Air Pollution

Acidification is one of the most serious environmental problems. Among the

effects of acid deposition, corrosion of constructions exposed to the atmosphere,

mainly by sulfur pollutants, is a well-known problem. The effects of

S02 on different materials have been subject to many laboratory and field

investigations, and the corrosive effects of polluted atmospheres on historic

and cultural monuments of sandstone or limestone are obvious. Current

knowledge of the effects of acidic air pollutants on atmospheric corrosion

is summarized in several reports (Swedish Ministry of Agriculture, 1982;

United Nations, 1984; Feenstra, 1984, Lanting and Moree, 1984). Among the parameters determining corrosion rate are the presence of a

moisture film on the surface and the pollutant content of this film. Relative

humidity, rain, dew, and temperature are determinants of the so-called time

of wetness, often defined as time with relative humidity> 80% at temperatures

> 0 oc. If mean values for time of wetness and temperature vary only

moderately, which is usually the case in the temperate climatic zone, then

the levels of air pollutants influence the corrosion rate decisively. In very

cold or dry regions, however, the time of wetness becomes a significant parameter.

The influence of high relative humidity and temperature, as found

in the tropics, is treated in a separate chapter.

The major pollutants, in urban and industrial areas are S02 and NOx;

chlorides are usually the dominant pollutant in marine regions. Most of the sulfur from burning fossil fuels is emitted as S02. During its

transport in the atmosphere S02 is oxidized to sulfuric acid in water droplets

or on moist particles. The sulfuric acid can then be partly neutralized by

ammonia to different forms of acid ammonium sulfates such as NH4HS04

and (NH4hH(S04h.

Dry deposition near emission sources in urban and industrial areas consists

largely of adsorption of S02 on surfaces, with the deposited amount

proportional to the content in air. The deposition rate is high at elevated

humidities, especially on some metals; e.g., steel and zinc (Sydberger