杏吧原创

FIFTY YEARS AGO, on 5 December 1952, Londoners received a rude awakening. They opened their curtains to find a choking dark cloud hanging over their city: a corrosive cocktail of fog mixed with smoke and gas from domestic fires and power stations. By a quirk of the weather the smog stayed put for the next four days, and that week there were more deaths in London than at the height of the cholera epidemic of 1866. Some 4000 people died of bronchitis after inhaling a concoction of smoke particles and acid that inflamed the lining of their lungs.

Until that week, Londoners thought they could live with air pollution. They didn鈥檛 much like the 鈥減ea-soupers鈥�, but they鈥檇 got used to them. The pea-souper of 1952 changed all that. Goaded by the public outcry, a committee of inquiry was set up by the government. When the Clean Air Act came into force in 1956, smokeless zones were established to reduce domestic sources of smoke. Only smokeless fuels could be burnt in these areas. In addition, urban power stations were closed down and cleaner types of coal were introduced.

Those four, dark, smoky days in 1952 marked a turning point in public and political thinking about pollution. One of the principal villains of the piece was sulphur dioxide. Levels of SO₂ in the air increased sevenfold, levels of smoke increased threefold鈥攁nd the peaks in these levels coincided with the peak in the number of deaths. Produced by coal burning, SO₂ is converted to acid in clouds by various oxidising agents, some natural, some not. But its effects are not only felt in smoggy cities鈥攖he wind can carry sulphur-laden clouds hundreds of miles before it falls as acid rain, lowering the pH of lakes and rivers, killing fish and invertebrates. And today there鈥檚 a new villain to contend with: nitric acid.

In the 19th and 20th centuries, SO₂ was the main source of acid rain in Europe and North America. Low levels of hydroxyl radicals鈥攈ighly reactive OH molecules that are naturally present in the air鈥攁ttack gaseous SO₂, slowly converting it to sulphuric acid. However, this acid forms much faster from dissolved SO₂ in cloud droplets: SO₂ + H₂O 鈫愨啋 H₂SO₃ (sulphurous acid) H₂SO₃ 鈫愨啋 H⁺ + HSO₃ HSO₃ + 陆 O₂ 鈫� H⁺ + SO₄² (sulphuric acid)

Traces of metals like iron and manganese, found in airborne fly ash from coal burning, catalyse the oxidation of SO₂ in droplets. Oxidation is even faster if carried out by dissolved ozone (O₃)鈥攏ow a commonplace pollutant in cities. Hydrogen peroxide (H₂O₂), naturally present in air, is another molecule that oxidises SO₂ in solution, and it becomes more effective under acidic conditions. However, it is only present in trace amounts.

Other gases also acidify rain. Hydrochloric acid (from hydrogen chloride gas produced by burning coal and wood) and nitric acid (produced from oxides of nitrogen in vehicle exhaust fumes) are very soluble in raindrops. And once in solution all these acidic substances soon fall to earth in rain.

Fire and brimstone

Industry creates a stink

The pollution that served up London鈥檚 pea-soupers had been brewing since the Industrial Revolution of the late 18th and 19th centuries. Industry, power production and domestic fires are major sources of SO₂. In Europe, sulphur emissions had increased so much by the middle of the 19th century that they were causing effects far away from their sources. Of course, nobody knew about acid rain, but the soot produced by burning coal was visible everywhere. Shepherds in upland Britain described the sooty coating on the fleeces of sheep as 鈥渕oorgrime鈥�. And as far away as Scandinavia, falls of 鈥渂lack snow鈥� were frequently blamed on coal burning in Britain.

Acid rain, as such, only became a prominent environmental issue in the 1980s following groundbreaking studies in the 1970s and campaigns by environmental groups such as Greenpeace and Friends of the Earth (鈥淎cid rain鈥�, Inside Science No. 2). The industrial and political climates in Europe and North America have changed radically since then. Dramatic falls in sulphur emissions have been achieved by switching to low-sulphur fuels, for example by using natural gas in power production. And in large coal-burning power plants, emissions can be reduced by scrubbing exhaust gases free of sulphur dioxide in a process known as flue gas desulphurisation.

As a result of these changes, public interest has shifted to issues such as holes in the ozone layer and the greenhouse effect. Nevertheless, acidification is still a very real threat in regions of the world where industries have failed to modernise and curb their sulphur emissions, such as China and parts of Eastern Europe and the former Soviet Union. Indeed, global sulphur emissions are set to continue rising at least until 2020 (see Figure).FIG-mg23437705.jpg

And there鈥檚 a new acid threat from the growing hordes of motor vehicles on our streets, which pump out oxides of nitrogen (NOx). These are rapidly converted to gaseous nitric acid, so even on cloudless days the air can be filled with this corrosive chemical.

It鈥檚 worth remembering that there are natural as well as human sources of acid rain. Rainfall is naturally slightly acidic, having a pH that is usually less than 5.6. This is due to carbon dioxide dissolving in water to form carbonic acid (by contrast, the pH of acid rain from pollution can be as low as 3). There are also many other sources of natural acidity in lakes, rivers and groundwater. For example, plankton in the oceans release dimethylsulphide gas, which is converted to sulphuric acid in the atmosphere. These minute droplets of sulphuric acid are actually an important part of natural weather patterns, as water condenses around them to form cloud droplets.

On land, volcanic eruptions are a significant source of sulphur compounds in the atmosphere, and hence sulphuric acid. In minor eruptions, especially from vents called fumaroles, the acid disperses over local land surfaces鈥攐ften damaging crops and making breathing difficult. In more violent eruptions, the gases are sent higher into the atmosphere. A large proportion end up in the stratosphere where droplets of sulphuric acid can linger for years, helping to thin the ozone layer. In addition, sulphate particles have an indirect cooling effect. They act as nuclei for the formation of clouds, which shade the planet. So, ironically, reducing sulphur emissions tends to promote global warming鈥攋ust one example of how tinkering with the complex chemistry set that is the atmosphere can have unforeseen consequences, both good and bad. But the cooling effects are slight and are a poor excuse for letting sulphur emissions run out of control.

Other natural sources of acid include lightning bolts. Nitric oxide created in the electrical discharge produces nitric acid. In some parts of the world, this is an important source of nitrate, fertilising soil that would otherwise be deficient in it. Large meteors also produce nitric oxide as they tear through the atmosphere. Some geologists have even argued that a corrosive rain of nitric acid following a massive meteor impact 65 million years ago was one factor leading to the extinction of the dinosaurs.

Forest fires send acids into the atmosphere, and modern, mechanised logging practices often allow such fires to spread more widely, as has happened in Malaysia in recent years. And, of course, they can be started deliberately, like the fires that menaced the suburbs of Sydney last year. Forest fires produce nitric, hydrochloric and sulphuric acids, along with organic acids such as acetic and formic acid that can make rainfall more acidic downwind of the conflagration.

In the first half of the 20th century, acid rain鈥攚hether natural or unnatural鈥攚as a hidden phenomenon. By the 1950s, however, its effects were becoming apparent to scientists in Scandinavia with the loss of fish from mountain lakes. A network of rainwater monitoring stations was set up across Europe, which very soon proved the existence of acid rain as a result of human activities. Acid rain was also charged with killing vast tracts of forest in Germany. The effects of long-range sulphur pollution were looking so worrying that the issue was placed before the world at the UN Conference on the Human Environment in Stockholm in 1972. The outcome was that states are now responsible for ensuring their activities do not damage the environment of other states. This paved the way for the development of programmes for reducing long-range air pollution. The most important of these was the Convention on Long-Range Transboundary Air Pollution (LRTAP) agreed in 1979 by the world鈥檚 leading economic nations.

Turning the supertanker

Strategies for curbing emissions

Over the final few decades of the 20th century a range of international agreements followed the LRTAP, including the Gothenburg Protocol in 1999 and various European Union directives tackling large emission sources. These have led to curbs on SO₂ and NOx emissions from large power stations and efforts to cut vehicle emissions. Catalytic converters, for example, reduce levels of carbon monoxide, hydrocarbons and NOx in car exhausts.

Sulphur emissions continue to fall across Europe and North America, though it will take several years to turn around the supertanker of sulphur pollution. Lakes can remain acid for years, but some are showing encouraging improvements (see Figure).

So when did acid rain first start to make its corrosive mark? The earliest widespread effect was the decline in fish in the lakes of north-western Europe from the beginning of the 20th century. But it鈥檚 possible to trace changes in acidity back further in time by looking at the sediments of lakes, in particular by examining the remains of single-celled algae called diatoms. Lakes high in acidity are dominated by species of diatom that can tolerate low pH levels. In Scotland, such work suggests that 鈥渟ensitive鈥� lochs were becoming more acid at the end of the 19th century. Lakes that are most vulnerable are those with a low 鈥渂uffer capacity鈥�, which usually means they contain a limited amount of alkaline material eroded from the surrounding rocks and soils.

Salmon, trout, minnow and roach seem most sensitive to acidification, with eels the least sensitive. Between 1978 and 1983, for example, 30 per cent of the brown trout in lakes in southern Norway died, and 12 per cent of the perch were lost. Acidic waters effectively make gills 鈥渓eaky鈥�, encouraging the loss of ions such as chloride and sodium as their membranes become more permeable. Fish need to maintain high concentrations of these ions in tissues like blood, otherwise water is sucked out through osmosis. One effect is reduced blood volume, eventually resulting in heart failure and death.

Another important change occurs with acidification: the solubility of aluminium in soil increases, washing the dissolved ion into rivers and lakes. This toxic metal damages gills so they clog with mucus, suffocating the fish. Aluminium tends to be more toxic when calcium concentrations in the water are low鈥攁s they often are in acidified lakes. Calcium is lost because the ions neutralise acid, forming insoluble salts.

In addition to these direct effects on fish, acidification may kill off the insects that they eat. In turn, the distribution of fish-eating birds can be affected as fish stocks decline. And birds like the dipper that eat insects such as mayflies, caddis flies and stoneflies also suffer from the reduced availability of their prey in rivers.

In fact, acidification triggers a whole range of ecological changes in surface waters. For example, the development of embryonic and larval stages of amphibians is acid-sensitive. The natterjack toad (Bufo calamita), in particular, has suffered in Britain and has been an endangered species since 1975. Acidification of its breeding sites is thought to be responsible.

All these effects can show a strong, seasonal pattern in temperate regions. This is because melting snow during the spring thaw can release large amounts of acid into rivers and lakes over a very short time. Even in areas that are relatively unpolluted, this pulse of acid can affect more sensitive, younger organisms that have hatched in the early spring.

On land, the death of forests became a major concern in the 1980s and led to dire predictions about an almost complete loss of trees, most particularly in parts of Germany. But acidic rainfall may not be the only culprit. Other factors include climate change鈥攊n particular an increased incidence of drought鈥攃hanging forestry practices and foliage diseases. In the US, for example, it has been difficult to link the loss of trees in hardwood forests to air pollution.

An open verdict

Was it the acid or the gases?

There were many early ideas about how acid rain would affect trees. Some of these considered the possibility that acids would be concentrated enough to directly damage leaves. Others identified indirect effects, such as acids leaching nutrient minerals from the soils, 鈥渟tarving鈥� the trees to death. It was also clear that acid rain could dissolve more toxic materials such as aluminium that would then poison the root system of trees. And there were concerns that acid rain increases trees鈥� susceptibility to disease and insect attack.

The picture has remained complex, but it鈥檚 no longer clear that acid rain is solely responsible for the death of forests. Attention has switched to the role of gaseous pollutants. Ozone and SO₂ rapidly break down chlorophyll, giving young conifer needles a bleached or reddish-brown appearance. Fortunately, levels of SO₂ gas in the air have declined significantly in Europe and North America and are now less of a threat. In China, however, SO₂ is still a major source of vegetation damage. Levels of ozone, meanwhile, have increased across Europe and North America. Unlike acid rain, ozone is associated with long, sunny and calm dry spells, since ultraviolet light promotes the conversion of NOx and hydrocarbons in car exhausts to ozone.

The dramatic reduction in sulphur emissions from Europe and North America have been achieved by switching from coal to low-sulphur fuels such as refined oils and gas, and the modernisation of eastern Europe. As a result, the amount of sulphur deposition from rain has generally fallen. By contrast, there have been large increases in emissions in Asia, with a projected rise from around 40 million tonnes of SO₂ in 1990 to around 110 million tonnes in 2020.

And there鈥檚 no room for complacency in the West, because there have been increases in other pollutants that cause acid rain. The most important are the nitrogen oxides released by large industrial plants and transport sources, which generate nitric acid as well as ozone. In Europe and North America, there are many regions where acid rain is essentially nitric rather than sulphuric.

One result has been that acid formation is more localised. This is because nitric acid is easily created in reactions between gases鈥攗nlike sulphuric, which only forms readily in solution. So nitric acid is less dependent on clouds for its production and generally forms closer to its sources than sulphuric acid.

And there鈥檚 more bad news. In the past a significant amount of the acid in rainfall was neutralised by alkaline materials, notably the calcium-rich minerals in windblown dusts. But there has been a slow decline in levels of alkaline materials in the air over the past few decades, probably as a result of decreased ash emissions from industry and fewer unpaved roads鈥攁n important source of dust in the past. Ammonia emissions, particularly from the manure associated with intensive pig and poultry farming, have been on the increase in many parts of the world. You might think this is a good thing because ammonia is alkaline, but it reacts with SO₂ to produce ammonium sulphate, which is converted to nitric acid in soil.

Nevertheless, there鈥檚 evidence of reduced deposition of acidic components in some areas (see Figure). There have been some improvements in the quality of freshwater ecosystems, with salmon and trout catches beginning to increase in Norwegian rivers, for example. The impact of acid rain on ecosystems is often discussed in terms of the critical load that the ecosystem can bear (see 鈥淥n the critical list鈥�).

The decrease in sulphur deposition has been so marked that crops in some parts of England and Germany, particularly oats, barley and oilseed rape, are often deficient in sulphur. Farmers now have to pay for sulphur to be added to their fields.

Lessons learned in temperate regions are not always easy to apply in Asia, where air pollution is on the increase. 杏吧原创s and regulators face a new problem, with entirely different ecosystems being confronted by acid rain. The rapid growth in emissions, most notably from coal use, will be combined with some novel factors found along the Asia-Pacific rim. Greater amounts of windblown alkaline dust, for example, offer the potential to neutralise acids in rainfall. Forest fires can produce acids, but they also liberate large amounts of alkaline ash that disperses along with the acids and leads to complicated patterns of acidity and alkalinity. The responses of tropical soils could be quite different to those found in Europe.

Acid rain now presents scientists in Asia with challenges every bit as demanding as those faced in Europe and North America at the end of the 20th century. In the West, meanwhile, pollutants such as NOx and ozone are the new threat. If car numbers keep rising at the current rate, there will be more than a billion on the roads by 2025. Our love affair with sulphur-rich fuels may be over, but our romance with the internal combustion engine is very much alive.