A new analysis of fine aerosol capture by raindrops at terminal velocities

• Diffusional deposition of submicron aerosols on a raindrop depends on Pe−1/2.
• Calculated efficiency curves by new correlations agree well with the test data.
• Flow field near a raindrop at its terminal speed depends on size of the drop.
• Boundary layer expansion is valid to simplify the flow field around raindrops.

The diffusional capture of submicron aerosols by a falling raindrop at its terminal velocity is theoretically analyzed by employing the thin concentration boundary layer theory. The flow fields around the raindrop are assumed to be viscous and potential flows for small and large drops, respectively. The mass transfer rate of submicron aerosols at the front stagnation point of the raindrop is determined by using the boundary layer expansion technique in the thin concentration boundary layer. Comparing the present results with experimental data and analytical results in previous studies, new empirical correlations are regressed and recommended. The collection efficiency of submicron aerosols by either a large or a small raindrop is proportional to Pe−1/2, and interception effect is negligible. The expressions are different from the earlier results for the same or similar problems, e.g., deposition on a hard spherical collector and the below-cloud scavenging process. In addition, the results show that the velocity components of both viscous and potential flows can be expressed as Taylor series with acceptable accuracies in the concentration boundary layer near the front stagnation point of the raindrop.

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