One of the main causes of acid rain is sulphur dioxide. Natural sources that emit this gas include volcanos, forest fires, sea spray and rotting vegetation.
The burning of fossil fuels, such as coal and oil are to blame for approximately half the emissions of the gas in the world.
When sulphur dioxide reaches the atmosphere, it oxidises to form sulphur ions
S + O2 SO2
2SO2 + O2 2SO3
Then it becomes sulphuric acid as it combines with hydrogen atoms in the air
SO3 + H2O H2SO4
The presence of ammonia and Ozone in heavily polluted clouds helps to catalyse this reaction.
Nitric oxide and nitric dioxide are also components of acid rain. Sources are mainly from power stations and exhaust fumes. Like sulphur dioxide the nitrogen oxides rise into the atmosphere and are oxidised in clouds to form nitric acid. These reactions are catalysed in heavily polluted clouds where iron, manganese, ammonia and hydrogen peroxide are present.
Effects:
Acid rain causes damage to thousands of lakes and rivers a year and contributes to the damage of trees and soils and a large number of breeding grounds for various species. As well as to the animals themselves as there have been examples of land animals’ feeding and breeding patterns have been disturbed. In addition acid rain accelerates the decay of building materials, paints, sculptures and statues.
Effects of Acid Rain on Human Health
The pollutants that cause acid rain (SO2 and NOx) can directly damage human health. These gases react with gases in the atmosphere to form sulphate and nitrate particles, which can travel to the lungs (by inhalation). They have been linked to respiratory problems such as dry coughs, headaches and eye/nose/throat irritations, as well as asthma, bronchitis and heart failure.
If there were less nitrogen oxide particles in the atmosphere, they would be less able to react with the volatile organic compound that from ozone, and this decreases the risk of the ‘Greenhouse Effect’. The ozone has also been linked with increased risk of lung inflammation (i.e. asthma and emphysema).
These chemicals present in acid rain can harm humans indirectly too, as fruits and vegetables absorb toxic metals dissolved in the water (rain), and therefore appear in animal tissue later on. This may not affect the animals themselves but it does affect humans when the animals are consumed. For example, mercury can accumulate in the organs and tissues of animals, and this has been linked to infant brain damage, nerve disorders, adult brain damage and death. Aluminium present in the organs of animals has been associated with kidney problems, and recently it has been suspected to be related to Alzheimer’s disease.
Effects of Acid Rain on Aquatic creatures and ecosystems
There are several ways in which acidic chemicals can enter lakes; some are dry particles and some enter as wet (e.g. rain, snow etc.) Acid rain that falls to earth carries toxic metals along with the soil’s nutrients, into the lakes. ‘Spring Acid Shock’ is what happens when snow rapidly melts in the spring and the acids and chemicals in the snow are released into the soils. These are then washed into lakes where they can affect aquatic ecosystems, which have no time to adjust.
In spring, aquatic species are vulnerable (due to reproduction of amphibians, fish and insects that may lay their eggs in the water to hatch). A sudden change in pH is dangerous, since acid can cause deformities and perhaps death of young aquatic creatures, (especially since they spend most of their lives in water). Sulphuric acid can affect fish directly, as it interferes with the fish’s ability to take in oxygen, salts and nutrients; vital for the fish to survive. Osmoregulation (the process of maintaining the balance of salts and minerals in tissues) is very important for the survival of freshwater fish, and Sulphuric acid disturbs this. Acid molecules can also cause mucus to form in the gills, preventing the absorption of oxygen and suffocating the fish. The disturbance in balance of salt (especially Calcium ions) in the fish’s tissue can cause poor reproduction (eggs are too brittle or weak). Calcium deficiencies also result in weak spines and deformities.
Eutrophication (when nitrogen from fertilisers is washed into lakes, causing an excess of algae to grow) results in an increase in oxygen production, but due to the increased death of fish from the acid rain, decomposition uses up most of this oxygen. This leaves very little for the surviving fish to take in. Indirectly, sulphuric acid dissociates heavy metals (such as Aluminium ions) from the soil, so it accumulates in lakes and subsequently kills the fish. This has a knock-on effect on the creatures further down the food chain, as a fish deficit results in malnutrition of another animal. This is a chart showing the effects of pH on aquatics creatures;
