Uptake of soluble gaseous pollutants by rain droplets in the atmosphere with nocturnal temperature profile

We analyze the uptake of gaseous pollutants by the rain droplets falling in the atmosphere with nocturnal temperature inversion. The rate of uptake of soluble trace gases by falling rain droplets is determined by solving energy and mass conservation equations. In the analysis we accounted for the accumulation of the soluble gas and energy in the bulk of the falling rain droplet. The problem is solved in the approximation of a thin concentration and temperature boundary layers in the vicinity of the droplet surface. It is assumed that the bulk of a droplet, beyond the diffusion boundary layer, is completely mixed and distributions of concentration of the absorbate and temperature are homogeneous and time-dependent in the bulk. The problem is reduced to a system of linear-convolution Volterra integral equations of the second kind which is solved numerically. Calculations are performed using available experimental data on nocturnal temperature profiles in the atmosphere. It is shown than if the concentration of gaseous pollutants in the atmosphere is homogeneous and the absolute temperature in the atmosphere increases with altitude, a droplet absorbs gas during all the period of its fall. In the case when the temperature-altitude curve comprises the nocturnal inversion and temperature fall segments, gas absorption by a falling rain droplet can be replaced by desorption and vice versa. Neglecting temperature inhomogeneity in the atmosphere caused by nocturnal temperature profile leads to significant underestimation of the concentration of gaseous pollutants inside a droplet on the ground. The calculations performed using temperature profiles measured by Corsmeier et al. (1997) showed that the underestimation of the concentration of gaseous pollutants in rain droplets at the ground can exceed 20%. (C) 2010 Elsevier B.V. All rights reserved.