Fig. 1
Conceptual diagram of the BC tracer method. The mass-equivalent diameter (Dtr) of each water-insoluble BC particle is an observable invariant during removal and transport processes. In a convective-type precipitating cloud system, the removal efficiency of aerosols in a moist air parcel uplifted from the boundary layer is determined mainly by the nucleation scavenging process within each localized supersaturated domain (LSD). The occurrence and activity of LSDs in a cloud system are complex functions of time and space. For simplicity, the figure illustrates a temporal snapshot of the spatial distribution of discrete LSDs (LSDk, k = 1, …, n). For each rainfall event, the average of SSlsd, weighted by the number of tracer particles scavenged in LSDk (k = 1, …, n), is estimated by comparing the Dtr-resolved number concentrations of initial tracer (in the surface air) and removed tracer (in rainwater) measured at a surface observation site. For each rainfall event, the rainwater sampling period was typically ~100−101 min (Table S1). The cloud system moves horizontally ~100−101 km during the rainwater sampling under the typical wind speed in the lower troposphere of ~10 ms–1. The number (n) of LSDs contributing to the scavenging of observed tracer depends on the sampling period as well as on the translational speed of the cloud system
