Acid deposition is a general term that includes more than simply acid rain. Acid deposition is primarily the result of emissions of sulphur dioxide (SO2) and nitrogen oxides (NOx) that can be transformed into dry or moist secondary pollutants such as sulphuric acid (H2SO4), ammonium nitrate (NH4NO3) and nitric acid (HNO3) as they are transported in the atmosphere over distances of hundreds to thousands of kilometres.
Acidic particles and vapours are deposited in two processes – wet and dry deposition. Wet deposition is acid rain, the process by which acids with a pH normally below 5.6 are removed from the atmosphere in rain, snow, sleet or hail. Dry deposition takes place when particles such as fly ash, sulphates, nitrates, and gases (such as SO2 and NOx), are deposited on, or absorbed onto, surfaces. The gases can then be converted into acids when they contact water.
Damage caused by acid deposition affects lakes, rivers, forest, soils, fish and wildlife populations and buildings. Prior to falling to the earth, acid-causing emissions (SO2 and NOx gases and the related acid particles) contribute to visibility degradation and impact public health. Acid deposition is a problem in eastern Canada because many of the waters (streams, rivers, ponds, lakes)and soils in this region lack natural alkalinity – such as a lime base – and therefore cannot neutralize acid naturally. Provinces that are part of the Canadian Precambrian Shield, like Ontario, Quebec, New Brunswick and Nova Scotia, are hardest hit because their water and soil systems cannot fight the damaging consequences of high levels of acid deposition. In fact, more than half of Canada consists of susceptible hard rock (i.e., granite) areas that cannot neutralize the effects of acid deposition. Lakes and soils found in areas of the Canadian Shield in northeastern Alberta, northern Saskatchewan and Manitoba, and parts of western British Columbia are as defenseless to acid rain as those in eastern Canada.
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