On the scavenging of SO2by cloud and raindrops: III. A theoretical study of SO2washout by rain falling through a pollution plume

✍ Scribed by C. J. Walcek; H. R. Pruppacher

• Publisher: Springer Netherlands
• Year: 1983
• Tongue: English
• Weight: 750 KB
• Volume: 1
• Category: Article
• ISSN: 0167-7764
• DOI: 10.1007/bf00058734

✦ Synopsis

Our previously described absorption model for the scavenging of trace gases by individual cloud and rain drops was applied to an ensemble of drops of given size distribution as found in typical atmospheric rainfalls. This study allowed: (1) determination of the redistribution which a pollution plume suffers as rain falls through it, and (2) determination of the amount of pollutant contained in the fallen rain. For the present application it was assumed that the pollution plume consisted of SO 2 only, and that its concentration varied vertically as a Gaussian distribution centered 200 m above the ground. Rain of various intensities and corresponding drop size distributions was allowed to pass through this pollution layer.

The results of our study show that the average height and concentration of the plume decrease in proportion to the total amount of rain which had passed through the pollution layer. The fractional plume washout rate (in % per mm of rain) was found to be inversely proportional to the SO 2 gas concentration, the plume height, and the plume thickness. By including the effects of oxidation inside the falling raindrops, it was noted that the removal of SO~ became enhanced by a factor of up to 10 times. For significant oxidation the rate with which SO 2 was washed out was found to be inversely proportional to the rainfall rate. For the case of light drizzle (2 mm h -t) and large oxidation rate constants (100 s-t), as much as 40% of the sulfur load could be removed by 1 mm of rain. For the case of a heavier rainfall (25 mm h -1) composed of correspondingly larger drops, the washout rate varied between 2 and 10% mm -1 over a wide range of oxidation rates.