1.1. Acid deposition phenomena
As a result of the equilibrium with atmospheric carbon dioxide, precipitations are naturally acidic. Natural
emissions of SO2 and other species such as nitrogen oxides (NOx) and chlorhydric acid (HCl) further increase
precipitations acidity. The most important gas which leads to acidification is SO2 and the main sources of generation
are fossil fuel combustion and industry, wildfires and volcanoes [1].
In the absence of human activity, natural long-term mean acidity would be of the order of pH 5.0. Thus, a strict
definition of acid rain might be “precipitation which is more acidic than this value“. However, the term refers not
only to precipitation but also to other deposition pathways [2]. The deposition of pollutants out of the atmosphere
appears in two ways: dry and wet deposition. Dry deposition can be due to adsorption of SO2 on metal surfaces or to
impaction of sulfate particles. Wet deposition can be produced by dissolution of atmospheric SO2 into rain or fog.
The primary cause of atmospheric corrosion is dry deposition, consisting mainly of the adsorption of SO2. The
amount deposited is proportional to the concentration in the atmosphere. Nevertheless, the atmospheric
concentration of pollutants is very volatile and, therefore, very difficult to predict [3]. Atmospheric reactions of acid
rain generation in which SO2 is involved are equations (1) to (3):