REMOVAL OF SULPHUR DIOXIDE AND SULPHATES FROM THE ATMOSPHERIC SURFACE LAYER DURING FROST FORMATION T. GEORGIADIS Z, V. STROCCHI 2, F. FORTEZZA 2, P. LUCIALLI 2, P. BONASONI 1, and G. GIOVANELLI 1 l FISBAT- C.N.R., Via Castagnoli 1, 40126 Bologna, Italy z PMP - USL 35, Via Alberoni, Ravenna, Italy (Received June 21, 1991; revised March 4, 1992) Abstract. In order to establish the amount of removal of SO 2 and sulphates from the lowest layer of the atmosphere during frost formation, a field measurement experiment was conducted in January 1990 in the Ravenna area. Frost can bring about removal of particles, gases and hydrosoluble substances. In common with areas of the Po Valley, Ravenna is frequently subjected to atmospheric conditions that can cause frost. During the winter of 1990, clear skies at night, that were associated with high pressure fields, strongly influenced the formation of frost. Close to industrial plants, characterized by wet emission of plumes, this frost has an adverse impact on soil, commonly known as 'chemical snow'. A large number of frost samples, collected in three locations representing typical but different features of the area (rural, urban and sea front sites), were analyzed. Analysis of the data reveals that SO2 is the major contributor SO4 - trapped in frost. 1. Introduction The ways by which pollutants reach the ground differ according to the meteorological and micrometeorological conditions. At the ground, deposition is accompanied by a decreasing of atmospheric concentration of pollutants near the surface. The process by which trace constituents are scavenged is controlled in part by their physical and chemical characteristics. The processes of pollutant removal from the atmosphere to the surface are usually considered under three main headings: dry deposition, wet deposition and 'special' events. Dry deposition is considered as a direct uptake from gas-phase at the earth's surface. Differing from the specific properties of the surface, the deposition velocities of SO2 are governed by such micrometeorological parameters as temperature gradient and wind speed profile. The dry deposition of sulphate aerosols through sedimen- tation and resuspension phenomena is controlled by the radius of the particles; this dependence can produce removal rates of 2 or 3 days for larger particles and up to several weeks for submicron fraction (Garland, 1978). Wet deposition involves complex chemical transformations of SO2 and S in presence of water, as in cloud, precipitation formation and in fog. The term 'acid rain' includes removal of acid materials, derived primarily form SO x and NOx, by precipitation, leading to deposition by wash-out and rain-out. Wash-out is the removal by hydrometers through impaction and interception of particles and gases; studies of the exchange of SOz between the atmosphere and falling rain have shown that the concentration decreases along with drop size (Garland, op. cit.; Hales, Water, Air, and Soil Pollution 66: 267-276, 1993. 9 1993 Kluwer Academic Publishers. Printed in the Netherlands.