Effect of rain scavenging on altitudinal distribution of soluble gaseous pollutants in the atmosphere

We suggest a one-dimensional model of rain scavenging of moderately soluble gaseous pollutants in the atmosphere. It is shown that below-cloud gas scavenging is determined by non-stationary convective diffusion equation with the effective Peclet number. The obtained equation was analyzed numerically in the case of log-normal droplet size distribution. Calculations of scavenging coefficient and the rates of precipitation scavenging are performed for wet removal of ammonia (NH3) and sulfur dioxide (SO2) from the atmosphere. It is shown that scavenging coefficient is non-stationary and height-dependent. It is found also that the scavenging coefficient strongly depends on initial concentration distribution of soluble gaseous pollutants in the atmosphere. It is demonstrated that in the case of linear distribution of the initial concentration of gaseous pollutants whereby the initial concentration of gaseous pollutants decreases with altitude, the scavenging coefficient increases with height in the beginning of rainfall. At the later stage of the rain scavenging coefficient decreases with height in the upper below-cloud layers of the atmosphere.

► Scavenging coefficient of soluble trace gases by rain depends on time and height.
► Scavenging coefficient at the ground increases with time.
► Below-cloud scavenging coefficient near the cloud decreases with time.
► Scavenging front propagates with wash-down velocity and is smeared by diffusion.
► Scavenging coefficient depends on the initial vertical distribution of trace gas.