Air Pollution

Loch Doilet (Scotland)Freshwater bodies in the uplands are subject to often excessive levels of atmospheric inputs, such as sulphur, nitrogen, heavy metals and POPs. These pollutants can have adverse effects on the ecological status of the water body. Reducing these adverse effects continues to be a major driver of air quality policy in the UK and internationally.

Acid rain

Acid rain is a widespread term used to describe a variety of forms of acidic precipitation, such as rain, snow and mist. Oxides of sulphur and nitrogen, particularly sulphur dioxide (SO42-), nitrogen oxide (NO) and nitrogen dioxide (NO2-) react with water in the atmosphere to form sulphuric and nitric acids. This leads to an increase in the acidity of moisture in the atmosphere which ultimately falls to earth as acid deposition.

Acid rain is not a new phenomenon - Robert Smith coined the term "Acid rain" in his role as Britain's first Air pollution officer following his discovery that all the rain was found to contain sulphuric acid in proportion as it approached the town (Smith, 1851 cited in Pearce, 1987). Smith outlined the effects acid rain might have on the urban fabric in his book "Air and Rain" (Smith, 1872), writing:

"When the air has so much acid that two to three grains are found in a gallon of the rain-water, or forty parts per million, there is no hope for vegetation... galvanized iron is valueless... stone and bricks of buildings crumble". (Smith, 1872, quoted in Pearce, 1987)

Despite knowledge of the potential deleterious effects of acidic precipitation it was only during the latter half of the 20th Century that acid rain became an object of international concern, following the introduction of tall chimney stacks that dispersed pollutants over many thousands of kilometers. The transboundary nature of the acid rain problem galvanised scientific and political debate in the wake of observations during the 1970s and 80s in Scandinavia of damage to trees and declines in fish stocks, and the realisation that a large proportion of the pollution causing this environmental damage was being transported from overseas.

The response

Over 25 years of scientific research has been undertaken since the extent of acid rain pollution was realised, and this has led to the introduction of numerous international agreements to curb emissions of acid-forming compounds to the atmosphere. As a consequence of the research on airborne pollutants and their effects on ecosystems we are now aware of a wide range of pollutants that have contaminated even the most remote areas of the globe - identifying new issues of concern, which in turn lead to further international efforts to reverse the environmental damage they may have caused.

The Freshwater Umbrella research programme has contributed extensively to our understanding of the effects of airborne pollutants on aquatic ecosystems. The research undertaken during the various Freshwater Umbrella programmes has influenced both UK and international policy on acid deposition. This section of the website aims to outline some of the science background behind acid deposition and its effect on aquatic ecosystems, as well as to describe some of the work undertaken on behalf of the Freshwater Umbrella.


  • Pearce, F. (1987) Acid Rain: what is it, and what is it doing to us? Penguin Books Ltd. Middlesex, England.
  • Smith R.A. (1872) Air and Rain. Longmans Green & Co., London.