| 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 | |
| Citation: | Luan Tian, Guo Xueliang, Zhang Tianhang, et al. The scavenging process and physical removing mechanism of pollutant aerosols by different precipitation intensities. J Appl Meteor Sci, 2019, 30(3): 279-291. DOI: 10.11898/1001-7313.20190303.
|
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.
Fig. 1 Relationship between PM2.5 mass concentration and hourly rainfall amount in Beijing from Mar 2014 to Jul 2016
Fig. 2 Relationship between rain case numbers and PM2.5 scavenging rate for different rain intensities in Beijing from Mar 2014 to Jul 2016
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)
Fig. 4 Mean raindrop size distributions of light, moderate and heavy rain events in Beijing
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
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)
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)
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)
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
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
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
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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Figures(11) / Tables(1)
