Below-Cloud Scavenging of an Irreversibly Soluble Gas

Equations (20.10) and (20.11) apply to the general case where a gas can be transferred both from the gas to the aqueous phase (when Cg Ceq), and vice versa (when Cg Ceq). However, in the limiting case when Cg Ceq, one can neglect the flux from the aqueous to the gas phase and assume that 

This simplification frees us from the need to estimate the aqueous-phase concentration Caq (z, t). This will be a rather good approximation for a very soluble species, that is, a species with sufficiently high effective Henry s law constant H. Nitric acid, with a Henry s law constant of 2.1 x 105 M atm-1, is a good example of such a species. Recalling that dissolved nitric acid dissociates to produce nitrate, we obtain 

001 ppb and as Cg lOppb the assumption Cg Ceq should hold under most ambient conditions. 

Let us calculate the scavenging rate of HN03(g) during a rain event. There are two ways to approach this problem  

To follow a falling raindrop calculating the amount of HN03(g) removed by it. 

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