Section Goals:

1. Define air pollution.
2. Correlate weather conditions with air quality.
3. Review concerning air pollutants.
4. Introduce the Clean Air Act of the United States.

Air Pollution

Air pollution occurs in many forms but can generally be thought of as gaseous and particulate contaminants that are present in the earth’s atmosphere. Chemicals discharged into the air that have a direct impact on the environment are called primary pollutants. These primary pollutants sometimes react with other chemicals in the air to produce secondary pollutants.

Air pollution  is typically separated into two categories: outdoor air pollution and indoor air pollution. Outdoor air pollution involves exposures that take place outside of the built environment. Examples include fine particles produced by the burning of coal, noxious gases such as sulfur dioxide, nitrogen oxides and carbon monoxide; ground-level ozone and tobacco smoke. Indoor air pollution involves exposures to particulates, carbon oxides, and other pollutants carried by indoor air or dust. Examples include household products and chemicals, out-gassing of building materials, allergens (cockroach and mouse dropping, mold, pollen), and tobacco smoke.

Sources of Air Pollution

A stationary source of air pollution refers to an emission source that does not move, also known as a point source. Stationary sources include factories, power plants, and dry cleaners. The term area source is used to describe many small sources of air pollution located together whose individual emissions may be below thresholds of concern, but whose collective emissions can be significant. Residential wood burners are a good example of a small source, but when combined with many other small sources, they can contribute to local and regional air pollution levels. Area sources can also be thought of as non-point sources, such as construction of housing developments, dry lake beds, and landfills.

A mobile source of air pollution refers to a source that is capable of moving under its own power. In general, mobile sources imply “on-road” transportation, which includes vehicles such as cars, sport utility vehicles, and buses. In addition, there is also a “non-road” or “off-road” category that includes gas-powered lawn tools and mowers, farm and construction equipment, recreational vehicles, boats, planes, and trains.

Agricultural sources arise from operations that raise animals and grow crops, which can generate emissions of gases and particulate matter. For example, animals confined to a barn or restricted area produce large amounts of manure. Manure emits various gases, particularly ammonia into the air. This ammonia can be emitted from the animal houses, manure storage areas, or from the land after the manure is applied. In crop production, the misapplication of fertilizers, herbicides, and pesticides can potentially result in aerial drift of these materials and harm may be caused.

Unlike the above mentioned sources of air pollution, air pollution caused by natural sources is not caused by people or their activities. An erupting volcano emits particulate matter and gases, forest and prairie fires can emit large quantities of “pollutants”, dust storms can create large amounts of particulate matter, and plants and trees naturally emit volatile organic compounds which can form aerosols that can cause a natural blue haze. Wild animals in their natural habitat are also considered natural sources of “pollution”.

Air Quality is Influenced by Weather

Some types of pollution are worse in the summer heat, while others are worse in cold winter weather. The same atmospheric conditions that create weather — air pressure, temperature, and humidity — also affect air quality.

Because air is almost always on the move, air pollution is easily transported from one area to another. For example, studies in the 1980s found that sulfur dioxide from coal burning in the Ohio Valley was carried across large distances by the wind. This caused acid rain to fall on regions of the eastern US and Canada up to a thousand miles away. Acid rain causes “dead lakes,” where the acidic lake conditions prevent fish eggs from developing. Similar effects from coal burning have been found in Europe.

In Asia, powerful spring winds carry clouds of industrial pollutants from China across the Gobi Desert (Figure 1). As the contaminated winds cross the desert, they pick up particle pollution as well, causing massive yellow dust storms across the Korean Peninsula and parts of Japan. These yellow dust storms reduce visibility, damage plants and soils, and pose significant health risks to humans.

Low-pressure systems bring wet and windy conditions. A passing storm front can wash pollutants out of the atmosphere or transport them to a new area, producing clear skies. It is important to note, however, that the pollutants are not really gone — rather, they have been moved to a new location.

The opposite is true of high-pressure systems, which can create stagnant air. When the air stops moving, pollutants such as vehicle and factory exhaust concentrate over an area.

Cold Weather

When the weather is cold, exhaust from vehicles, chimneys, and smokestacks is more visible. Does this mean that there are more pollutants in the air, or just that the warm vapor exhaust is more visible? Typically, both are true. While industrial emissions remain mostly constant throughout the year, particulate matter and carbon monoxide pollutants from wood burning increase during the cold winter months. Idling cars to defrost or keep them warm increases the amount of air pollution as well.

Typically, warm rising air near the ground lifts pollution away, but during the winter the layer of warm air acts like a lid, keeping cold air at the surface. This creates a thermal inversion, which forms when a layer of warm air above traps cool air and pollution close to the ground (Figure 2). Thermal inversions are more common above cities where cold, dense air gets trapped in mountain basins or valleys, such as Los Angeles, Denver, and Mexico City.

Hot Weather

Some pollution, such as ground-level ozone, is made more efficiently in sunny, hot weather. The reactions that create harmful ozone in our atmosphere require sunlight. In the summers and especially during extreme heat waves, ozone often reaches dangerous levels in cities or nearby rural areas.

Thankfully, humidity can help to decrease ozone pollution. Afternoon thunderstorm clouds block sunlight, causing ozone production to slow down for the day, while moisture from the storm destroys the ozone that has formed.

Heat waves often lead to poor air quality. The extreme heat and stagnant air during a heat wave increase the amount of ozone pollution and particulate pollution. Drought conditions can also occur during a heat wave, meaning that soils are very dry. During a drought, forest fires are more common. Fires add carbon monoxide and particle pollution to the atmosphere.

Six Common Air Pollutants

The most commonly found air pollutants are particulate matter, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. These pollutants can harm health and the environment, and cause property damage. Of the six pollutants, particle pollution and ground-level ozone are the most widespread health threats. The U.S. Environmental Protection Agency (EPA) regulates them by developing criteria based on considerations of human and environmental health.

Ground-level ozone is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC. Breathing ozone can trigger a variety of health problems, particularly for children, the elderly, and people of all ages who have lung diseases such as asthma. Ground level ozone can also have harmful effects on sensitive vegetation and ecosystems. Ground-level ozone should not be confused with the ozone layer, which is high in the atmosphere and protects Earth from ultraviolet light; ground-level ozone provides no such protection.

Particulate matter, also known as particle pollution, is a complex mixture of extremely small particles and liquid droplets. Particle pollution is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles. The size of particles is directly linked to their potential for causing health problems. EPA is concerned about particles that are 10 micrometers in diameter or smaller because those are the particles that generally pass through the throat and nose and enter the lungs. Once inhaled, these particles can affect the heart and lungs and cause serious health effects.

Carbon monoxide (CO) is a colorless, odorless gas emitted from combustion processes.  Nationally and, particularly in urban areas, the majority of CO emissions to ambient air come from mobile sources.  CO can cause harmful health effects by reducing oxygen delivery to the body’s organs (like the heart and brain) and tissues. At extremely high levels, CO can cause death.

Nitrogen dioxide (NO₂) is one of a group of highly reactive gasses known as “oxides of nitrogen,” or nitrogen oxides (NOx). Other nitrogen oxides include nitrous acid and nitric acid. EPA’s National Ambient Air Quality Standard uses NO₂ as the indicator for the larger group of nitrogen oxides. NO2 forms quickly from emissions from cars, trucks and buses, power plants, and off-road equipment. In addition to contributing to the formation of ground-level ozone, and fine particle pollution, NO2 is linked with a number of adverse effects on the respiratory system.

Sulfur dioxide (SO₂) is one of a group of highly reactive gasses known as “oxides of sulfur.”  The largest sources of SO₂ emissions are from fossil fuel combustion at power plants (73%) and other industrial facilities (20%).  Smaller sources of SO₂ emissions include industrial processes such as extracting metal from ore, and the burning of high sulfur containing fuels by locomotives, large ships, and non-road equipment. SO₂ is linked with a number of adverse effects on the respiratory system.

Lead is a metal found naturally in the environment as well as in manufactured products. The major sources of lead emissions have historically been from fuels in on-road motor vehicles (such as cars and trucks) and industrial sources.  As a result of regulatory efforts in the U.S. to remove lead from on-road motor vehicle gasoline, emissions of lead from the transportation sector dramatically declined by 95 percent between 1980 and 1999, and levels of lead in the air decreased by 94 percent between 1980 and 1999. Today, the highest levels of lead in air are usually found near lead smelters. The major sources of lead emissions to the air today are ore and metals processing and piston-engine aircraft operating on leaded aviation gasoline.

Indoor Air Pollution (Major concerns in developed countries)

Most people spend approximately 90 percent of their time indoors. However, the indoor air we breathe in homes and other buildings can be more polluted than outdoor air and can increase the risk of illness. There are many sources of indoor air pollution in homes. They include biological contaminants such as bacteria, molds and pollen, burning of fuels and environmental tobacco smoke, building materials and furnishings, household products, central heating and cooling systems, and outdoor sources. Outdoor air pollution can enter buildings and become a source of indoor air pollution.

Sick building syndrome is a term used to describe situations in which building occupants have health symptoms that are associated only with spending time in that building. Causes of sick building syndrome are believed to include inadequate ventilation, indoor air pollution, and biological contaminants. Usually indoor air quality problems only cause discomfort. Most people feel better as soon as they remove the source of the pollution. Making sure that your building is well-ventilated and getting rid of pollutants can improve the quality of your indoor air.

The Clean Air Act in the United States

In 1970, fueled by persistent visible smog in many U.S. cities and industrial areas and an increase in health problems caused by air pollution, the Clean Air Act paved the way for numerous efforts to improve air quality in the United States. The Clean Air Act requires the Environmental Protection Agency (EPA) to set air quality standards for several hazardous air pollutants reported in the Air Quality Index (AQI), requires states to have a plan to address air pollution and emissions reduction, and also addresses problems such as acid rain, ozone holes, and greenhouse gas pollution which is causing the climate to warm.

Attribution

Essentials of Environmental Science by Kamala Doršner is licensed under CC BY 4.0. Modified from the original by Matthew R. Fisher and Joni Baumgarten.

Air Quality Solutions by UCAR Center for Science Education is licensed for non-commercial, non-profit research or educational purposes only.

How Weather Affects Air Quality by UCAR Center for Science Education is licensed for non-commercial, non-profit research or educational purposes only.