The Scavenging of Gases by Cloud and Rain Drops

For ideal solutions in which solute interactions can be neglected, the mole fraction xs of a substance dissolved in water and its equilibrium vapor pressure p in the gas phase above the solution are related by Henry s law, 

cs denotes the concentration in mol/kg (molality scale), and [s] is the concentration in mol/liter (molarity scale). Both units are related in that [s] = pwcs where pw= 1 kg/dm3 is the density of water. Its variation with temperature causes the molarity scale to depend on temperature, whereas the molality scale does not. In the temperature range 0-25°C, however, the density of water differs from unity by less than 0.3%, so that [s] = cs with reasonable accuracy. Most Henry coefficients are less well known. From the definitions in Eqs. (8-7) and (8-8), the coefficients involved are related by 

In order to determine the gas-liquid partitioning of a water-soluble atmospheric component in a cloud, consider a fixed volume Vc of cloud-filled air containing v0 number of moles of the substance in question. Of this, v% moles reside in solution and ve moles in the gas phase. We make use of the ideal gas law [Eq. (1-1)] and set 

The fraction of the substance that is incorporated into solution is given by eg = vjv0= 1/(1+H MJRgTL) 

pair is the atmospheric pressure at the condensation level, and m0 is the mixing ratio of the substance in the gas phase before condensation took place. For eg 1, one has HMJRtTL 1, and the concentration simplifies to 

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