Scavenging of aerosol particles by large water drops: 3. Washout coefficients, half‐lives, and rainfall depths

Abstract

Using the average values of collection efficiencies obtained from our experiments [Pranesha and Kamra, 1996, this issue] washout coefficients for the drops in the diameter (D) range 3.6≤D≤5.0 mm collecting micron‐sized aerosol particles have been calculated when the drops are neutral, charged, or falling in an electric field. Compared with the neutral case, the values of washout coefficients are higher in both electrical cases, the increase being more pronounced for smaller particles. Washout coefficients show a maximum for a drop charge of 10−12 to 10−11 C. With an increase in electric field, the washout coefficients increase linearly for ∼1‐μm particles, show a maximum for ∼4‐μm particles, and change insignificantly for ∼7‐μm particles. Combining our experimental values of collection efficiencies with the theoretical collision efficiencies of McGann and Jennings [1991] for smaller drops, washout coefficients, half‐lives, and rainfall depths have been computed for the raindrop size distribution extending from 0.1‐ to 5 mm‐diameter. Results show that raindrops of diameter >1 mm contribute dominantly in removing particles of diameter 1–2 μm and their contributions increase with the rainfall rate. When the effect of the raindrops of diameter >1 mm is included, the values of washout coefficient increase by about 2 orders of magnitude for particles of diameter 1–2 μm and by about 1 order of magnitude for particles of diameter >2 μm. It can be concluded from the estimates of rainfall depth that a heavy rainfall over a short duration is more efficient in removing the particles of diameter <2.2 μm, whereas a lower rainfall spread over a longer duration is more efficient in removing the particles of diameter >2.2 μm.