In the presence of water, (SO3) is converted rapidly to
SO3(g) + H2O(l) → H2SO4(l)
is formed by the reaction of OH with NO2 + OH· → HNO3
Acid deposition occurs as both wet and dry deposition. Wet deposition of acid occurs when rain, snow, fog etc are formed from acids such as nitric and sulphuric acid when gases in the atmosphere react with water as outlined previously. These acids are delivered to the Earth’s surface either by deposition of acids produced in the raindrops or by precipitation removing the acids in clouds or below clouds.
Dry deposition occurs in the absence of precipitation. This has been calculated to be responsible for up to 60% of acid deposition in some areas. Here, particle and gases such as NOx and SO2 stick to the ground, trees and buildings. This is combined with water from rain and other precipitates which causes acid formation at a local level.
Effects of Acid Rain
Acid rain has been shown to adversely affect water systems, vegetation and soils.
When acid rain is deposited onto the ground it can cause major changes to occur in soil chemistry and therefore biology. Increasing amounts of acidity in soil can mobilise aluminium ions which are normally present in the form of insoluble, non-toxic aluminium hydroxide. When the pH of soil is at pH 5 or lower, aluminium ions are dissolved and so are able to leach into water systems and directly affect the soil biota. Lower soil pH and aluminium mobilisation can reduce populations of soil microorganisms. Decomposers such as bacteria, fungi, earthworms and other insects break down the dead and decaying leaves and other debris on the forest floor. This in turn releases nutrients such as calcium, magnesium, phosphates and nitrates into the soil. If this process is stopped, as occurs with the change in pH due to the deposition of acid rain then the land becomes sterile and lifeless. Higher acidity levels may also cause other toxic metals to go from the insoluble to the soluble ion forms in the same way as aluminium. These include lead, mercury, zinc, copper, cadmium, chromium, manganese, and vanadium. This combination of toxic metals may also adversely affect the growth of soil bacteria, mosses, algae, fungi, earthworms and other organisms in the soil.
Related to the effect which acid rain has on the soil chemistry is the effect which it has on plant life. Many plants are unable to grow in low pH conditions and even those which are able to tolerate slightly acidic soil such as the Spruce may succumb to acid rain damaging leaves/spines, which prevents the trees from photosynthesising. The bark is also affected which leaves the trees more susceptible to the elements. Also, aluminium ions can prevent root growth and stop the uptake of calcium from the soil. Trees derive their nutrition primarily from element ions such as calcium, magnesium and potassium that have dissolved from rocks into the soil. Acid deposition adds hydrogen ions, which displace these ions in a process called leaching. When leaching occurs, the ions are washed deeper into the subsoil or washed out of the top soil. If ions are leached from the soil, they are no longer available to the roots of the plants, again preventing plant growth.
Acid rain directly affects water systems. For example, when acid rain is deposited into freshwater bodies of water the pH of that body is lowered. This is a major issue as most aquatic life such as fish and insects cannot survive in acidic conditions. Adult populations of fish such as the brook trout in the Adirondack Mountains of the Unites States have been completely eliminated. Furthermore at low pHs most fish eggs will not hatch and insect larvae will not develop into adults. Toxic metals, leached into groundwater or runoff into streams can also have an adverse affect, poisoning aquatic life. Nutrient leaching can also cause massive algal blooms which leads to a decrease in oxygen levels in the water. Nitrogen deposition from acid rain also damages coastal waters and estuaries. Nitrogen-rich water supports and algal blooms. Bacteria decompose the dead algae, flourish themselves and soak up the water's available oxygen. Fish, shellfish, sea grass beds and coral reefs die in the algae-choked, oxygen-depleted waters.
Solutions to Acid Rain
The first and foremost solution to acid rain is the reduction of SO2 and NOx emissions. One way is to rely on electricity generation from renewable resources or nuclear power rather than of fossil fuel burning power stations. Switching to fuels which do not produce or have low SO2 content upon burning is another solution. For those power stations which do produce electricity by these means, the use of flue gas desulphurisation units (FGDs) to remove sulphur containing gasses is another way to reduce acidic deposition. Wet ‘scrubbers’ are also used to remove SO2 from the stack gasses in power stations. Here, a lime or limestone slurry is injected into the stacks and reacts with the SO2 to produce sulphate compounds. This is then removed and either sold as gypsum where the calcium sulphate is high quality or buried in landfill sites.
As previously mentioned, the use of catalytic converters in cars since the 1980s has already reduced the emissions of NOx compounds into the atmosphere, however a move to hydrogen and electric powered cars would reduce these emissions further. Also fitting catalytic convertors to older cars would help with the reduction of emissions, especially in developing countries, where the use of older cars is prevalent.
To combat the effects of acidic deposition on the soil, especially soil used for crops, lime is added to the ground to combat acidity and raise the pH. The increase in pH encourages the return of soil invertebrates and microorganisms and with the addition of fertilisers gives the chance for vegetation to grow.
The addition of lime is also used to repopulate lakes and water courses which have been acidified. Large quantities or hydrated lime, quick lime or soda is added to the body of water and this causes a raise in pH. This method has seen great success, including acidic lakes in Wales where the pH was raided from pH 5 to pH 7. This enabled the repopulation of the lakes with Brown Trout. Aquatic plant life was also successfully introduced which caused many species of invertebrates to repopulate.