Below Cloud Scavenging

Removal of fine particles by precipitation (below-cloud scavenging) is one of the most important natural processes in the transformation of fine particles. Both liquid (fog, cloud droplets, and raindrops) and solid (ice crystals, snow, graupel, and hail elements) precipitation can be involved in this removal process. The precipitation scavenging process is depicted in Figure 4.

Particles can be removed from the atmosphere through the following processes:

1. Brownian diffusion through coagulation of fine particles with liquid or solid precipitation;

Figure 4 The precipitation scavenging process. Reproduced from Pacyna JM and Ottar B (eds.) (1989) Control and Fate of Atmospheric Trace Metals. Dordrecht: Kluwer Academic Publishers, with permission from Springer Science and Business Media.

Figure 4 The precipitation scavenging process. Reproduced from Pacyna JM and Ottar B (eds.) (1989) Control and Fate of Atmospheric Trace Metals. Dordrecht: Kluwer Academic Publishers, with permission from Springer Science and Business Media.

2. collision of fine particles with liquid or solid precipitation caused by phoretic forces (thermophoresis and diffusiophoresis);

3. inertial impaction of fine particles on precipitation elements, caused by the hydrodynamic interaction of the fine particles with either liquid or solid precipitation, with both falling free under gravity, and

4. electrical forces which can promote precipitation.

Scavenging rate for fine particles due to the above-mentioned processes depends on parameters that vary for different types of the scavenging process. This rate is, however, a function of particle size in all cases. This function is presented in Figure 5 for Brownian diffusion, inertial capture, and thermo- and diffusiophoresis. This function has been studied in a number of theoretical and experimental works. A full numerical treatment of the

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Figure 5 Scavenging rate (h-1) as a function of aerosol particle size for Brownian diffusion, inertial capture, and thermo- and diffusiophoresis; [37] AT = T^- T = 30C, precipitation rate R = 10 mm h -1; raindrop size distribution n(a)da = (10-4R/ 6namax)a2 exp(-2a/amax) da, with R in cm s-1; drop terminal velocity V^ = 8000a (s-1) with a in cm. Reproduced with permission from Atmospheric Particles, Harrison RM and van Grieken R (eds.) Copyright 1998. © John Wiley & Sons Limited.

capture of particles within various sizes (from 1.0 to 20.0 mm in diameter) due to combined action of inertial, phoretic, and electrical forces has been developed. Extended estimates have been made in this approach for collision efficiencies for fine particles and droplets over a large number of drop sizes.

In another approach suitable for finer particles, particle capture due to combined action of Brownian motion, phore-tic forces, and electrical charge was presented in a numerical model through solving the convective diffusion equation.

The scavenging of particles by solid precipitation has not been studied as much as compared to liquid precipitation scavenging. The efficiency with which particles are collected by ice crystal plates has been analyzed for particles in size range from 0.001 to 10 mm in diameter. Models were developed to calculate the collection efficiency of fine particles by columnar ice crystals.

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