Introductory Note BY R. LESSING In the Faraday Society’s General Discussion on ‘‘ Disperse Systems in Gases : Dust, Smoke and Fog ” at Eeeds in 1936, the opening sentence of my introductory paper on the industrial aspects read : “ Disperse systems in gases in their relation to the problems of industry and the amenities of life have not yet received the close study which their ubiquity and practical importance demand.” Whilst much research has been done since then, largely stimulated by the fog disasters in the Meusc Valley in 1930, at Donora, Pennsylvania, in 1948, by the great London fog in December, 1952, and the continuing smog nuisance of Los Angeles, many problems of air pollution still remain unsolved. As far as fog formation and the dispersion of air masses containing natural nuclei such as salt spray from the oceans or dust from volcanic eruptions, or man- made pollutants from the combustion of fuels are concerned, much closer co- operation between meteorologist and physical chemist is needed in the future.In this brief note I will only refer to what I consider the most pressing among the many complex problems of air pollution still awaiting intensive study by the physical chemist, that of sulphur. Great Britain discharges into the atmosphcre a sulphuric acid equivalent of about 12 million tons a year derived from the combustion of coal and oil and their products, the bulk in form of sulphur dioxide with a small proportion about 2-4 % of sulphur trioxide formed-by a still unexplained mechanism-in high- temperature firing.Relatively small quantities of hydrogen sulphide are emitted in the inefficient burning of coal in domestic grates and old boiler plants. In addition an unknown quantity of sulphur compounds, perhaps hydrogen sulphide, arises from decaying vegetation. Some of the sulphur dioxide is blown out to sea and, incidentally, is identifiable in the western part of Scandinavia, but the majority reaches the ground, being deposited in combination with particulate matter or being washed out by rain. Ths vast quantity of acidic material is the main contributor to the damage caused by air pollution assessed by the Beaver Committee 1 at &250 million a year-probably an under-estimate. Whilst the final effects of the reactions of suiphur dioxide with metals, stones, textiles, vegetation and the human and animal body are well known, it is by no means clear whether they or some of them are caused by direct reaction or through the medium of sulphuric acid.Eventually the end products of the attack by atmospheric sulphur oxides are sulphates, but the mechanisms of the oxidation of sulphur dioxide or its salts still require elucidation. Practical experience shows that the interaction of sulphur oxides with dusts and liquid aerosols begins in the air. I venture to present a picture of the com- plex phenomena involved suggesting pointers for future individual researches rather than to record achievements attained. Much, if not most, of the dust in the air over Britain is part o€ the ash emitted from coal-fired furnaces. The particles vary widely in chemical composition.:! They may be oxides or carbonates of bases, silicates, occasionally fused to spheres, free silica, or carbonaceous matter ranging from tarry or oily soot to coke.They 78 AEROSOL POLLUTANTS OF THE ATMOSPHERE vary in shape, structure, surface, porosity and size distribution. They will there- fore differ in their capacity of adsorbing and concentrating sulphur dioxide molec- ules. When the particle is in collision with pre-existing SO3 or H2SO4, the acid film formed tends to collect more S02. On deposition, such acid-coated particles have bcen found to initiate severe corrosion of metals. Whether the oxidation of sulphur dioxide, with or without the intervention of pre-formcd sulphuric acid, is promoted by catalysis and to what extent, will require the examination of different types of dust particles.These might be separated from the mixed ash dust from pulverized coal.2 The acid boundary layer adsorbed on particulates is affected in its reactivity, hygroscopicity and resistance to diffusion by the presence in the air of minor quantities of ammonia liberated in the carbonization stage of the inefficient burning of coal or derived from animal and vegetable sources. The behaviour of sulphur oxides in combination with organic aerosols calls for special and urgent study in view of the increasing rate of pollution by the exhaust from motor vehicles and the emission from oil-fired furnaces. Although the sulphur content of petrol and diesel oil is relatively low, the discharged mixture of hydrocarbons and aldehydes in the former and of oily soot in the latter also contains oxides of sulphur and nitrogen which affect the stability of the aerosols formed.To what extent they are affected by collision with SO;! and SO3 after discharge is an open question. The formation of acid " smuts " when burning fuel oil still remains unexplained. Their corrosive and soiling effect is particularly harmful as shown by deposits on motor cars standing in the lee of oil-fired installations. The reactions involved in the oxidation of sulphur dioxide and leading to its final and stable form of sulphuric acid and sulphates may be pronioted by catalysis as suggested. At the same time the photo-chemical effects of sunlight must also be considered. The indication that olefines under the influence of irradiation are more liable to aerosol formation 3 than other hydrocarbons suggests that sulphur dioxide adsorbed by solids may exhibit similar differences. I submit this note, being convinced that the problem of the pollution of the air with its dire consequences can only find a practical solution when the physical chemist can satisfactorily explain the fundamental reactions involved and give a lead to the chemical engineer to devise appropriate remedial measures. 1 Report of Committee on Air Pollution (H. M. Stationery Office), Cmd. 0 322, 1954. 2 Lessing, 2nd World Power Con$ Trans., 1930, 4, 174 ; Fuel in Science and Practice, 3 Renzetti and Doyle, Int. J. Air Pollution, 1960, 2, 327. Prager, Stephens and Scott 1930, 9, 348. Ind. Eng. Chem. 1960, 52, 521.