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The Scavenging Process and Physical Removing Mechanism of Pollutant Aerosols by Different Precipitation Intensities
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摘要: 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PM2.5和降水数据，研究了不同降水强度对PM2.5的清除率，以及雨滴谱、风速和降水持续时间对PM2.5清除率的影响。研究表明：降水强度越大，对PM2.5清除效率越高。小雨、中雨和大雨对PM2.5清除率平均值分别为5.1%，38.5%和50.6%。小雨不但对PM2.5的清除率最低，而且对PM2.5的清除效果也存在很大差异，约50%的小雨个例中PM2.5质量浓度出现减小情况，而另外50%的小雨个例中，PM2.5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，PM2.5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PM2.5，因此，对降水的持续时间和风速大小不敏感。Abstract:
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. PM2.5 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 PM2.5 scavenging rate are studied. Results show that stronger precipitation is more efficient in removing polluted aerosols in atmosphere. The mean PM2.5 scavenging rate is 5.1%, 38.5% and 50.6% for light, moderate and heavy rain, respectively. However, PM2.5 scavenging rate by light rain has large difference. In about 50% light rain cases, PM2.5 mass concentration decreases, while in the other 50% light rain cases, PM2.5 mass concentration increases. In all moderate and heavy rain cases, PM2.5 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, PM2.5 scavenging rate by light rain is much lower, while moderate and heavy rain usually have wider size distribution and more larger drops, so that PM2.5 scavenging rates by these precipitation are much higher. In addition, further investigations indicate that PM2.5 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 PM2.5 scavenging rates are relatively smaller for moderate and heavy rain. This is because that the moderate and heavy rain can scavenge most of PM2.5 in a short time. The size distribution of raindrops is not an important factor to cause the different PM2.5 scavenging rate for the same rain intensity.
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Key words:
- PM2.5;
- precipitation scavenging;
- precipitation intensity;
- impact factors
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图 1 2014年3月—2016年7月北京降水过程中小时降水量与PM2.5质量浓度之间的对应关系
Fig. 1 Relationship between PM2.5 mass concentration and hourly rainfall amount in Beijing from Mar 2014 to Jul 2016
图 2 2014年3月—2016年7月北京降水频次与PM2.5清除率的关系
Fig. 2 Relationship between rain case numbers and PM2.5 scavenging rate for different rain intensities in Beijing from Mar 2014 to Jul 2016
图 3 2014年3月—2016年7月北京降水后PM2.5质量浓度减小个例中不同小时降水量对应的PM2.5清除率
(黑线代表中值，圆圈代表平均值，矩形框的上下边分别代表上下四分位数，上下触须线分别代表最大值和最小值)
Fig. 3 Relationship between PM2.5 scavenging rate and hourly rainfall amount in rain events with PM2.5 mass concentration decreased after the rain in Beijing from Mar 2014 to Jul 2016
(black lines in the box indicate median values, circles indicate average values, box boundaries indicate the first and third quartiles, lines above and under the box indicate the maximum and minimum values)
图 4 北京小雨、中雨和大雨的平均雨滴谱分布
Fig. 4 Mean raindrop size distributions of light, moderate and heavy rain events in Beijing
图 5 北京小雨和中雨降水后PM2.5质量浓度减小个例和降水后PM2.5质量浓度增加个例的平均雨滴谱分布
Fig. 5 Mean raindrop size distributions for both cases with PM2.5 mass concentration decreased and increased of light and moderate rain events in Beijing
图 6 2014年3月—2016年7月北京降水后PM2.5质量浓度减小个例中降水持续时间与PM2.5清除率的箱形图
(黑线代表中值，圆圈代表平均值，矩形框的上下边分别代表上下四分位数，上下触须线分别代表最大和最小值，加号代表异常值)
Fig. 6 Variations of PM2.5 scavenging rate with rain duration in rain events with PM2.5 concentration decreased after the rain in Beijing from Mar 2014 to Jul 2016
(black lines in the box indicate median values, circles indicate average values, box boundaries indicate the first and third quartiles, lines above and under the box indicate the maximum and minimum values, plus signs indicate the outlier)
图 7 2014年3月—2016年7月北京降水后PM2.5质量浓度增加个例中降水持续时间与PM2.5清除率的箱形图
(黑线代表中值，圆圈代表平均值，矩形框的上下边分别代表上下四分位数，上下触须线分别代表最大和最小值，加号代表异常值，|ΔC|为PM2.5清除率的绝对值)
Fig. 7 Variations of PM2.5 scavenging rate with rain duration in rain events with PM2.5 mass concentration increased after the rain in Beijing from Mar 2014 to Jul 2016
(black lines in the box indicate median values, circles indicate average values, box boundaries indicate the first and third quartiles, lines above and under the box indicate the maximum and minimum values, plus signs indicate the outlier, |ΔC| is the absolute value of PM2.5 scavenging rate)
图 8 2014年3月—2016年7月北京PM2.5清除率和降水前后风速差的对应关系
(降水前后风速差为降水过程中的平均风速减去降水前的平均风速)
Fig. 8 Relationship between PM2.5 scavenging rate and the difference of wind speed in Beijing from Mar 2014 to Jul 2016
(the wind speed during the rain minus that before the rain)
图 9 2014年4月25—26日北京小雨过程PM2.5质量浓度和小时降水量(a)以及风速和风向(b)随时间变化
Fig. 9 Temporal variations of PM2.5 mass concentration and hourly rainfall amount(a), wind speed and wind direction(b) of light rain event in Beijing during 25-26 Apr 2014
图 10 2014年9月12日北京小雨过程PM2.5质量浓度和小时降水量(a)以及风速和风向(b)随时间变化
Fig. 10 Temporal variations of PM2.5 mass concentration and hourly rainfall amount(a), wind speed and wind direction(b) of light rain event in Beijing on 12 Sep 2014
图 11 2015年9月17—18日北京小雨过程PM2.5质量浓度和小时降水量(a)以及风速和风向(b)随时间变化
Fig. 11 Temporal variations of PM2.5 mass concentration and hourly rainfall amount(a), wind speed and wind direction(b) of light rain event in Beijing during 17-18 Sep 2015
表 1 2014年3月—2016年7月北京不同降水强度的PM2.5清除率
Table 1 PM2.5 scavenging rates for different rain intensities in Beijing from Mar 2014 to Jul 2016
类别 降水强度 个例数量 PM2.5清除率/% 平均值 最大值 最小值 小雨 48 23.4 78.8 0.4 PM2.5质量浓度减小个例 中雨 12 46.0 83.9 5.6 大雨 9 50.6 70.9 10.5 PM2.5质量浓度增加个例 小雨 46 -14.0 -0.2 -50.9 中雨 2 -6.7 -2.9 -10.4 小雨 94 5.1 78.8 -50.9 总个例 中雨 14 38.5 83.9 -10.4 大雨 9 50.6 70.9 10.5 
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