Below-Cloud Scavenging of Gases

Note that according to the definitions of (20.7) and (20.8) the relation between the wet deposition velocity uw and the washout ratio wr is 

During rain, soluble species that exist below clouds dissolve into falling raindrops and are removed from the atmosphere. We would like to estimate the rate of removal of these species based on rain event characteristics (rain intensity, raindrop size) and species physical and chemical properties. 

The rate of transfer of a gas to the surface of a stationary or falling drop can be calculated by 

Both Cg and Ceq are gas-phase concentrations and therefore have units of moles of A per unit volume of air, whereas W, has units of moles of A transferred per unit surface area per time. Using Henry s law, Ceq = Caq/ H RT), where H is the effective Henry s law coefficient of the species, and Caq is its aqueous-phase concentration (mol A per unit volume of droplet). Therefore (20.10) can be written as 

The mass transfer coefficient of a gaseous molecule to a sphere can be calculated by the empirical correlation (Bird et al. 1960) 

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BELOW CLOUD SCAVENGING OF ACIO GASES AND FINE PARTICLES... 

Typical values of scavenging ratio lie within the range 300-2000. Scavenging ratios are rather variable, dependent upon the ehemieal nature of the trace substance (particle or gas, soluble or insoluble, etc) and the type of atmospheric precipitation. Incorporation of gases and particles into rain can occur both by in-cloud scavenging (also termed rainout) and below-cloud scavenging (termed washout). 

Species can be incorporated into cloud and raindrops inside the raining cloud. These processes determine the initial concentration Caq° of the raindrops, before they start falling below the cloud base, and have been discussed previously. Let us summarize the rates of in-cloud scavenging for gases and aerosols. 

Scavenging of particles or gases may take place in clouds (rainout) by cloud droplets or below clouds(washout) by precipitation. A scavenging ratio or washout ratio W can be defined as... 

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