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Peter H. Raven


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Air Pollution

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uring the 20th century air pollution, once a localized problem, became a global one. Nowhere is immune from toxic fallout or changes to the planet's atmospheric chemistry. Even so, the most intense effects on both ecosystems and human health are local. [Add]

Atmospheric chemistry

Humankind has been manipulating atmospheric chemistry on a small scale, usually accidentally, for many centuries. Urban areas contain enough heat-absorbing construction material to keep cities warmer than surrounding areas. Sulfate particles in urban smog, on the other hand, reduce solar heating. Deforestation has upset the hydrological cycle, often reducing rainfall downwind. For half a century scientists have attempted, generally unsuccessfully, to "seed" clouds with tiny particles to encourage the formation of raindrops. In recent decades evidence has grown that humans are altering climate on the global scale by adding greenhouse gases to the atmosphere. [Add]

Approximately half of the world's population now lives in urban areas, and half of all the world's urban residents are exposed to potentially harmful amounts of sulfur dioxide (SO2), ozone and particulate matter in "smogs" [1]. The chemistry of smogs takes different forms. Winter smogs largely arise from burning coal to warm buildings during cold weather. When the smoke and SO2 combine with fog in windless weather they create a pollution cap that the sun is not strong enough to clear.

Some 4 000 people died from lung and heart conditions during a London "peasouper" smog in December 1952. Similar smogs now occur regularly in northern Chinese and Indian cities, including Delhi and Beijing. China's smogs cause more than 50 000 premature deaths and 400 000 new cases of chronic bronchitis a year in 11 of its largest cities alone [2].

Summer smogs, first reported in Los Angeles, involve pollutants -- mainly from vehicle exhausts -- that undergo photochemical changes in bright sunlight, creating substances such as ozone, a gas that can trigger asthma attacks. Conditions are worst in thin air at higher altitudes and if the air is trapped inside a valley. Both situations apply in Mexico City, the world's second largest urban agglomeration, where smog alerts close factories and force cars off city streets several times a year. Globally, some 50 percent of cases of chronic respiratory illness are now thought to be associated with air pollution [3]. [Add]

A particularly toxic component in some urban air is lead, the heavy metal which has for many years been added to gasoline to raise octane levels and help engines run more smoothly. It is emitted as tiny particles in exhausts, contaminating both air and food. Elevated lead levels are widespread among children in cities where leaded petrol is sold. Lead damages the neurological development of children, lowering IQ and causing attention and behavioral problems. Many nations have reduced or banned lead additives. Elsewhere, urban areas can have high lead contamination even with relatively low vehicle numbers. Lead levels in the air of large African cities such as Cairo, Cape Town and Lagos are up to ten times those typical of European cities [4].

Analysts at the World Bank argue that exposure to lead is due less to urban demographics, vehicle numbers or national wealth and more to direct political choice. The Bank says that removing lead from gasoline is one of the most cost-effective ways of improving both the urban environment and human health [5]. [Add]

Smogs are generally very acidic. Some of the pollutants they contain travel long distances on the winds, causing acid deposition in surrounding countryside and even in neighboring countries. In the 1980s, "acid rain" was identified as a major international environmental problem, spilling over from densely populated and heavily industrialized areas of both Europe and North America into prime agricultural areas. Mountain regions suffered worst because their higher rainfall increased the volume of acid deposition, and their often thin soils could not neutralize the acid. Lakes and streams in "pristine" parts of Scandinavia and Scotland became acidified, losing fish over large areas. The most intense fallout occurred in the "black triangle" bordering Germany, the Czech Republic and Poland. [Add]


[industrial facilities]


[most polluted cities]

Since 1985, international treaties and heavy investment by power station operators in "desulfurization" equipment have cut sulfur pollution in Europe and North America by as much as 80 percent. Meanwhile nitrogen emissions from vehicles have stabilized, with the impact of cleaner cars counterbalanced by increased car use. Critical loads for acidification are still being exceeded in 10 percent of the land area of Western and Central Europe [6]. In some places, acidified soils and surface water are recovering. But in others the large amounts of acid accumulated in soils mean recovery could take decades [7]. The 1998 Forest Condition Survey of Europe by the UN Economic Commission for Europe found a quarter of the continent's trees were missing more than a quarter of their leaves. Air pollution was the main cause [8]. [Add]

As more countries industrialize, acidification of the environment is becoming a global problem. Asian emissions of SO2 were expected to exceed those of Europe and North America combined in the year 2000. The largest source is China, which emits 18 million tons of SO2 a year. China's losses to crops and forests from acid deposition stand at US$5 billion a year [9]. Japan, which invested heavily to clean up its own emissions, is now suffering cross-border pollution from its neighbors [10]. Modelling studies suggest that without a clean-up, acid fallout over large areas of China will by 2020 exceed the levels reached in Central Europe in the 1970s [11].

Under certain meteorological conditions, smogs can spread very large distances to remote, unpopulated areas. In winter, weather systems take smog from Russian industrial centres north into the Arctic, where it lingers for many months -- a phenomenon known as Arctic haze [12]. Similarly, Asian smogs sometimes travel on westerly winds across the Pacific to North America in spring [13].

The smoke from some forest fires can also be categorized as human-induced pollution, and can spread thousands of kilometers. In late 1997, Indonesian forest fires polluted neighboring countries, causing plane and shipping crashes as well as thousands of hospital admissions for lung and eye complaints. Health costs from the fires were later put at US$940 million [14]. [Add]

Researchers have only quite recently realized the importance of biomass burning in overall emissions of greenhouse gases. More than half of the carbon released into the atmosphere comes from biomass burning, the remainder being produced by fossil-fuel burning, cement manufacture and gas flaring. [Add]

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