C. Acid Rain
As the name suggests, acid rain is just rain that is acidic. The rain becomes acidic because of gases which dissolve in the rain water to form various acids. Rain is naturally slightly acidic because of the carbon dioxide dissolved in it (which comes from animals breathing), and to a lesser extent from chlorine (which is derived from the salt in the sea). These gives rain a pH (the measure of acidity) of around 5.0, and in some parts of the world it can be as low as 4.0 (this is typical around volcanoes, where the sulfur dioxide and hydrogen sulfide form sulfuric acid in the rain). The lower the pH, the greater the acidity. Before the Industrial Revolution, the pH of rain was generally between 5 and 6, so the term acid rain is now used to describe rain with a pH below 5.
About 70% of acid rain comes from sulfur dioxide (SO2), which dissolves into the water to form sulfuric acid. The rest comes from various oxides of nitrogen (mainly NO2 and NO3). (These figures are for Scandinavia - Scotland has a very similar ratio, while the north-eastern USA has 62 % sulfuric acid, 32% nitric acid and 6% hydrochloric acid). These gases are produced almost entirely from burning fossil fuels, mainly in power stations and road transport.
Acid rain causes damage to:
stage where it contained enough copper to turn you hair green! Slightly more worrying, that much copper can also cause diarrhea in young children, and can damage livers and kidneys.Lakes and rivers: Acid rain causes lakes and rivers to become acidic, killing off fish - all the fish in 140 lakes in Minnesota have been killed, and the salmon and trout populations of Norway's major rivers have been severely reduced because of the increased acidity of the water. Short-term increases in acid levels kill lots of fish, but the greatest threat is from long-term increases, which stop the fish from reproducing. The extra acid also frees toxic metals that were previously held in rocks, especially aluminum, which prevents fish from breathing. Single-celled plants and algae in lakes also suffer from increased acid levels, with numbers dropping off quickly once the pH goes below 5, and by the time the pH gets down to 4.5, virtually everything is dead.
Trees: A very highly publicized problem is the effect of acid rain on trees. Conifers appear to be particularly affected, with needles dropping off, and seedlings failing to produce new trees. The acid also reacts with many nutrients the trees need, such as calcium, magnesium and potassium, which starve the trees. The trees are then much more susceptible to other forms of damage, such as being blown down, or breaking under the weight of snow.
People: Rather surprisingly, the effects of acid rain on trees have overshadowed the effects on people. Many toxic metals are held in the ground in compounds. However, acid rain can break down some of these compounds, freeing the metals and washing them into water sources such as rivers. In Sweden, nearly 10,000 lakes now have such high mercury concentrations that people are advised not to eat fish caught in them. As the water becomes more acidic, it can also react with lead and copper water pipes, contaminating drinking water supplies. In Sweden, the drinking water reached a
Buildings: A less serious problem is the damage acid rain causes to certain materials, particularly limestone and marble. The acid dissolves the calcium carbonate in the stone, and this solution evaporates, forming crystals within the stone. As these crystals grow, they break apart the stone, and the structure crumbles.
Because rain travels over long distances in clouds, acid rain is a global problem. In Britain, the prevailing winds come in from the Atlantic, so they are unpolluted. This means that 87% of the sulfur dioxide in the air in Britain is produced in North America (only about 1% is produced naturally). Other countries are much less fortunate, especially the central European countries. They have the additional problem of very poor environmental control in many of their Eastern European neighbors. However, acid rain does not just affect Europe. Every country with a power station or a significant number of road vehicles helps to generate the gases that cause acid rain. Acid rain is a global problem, and needs a global solution.
The best approach to acid rain is to reduce the amount of nitrous oxides and SO2 being released into the atmosphere. Fitting a catalytic converter to a car can reduce the emissions of nitrous oxides by up to 90%, but they are very expensive, and cause more carbon dioxide to be released, which contributes to the greenhouse effect. SO2 emissions from power stations can be reduced before, during, or after combustion.
If a fuel with a low sulfur content (such as North Sea gas or oil) is burned, not much sulfur dioxide will be formed. However, low sulfur fuels are more expensive because they are in greater demand, and although high-sulfur fuels can be treated to reduce their sulfur content, this is very expensive.
The SO2 created during combustion can be absorbed if an appropriate chemical (such as limestone) is present as the fuel burns.
Once the fuel has been burned, the SO2 can be removed from the exhaust gases. Most systems spray a mixture of limestone and water onto the gases. This mixture reacts with the SO2 to form gypsum, a useful building material.
Another option is not to burn fossil fuels, but to use alternative energy sources (such as solar, wind, biomass fuels, etc.).
All these methods for reducing acid gases are expensive, and have drawbacks, so laws have been passed to force businesses to use them. The best way to reduce them is not to use as much energy in the first place. You can help in lots of ways:
Turn off lights when you leave a room
- If you have a car, don't use it for short journeys
- Get your parents to insulate their house properly
- Basically, anything at all that uses less energy
Assignment
:Answer the following questions on a separate sheet. You must copy out the question, and neatly put the answer below each question. (12)
- How does rain become acidic?
- What is pH?
- Describe some problems associated with have acidic rain?
- Does acid rain only occur in some parts of the world?
- What are some things we can do to help control the problem of acid rain?
- What will eventually happen to the Earth if this problem is not overcome?