I. Introduction
The content of impurities in rainwater is an essential indicator of air pollutions. Besides direct testing of air quality, an essential part of information can be obtained from rainwater assays. Water vapor in the air is adsorbed on solid particles of industrial dust, which is formed as a result of mechanical treatment of various materials, thermal processes of combustion or smelting, and transportation of leaky bulk materials, and falls as rain. For instance, rain with an intensity of 1 mm/h in 45 minutes washes out of air 28% of fine dust particles with a diameter of about