快猫短视频

IT GETS everywhere鈥�in your eyes, up your nose, deep into your lungs. And given the right conditions, dust can hitch a ride on the wind and travel the globe. In April this year, skiers in the Swiss Alps got a dramatic reminder of just how mobile it can be when some 80,000 tonnes of fine sand fell over Geneva and the resorts of Zermatt and Verbier, turning the snow a reddish-brown colour. The sand hailed from the Sahara, thousands of kilometres away in North Africa.

This extraordinary mobility makes dust a formidable pollutant. Last month, the UN drew the world鈥檚 attention to the 鈥渂rown cloud鈥� hanging over Asia, a noxious mix of particles and gases from forest fires, vehicle exhausts and millions of small, inefficient cookers burning wood and cattle dung. The pollution is thought to kill thousands a year in the region and is probably disrupting its climate.

Dust and other particles suspended in the atmosphere 鈥� collectively known as particulates 鈥� come in a bewildering range of sizes and compositions, from minuscule particles of partially burnt fuel spewed from the exhaust pipes of cars, buses and lorries, to relatively massive grains of pollen, particles of rock dust blown from quarries, and flecks of soot or 鈥渇ly ash鈥� from coal-burning. Some particulates are simply a nuisance, like the films of red dust 鈥� also from the Sahara 鈥� that people living in London sometimes find on the windscreens of their cars. But some are downright dangerous.

Over the past decade, doctors have become increasingly concerned about how the tiniest particulates affect health. They may be responsible for up to 10,000 premature deaths in Britain each year. People with lung and heart disease are the most vulnerable. When breathed in, fine dust is carried deep into the lungs, where it can exacerbate inflammation in lung disease and even precipitate a heart attack in people with cardiovascular disease (see 鈥淓nvironmental lung disease鈥�, Inside Science No. 84). During the notorious London smogs of the 1950s, poisonous mixtures of fog and smoke from coal-burning power stations and domestic fires are thought to have killed at least 4000 people. And as recently as 1991, the number of deaths in London from respiratory disease shot up by 22 per cent during a bad four-day smog. We can鈥檛 choose the air we breathe: keeping it clean is vital.

快猫短视频s have been observing airborne particles for more than 150 years. In 1833, sailing on HMS Beagle off the west coast of Africa, Charles Darwin noted that 鈥渢he atmosphere was often hazy, and a very fine dust was almost constantly falling, so that astronomical instruments were roughened.鈥� In the 1890s, the American geologist J. A. Udden carried out some of the first laboratory and field experiments on windborne sand and dust. His scientific legacy lives on today, though his successors take their labs with them into the field on major projects. In the US, they鈥檝e even been known to erect huge wind tunnels in the prairies to study the movement of dust.

Increasingly sophisticated instruments have allowed finer and finer particles to be monitored. This has helped shift the focus of research away from coarser nuisance dust like that billowing around a building site 鈥� which might dirty washing or the windows of nearby houses but doesn鈥檛 pose a health risk 鈥� to the microscopic particulates that certainly are a threat.

But particulates are complex beasts. They can be made of more than one type of material. One particularly nasty confection is diesel-coated pollen. And some are aerosols 鈥� tiny liquid droplets suspended in the air that originate in breaking waves and bursting bubbles at sea, in industrial smoke and fumes from hot surfaces and even from electric fires and cookers. Solid particulates include grit from quarries and building sites and soot in the fumes belched from industrial chimneys and vehicle exhausts. This diversity is a nightmare for scientists trying to understand the behaviour of particulates in the atmosphere and for those who assess air pollution, because each type behaves differently and each has a different effect on human health.

On a global scale, most particulates come from natural sources. Salty aerosols from the oceans have a huge input (see Graph). Large quantities of ash are hurled into the air by volcanoes 鈥� 500 million tonnes from the eruption of Mount St Helens in Washington state in 1980. And desert storms sweep fine sand high into the atmosphere. We top this up with artificial particulates from industry, fires and vehicles. These, which go hand-in-hand with sulphur dioxide emissions, not only harm human health but also contribute to acid rain, damaging buildings and freshwater ecosystems (see 鈥淎cid drops鈥�, Inside Science No. 150).

Then there are the vast quantities of secondary particulates created from gases in the air. Gases like SO2 are oxidised in the atmosphere to form compounds such as ammonium sulphate and sulphuric acid, while nitrogen oxides are converted to particulate ammonium salts and nitrates. Secondary particulates are usually less than 10 micrometres (thousandths of a millimetre) across. Some originate from combustion sources such as vehicles, but most are natural. An estimated 2 billion tonnes of natural secondary particulates form in the Earth鈥檚 atmosphere every year 鈥� a whopping amount compared with the 300 million tonnes we add.

To put it simply, natural sources are the biggest contributor to dust in the air, releasing 10 times as much particulate matter into the atmosphere as humans do. This isn鈥檛 necessarily bad news: natural particulates such as mineral dusts tend to be inert and are thought to be less damaging to health. And the deadliest particulates are the finest, and we鈥檙e responsible for pretty much all of them (see Graph). The bottom line is that we can exert some measure of control over how much we humans pollute, and try to mitigate the damage.

So what is it about the smallest particulates that makes them so dangerous? In general, particles greater than 10 micrometres across are deposited in the nose and throat, which are well protected with mucus. Those between 4 and 10 micrometres are trapped by the mucus coating the airways, which is continuously driven by billions of tiny hairs towards the mouth, where it is swallowed. Particles less than 4 micrometres across, however, can reach the naked gas-exchange surfaces of the tiny air sacs called alveoli (see Figure). It鈥檚 not entirely clear why people with lung and heart disease are so susceptible. But in asthmatics, for example, dust may exacerbate inflammation in the lungs. After prolonged exposure, the immune system鈥檚 mast cells in the lining of bronchioles become sensitised to particulates. Further exposure then provokes these cells to release histamine and other inflammatory mediators. Histamine makes the smooth muscles that line the airways contract, narrowing the tubes, and increases mucus production. It also dilates capillaries and makes them more permeable, which causes tissue swelling or oedema (see 鈥淏eware! Allergens鈥�, Inside Science No. 127).

Recent research in Canada also suggests that our arteries may narrow slightly when we breathe in the sort of cocktail of traffic pollutants found in urban areas during rush hour. The small restriction in blood flow may not be a problem for healthy people, but it could be fatal for those with cardiovascular disease. Researchers at the University of Toronto asked healthy volunteers to inhale a mixture of ozone and particles less than 2.5 micrometres across for two hours. The width of their brachial artery, a large vessel in the arm, reduced by between 2 and 4 per cent. Breathing ozone or particulates by themselves, or breathing filtered air, did not cause constriction.

It鈥檚 unclear how this effect is mediated, but there can be no doubt that heart disease and airborne pollution are deadly allies. In the US, for example, the Environmental Protection Agency has estimated that air pollution contributed to 60,000 heart-related deaths in 1996.

With the health stakes so high, environmental scientists like myself have been given the task of measuring levels of different particulates in the atmosphere, and tracking them back to their source. A particle鈥檚 behaviour is partly determined by its density. But its size is the most important factor, and the smaller particulates are, the more potentially dangerous they are to health. Their size range is very wide. Particles can be as large as 2 millimetres across, but only the finest become true globetrotters. The Saharan sand that fell over the Swiss Alps earlier this year would have been less than 5 micrometres in diameter, but still clearly visible. But the finest particulates, such as those in diesel exhaust fumes, may measure only a few nanometres and are invisible to the naked eye.

To simplify matters, scientists talk about dust in terms of size fractions. So, for example, all particles less than 10 micrometres in diameter are known as PM10s, and all those less than 2.5 micrometres are known as PM2.5s.

In rainy Britain, fine particles are likely to be removed from the lower atmosphere in about 10 days, regardless of the season. In drier parts of the world, however, such particles and larger ones about 10 micrometres across are likely to stay airborne for 10 to 20 hours. In really dry conditions, finer particles, from around 2.5 to 0.1 micrometres, can blow about in the atmosphere for as long as a thousand days.

The mean wind speed about 6 kilometres above the Earth鈥檚 surface in the lower troposphere is 7 metres per second, so larger particles can travel some 20 to 30 kilometres, while the smaller ones may travel several thousand kilometres.

By hitching a ride on a series of weather systems, particles can embark on a veritable odyssey. Shifting local and global weather patterns serve up a continuously changing selection of particles. On one day, the air might be loaded with far-travelled desert dust, while on the next it could be filled with a fog of salt-laden aerosol particles from the ocean. Overnight, winter weather conditions might trap soot from vehicle exhaust emissions due to a temperature inversion (when a layer of hot air created by sun-warmed hilltops or high-rise buildings during the day seals in colder air beneath), or far-travelled nitrates and sulphates from industrial pollution. Or several of these scenarios could coincide. When hospital admissions for asthma and heart conditions start to go up, this complexity gives environmental scientists quite a headache.

Making models out of thin air

To ease the confusion, they turn to computers and a technique called back trajectory computer modelling. With this they can trace far-travelled particulates that have crossed national borders back to their origins. The method involves plugging meteorological data into a model of how particulates move through the atmosphere to give estimates of particles鈥� latitude, longitude and height above the ground over five days or so. This kind of modelling is something of a black art, however, so when it comes to finding the source of a pollutant, there鈥檚 no substitute for actually monitoring particulate levels in the field (see 鈥淕auges galore鈥�).

In the past, moves to clean up the atmosphere have been driven by major environmental disasters such as the catastrophic London smog of 1952, which led to the 1956 Clean Air Act banning smoky fuels in urban areas and culminated in the closure of coal-burning power stations in towns and cities. Meanwhile, diesel and petrol engines have become increasingly efficient, reducing the amount of unburnt fuel in exhaust fumes. Catalytic converters remove hydrocarbons and nitrogen oxides from exhaust: gases that contribute to secondary particulate formation. Diesel engines emit more particulates than petrol engines, but fitting them with particulate traps can reduce PM10 emissions by up to 90 per cent.

At the same time, conventional fuels have become much cleaner in recent years, and 鈥渄esigner鈥� fuels such as ultra low-sulphur diesel 鈥� which can reduce particulate emissions by up to 40 per cent 鈥� are increasingly available. In addition, governments have imposed strict emission testing on older vehicles. In Britain, for example, all cars over three years old are given an MOT test every year, which includes a visual check for excessive exhaust smoke. The engines of poorly performing cars are retuned. Diesel cars and lorries face particularly stringent tests.

Rather like London鈥檚 infamous smogs, air pollution in cities like Los Angeles and San Francisco has concentrated minds in the US. Particulate pollution is thought to kill as many as 9000 people in Southern California each year. But the US Environmental Protection Agency is now leading the way internationally in setting acceptable air quality standards and approved methods for monitoring. Similarly, European Union legislation is forcing member countries to consider the effects of pollution on their neighbours, and adopt tighter and tighter limits.

The EU has named five main air pollutants as having short-term health effects that need monitoring: sulphur dioxide, nitrogen dioxide, ozone, carbon monoxide and particulates. The range of particulates it monitors now includes benzene aerosols, lead particles and the petroleum by-product 1,3-butadiene, as more evidence of their effect on health emerges. Governments of developing countries are also setting stringent standards. But they have an especially daunting task ahead. Their industries are encumbered with technologies that are less than green, yet they need to avoid the mistakes already made by the Western world.

叠谤颈迟补颈苍鈥檚 National Air Quality Strategy sets safety limits for eight key pollutants thought to damage health, and puts the onus on local authorities to identify and tackle breaches. Even seemingly small measures such as limiting vehicle speeds on quarry roads can reduce particulate emissions. The strategy also sets ambitious long-term goals for reducing air pollution.

To provide the British public with information about air quality, data on the five types of pollutant named by the EU is gathered every hour from 110 automatic monitoring sites around the country. The data is simplified by assigning each pollutant a score from 1 to 10 and giving it a risk rating. So a score of 1 to 3 is a low risk, 4 to 6 is moderate, 7 to 9 is high and 10 is very high. Whichever of the five pollutants is highest is used in the forecast or summary for that day. This is relayed to television sets via Teletext, posted on the Internet, and on a freephone service. A 鈥渉igh鈥� rating for any of these pollutants would alert asthmatics, for instance, to spend less time outdoors and adjust their medication.

Apart from health, particulates have another worrying effect:they can change the weather. The vast blanket of ash, soot and aerosols in Asia鈥檚 鈥渂rown cloud鈥� extends to some 3 kilometres high and stretches from the Arabian peninsula across India, South-East Asia and China. It cuts the amount of sunlight reaching the ground by between 10 and 15 per cent, possibly reducing evaporation and rainfall, and affecting crop yields. Dust in the stratosphere alters the heat balance of the globe by reflecting sunlight away, cooling the surface, and by reflecting heat rising from the Earth back downwards, warming it (see 鈥淭he greenhouse effect鈥�, Inside Science No. 92).

But the overall effect of dust clouds seems to be to cool the Earth. After the eruption of Mount Pinatubo in the Philippines in 1991, the world cooled by about half a degree Celsius over the ensuing 18 months.

In December, issues like these will be hot topics when environmental scientists meet in London at the Houses of Parliament to mark the 50th anniversary of the notorious smog of 1952. They will review the progress Britain has made in improving air quality over the past half-century and look closely at the problems and priorities ahead. A number of pressing questions will be addressed. Are naturally occurring particulates relatively harmless? How small are the real killer particles and what are they made of?

There is some cause for optimism. Indicators such as the Air Quality Headline Indicator, based on the average number of days per year on which one of the five main pollutants is moderate or higher, suggest a long-term improvement in Britain. Much of this is due to successive rounds of pollution regulations and advances in technology that have significantly cut emissions from vehicles. Similar improvements in particulate levels have been noted elsewhere in the developed world. The challenge now is to sustain this progress and pass on what we have learned about these tiny, silent killers.