Hot air acts like a lid on the atmosphere

During the heat of summer, a layer of hot air in which the temperature increases with height often forms high above the Earth. The layer of hot air prevents polluted air near the surface from mixing with cleaner air above it. To make matters worse, the heat helps trap pollution from automobile exhaust and factory emissions.

Acid Rain

As the name suggests, acid rain is just rain which is acidic. The rain becomes acidic because of gases which dissolve in the rain water to form various acids.

About 70 percent of acid rain comes from sulphur dioxide (SO2), which dissolves into the water to form sulphuric acid. The rest comes from various oxides of nitrogen (mainly NO2 and NO3, collectively called NOx). (These figures are for Scandinavia - Scotland has a very similar ratio, while the north-eastern USA has 62 percent sulphuric acid, 32 percent nitric acid and 6 percent hydrochloric acid). These gases are produced almost entirely from burning fossil fuels, mainly in power stations and road transport.

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 reproducing. The extra acid also frees toxic metals which were previously held in rocks, especially aluminium, which prevents fish from breathing. Single-celled plants and algae in lakes also suffer from increased acid levels.

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 starves 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.

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 stage where it contained enough copper to turn you hair green! Slightly more worryingly, that much copper can also cause diarrhoea in young children, and can damage livers and kidneys.

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 solutions evaporates, forming crystals within the stone. As these crystals grow, they break apart the stone, and the structure crumbles. This picture shows how much the gargoyle on the left has been damaged by acid rain - the gargoyle on the right has been reconstructed.

Because rain travels over long distances in clouds, acid rain is a global problem. This map shows just how much of a problem it is across Europe:

The best approach to acid rain is to reduce the amount of NOx and SO2 being released into the atmosphere. Fitting a catalytic converter to a car can reduce the emissions of NOx by up to 90 percent, 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 sulphur content (such as North Sea gas or oil) is burned, not much sulphur dioxide will be formed. However, low sulphur fuels are more expensive because they are in greater demand, and although high-suphur fuels can be treated to reduce their sulphur 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.

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

Copyright © Malta Weather Services 2003