Scavenging of Soluble Gases by Slowly Evaporating Droplets in Inhomogeneous Atmosphere

We present a one-dimensional model of scavenging of soluble gaseous pollutants by evaporating droplets which is valid for arbitrary initial vertical distribution of soluble trace gases in the atmosphere. It is shown that for low gradients of soluble trace gases in the atmosphere gas scavenging is determined by a linear wave equation that describes propagation of a wave in one direction. The derived equation is solved by method of characteristics. Calculations of scavenging coefficient are performed for wet removal of sulfur dioxide (SO2) from the atmosphere for available in the literature initial distribution of trace gases and for different values of humidity in air. It is shown that for arbitrary initial vertical distribution of soluble trace gases in the atmosphere scavenging coefficient is non-stationary and height-dependent. For exponential initial distribution of soluble trace gases in the atmosphere scavenging coefficient in the region between the ground and the location of a scavenging front is proportional to rainfall rate at a bottom of a cloud, solubility parameter and the growth constant in the formula for the initial profile of a soluble trace gas in the atmosphere. This expression yields the same estimate of scavenging coefficient for sulfur dioxide as field estimates presented in McMahon and Denison (1979). It is demonstrated that the smaller the slope of the concentration profile the higher is the value of a scavenging coefficient.