APES Chapter 15: Air Pollution and Stratospheric Ozone Depletion

Explain the benefits of stratospheric ozone and how it forms.

The Sun radiates energy at many different wavelengths. The ultraviolet wavelengths are further classified into three groups: UV-A, or low-energy ultraviolet radiation, and the shorter, higher-energy UV-B and UV-C wavelengths. UV radiation of all types can damage the tissues and DNA of living organisms. Exposure to UV-B radiation increases the risk of skin cancer and cataracts, and suppresses the immune system in humans. It is also harmful to the cells of plants, and it reduces their ability to convert sunlight into usable energy. UV-B exposure can therefore harm entire biological communities. A layer of ozone in the stratosphere absorbs ultraviolet radiation, filtering out harmful UV rays from the sun. It is easy to confuse stratospheric ozone with tropospheric, or ground-level, ozone. However, stratospheric ozone occurs higher in the atmosphere where its ability to absorb ultraviolet radiation and thereby shield the surface below makes stratospheric ozone critically important to life on Earth. When solar radiation strikes O2 in the stratosphere, a series of chemical reactions begins that produces a new molecule: ozone. In the first step, UV-C radiation breaks the molecular bond holding an oxygen molecule together. This happens to only a few oxygen molecules at any given time. In the second step, a free oxygen atom produced in the first reaction encounters an oxygen molecule, and they form ozone. Both UV-B and UV-C radiation can break a bond in this new ozone molecule, forming molecular oxygen and a free oxygen atom once again. Thus formation in the presence of sunlight and its subsequent breakdown is a cycle that can occur indefinitely as long as there is UV energy entering the atmosphere. Under normal conditions, the amount of ozone in the stratosphere remains at steady state. For many years, the same chemicals that made refrigeration and air conditioning possible were also used in a host of other consumer items, including aerosol spray cans and products such as Styrofoam. These chemicals, called chlorofluorocarbons, or CFCs, were considered essential to modern life. CFCs were considered "safe" because they are both nontoxic and nonflammable. But it turned out that these chemicals had adverse effects in one part of the upper atmosphere, the stratosphere, by promoting the breakdown of ozone. CFCs introduce chlorine into the stratosphere. When chlorine is present, it can attach to an oxygen atom in an ozone molecule, thereby breaking the bond between that atom and the molecule and forming chlorine monoxide and O2. Subsequently, the chlorine molecule reacts with a free oxygen atom, which pulls the oxygen from the ClO to produce free chlorine again. Chlorine starts out and ends up as a free Cl atom. In contrast, an ozone molecule and a free oxygen atom are converted into two oxygen molecules. A single chlorine atom can catalyze the breakdown of as many as 100,000 ozone molecules until finally one chlorine atom finds another and the process is stopped. In the process, the ozone molecule are no longer available to absorb incoming UV-B radiation. As a result, the UV-B radiation can reach Earth's surface and cause harm to biological organisms.

Explain how photochemical smog forms and why it is still a problem in the US.

Although sulfur, nitrogen, and carbon monoxide pollution have been reduced well below the specified standards since the Clean Air Act was implemented, photochemical smog and ozone present especially difficult challenges. The reason lies in the chemistry of smog formation and the behavior of the atmosphere during changing weather conditions. These factors make smog formation very complex and difficult to predict. The term smog was originally used to describe the combination of smoke, fog, and sometimes sulfur dioxide that used to occur in cities that burned a lot of coal. A number of pollutants are involved and they undergo a series of complex transformations in the atmosphere that involve sunlight, water, and the presence of VOCs. The first part of the process takes place during the day, in the presence of sunlight. When an abundance of nitrogen oxides are present in the atmosphere, with a very few VOCs present, nitrogen dioxide splits to form nitrogen oxide and a free oxygen atom. In the presence of energy inputs from sunlight, this free oxygen atom combines with diatomic oxygen to form ozone. With abundant nitrogen dioxide and abundant sunlight, ozone can accumulate in the atmosphere. A few hours later, when sunlight intensity decreases and with nitrogen oxide still present in the atmosphere, the ozone combines with nitrogen oxide, and reforms into O2+NO2. This is referred to as ozone destruction. Volatile organic compounds come from human activity such as spilling of gasoline on pavement and from natural sources. When volatile organic compounds are absent or in small supply, the cycle of ozone formation and destruction generally takes place on a daily basis and relatively small amounts of photochemical smog form. A different scenario occurs when VOCs are present. The first part is the same. However, because VOCs have combined with nitrogen oxide in a strong bond, nitrogen oxide is no longer available to combine with ozone. Since the nitrogen oxide is not available to break down ozone by recombining with it, a larger amount of ozone accumulates. Although smog is associated with urban areas, it is not limited to such areas. Trees and shrubs in rural areas produce VOCs that can contribute to the formation of photochemical smog. Atmospheric temperature influences the formation of smog in several important ways. Emissions of VOCs from vegetation, as well as from evaporation of volatile liquids, increase as the temperature increases. NOx emissions from electric utilities are also greater with air-conditioning demands for electricity increasing on the hottest days. Moreover, many of the chemical reactions that form ozone and other photochemical oxidants proceed more rapidly at higher temperatures. These and other factors increase smog concentrations when temperatures are higher. Temperature also influences air pollution conditions in more complex ways. The warmest air is closest to Earth. This warm air, which is less dense than the colder air above it, can easily rise, dispersing pollutants. This allows pollutants from the surface to be reduced or diluted. However, during a thermal inversion, a relatively warm layer of air at mid-altitude covers a layer of cold, dense air below it. Because the air closest to the surface of Earth is denser than the air above it, the cool air and the pollutants within it do not rise. Thus, the inversion layer traps emissions that then accumulate beneath it, and these trapped emissions can cause a severe pollution event. Thermal inversions are particularly common in some cities, where high concentrations of vehicle exhaust and industrial emissions are easily trapped by the inversion layer. Thermal inversions can lead to other forms of pollution.

Explain strategies and techniques for controlling sulfur dioxide, nitrogen oxides, and particulate matter.

As with other types of pollution, the best way to decrease air pollution emissions is to avoid them in the first place. This can be achieved through the use of fuels that contain fewer impurities. Use of a low-sulfur coal or oil is certainly one of the best means of controlling air pollution, although typically low-sulfur coal or oil is more expensive to purchase than coal or oil containing higher sulfur concentrations. Other ways to reduce air pollution include increased efficiency and conservation. While these measures reduce emissions by a certain amount, wherever fuel is combusted, pollution will be emitted. So ultimately, most attempts to reduce air pollution will depend on the control of pollutants after combustion. Sulfur and nitrogen oxides are common air pollutants in the US and they cause a variety of environmental problems. A substantial number of air pollution control measures have been directed toward sulfur and nitrogen oxides. Sulfur dioxide emissions from coal exhaust can be reduced by a process known as fluidized bed combustion. In this process, granulated coal is burned in close proximity to calcium carbonate. The heated calcium carbonate absorbs sulfur dioxide and produces calcium sulfate. Some of the sulfur oxide that does escape the combustion process can be captured b other methods after combustion. Nitrogen oxides are produced in virtually all combustion processes. Hotter burning conditions and the presence of oxygen allow proportionally more nitrogen oxide to be generated per unit of fuel burned. In order to reduce nitrogen oxide emissions, burn temperatures must be reduced and the amount of oxygen must be controlled. However, lowering temperatures and oxygen supply can result in less-complete combustion, which reduces the efficiency of the process and increases the amount of particulates and carbon monoxide. Finding the exact mix of air, temperature, oxygen, and other factors is a significant challenge. Nitrogen oxide emissions from automobiles have also reduced significantly in the US. All new automobiles sold in the US were required to include a catalytic converter, which reduces the nitrogen oxide and carbon monoxide emissions. In order to operate properly, the precious metals in the catalytic converter cannot be exposed to lead. Therefore, gasoline could no longer contain lead. The removal of particulate matter is the most common means of pollution control. Sometimes the process of removing particulate matter also removes sulfur. There are a variety of methods used to remove particulate matter. The simplest is gravitational settling, which relies on gravity as the exhaust travels through the smokestack. The particles simply settle out to the bottom. The ash residue that accumulates must be disposed of in a landfill. Depending on the fuel that was burned, the ash may contain sufficiently high concentrations of metals that require special disposal. Pollution control devices remove particulate matter and other compounds after combustion. Each has its advantages and disadvantages and all of them use energy, which generates additional pollution. Fabric filters are a type of filtration device that allow gases to pass through them but remove particulate matter. Certain fabric filters can remove almost 100% of the particulate matter emissions. Electrostatic precipitators use an electrical charge to make particles coalesce so they can be removed. Polluted air enters the precipitator and the electrically charged particles within are attracted to negative or positive charges on the sides of the precipitator. The particles collect and relatively clean gas exits the precipitator. A scrubber uses a combination of water and air that actually separates and removes particles. Particles are removed in the scrubber in a liquid or sludge form and clean gas exits. Borrowing from the concept utilized in the electrostatic precipitator, particles are sometimes ionized before entering the scrubber to increase its efficiency. Scrubbers are also used to reduce the emissions of sulfur dioxide. All three types of pollution control devices, because they use additional energy and increase resistance to air flow in the factory or power plant, require the use of more fuel and result in increased carbon dioxide emissions. Devices such as the electrostatic precipitator and the scrubber have helped reduce pollution significantly before it is released into the atmosphere. It is much harder to remove pollutants from the environment after they have been dispersed over a wide area. Because the main component of photochemical smog is a secondary pollutant, control efforts must be directed toward reducing the precursors, or primary pollutant. Historically, most local smog reduction measures have been directed primarily at reducing emissions of VOCs in urban areas. With fewer VOCs in the air, there are fewer compounds to interact with nitrogen oxides, and thus more nitrogen oxide will be available to recombine with ozone. More recently, regional efforts to control ozone have focused on reducing nitrogen oxide emissions, which appears to be a more effective method of controlling smog in areas way from urban centers.

Identify and describe the major air pollutants.

One of the major repositories for air pollutants is the atmosphere. Evidence appears to link air pollution across long distances. The air pollution system has many inputs, which are the sources of air pollution. It also has many outputs, which are components of the atmosphere and biosphere that remove air pollutants. Air pollution inputs can come from automobiles on the ground, airplanes in the sky, or vegetation. Similarly, air pollution can be removed or altered by vegetation, soil, and components of the atmosphere such as clouds, particles, or gases. Both the specific definition of pollution and the classification of a substance as a pollutant have evolved. The atmosphere is a public resource- in effect, a global commons- and consequently the science of air pollution is closely intertwined with political and social perspectives. The original Clean Air Act identified six pollutants that significantly threaten human well-being, ecosystem, and structures: sulfur dioxide, nitrogen oxides, carbon monoxide, particulate matter, tropospheric ozone, and lead. These were called criteria air pollutants because under the Clean Air Act, the EPA must specify allowable concentrations of each pollutant. Although carbon dioxide was not included among the major air pollutants identified in the 1970s, today it is widely accepted that carbon dioxide is altering ecosystems in a substantial way. In addition, a volatile organic compounds and mercury, though not officially listed, are commonly measured air pollutants that have the potential to be harmful. Sulfur dioxide is a corrosive gas that comes primarily from combustion of fuels. It is a respiratory irritant and can adversely affect plant tissue as well. Sulfur dioxide is also released in large quantities during volcanic eruptions and can be released, though in much smaller quantities, during forest fires. There are two nitrogen oxides. Nitrogen oxide is a colorless, odorless gas, and nitrogen dioxide is a pungent, reddish-brown gas. Motor vehicles and stationary fossil fuel combustion are the primary anthropogenic sources of nitrogen oxides. Natural sources include forest fires, lightning, and microbial action in soils. Atmospheric nitrogen oxides play a role in forming ozone and other components of smog. There are two carbon oxides. Carbon monoxide is a colorless, odorless gas that is formed during incomplete combustion of most matter, and therefore is a common emission in vehicle exhaust and most other combustion processes. Carbon monoxide can be a significant component of air pollution in urban areas. It also can be a dangerous indoor air pollutant when exhaust systems on natural gas heaters malfunction. Carbon monoxide is a particular problem in developing countries. Carbon dioxide is a colorless, odorless gas that is formed during the complete combustion of most matter. The complete combustion of matter that produces carbon dioxide is more desirable than the incomplete combustion that produces carbon monoxide and other pollutants. However, burning fossil fuels has contributed additional carbon dioxide to the atmosphere and led to its becoming a major pollutant. Particulate matter comes from the combustion of wood, animal manure and other biofuels, coal, oil, and gasoline. It is most commonly known as a class of pollutants released from the combustion of fuels. Diesel-powered vehicles give off more particulate matter than gasoline-powered vehicles. Particulate matter can also come from road dust and rock-crushing operations. Volcanoes, forest fires, and dust storms are important natural sources of particulate matter. Particulate matter also scatters and absorbs sunlight. Reduced visibility, also known as haze, occurs primarily when particulate matter from air pollution scatters light. Oxides are reactive compounds that remove electrons from other substances. Photochemical oxidants are a class of air pollutants formed as a result of sunlight acting on chemical compounds. There are many photochemical oxidants, and they are generally harmful to plant tissue, human respiratory tissue, and construction materials. However, environmental scientists frequently focus on ozone. It is harmful to both plants and animals and impairs respiratory function. In the presence of sulfur and nitrogen oxides, photochemical oxidants can enhance the formation of certain particulate matter. The resulting mixture is called smog. Smog is partly responsible for the hazy view and reduced sunlight observed in many cities. Smog can be divided into two categories. Photochemical smog is dominated by oxidants such as ozone. Sulfurous smog is dominated by sulfur dioxide and sulfate compounds. Atmospheric brown cloud is a relatively new descriptive term that has been given to the combination of particulate matter and ozone. Derived primarily from combustion of fossil fuel and burning biomass. The brownish tint that characterizes these clouds of pollution is typically caused by the presence of black or brown light absorbing carbon particles and/or nitrogen dioxide. In addition to human health problems, particulate matter and photochemical oxidants also cause economic harm. Lead is a trace metal that occurs naturally in rocks and soils. It is present in small concentrations in fuels. Lead compounds were added to gasoline for many years to improve vehicle performance. During that time, lead compounds released into the air traveled with the prevailing winds and were deposited on the ground by rain or snow. They became pervasive around the globe. Lead was phased out as a gasoline additive, and since then its concentration in the air has dropped considerably. Another persistent source of lead is lead-based paint in older buildings. Mercury is also found in coal and oil and, like lead, is toxic to the central nervous system of humans and other organisms. As a result of the release of mercury into the air, primarily from the combustion of fossil fuels, especially coal, the concentrations of mercury in both air and water have increased dramatically in recent years. Mercury concentrations in some fish have also increased. People who eat these fish increase their own mercury concentrations. Over the past 20 years, mercury emissions in the US from waste incinerators have been reduced substantially. Because coal-fired electricity generation plants remain the largest uncontrolled source of mercury, emissions standards for coal plants will likely be the focus of future regulations. Many VOCs are hydrocarbons. Compounds that give off a strong aroma are often VOCs since the chemicals are easily released into the air. VOCs play an important role in the formation of photochemical oxidants. VOCs are not necessarily hazardous; many cause no direct harm. VOCs are not currently considered a criteria air pollutant, but because they can lead to the formation of photochemical oxidants, they have the potential to be harmful and are therefore of concern to air pollution scientists. They include CO, CO2, SO2, NOx, and most suspended particulate matter. Many VOCs are also primary pollutants. Secondary pollutants are primary pollutants that have undergone transformation in the presence of sunlight, water, oxygen, or other compounds. Because solar radiation provides energy for many of these transformations, and because water is usually involved, the conversion to secondary pollutants occurs more rapidly during the day and in wet environments. Ozone is an example of a secondary pollutant. While trying to control secondary pollutants, it is necessary to consider the primary pollutants that create them, as well as factors that may lead to the breakdown or reduction in the secondary pollutants themselves.

Describe innovative pollution control measures.

A number of cities around the world, including those in China, Mexico, and England, have taken innovative and often controversial measures to reduce smog levels. Both urban and suburban areas have taken additional actions such as calling for a reduction in the use of wood-burning stoves or fireplaces that would reduce emissions of not only nitrogen oxide but also particulate matter, VOCs, and carbon monoxide. Since cars are responsible for large emissions of nitrogen oxides and VOCs in urban areas, and these two compounds are the major contributors to smog formation, some municipalities have tried to achieve lower smog concentrations by restricting automobile use. Limiting automobile use has also helped to reduce other air pollutants. Carpool lanes, available in many areas, reduce the number of cars on the road by encouraging two or more people to share one vehicle. Improving the quality and accessibility of public transportation encourages people to leave their cars at home. In 1990 and again in 1995, scientists, policy makers, and academics collaborated on amendments to the Clean Air Act that would allow the free market to determine the least expensive ways to reduce emissions of sulfur dioxide. The free-market program was implemented in two phases between 1995 and 2000. One of the most innovative aspects of the Clean Air Act amendments was the provision for the buying and selling of allowances that authorized the owner the owner to release a certain quantity of sulfur. Each allowance authorizes a power plant or industrial source to emit one ton of SO2 during a given year. At the end of a given year, the emitter must possess a number of allowances at least equal to its annual emissions. Sulfur allowances can be brought and sold on the open market by anyone. If emitters wanted to exceed their allowance, they would be required to purchase more allowances from another source. If, on the other hand, a company decreased its sulfur emissions more than it needed to in order to comply with its allowance amount, it could sell any unused sulfur emission allowances.

Describe how acid deposition forms and why it has improved in the US and become worse elsewhere.

All rain is naturally somewhat acidic; the reaction between water and atmospheric carbon dioxide lowers the pH of precipitation from neutral 7.0 to 5.6. Acid deposition is largely the result of human activity, although natural processes may also contribute to its formation. Nitrogen oxides and sulfur dioxide are released into the atmosphere by natural and anthropogenic combustion processes. Through a series of reactions with atmospheric oxygen and water, these primary pollutants are transformed into the secondary pollutants nitric acid and sulfuric acid. These latter compounds break down further, producing nitrate, sulfate, and hydrogen ions that generate the acidity in acid deposition. These transformations occur over a number of days. Eventually, these secondary acidifying pollutants are washed out of the air and deposited either as precipitation or in dry form on vegetation, soil, or water. Acid deposition has been reduced in the US as a result of lower sulfur dioxide and nitrogen oxide emissions. It had a variety of effects on materials, on agricultural, and on both aquatic and terrestrial natural habitats. Effects of acid deposition may be direct, such as a decrease in the pH of lake water, or indirect. It is often difficult to determine whether an effect is direct or indirect, making remediation challenging. The greatest effects of acid deposition have been on aquatic ecosystems. Lower pH of lakes and streams has caused decreased species diversity of aquatic organisms. Lower pH can also lead to mobilization of metals, an indirect effect. When this happens, metals bound in organic or inorganic compounds in soils and sediments are released into surface water. Because metals such as aluminum and mercury can impair the physiological functioning of aquatic organisms, exposure can lead to species loss. Decreased pH can also affect the food sources of aquatic organisms, creating indirect effects at several trophic levels. People are not harmed by direct contact with precipitation at the acidities commonly experienced in the US or elsewhere in the world because human skin is a sufficiently robust barrier. Human health is more affected by the precursors to acid deposition such as sulfur dioxide and nitrogen dioxides. Acid deposition can, however, harm human-built structures such as statues, monuments, and buildings. The damage happens because acid deposition reacts with building materials. When the hydrogen ion in acid deposition interacts with limestone or marble, the calcium carbonate reacts with H+ and gives off Ca2+. In the process, the calcium carbonate material is partially dissolved. The more acidic the precipitation, the more hydrogen ions there are to interact with the calcium carbonate.

Explain how indoor air pollution differs in develop and developed countries.

Biomass and coal are usually burned in open-pit fires that lack the proper mix of fuel and air to allow complete combustion. Usually, there is no exhaust system and little or no ventilation available in the home, which makes indoor air pollution from carbon monoxide and particulates a particular hazard in developing countries. Exposure to indoor air pollution from cooking and heating increases the risk of acute respiratory infections, pneumonia, bronchitis, and even cancer. There are a number of factors that have caused the quality of air in homes in developed countries to take on greater importance. First of all, people in much of the developed world have begun to spend more and more time indoors. Although, improved insulation and tighlty sealed building envelopes reduce energy consumption, these tightly sealed buildings also keep existing air in contact with the inhabitants of homes, schools, and offices for greater amounts of time. Finally, an increasing number of materials in the home and office are made from greater plastics and other petroleum-based materials that can give off chemical vapors.

Explain efforts to reduce ozone depletion.

In response to the decrease in stratospheric ozone, 24 nations in 1987 signed the Montreal Protocol on Substances That Deplete the Ozone Layer. This was commitment to reduce CFC production by 50% by the year 2000. Global CFC exporters like the US appeared in some ways to prioritize the protection of the global biosphere over their short-term economic self-interest. The protocol addressed 96 ozone-depleting compounds. Because of these efforts, the concentration of chlorine in the stratosphere has stabilized at about 5 ppb and should fall to about 1 ppb by 2100. The chlorine concentration reduction process is slow because CFCs are not easily removed from the stratosphere. However, with the leveling off of chlorine concentrations, stratospheric ozone depletion should decrease in subsequent decades. The number of additional skin cancers should eventually decrease as well, although this effect will take some time due to the long time it takes for these cancers to develop.

Describe the depletion of stratospheric ozone.

In the mid-1980s, atmospheric researchers noticed that stratospheric ozone in Antarctica has been decreasing each year. Depletion was greatest at the poles, but occurred worldwide. Researchers also determined that, in the Antarctic, ozone depletion was seasonal: each year the depletion occurred from roughly August through November. The depletion caused an area of severely reduced ozone concentrations over most of Antarctica, creating what has come to be called the "ozone hole". The depletion of ozone also occurs over the Arctic in January through April, but it is not as severe, varies more from year to year, and does not cause a "hole" as in the Antarctic. The cause of the formation of the ozone hole, which has received a great deal of media attention and has been studied intensively, is complex. It appears that extremely cold weather conditions during the polar winter cause a buildup of ice crystals mixed with nitrogen oxide. This in turn provides the perfect surface for the formation of the stable molecule Cl2, which accumulates as atmospheric chlorine interacts with the ice crystals. When the sun reappears in the spring, UV radiation breaks down this molecule into Cl again, which in turn catalyzes the destruction of ozone. Because almost no ozone forms in the dark of the polar winter, a large "hole" occurs. Only after the temperatures warm up and the chlorine gets diluted by air coming from outside the polar region does the hole diminish. In contrast, the overall global trend of decreasing stratospheric ozone concentration is not related to temperature, but is caused by the breakdown reactions that result from increased concentration of chlorine in the atmosphere. Decreased stratospheric ozone has increased the amount of UV-B radiation that reaches the surface of Earth. For plants, both on land and in water, increasing exposure to UV-B radiation can be harmful to cells and can reduce photosynthetic activity, which could have an adverse impact on ecosystem productivity, among other things. In humans, particularly those with light skin, increasing exposure to UV-B radiation is correlated with increased risks of skin cancers, cataracts, and other eye problems, and with a suppressed immune system.

Describe the major indoor air pollutants and the risks associated with them.

Many other consumer products such as detergents, dry-cleaning fluids, deodorizers, and solvents may contain VOCs and can be harmful if inhaled. Plastics, fabrics, construction materials, and carpets may also release VOCs over time. In newer buildings in developed countries in the temperate zone, more and more attention is given to insulation and prevention of air leaks in order to reduce the amount of heating or cooling necessary for a comfortable existence. This reduces energy use but may have the unintended side effect of allowing the buildup of toxic compounds and pollutants in an airtight space. Because new buildings contain many products made with synthetic materials and glues that may not have fully dried out, a significant amount of off-gassing occurs, which usually means that the indoor levels of VOCs, hydrocarbons, and other potentially toxic materials are quite high. Sick building syndrome has been observed particularly in office buildings, where large number of workers have reported a variety of maladies such as headaches, nausea, throat or eye irritations, and fatigue. The EPA has identified four specific reasons for sick building syndrome: inadequate or faulty ventilation; chemical contamination from indoor sources; chemical contamination in the building from outdoor sources; and biological contamination from inside or outside.

Describe the sources of air pollution.

Volcanoes, lightning, forest fires, and plants both living and dead all release compounds that can be classified as pollutants. Volcanoes release sulfur dioxide, particulate matter, carbon monoxide, and nitrogen oxides. Lightning strikes create nitrogen oxides from atmospheric nitrogen. Forest fires release particulate matter, nitrogen oxides, and carbon monoxide. Living plants release a variety of VOCs, including ethylene and terpenes. Long before anthropogenic pollution was common, the natural VOCs from plants gave rise to smog and photochemical oxidant pollution. Large nonindustrial areas such as agricultural fields can give rise to particulate matter when they are plowed. The effects of these various compounds, especially when major natural events occur, depend in part on natural conditions. In contrast to natural emission, emissions from human activity are monitored, regulated, and in many cases controlled. On-road vehicles, also referred to as the general category of transportation, are the largest sources of carbon monoxide and nitrogen oxides. Electricity generation is the major source of anthropogenic sulfur dioxide. Particulate matter comes from a variety of sources including natural and human-made fires, road dust, and the generation of electricity. The Clean Air Act and its various amendments require that the EPA establish standards to control pollutants that are harmful to "human health and welfare". The term human health means the health of the human population and includes the elderly, children, and sensitive populations. The term welfare refers to visibility, the status of crops, natural vegetation, animals, ecosystems, and buildings. Through the National Ambient Air Quality Standards, the EPA periodically specifies concentrations limits for each pollutants. Each year, the EPA issues a report that shows the national level of the six criteria air pollutants relative to the published standards.