Environmental Engineering Reference
In-Depth Information
Figure 1
Schematic representation of the
formation, distribution and impact of acid rain.
Pure rainwater is slightly acidic because it absorbs some of the carbon dioxide from
the atmosphere to form dilute carbonic acid. Levels of pollutants in the atmosphere have
increased greatly as a result of human activities, particularly the burning of fossil fuels.
Large quantities of sulphur and nitrogen oxides are added to the atmosphere (Figure 1).
These gases react with water vapour and sunlight to produce nitrates and sulphates. Some
of the pollutants are deposited directly from the atmosphere as dry deposition and some
are absorbed into the precipitation process and reach the ground as rain or wet deposition.
It is the wet deposition which is measured as acid rain, with pH values as low as 2, but
both processes add acidity to the ground surface.
The source of much of this pollution is industry and urban areas. However, it is not
just urban areas which receive acid rain. The gases released are dispersed by the winds,
and levels of high rainfall acidity extend over considerable areas. Over Europe the
highest levels of acidity are to the centre and east, with mean values below 4. It is
believed that heavy industry and lignite burning are the main contributors to this peak. As
the prevailing winds in Europe are from the west, it is not surprising that the most acid
rain occurs towards the east (Figure 2). Wind is not the only factor in dispersal.
Atmospheric stability will determine whether pollutants remain concentrated or are
dispersed. The westerly winds tend to be relatively unstable and so allow greater vertical
mixing; south-easterly winds, though less common, are often relatively stable, and
polluwhich may start as a local problem can cross national boundaries to become a
regional issue.
The effect of this increased acidity has been debated. It has been argued that the
biosphere, human health and building materials can suffer from its effects. What does
seem clear is that the ecological effects of acid rain will depend upon the ability of an
