What is Acid Rain?

Rain is naturally slightly acidic (around pH 5) because it contains acids formed when carbon dioxide and chlorine gases react with moisture in the atmosphere. If it has a pH lower than 5, it is considered acid rain.

Acid rain is caused primarily by two common air pollutants—sulphur dioxide (SO₂) and nitrogen oxides (NO_x)—that are produced by the burning of fossil fuels. Most sulphur dioxide emissions come from smelters and power stations, while most nitrogen oxide emissions are produced by motor vehicles. These pollutants can travel thousands of kilometres in the atmosphere, where they mix with water vapour to form a mild solution of sulphuric and nitric acid. Rain, snow, hail, fog and other precipitation wash this solution down to earth as acid rain. Acids can also be transformed chemically into sulphur dioxide gas or sulphur and nitrogen salts that are deposited dry in dust or other particles.

Acid rain can affect virtually anything it contacts—including soil, water, plants and building materials. In North America, most acid rain falls on the eastern part of the continent, because most acidic air pollutants are produced there, and winds tend to blow toward the east.

Effects of Acid Rain

Over the last two decades, Canada has cut its sulphur dioxide emissions by more than half, and reductions are also underway in the United States. Despite this progress, however, the recovery of natural ecosystems has been much slower than anticipated. Acid rain continues to affect our lakes, forests, wildlife and even our health.

Lakes

The more acidic a lake becomes, the fewer species it can support. Plankton and invertebrates are among the first to die from acidification, and when the pH of a lake drops below 5, more than 75 per cent of its fish species disappear. This causes a ripple effect in the food chain, and has a significant impact on fish-eating birds, such as loons.

Not all lakes exposed to acid rain become acidified. Bases found in certain types of rock and soil help to neutralize acidity. Unfortunately, most acid rain falls in eastern Canada, where coarsely textured soil and granite bedrock have little ability to neutralize acid. Models predict that even after 2010 emissions targets are reached, up to one quarter of the lakes in eastern Canada will remain chemically damaged.

Forests

Acid rain dissolves nutrients and helpful minerals in the soil and washes them away before trees can use them to grow. It also releases toxic chemicals, such as aluminum, which interferes with the uptake of nutrients. Nutrient-starved trees can experience stunted growth and loss of leaves, and are more vulnerable to climatic stresses, pests and disease.

Like lakes, a forest’s ability to withstand acid rain depends on the neutralizing capacity of its soil. Forests in eastern Canada receive roughly twice the level of acid they can tolerate without long-term damage. Forests in coastal and upland areas also experience damage from acidic fog.

Human Health

The sulphur dioxide that contributes to acid rain can also react with other chemicals in the air to form tiny sulphate particles, which can lodge deep within the lungs and cause respiratory problems.

Corrosion

Acid rain can also accelerate the corrosion of materials such as limestone, sandstone, marble, brick, concrete and metal, causing serious concerns for older buildings and outdoor sculptures and monuments. Acid rain damages stonework because it dissolves calcium carbonate, leaving behind crystals in the rock when it evaporates. As the crystals grow they break apart the stone.

Acid Rain and Other Pollutants

The interactions between acid rain, ultraviolet (UV) radiation, climate change and other human-related stresses can magnify their impacts. For example, because acidity reduces the amount of dissolved organic matter in lake water, acidic lakes are clearer and therefore more vulnerable to the effects of increased UV levels.

Climate change also affects acid levels in lakes, because hot, dry conditions convert harmless sulphur compounds that have accumulated in wetlands into acid-forming sulphates. When it rains, these sulphates are flushed into surrounding lakes, boosting their acid levels.