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Air pollution is a broad term applied to all physical (particulate matter), chemical, and biological agents that modify the natural characteristics of the atmosphere. Some definitions also consider physical perturbations such as noise pollution, heat, radiation or light pollution as air pollution. Definitions commonly include the term harmful as a requisite to consider a change to the atmosphere as pollution. |
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Air Pollutants are classified as either Primary or Secondary. A primary air pollutant is one that is emitted directly to the air from a given source. Carbon monoxide is an example of a primary air pollutant because it is produced as a byproduct of combustion. A secondary air pollutant is formed in the atmosphere through chemical reactions involving primary air pollutants. The formation of ozone in photochemical smog is an example of a secondary air pollutant. The atmosphere is a complex, dynamic and fragile system. Concern is growing about the effects of air pollutant emissions in a global context, and the inter-linkage of these emissions with global warming, climate change and stratospheric ozone depletion. Deaths It is estimated that three million people may die of air pollution each year worldwide. 2.8 million of the 3 million mortalities may be due to indoor air pollution. 90% of the 3 million estimated deaths are in developing nations. 70,000 die each year in the U.S. (Some estimates are as low as 50,000 or as high as 100,000). Deaths from air pollution are compared to deaths from second hand smoke and chemical weapons. In the U.S, more people die from air pollution than from car accidents. They die specifically from agitated asthma, bronchitis, emphysema, lung and heart diseases, and other respiratory allergies. The EPA estimates that a proposed set of changes in diesel fuel technology (Tier 2) could result in 12,000 fewer premature mortalities, 15,000 fewer heart attacks, 6,000 fewer emergency room visits by children with asthma, and 8,900 fewer respiratory-related hospital admissions each year in the US. The worst short-term civilian event from pollution in India was the 1984 Bhopal Disaster. Leaked industrial vapors killed more than 2,000 people outright and injured anywhere from 150,000 to 600,000 others, some 6,000 of whom would later die from their injuries. The worst single incident of air pollution to occur in the United States of America occurred in Donora, Pennsylvania in late October, 1948, when 20 people died and over 7,000 were injured. The United Kingdom suffered its worst air pollution event when the December 4th Great Smog of 1952 formed over London. In six days more than 4,000 died, and 8,000 more died within the following months. An accidental leak of anthrax spores from a biological warfare laboratory in the former USSR in 1979 near Sverdlovsk is believed to have been the cause of hundreds of civilian deaths. Intentional air pollution in combat is called chemical warfare. Poison gas as a chemical weapon was principally used during World War I, and resulted in an estimated 91,198 deaths and 1,205,655 injuries. Various treaties have sought to ban its further use. Non-lethal chemical weapons, such as tear gas and pepper spray, are widely used. Pollution Sources
Other Anthropogenic Sources
Natural Sources
Contaminants Contaminants of air can be divided in particulates and gases. Particulates are small, solid particles, classified by their sizes. Atmospheric particles are usually measured as TSP, PM10 or PM2.5. TSP stands for Total Suspended Particulates. The PM10 fraction consists of particles with an aerodynamic diameter of less than 10 micrometres; these are more dangerous to humans than TSP, because they can be breathed deep into the respiratory tract and reach the lungs. PM2.5 particles are even more dangerous because they can pass through the upper airway filtering and into the alveoli, where they can cross the lung/blood stream barrier and transport into the blood. Increasing attention is now focusing on the health impacts of even smaller particles- the so-called 'nanoparticles'. Smaller particles tend to be more toxic than larger particles and can stay airborne as an 'aerosol' for longer than larger particles, which settle out more quickly. Important pollutant gases include:
Control Devices The following items are commonly used as pollution control devices by industry or transportation devices. They can either destroy contaminants or remove them from an exhaust stream before it is emitted to the atmosphere.
Indoor air pollution The lack of ventilation indoors concentrates air pollution where people are most exposed to them. Background pollution comes from such mundane sources as shower water mist containing arsenic or manganese, both of which are damaging to inhale. The arsenic can be trapped with a shower nozzle filter. Radon gas, a carcinogen, is exuded from the earth and trapped inside houses. Researchers have found that radon gas is responsible for over 1,800 deaths annually in the United Kingdom. These natural radon emissions can be blocked by a layer of aluminum foil under the carpet (according to the U.S. Department of Air Quality Management). Building materials including carpeting and plywood emit formaldehyde gas. Paint and solvents give off volatile organic compounds (VOCs) as they dry. Lead paint can degenerate into dust and be inhaled. Asbestos insulation was commonly used in many application and can be carcinogenic in the lungs. Intentional air pollution is introduced with the use of air fresheners, incense, and other scented items. Controlled wood fires in stoves and fireplaces can add significant amounts of smoke particulates into the air, inside and out. Clothing emits perchloroethylene for days after dry cleaning. Deaths are often caused by using pesticides and other chemical sprays indoors without proper ventilation, and many homes have been destroyed by accidental pesticide explosions. Second-hand tobacco smoke is now recognized as an indoor air pollutant which accounts for an estimated 3,000 lung cancer deaths annually in the US. Carbon monoxide poisoning is a quick and silent killer, often caused by faulty vents and chimneys, or by the burning charcoal indoors. 56,000 Americans died from CO in the period 1979-1988. Chronic carbon monoxide poisoning can result even from poorly adjusted pilot lights. Smoke inhalation is a common cause of death in victims of house fires. Traps are built into all domestic plumbing to keep deadly sewer gas, hydrogen sulfide, out of interiors. Biological sources of air pollution can also be found indoors, and include gases, particulates, allergens, and microbes. Pets produce dander, bed mites deposit shells and microscopic droppings, inhabitants emit methane, mold can form in walls and generate spores, air conditioning systems can incubate Legionnaires disease, toilets can emit feces-tainted mists, and houseplants and surrounding gardens can produce pollen, dust, and mold spores. Greenhouse effect
The greenhouse effect, first discovered by Joseph Fourier in 1824, is the process by which an atmosphere warms a planet. Mars, Venus and other celestial bodies with atmospheres (such as Titan) have greenhouse effects, but for simplicity the rest of this article will refer to the case of Earth. The term greenhouse effect may be used to refer to two different things in common parlance: the natural greenhouse effect, which refers to the greenhouse effect which occurs naturally on earth, and the enhanced (anthropogenic) greenhouse effect, which results from human activities (see also global warming). The former is accepted by all; the latter is accepted by most scientists, although there is some dispute. The natural greenhouse effect Process The Earth receives an enormous amount of solar radiation. Just above the atmosphere, the solar power flux density averages about 1366 watts per square meter, or 1.740×1017 W over the entire Earth. This figure vastly exceeds the power generated by human activities. The solar power hitting Earth is balanced over time by a roughly equal amount of power radiating from the earth (as the amount of energy from the Sun that is stored is small). Almost all radiation leaving the Earth takes two forms: reflected solar radiation and thermal blackbody radiation. Reflected solar radiation accounts for 30% of the Earth's total radiation: on average, 6% of the incoming solar radiation is reflected by the atmosphere, 20% is reflected by clouds, and 4% is reflected by the surface. The remaining 70% of the incoming solar radiation is absorbed: 16% by the atmosphere (including the almost complete absorption of shortwave ultraviolet over most areas by the stratospheric ozone layer); 3% by clouds; and 51% by the land and oceans. This absorbed energy heats the atmosphere, oceans, land and powers life on the planet. Like the Sun, the Earth is a thermal blackbody radiator. So because the Earth's surface is much cooler than the Sun (287 K vs 5780 K), Wien's displacement law dictates that Earth must radiate its thermal energy at much longer wavelengths than the Sun. While the Sun's radiation peaks at a visible wavelength of 500 nanometers, Earth's radiation peak is in the longwave (far) infrared at about 10 micrometres. The Earth's atmosphere is largely transparent at visible and near-infrared wavelengths, but not at 10 micrometres. Only about 6% of the Earth's total radiation to space is direct thermal radiation from the surface. The atmosphere absorbs 71% of the surface thermal radiation before it can escape. The atmosphere itself behaves as a blackbody radiator in the far infrared, so it re-radiates this energy. The Earth's atmosphere and clouds therefore account for 91.4% of its longwave infrared radiation and 64% of Earth's total emissions at all wavelengths. The atmosphere and clouds get this energy from the solar energy they directly absorb; thermal radiation from the surface; and from heat brought up by convection and the condensation of water vapor. Because the atmosphere is such a good absorber of longwave infrared, it effectively forms a one-way blanket over Earth's surface. Visible and near-visible radiation from the Sun easily gets through, but thermal radiation from the surface can't easily get back out. In response, Earth's surface warms up. The power of the surface radiation increases by the Stefan-Boltzmann law until it (over time) compensates for the atmospheric absorption. The surface of the Earth is in constant flux with daily, yearly, and ages long cycles and trends in temperature and other variables from a variety of causes. The result of the greenhouse effect is that average surface temperatures are considerably higher than they would otherwise be if the Earth's surface temperature were determined solely by the albedo and blackbody properties of the surface. It is commonplace for simplistic descriptions of the "greenhouse" effect to assert that the same mechanism warms greenhouses but this is an incorrect oversimplification: see below. Limiting factors The degree of the greenhouse effect is dependent primarily on the concentration of greenhouse gases in the planetary atmosphere. The carbon dioxide-rich atmosphere of Venus causes a runaway greenhouse effect with surface temperatures hot enough to melt lead, the atmosphere of Earth creates habitable temperatures, and the thin atmosphere of Mars causes a minimal greenhouse effect. The use of the term runaway greenhouse effect to describe the effect as it occurs on Venus emphasises the interaction of the greenhouse effect with other processes in feedback cycles. Venus is sufficiently strongly heated by the Sun that water is vaporised and so carbon dioxide is not reabsorbed by the planetary crust. As a result, the greenhouse effect has been progressively intensified by positive feedback. On Earth there is a substantial hydrosphere and biosphere which respond to higher temperatures by recycling atmospheric carbon more quickly (in geologic terms; the timescale for the ocean/biosphere to remove a CO2 perturbation is on the order of several hundred years). The presence of liquid water thus limits the increase in the greenhouse effect through negative feedback. This state of affairs is expected to persist for at least hundreds of millions of years, but, ultimately, the warming of an aging Sun will overwhelm this regulatory effect. The average surface temperature would be -18°C without a greenhouse effect or 72°C with just the greenhouse effect and no convection, but in reality this temperature is closer to 15°C due to convective flow of heat energy within the atmosphere and partly above much of the thermal IR absorbence of the atmosphere. [2] Recent measurements of carbon dioxide amounts from Mauna Loa observatory show that CO2 has increased from about 313 ppm (parts per million) in 1960 to about 375 ppm in 2005. The current observed amount of CO2 exceeds the geological record of CO2 maxima (~300 ppm) from ice core data (Hansen, J., Climate Change, 61, 269, 2005). This suggests that the CO2 production rate from increased industrial activity (automobile use and fossil fuel generation) and other human activities has overwhelmed the normal feedback control mechanisms. Global climate model calculations indicate that the elevated CO2 levels are likely to lead to global warming. There has been an observed global average temperature increase of about 0.5oC since 1960 (Science 308, 1431, 2005). There is still some public controversy about human activities, CO2 increases, and global warming. The greenhouse gases Water vapor (H2O) causes about 60% of Earth's naturally-occurring greenhouse effect. Other gases influencing the effect include carbon dioxide (CO2) (about 26%), methane (CH4), nitrous oxide (N2O) and ozone (O3) (about 8%). Collectively, these gases are known as greenhouse gases. The greenhouse effect due to carbon dioxide is specifically known as the Callendar effect. The wavelengths of light that a gas absorbs can be modelled with quantum mechanics based on molecular properties of the different gas molecules. It so happens that heteronuclear diatomic molecules and tri- (and more) atomic gases absorb at infrared wavelengths but homonuclear diatomic molecules do not absorb infrared light. This is why H2O and CO2 are greenhouse gases but the major atmospheric constituents (N2 and O2) are not. Between the absorptions of water vapor and those of carbon dioxide, there is an atmospheric window where, prior to the industrial era, no infrared radiation was trapped, lying between 8 and 15 micrometres. Compounds such as perflurocarbons (CF4, C2F6 etc.), chlorofluorocarbons, halons and SF6 absorb very strongly in this window. This means that they are extremely potent greenhouse gases, especially given the absence of natural sinks to remove them. Perfluorocarbons can have a lifetime of 50,000 years, possibly longer. Effects of various gases It is hard to disentangle the percentage contributions to the greenhouse effect by different gases, because their respective infrared spectrums overlap. However, one can calculate the percentage of trapped radiation remaining, and discover:
Water vapor effects Water vapor is the major contributor to Earth's greenhouse effect. Its effects vary due to localized concentrations, mixture with other gases, frequencies of light, different behavior in different levels of the atmosphere, and whether positive or negative feedback takes place. High humidity also affects cloud formation, which has major effects upon temperature but is distinct from water vapor gas. The IPCC TAR (2001; section 2.5.3) reports that, despite non-uniform effects and difficulties in assessing the quality of the data, water vapor has generally increased over the 20th Century. Estimates of the percentage of Earth's greenhouse effect due to water vapor:
Including clouds, the table above would suggest 50%. For the cloudless case, IPCC 1990, p 47-48 estimate water vapor at 60-70% whereas Baliunas & Soon estimate 88% considering only H2O and CO2. Water vapor in the troposphere, unlike the better-known greenhouse gases such as CO2, is essentially passive in terms of climate: the residence time for water vapor in the atmosphere is short (about a week) so perturbations to water vapor rapidly re-equilibriate. In contrast, the lifetimes of CO2, methane, etc, are long (hundreds of years) and hence perturbations remain. Thus, in response to a temperature perturbation caused by enhanced CO2, water vapor would increase, resulting in a (limited) positive feedback and higher temperatures. In response to a perturbation from enhanced water vapor, the atmosphere would re-equilibriate due to clouds causing reflective cooling and water-removing rain. The contrails of high-flying aircraft sometimes form high clouds which seem to slightly alter the local weather. Real greenhouses The term 'greenhouse effect' originally came from the greenhouses used for gardening, but it is a misnomer since greenhouses operate differently. A greenhouse is built of glass; it heats up primarily because the Sun warms the ground inside it, which warms the air near the ground, and this air is prevented from rising and flowing away. The warming inside a greenhouse thus occurs by suppressing convection and turbulent mixing. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. It has also been demonstrated experimentally (Wood, 1909): a "greenhouse" built of rock salt (which is transparent to IR) heats up just as one built of glass does. Greenhouses thus work primarily by preventing convection; the greenhouse effect however reduces radiation loss, not convection. It is quite common, however, to find sources that make the "greenhouse" analogy. Although the primary mechanism for warming greenhouses is the prevention of mixing with the free atmosphere, the radiative properties of the glazing can still be important to commercial growers. With the modern development of new plastic surfaces and glazings for greenhouses, this has permitted construction of greenhouses which selectively control radiation transmittance in order to better control the growing environment.
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