Acid rain: causes and effects

Acid rain occurs when acid-containing precipitation falls onto the earth’s surface. Precipitation comes in the form of rain, snow, sleet, or hail. Precipitation collects acidic particles and gases and becomes acidic. These particles will have a pH level below 5.6.

There are two types of deposition processes: wet and dry. Acid rain is wet deposition. Wet deposition causes erosion that affects ecosystems. Dry deposition occurs when small acid particles and gases fall onto the earth’s surface. Gases such as sulphur dioxide and nitrogen oxides change into acids when they contact water.

Acid deposition occurs when sulphur dioxide and nitrogen oxide emissions transform into secondary pollutants. Examples of such pollutants are sulphuric acid, ammonium nitrate, and nitric acid. These pollutants then fall onto land, water, vegetation, or structures. 

Acid deposition can damage:

  • lakes and rivers
  • forests
  • soils
  • fish and wildlife populations
  • buildings

Before falling to the earth, acid-causing emissions (sulphur dioxide and nitrogen oxide gases and the related acid particles) contribute to haze and smog and affect public health. Acid deposition is a problem in many parts of Canada since emissions that contribute to acid rain can travel thousands of kilometres from their source.

More than half of Canadian geology consists of vulnerable hard rock (i.e., granite) areas that offer poor natural defenses from the damaging effects of acid deposition. Many of the waters (streams, rivers, ponds, and lakes) and soils in Canada lack natural alkalinity, such as a lime base, and cannot neutralize acid naturally.

Provinces that are part of the Canadian Precambrian Shield, such as Ontario, Quebec, New Brunswick, and Nova Scotia, are most affected. Lakes and soils found in areas of the Canadian Shield in northeastern Alberta, northern Saskatchewan and Manitoba, and parts of western British Columbia, are also sensitive to acid deposition.

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