或地面风速增大的情况下,小雨也表现出较高的清除率。在中雨和大雨情况下,PM_(2.5)质量浓度均出现明显减小情况。但降水持续时间和风速对中雨和大雨的清除率影响较小,这是由于中雨和大雨一般在较短时间内即可清除大部分PM_(2.5),因此,对降水的持续时间和风速大小不敏感。

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.