When a rainstorm hit Pitlochry, in Scotland, on April 10, 1974, it beat the world record – not for volume, but for acidity. The rain that fell that day was roughly equivalent to lemon juice, and more acidic than vinegar. It was several hundred times more acid than normal rain should be.
While the Pitlochry figures were exceptional, many places in Europe and North America have rainfall that is tens or hundreds of times more acidic than it ought to be. Acid rain rots buildings, damages soil, kills fish in lakes and helps to destroy trees, which are dying in a wide swathe across Europe.
Acid rain is an environmental problem that knows no boundaries. The atmospheric pollution that causes it is carried by the prevailing winds from major industrial areas to the mountains, lakes and forests that lie to the east.
Not even the Arctic is free of the air pollution that causes acid rain.
Where does the acid come from? There is now no doubt that most comes from man’s activities – from cars, homes, factories and power stations. There has always been some acid in rain, coming from volcanoes, swamps and plankton in the oceans, but scientists know that it has increased very sharply over the past 200 years. Ice formed before the Industrial Revolution and trapped in glaciers has been measured, and found to be just mildly acid, consistent with natural sources.
Rain is made acid mainly by two elements, sulphur and nitrogen. Sulphur is found in coal and oil. When burned, it turns to sulphur dioxide, which mixes with the water droplets in clouds and is converted into sulphuric acid. Nitrogen, from the air and also in the fuel itself, is turned into oxides of nitrogen by burning, and then reacts with water molecules to form nitric acid. Some of the sulphuric and nitric acid falls locally, while the rest can be carried thousands of kilometres.
Since the 1950s, chimneys 500ft (150m) high have been built to carry pollution away from urban areas, but their effect has been to spread it more thinly and more widely. This, coupled with the big increase in the amount of pollution, especially from power station in recent decades, has resulted in places like Scandinavia being affected by pollution from factories in countries thousands of kilometres away. Swedish scientists have estimated that 70 per cent of the sulphur in the air over Sweden comes from fuel burning, and that most of it comes from outside Sweden, particularly from eastern Europe.
To discover if any of the acid rain was coming from Britain, samples of air were collected by aircraft and tested by British scientists. On one flight it was found that air reaching the west coast of Britain across the Atlantic on the prevailing wind contained less than half as much sulphur and a quarter as much nitrate as air along the east coast. As it blew across Britain it had picked up the pollutants that it then carried to Scandinavia.
It was even possible to trace the ‘plumes’ of pollution from a particular power station by releasing a chemical, sulphur hexafluoride, from its chimneys. Instruments on the aircraft could tell when it was flying through that particular plume, and take measurements.
Picture Credit : Google