Acid rain blights our woodlands

By Md. Asadullah Khan

IN Asia and much of the Third World, trees are destroyed the old-fashioned way: they are cut down for fuel and cropland. But in Europe there is a new and potentially more deadly culprit known as "dying forest syndrome". Now, this mysterious malady, first observed in Germany back in 1980, has raced across the world blighting woods in countries as far apart as Sweden, Germany, Italy, the Philippines, Indonesia, India and Bangladesh. In all parts of Europe, needle trees such as fir, spruce and pine as well as leaf trees such as beach and oak have been affected. Europe now faces one of the most serious environmental catastrophes of modern times. Reports available till the end of 1990 reveal that in France 5,000 hectares of woodland have been seriously damaged, but 30,000 hectares are showing signs of deterioration. The problem is especially dramatic in West Germany. About 7.7 per cent of 7.4 million hectares of forest were visibly damaged till 1990. At the same time 34 per cent of its trees had suffered some needle and leaf loss and discoloration. In Southwestern Germany, home of the Black Forest, 75 per cent of the trees have been killed or damaged. In East Germany, an estimated 86 per cent of three million hectares of woodland has been visibly lost. In Czechoslovakia, now split into the Czech Republic and Slovakia, approximately 960,000 hectares of woodland, more than 20 per cent of the total, are now irreversibly damaged. The epidemic of dying trees has swept all parts of the world, especially Europe quite mercilessly and in ways that are just as mysterious.

World geographical records show that forest occupy about 32 per cent of land area and encouragingly Asian countries account for 14 per cent of the total woodlands in the global map. The forest covering in Bangladesh is not however, anything at all encouraging. The Sunderbans covering a total area of 10,000 square kilometres, making it the largest block of mangrove forests that exists in the world today, is on the verge of extinction. Most of the trees there have been affected with a strange plague known as "top dying disease."

Explanations for such epidemic range from a cyclic change in the environment to a baffling form of tree cancer. But the most convincing evidence points to air pollution, especially sulphur dioxide and oxides of nitrogen which are spewed into the air by the tonne from the electrical generating plants, industrial boilers, smelting plants and automobiles. The problem of free-floating pollution, largely a scientific phenomenon, now wreaks havoc. One country may have only a handful of factories that emit culprit chemicals, but it could nevertheless suffer forest blight because of a neighbour's pollution. It is now known that trees in the Netherlands, for example, are no less tainted than those in Germany, where smokestacks tend to be taller. Likewise Italy gets more pollution than it gives. Some two million tonnes of sulphur dioxide drift into the country from Switzerland, Austria and France while Italy in exchange sends those nations 1.7 million tonnes of pollution over the Alps. Whether a nation is a sinner or a victim tends to dictate its position on international pollution standards. Those are sinned against, notably the Scandinavian countries, have worked for years to make acid rain an international issue. The sinners, like Britain, have resisted putting teeth into pollution control regulations since they must bear the costs but enjoy few of the benefits.

Acidity is measured using pH scale with the number 7 being neutral. Measured on a chemical scale of pH from 0 to 14 (most acidic to most alkaline), acid rain is defined as precipitation below 5.6. In most of the industrialised areas of Europe, rainfall now has a pH between 4.5 and 5.5. In some parts of Italy, it has been recorded as low as 2.6 or more acidic than table vinegar which has a pH of about 2.9. It is worth mentioning that the pH scale is logarithmic, that is, a substance of pH of 6 is 10 times more acidic than another with a pH of 7. The pH of 5.6 has been used as the baseline in identifying acid rain.

One of the main causes of acid rain is sulphur dioxide. Natural sources which emit this gas are volcanoes, sea spray, rotting vegetation and plankton. However, the burning of fossil fuels, such as coal and oil, are largely to be blamed for approximately half of the emissions of this gas in the world. When sulphur dioxide reaches the atmosphere, it oxidises first to form a sulphate ion. It then becomes sulphuric acid as it joins with hydrogen atoms in the air and falls back down to earth. Oxidation occurs the most in clouds and especially in heavily polluted air where other compounds such as ammonia and ozone help to catalyse the reaction, converting more sulphur dioxide to sulphuric acid. However, not all of the sulphur dioxide is converted to sulphuric acid. In fact, a substantial amount can float up into the atmosphere, move over to another area and return to earth unconverted.

Nitric oxide and nitrogen dioxide are also components of acid rain. Its sources are mainly from power stations and exhaust fumes. Like sulphur dioxide, these nitrogen oxides rise into the atmosphere and are oxidised in clouds to form nitric acid. These reactions are also catalysed in heavily polluted clouds where iron, manganese, ammonia and hydrogen peroxide are present.

Source: The Daily Star,December 08,2000