Geographical Magazine
Rain Of Terror

There have been numerous reigns of terror in human history but if planet Earth could speak it might refer to recent decades in terms of rains of terror. When a traditional life-giver like water becomes a poison there's cause for worry, and acid rain is responsible for decimating plant and animal ecosystems around the globe.

A random flick through the damage report would come up with around 150 Minnesota lakes now totally devoid of fish, severe damage to European forests from Sweden in the north to France in the south, and 10,000 Swedish lakes with such high mercury concentrations that their fish are very much off the menu.

While we all have a general idea about the corrosiveness of things like sulphuric or hydrochloric acids, chemists define acid more mundanely as a substance that liberates hydrogen ions in solution. Strong acids are those which have become completely dissociated into ions in solution, while weak acids like vinegar (acetic acid) are only partly dissociated in solution.

Acidity isn't bad in itself. The corrosiveness of strong acids is useful for many industrial processes, while even a nice, homely thing like milk is mildly acidic. Ironically -- or should that be ionically -- even `normal' rainwater is acidic (pure water is defined as neutral), falling somewhere between the acidity of milk and tomato juice. So what exactly is `acid rain' and why is it so nasty?

Simply put, rain becomes more acidic when it reacts with chemicals in air pollution, in particular atmospheric compounds created from sulphur and nitrogen belched from factories, power stations and vehicles. Around 65 per cent of acid rain comes from sulphur dioxide which dissolves in water to become sulphuric acid; 35 per cent from various oxides of nitrogen (known as NO) that form nitric acid, while a few per cent derive from chlorine compounds, which form hydrochloric acid.

While `normal', slightly acidic rain is safely neutralised by alkaline chemicals present throughout the environment (derived from bedrock), more acidic rain disrupts that thing Nature loves so much -- balance. The more acidic the rain that falls, the faster acid-buffering chemicals are used up before they can be replaced, eventually disappearing and destroying the natural balance. And when the environment can't neutralise acid rain anymore, the destruction of forests, crops, lakes and rivers begins.

In aquatic systems, when the pH of water approaches six (neutral is seven), crustaceans, insects and some plankton begin to disappear. As pH approaches five, major changes in plankton occur and more acid-sensitive fish populations begin to die. Below pH five, most water life is dead, mosses take over shore areas and terrestrial animals dependent on aquatic systems also suffer. The situation is worsened by acid rain's ability to make toxic elements such as aluminium more soluble; an increase in aluminium (freed from rocks) stops fish from breathing.

Humans shouldn't think that bunging up an umbrella is enough for us to get off. Acid rain's increase in the solubility of metals such as aluminium, copper and lead can result in these seeping from water pipes into drinking water. In some parts of Sweden, copper levels in water have reached levels high enough to turn hair green, as well as being a cause of kidney and liver damage.

Unlike some other environmental nasties, acid rain roams far and wide, as air currents carry affected clouds long distances. This is the reason why acid rain is one of the least welcome exports in the world. Thus pollutants rising into the dirty skies of Britain are carried by prevailing winds to fall as acid rain on the once pristine lakes and forest of Scandinavia, while across the Atlantic, the airborne muck of the industrial midwest and central states of Canada falls back to Earth as acid rain on the much cleaner areas of southeast Canada and northeast USA. The USA plays a part in this particular trade, though, with an estimated 50 per cent of the sulphuric acid rain deposited in Canada having `made in America' stamped on each drop.

The human cost of acid rain extends beyond the threat to drinking water supplies posed by toxic metals leached, or dissolved, from pipes. Communities which rely on fisheries, forestry and agriculture are most obviously in the frontline, with ecological damage translating into loss of income and ultimately loss of livelihoods in places such as Canada and northern Europe. There is also increasing evidence linking acid deposition and respiratory problems among children and asthmatics, though the precise mechanisms are still not clear.

Acid deposition also, of course, attacks buildings, accelerating corrosion, fracturing and discolouring different structures (things like statues are often the most high profile victims), levying both an aesthetic and monetary cost in terms of damaged masterpieces and architectural restoration. Acid rain is, however, a problem with clear solutions.

The best method of attack is reducing sulphur dioxide emissions (primarily from power stations), and there are various ways of doing this. Using fuels with low sulphur content -- such as North Sea oil and gas -- is one, though the higher cost of such fuels explains why companies are often loathe to use them unless compelled by legislation. It is also possible to treat fuels to reduce their sulphur content, but since this approach falls into the very expensive category, once again the heavy hand of environmental law looks like a necessary part of the equation.

A better solution, perhaps, is to remove the sulphur either during the fuel burn or afterwards, using something like limestone which reacts favourably with the unwanted pollutant. Limestone and water sprayed onto the sulphur gas, for example, produces gypsum, a useful building material.

Another option, of course, is simply not to burn fossil fuels - but until wind, wave and solar power get the funds they need that tends to mean using nuclear and hydro power, both of which come with their own drawbacks. The easiest option is to use less energy in the first place. Turn off those lights when you leave a room, use your car less, insulate your house better. That way, you've a better chance of staying happy, even when it rains.