Below-Cloud Scavenging of a Reversibly Soluble Gas

For a reversibly soluble gas, one needs to retain both the flux from the gas to the aqueous phase and the inverse flux. The mass balance of (20.15) is still valid, so combining it with (20.11), we get 

This equation indicates that the driving force for scavenging is the difference between the gas-phase concentration of the species and the concentration at the drop surface (droplet equilibrium concentration). As the aqueous-phase concentration increases during the droplet fall, the driving force will tend to decrease. 

Let us assume that the rain droplet size and droplet pH remain constant during its fall and that the atmosphere is homogeneous so that Cg(z, t) = Cg(t). Then (20.27) can be integrated using Caq 0, t) — to give 

The overall below-cloud scavenging rate, Wbc, can be calculated once more by multiplying the per droplet scavenging rate by the number of droplets per volume of air 

Because of the reversibility of the scavenging process, one can define a scavenging coefficient only if CgH RT This is valid when the initial raindrop species concentration is much lower than the equilibrium concentration corresponding to the below-cloud atmospheric conditions. If this is valid, then 

---