20-4 Air Pollution from Acid Deposition
What Is Acid Deposition, and Where Does It Occur? Acids Falling on Your Head
Most coal-burning power plants, ore smelters, and other industrial plants in developed countries use tall smokestacks to emit sulphur dioxide, suspended particles, and nitrogen oxides high into the troposphere where wind can mix, dilute, and disperse them.
These tall smokestacks reduce local air pollution, but they can increase regional air pollution downwind. This occurs because the primary pollutants, sulphur dioxide and nitrogen oxides, emitted into the atmosphere above the inversion layer are transported as much as 1 000 kilometres (600 miles) by prevailing winds.
These acidic substances remain in the atmosphere for 2–14 days, depending mostly on prevailing winds, precipitation, and other weather patterns. During this period, they descend to the Earth’s surface in two forms:
One is wet deposition as acidic rain, snow, fog, and cloud vapour with a pH less than 5.6
The other is dry deposition as acidic particles. The resulting mixture is called acid deposition, sometimes termed acid rain.
Acid deposition is a regional air pollution problem in most parts of the world that are downwind from coal-burning facilities and from urban areas with large numbers of cars. Two types of sensitive areas are vulnerable to acid deposition because they lack the buffering capacity needed to neutralize the acidic compounds falling from the sky:
One has areas with thin acidic soils derived mostly from granite rock without such natural buffering
The other is areas where the buffering capacity of soils has been depleted by decades of acid deposition
What Are Some Harmful Effects of Acid Deposition? Lung Disease, Corrosion, Haze, and Dead Fish
Acid deposition has a number of harmful effects. It contributes to human respiratory diseases such as bronchitis and asthma, and can leach toxic metals (such as lead and copper) from water pipes into drinking water. It also damages statues, national monuments, buildings, metals, and car finishes.
Large amounts of money are spent each year to clean and repair monuments and buildings that have been attacked by acid deposition. Acid deposition also decreases atmospheric visibility, mostly because of the sulphate particles it contains.
Because of excess acidity, several thousand lakes in Norway and Sweden contain no fish, and many more lakes there have lost most of their acid-neutralizing capacity. In the United States, several hundred lakes (mostly in the northeast) are threatened with excess acidity
What Are the Effects of Acid Deposition on Plants and Soils? Depleting Nutrients and Damaging and Weakening Plants
Acid deposition (often along with other air pollutants such as ozone) can harm forests and crops, especially when the soil pH falls below 5.1.
At first, sustained acid precipitation adds nitrogen and sulphur to the soils, which stimulates plant growth. But continued acid inputs can cause several problems. One is that the acids leach essential plant nutrients and calcium and magnesium salts (which can reduce acidity) from soils. This reduces plant productivity and the ability of the soils to buffer or neutralize acidic inputs.
Another problem is that calcium deficiencies in plants produced in such nutrient-depleted soils can be passed on to herbivores.
In addition, some air pollutants can harm some types of trees through synergistic effects. For example, studies show that no visible injury occurs to white pine seedlings when they are exposed individually to low concentrations of sulphur dioxide or ozone. However, if the seedlings are exposed to the same concentrations of both pollutants simultaneously, visible damage occurs, apparently because the two pollutants interacted synergistically.
Acid inputs can also dissolve insoluble soil com- pounds and release ions of metals such as aluminum, lead, cadmium, and mercury. When absorbed from soil, these ions are highly toxic to plants and animals.
Case Study: Transforming Sudbury from Black to Green
Sudbury Environmental History Summary:
Original Landscape: Once a mixed forest ecosystem typical of the Great Lakes–St. Lawrence region, with maples, birch, pine, and other native trees.
Mining Impact: Nickel and copper were discovered during CPR construction in 1883. Early ore-processing used open-air roast beds, which created local acidified, metal-contaminated soils.
Industrial Pollution: Companies like Inco and Falconbridge later introduced large-scale smelting, worsening pollution despite chimney use. Around 850 km² of land became degraded, including 150 km² of barren "industrial barrens."
Air Quality Issues: Main pollutants were SO₂ and heavy metals from local smelters, as well as SO₂ transported from the U.S. The 1972 Inco superstack helped disperse local emissions over a wider area.
Pollution Control: Technological upgrades since the 1970s (e.g., better filters, improved smelting, gas recovery) led to a 90% drop in SO₂ emissions. Canadian and U.S. air quality programs further reduced SO₂ levels.
Current Air Quality: Now better than cities like Hamilton and comparable to other small northern Ontario towns. Emissions are adjusted based on weather to prevent pollution buildup.
Ecosystem Recovery: Starting in the 1960s, scientists, citizens, and governments worked to restore damaged ecosystems. Recovery methods included liming soils, planting metal-tolerant grasses, and reforestation. Natural succession is also contributing.
Lake Recovery: Water quality in Sudbury lakes is improving due to lower emissions. Recovery of aquatic life is slower, depending on species’ sensitivity and dispersal. Some lakes required fish restocking or liming.
What Can Be Done to Reduce Acid Deposition? Plenty, but There Can Be Complications
According to most scientists studying the problem, the best solutions are prevention approaches that reduce or eliminate emissions of SO2, NOx, and particulates.
To help reduce SO2 emissions and thus acid deposition, some coal-burning power plants increased their use of low-sulphur lignite coal (Figure 17-23, p. 401). However, because low-sulphur lignite coal has a low heating value, more coal must be burned to generate the same amount of electricity. This increases air pollution by emitting more CO2, toxic mercury, and radioactive particles into the troposphere.