The variation of particle number size distribution during the rainfall: wet scavenging and air masses changing

Abstract. Below-cloud wet scavenging is an important pathway to remove atmospheric aerosols. The below-cloud wet scavenging coefficient (BWSC) is the value to describe the ability of rainfall to remove aerosols. The reported BWSCs obtained from the field measurements are much higher than the theory, but the reason for this remains unclear. In this study, based on the long-term field measurements in the Yangtze River Delta of eastern China, we find 28 % of the rainfall events are high BWSC events. The high BWSC events show the sudden decrease of particle number concentration in all size bins near the end of rainfall. By investigating the circulation patterns, backward trajectories and the variations of simultaneously observed atmospheric components, we find the cause of the high BWSC events is the air masses changing but not the wet scavenging. The change of air masses is always followed by the rainfall processes and cannot be screened out by the traditional meteorological criteria, which would cause the overestimation of BWSC. After excluding the high BWSC events, the observed BWSC is close to the theory and is correlated with the rainfall intensity and particle number concentrations prior to rainfall. This study highlights the discrepancy between the observed BWSC and the theoretical value may not be as large as it is currently believed. To obtain reasonable BWSCs and parameterization from field measurements, the effect of air masses changing during rainfall needs to be carefully considered.