The chemical content of raindrops as a function of drop radius—II. Field experimental study on the scavenging of marked aerosol particles by raindrops sampled as a function of drop size

The present work attempts to explain the relation between the concentration of a chemical species in raindrops and the drop radius. Field experiments were performed to investigate the influence resulting from the impaction scavenging of aerosol particles by raindrops. “Large” and “giant” aerosol particles were injected into the air during several rain events. These particles were produced by atomisation of concentrated solutions of Al3+ and Mn2+ salts. The scavenging of the aerosol particles took place under natural environmental conditions. Raindrops which passed through the volume of air containing the marked aerosol particles were collected and analysed. A special sampling technique, i.e. the Guttalgor method, was used to get size fractionated raindrops. Chemical analyses were done by atomic absorption spectroscopy (AAS). It was shown that the scavenging of “giant” aerosol particles (4 μm mean radius) leads to a continuous decrease in concentration with increasing radius of the raindrops. “Large” aerosol particles (0.8 μm mean radius) showed a maximum concentration at a specific drop radius (0.225 mm). Our earlier field experiments showed similar results. Thus, the dependence of concentration on drop radius for natural Fe, Al and Si gave a continuous decrease in the concentration with increasing drop radius, while for Mn and Pb a maximum concentration was obtained at an intermediate radius. The old and new data with marked aerosol particles suggest that the different behaviours are caused by the size of the scavenged aerosol particles. An attempt has been made to explain this behaviour on the basis of the collision efficiency as a function of drop size and particle size.