东莞地区PM_(2.5)湿清除的观测研究

Observational study of PM_(2.5) scavenging by rain in Dongguan

通过对东莞市2013—2017年降水、PM_(2.5)观测数据统计研究发现,降雨及PM_(2.5)浓度存在负相关关系,降雨对PM_(2.5)具有一定的清除效果.PM_(2.5)浓度下降的降水过程有376次,浓度上升的降水过程有333次,部分降水过程浓度出现明显反弹,应与气溶胶再悬浮机制有关.在所有PM_(2.5)初始浓度低于40μg·m^(-3)的个例中,浓度下降的个例占比约49.4%,而当PM_(2.5)初始浓度高于40μg·m^(-3)时,浓度下降的个例占比约71.2%,说明PM_(2.5)粒子数量较多时,雨滴对粒子的捕获能力较强,降雨前PM_(2.5)的初始浓度可以作为判断降雨后PM_(2.5)浓度是否下降的预报依据.雨强小,也是造成雨后PM_(2.5)浓度反弹的原因.对于有效清除个例,雨强与清除率存在正相关关系,雨强越强,整体的清除作用有所加强.降水持续时间是影响PM_(2.5)清除作用的重要因素,持续时间超过6 h的小雨降水过程能达到40%的清除率效果,中雨以上降水过程持续时间越长,PM_(2.5)的清除效果越好.在浓度反弹的个例中,在低的初始浓度条件下,降水强度与降水持续时间对PM_(2.5)浓度反弹影响均较小,在初始浓度大于30μg·m^(-3)的条件下,平均反弹幅度收窄,初始浓度的增加,使得雨滴与颗粒物之间的碰并作用加强,降水强度的增大,使这种碰并作用进一步加强,进而制约了PM_(2.5)浓度反弹的幅度.降雨末期,细粒径颗粒物数浓度回升明显,小雨滴蒸发使细粒子脱离再次在空中悬浮,是造成雨后颗粒物浓度出现回升的主要原因.

The precipitation scavenging of PM_(2.5) in Dongguan was investigated using hourly ground level meteorological and PM_(2.5) mass concentration data during 2013—2017. The negative correlation between rainfall and PM_(2.5) indicated rainfall had removal effect on PM_(2.5). Decreases of PM_(2.5) mass concentration were observed in 376 rain events. While in 333 rain events, PM_(2.5) mass concentration exhibited increasing tends. The increased PM_(2.5) was associated with aerosol resuspension. The PM_(2.5) mass concentration was decreased in 49.4% of the samples with an initial concentration lower than 40 μg·m^(-3), while the concentration was decreased in 71.2% of the samples with an initial concentration that higher than 40 μg·m^(-3). The ratio increased in samples with a higher initial concentration, indicating that more PM_(2.5) had a more significant effect on the particulate removal, therefore, the initial concentration could be used as the important factor for predicting whether the PM_(2.5) will decrease after rainfall. The weak rain intensity is one of the reasons for the concentration increase after rain. In the samples with decreased concentration, the correlation between scavenging rate and rain intensity was positive. A higher rainfall intensity led to more efficient scavenging. The duration of rainfall is an important factor affecting the removal of PM_(2.5). The light rain lasting more than 6 hours can remove 40% of PM_(2.5). The longer duration of precipitation in moderate rain led to a higher removal rate in PM_(2.5). In the cases of increased PM_(2.5) mass concentration, when the initial concentration was at a low level, rainfall intensity and duration had weak effects on PM_(2.5) concentration. When the initial concentration higher than 30 μg·m^(-3), a higher rainfall intensity led to a lower rebound range of PM_(2.5) concentration. The number concentration of fine particle increased significantly in the late rainfall, which caused by the evaporation of small raindrops. The evaporation of small raindrops made the fine particle suspend in the air again, which was the main reason for the rise of particle concentration after rain.