Particulates can be removed from smokestack emissions by which of the following methods?

Air pollutants cause air pollution in high concentrations. They can take the form of solid particles (such as particulate matter, which is composed of solid or liquid particles from unburnt matter emitted from combustion), liquid droplets (such as acid rain), gases (such as sulfur dioxide and carbon monoxide), or a combination thereof (including some forms of particulate matter that are made up of solid or liquid molecules suspended in gas). Air pollutants come from natural and human sources, and natural and human activity can increase air pollutant concentrations to a level that may cause visibility problems, health problems in people, and/or damage natural ecosystems.[1]

Naturally occurring pollutants include ash, soot, sulfur dioxide, ground-level ozone (also known as smog), salt spray, volcanic and combustion gases, and radon. These pollutants are released during volcanic eruptions, forest fires, and grass fires. Human-caused pollutants include naturally occurring pollutants released from human activity: carbon monoxide, sulfur dioxide, nitrogen dioxide, and particulate matter. They are released from coal, oil, and natural gas use, electric power plants, factories, and motor vehicles.[1]

Background

Definition

In atmospheric science, air pollutants are classified as either primary or secondary pollutants. Primary air pollutants come from a specific process and source. Examples include ash and sulfur dioxide from a volcanic eruption, carbon monoxide from motor vehicles, or nitrogen oxide or sulfur dioxide from industrial activities. Secondary pollutants are not emitted directly but are formed when primary pollutants interact with each other and other substances in the air. For example, ground-level ozone (also known as smog) is a prominent secondary pollutant created when nitrogen oxides combine with volatile organic chemicals—gases produced by burning coal, oil, or wood—and sunlight. Ground-level ozone differs from atmospheric ozone, which shields the Earth from the sun’s ultraviolet radiation.[1]

A complete description of various air pollutants can be found below.

Types of pollutants

Six air pollutants, known as criteria pollutants, are regulated by the federal government regulates under the Clean Air Act: ground-level ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, lead, and particulate matter (known as fine particle pollution).

• Ozone is a naturally occurring gas in the Earth's stratosphere. Ozone absorbs ultraviolet radiation from the sun. Without ozone to absorb ultraviolet radiation, all animal life, including human life, would be more at-risk for cancer, damaged immune systems, and eye problems. Ozone is created when oxygen is split apart by sunlight into single atoms. These atoms rejoin with other oxygen molecules to make ozone. The natural level of ozone in the stratosphere is a result of the balance between sunlight that creates ozone and the natural and human-made chemical reactions that destroy it. While ozone in the atmosphere shields life on Earth from harmful radiation, ground-level ozone (commonly known as smog) can harm plants, animals, and human beings. Ground-level ozone forms when nitrogen oxide gases from motor vehicles and industrial emissions react with volatile organic compounds, which are carbon-containing chemicals that evaporate into the air. Short-term exposure to a higher concentration of ground-level ozone (an average of one hour) can temporarily affect eyes, lungs, and the respiratory tract while long-term exposure (6 hours or more) to higher concentrations of ozone (80 parts per billion or higher) can reduce lung function.[2][3][4]

A view of smog in southern Los Angeles in September 2011.

• Carbon monoxide is a colorless, odorless, and tasteless gas. It is present in small amounts as a product of volcanic activity. It is also a product of natural and human-made fires (such as forest fires and bushfires). Human-made carbon monoxide is produced during incomplete combustion, a process where fuel such as gasoline, kerosene, wood, oil, coal, and charcoal is partially burned primarily due to a lack of oxygen. Internal combustion engines such as portable generators, cars, lawn mowers, and power washers can also produce carbon monoxide. At concentrations above 35 parts per million (ppm), carbon monoxide is toxic to animals, including human beings, which have hemoglobin in their red blood cells. When combined with hemoglobin, carbon monoxide produces carboxyhemoglobin, which takes over the space normally occupied by oxygen. A level of 50 percent carboxyhemoglobin in the blood can produce seizures, comas, and fatalities.[5][6]
• Nitrogen dioxide is a naturally occurring light brown gas that is one of several nitrogen oxides, which are binary compounds of oxygen and nitrogen. It is introduced into the atmosphere by bacteria, lightning, and volcanoes. Nitrogen dioxide absorbs sunlight and regulates the chemistry of the troposphere, which helps determine ozone concentrations. Air pollution can occur when nitrogen dioxide in the air combines with ozone and particulate matter. Nitrogen dioxide at high concentrations can irritate the human respiratory system. Long-term exposure to nitrogen dioxide may decrease lung function and increase the risk of respiratory symptoms, such as acute bronchitis. Human-made sources of nitrogen dioxide include power plants, trucks, cars, and buses, home heaters, and gas stoves.[7][8][9][10][11][12]

A sulfur dioxide plume at night

• Sulfur dioxide is a heavy, colorless, and poisonous gas or liquid. It is naturally occurring and found in volcanoes and in the waters of some hot springs. Sulfur dioxide is produced by the burning of sulfur during industrial activities, including the burning of coal, oil, and other fossil fuels, and by the smelting of mineral ores that contain sulfur. Sulfur dioxide is released into the atmosphere by electric utilities that use coal or petroleum. Other sources include petroleum refineries, paper pulp manufacturing, cement manufacturing, and metal smelting and processing facilities. In a home, sulfur dioxide can be produced from improperly vented gas appliances such as stoves, furnaces, clothes dryers, wood stoves, and automobile exhaust. In the atmosphere, sulfur dioxide can combine with water vapor to produce sulfuric acid, a major component of acid rain.[13][14]
• Lead is a heavy metal found naturally in the Earth’s crust. It is also found in manufactured products. Lead is released naturally into the air as particles and into the soil and water through erosion, volcanic eruptions, bushfires, and sea spray. The natural concentration of lead in the air is less than 0.1 microgram per cubic meter. Human-made sources of lead air emissions include lead smelters, waste incinerators, utilities, metal processing, copper smelters, glass manufacturers, and industrial boilers, among others. Human beings can be exposed to lead emissions directly through inhalation of lead from the air itself or the ingestion of lead that has settled onto solid surfaces. Once it is taken into the body, lead distributes itself throughout the human body and accumulates in the bones. At high levels of exposure, lead can harm the nervous system, the immune system, and the cardiovascular system. The harmful effects of lead include neurological effects in children, which can contribute to learning disabilities, lowered IQ, and behavioral problems. Adults may experience high blood pressure and heart disease when exposed to high levels of lead.[15][16]

The image above compares the size of particulate matter to fine beach sand and a strand of human hair (click to enlarge).

• Particulate matter is a mix of liquid droplets and small particles, which can be made up of metal, dust, soil, organic chemicals, or acids. Particles come in different sizes. The two federally regulated particles include PM10 and PM2.5. PM10 consists of particles that are less than or equal to 10 micrometers in diameter. PM2.5 is made up of fine particles that are less than or equal to 2.5 micrometers in diameter. As a comparison, the average diameter of human hair is 70 micrometers. Particulate matter is directly emitted into the air during combustion processes or by windblown dust. It can also be formed in the atmosphere when particles react with sulfur dioxide and nitrogen dioxide. The health effects of particulate matter are related to the size of its particles. Small particles that are 2.5 micrometers in diameter or less can be inhaled deeper into the lungs. High concentrations of these particles have been linked to decreased lung function, aggravated asthma, irregular heartbeats, increased respiratory problems such as coughing or difficulty breathing, and premature death in people with lung or heart disease. Particulate matter can contribute to haze, which reduces visibility. The particulate matter absorbs and reflects sunlight, reducing air clarity. Particulate matter in the air can deposit on soil and water and may harm soil, crops, and ecosystems.[17][18]

Pollution over time

From 1970 to 2014, aggregate national emissions of the six criteria pollutants (carbon monoxide, ground-level ozone, lead, nitrogen dioxide, particulate matter, and sulfur dioxide) dropped by an average of 69 percent, according to the U.S. Environmental Protection Agency (EPA).[19]

The nationwide average levels of nitrogen dioxide, sulfur dioxide, and carbon monoxide declined from 1984 to 2007. During this period, nitrogen dioxide declined by 31 percent, sulfur dioxide by 61 percent, and carbon monoxide by 70 percent.[20]

Nitrogen dioxide pollution, averaged yearly from 2005-2011, has decreased across the United States, including New York and New Jersey.

In a 2014 study, the National Aeronautics and Space Administration (NASA) found that nitrogen dioxide pollution, averaged yearly from 2005-2011, decreased across the United States. The study found that areas with a high level of nitrogen dioxide had decreased by an average of 40 percent during that period. Satellite data from between the 2005-2007 and 2009-2011 periods showed that several U.S. cities saw a decline in nitrogen dioxide, including Atlanta, Georgia (42 percent decrease), New York City (32 percent decrease), and Denver, Colorado (22 percent decrease). NASA scientists attributed the decline to more efficient motor vehicles and improved technology at industrial facilities, such as scrubbers on smokestack emissions that remove toxins.[21][22]

Ground-level ozone (smog) levels declined from the 1970s to 2006. At the end of 2006, 94 percent of U.S. monitoring locations complied with the federal one-hour ozone standard, which was up from the approximately 20 percent attainment rate during the late 1970s. At the end of 2006, 85 percent of U.S. monitors complied with the 8-hour standard, which was up from 20 percent in the late 1970s. Though ground-level ozone has decreased since the 1970s, levels declined at a slower pace during the 1990s and sometimes increased in some areas, particularly in California.[20][23][24]

Though overall air pollution decreased from the 1970s to 2007, other reports concluded that a sizable portion of the U.S. population is susceptible to unhealthy levels of ground-level ozone or particle pollution. In its annual study entitled State of the Air, the American Lung Association (ALA), who stated mission is "to save lives by improving lung health and preventing lung disease through education, advocacy and research," reported in 2016 that 52.1 percent of the U.S. population—approximately 166 million people—"live in counties that have unhealthful levels of either [ground-level] ozone or particle pollution." In 2015, the ALA reported the number as nearly 138.5 million people—around 44 percent of the U.S. population.[25][26]

Critics of the ALA's study argue that the organization's reports misconstrue actual air pollution levels and trends by using a faulty methodology. According to a study on air quality in the United States from American Enterprise Institute, whose stated mission is "making the intellectual, moral, and practical case for expanding freedom, increasing individual opportunity, and strengthening the free enterprise system in America and around the world," the ALA inflates the number of people affected by air pollution by "counting everyone in a county as breathing air that exceeds federal standards, even if most of the county has clean air." One example is found in the ALA's 2007 report. ALA's report found that 10 million people in Los Angeles County were exposed to polluted air, even though six million residents lived in areas that were at or below the eight-hour federal ozone standard. Another example is found in the ALA's 2006 report. The ALA gave Cook County, Illinois and Maricopa County, Arizona, failing grades for their ozone levels even though the counties complied with the eight-hour ozone standard in 2004 and maintained compliance in 2005.[20][27]

Pollution reduction technologies

An irrigated cyclone scrubber is an example of an air pollution control device.

Air pollution control technologies are used to control, capture, or otherwise reduce emissions at power plants, factories, steel mills, cement plants, refineries, and other facilities. The following methods are used to control or reduce emissions:[28][29]

• Thermal or catalytic combustion, which destroys pollutants by incinerating them at high temperatures
• Use of chemical reactions to convert pollutants into less harmful forms, such as the use of ammonia to change nitrogen oxides into water and nitrogen
• Scrubbing, which involves using liquid to remove particles, vapors, or gases from exhaust
• Adsorption, which involves attracting vapors, particles, or gases onto solid surfaces that are then stripped of pollutants using heat and/or steam
• Use of fabric filters, which filter particle-filled gases and capture particles on a fabric surface
• Use of cylinders (also known as cyclones), which create centrifugal forces to throw particles against walls and to drop the particles onto a surface below

Regulation

Air pollution regulation was a state and local issue for most of the 20th century. Prior to 1955, state and local governments enacted laws aimed at reducing air pollution caused by industrialization and growing communities. In 1881, Chicago, Illinois, and Cincinnati, Ohio, enacted smoke control ordinances targeting smoke emissions from industrial facilities and the burning of coal. In 1904, Philadelphia, Pennsylvania, passed an ordinance regulating smoke from chimneys and smokestacks. In 1947, California enacted the Air Pollution Control Act, which established air pollution control districts in each county in the state.[30][31][32]

1955: Air Pollution Control Act

In 1955, Congress passed the Air Pollution Control Act. The legislation stated that air pollution was a problem and emphasized that the issue should be handled by state and local governments. The law did not create any federal regulatory measures or controls on air pollution. In addition, the law allowed the federal government to conduct research to understand the causes and effects of air pollution. State and local governments received technical assistance to address pollution, and the federal government provided $5 million per year for five years for federal research on air pollution.[30]

1960s: Federal air pollution regulation

President Lyndon B. Johnson (D) signs the Air Quality Act of 1967.

At the request of President John F. Kennedy (D), Congress passed the Clean Air Act in 1963. The 1963 statute gave the federal government authority to regulate interstate air pollution caused primarily by the combustion of petroleum and coal. Under the act, state and local governments received a total of $96 million over three years to conduct air pollution research and implement local pollution control programs.[30]

Federal involvement in air pollution matters continued in the 1960s. In 1965, Congress passed the Motor Vehicle Pollution Control Act, which established federal emissions standards for new motor vehicles. The Air Quality Act of 1967 required the federal government to establish emissions standards for stationary sources of air pollution, such as industrial facilities. The 1967 act further required the federal government to divide the United States into regions in order to plan, monitor, and control air pollution in specific areas.[30]

Congress passed the National Environmental Policy Act in 1969. The statute set forth a national environmental policy agenda focusing on "the critical importance of restoring and maintaining environmental quality to the overall welfare and development of man" by using "all practicable means and measures. ... to create and maintain conditions under which man and nature can exist in productive harmony." In 1970, President Richard Nixon (R) issued an executive order to create the U.S. Environmental Protection Agency (EPA). At the time, the EPA received all existing environmental regulatory powers found in other federal agencies and departments.[30][33]

1970 and 1977: Clean Air Act Amendments

In 1970, Congress passed the Clean Air Act Amendments, which revised the original Clean Air Act. The 1970 amendments required the EPA to set National Ambient Air Quality Standards (NAAQS), which are nationwide federal standards for six air pollutants: particulate matter, ground-level ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide, and lead. The EPA also set emissions standards for hazardous air pollutants. The amendments required that states submit a plan for attaining and maintaining federal air quality standards. The Clean Air Act Amendments of 1977 allowed the EPA to review and update federal air pollution standards every five years.[30]

1990: Clean Air Act Amendments

The Clean Air Act was amended 1990. The 1990 amendments gave the federal government authority to issue regulations related to acid rain, motor vehicle emissions, and hazardous air pollutants. The federal government established statutory deadlines for reducing smog in certain areas. The amendments also required new emissions standards for motor vehicles beginning with the 1995 model year. In addition, the federal government issued regulations targeting hazardous air pollutants at previously unregulated industries and activities.[30]

1. ↑ 1.0 1.1 1.2 Synonym, "What Is the Difference Between Human & Natural Air Pollution?" accessed September 18, 2016
2. ↑ National Oceanic Atmospheric Administration, "National Air Quaility Forecast Capacity FAQ," accessed October 24, 2016
3. ↑ National Aeronautics and Space Administration, "What is Ozone?" accessed September 19, 2016
4. ↑ University of Utah, "Ozone in the Atmosphere," accessed October 24, 2016
5. ↑ Universal Security Instruments, "What are some common sources of carbon monoxide (CO)?" accessed September 16, 2016
6. ↑ U.S. Consumer Product Safety Commission, "Carbon Monoxide Questions and Answers," accessed September 16, 2016
7. ↑ U.S. Environmental Protection Agency, "Integrated Science Assessment for Oxides of Nitrogen – Health Criteria (2016 Final Report)," accessed October 24, 2016
8. ↑ World Health Organization, "Air Quality Trends – Global Update 2005," accessed October 24, 2016
9. ↑ Ontario Ministry of the Environment and Climate Change, "Nitrogen dioxide," accessed September 16, 2016
10. ↑ Georgia Department of Natural Resources, "Information about Nitrogen Dioxide (NO2)," accessed September 16, 2016
11. ↑ Green Facts, "Air Pollution Nitrogen Dioxide," accessed September 15, 2016
12. ↑ Australian Department of Environment and Energy, "Nitrogen dioxide (NO2)," accessed September 15, 2016
13. ↑ Encyclopedia Britannica, "Sulfur dioxide," accessed September 16, 2016
14. ↑ Wisconsin Department of Health Services, "Sulfur dioxide," accessed September 16, 2016
15. ↑ Australian Department of Environment and Energy, "Lead," accessed September 16, 2016
16. ↑ Texas Commission on Environmental Quality, "Air Pollution from Lead," accessed September 16, 2016
17. ↑ Texas Commission on Environmental Quality, "Air Pollution from Particulate Matter," accessed September 16, 2016
18. ↑ Green Facts, "Air Pollution Particulate Matter," accessed September 16, 2016
19. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named EPAstats
20. ↑ 20.0 20.1 20.2 American Enterprise Institute, "Air Quality in America," accessed September 20, 2016
21. ↑ National Aeronautics and Space Administration, "New NASA Images Highlight U.S. Air Quality Improvement," accessed September 20, 2016
22. ↑ CNN, "NASA data shows nationwide air improvement -- but still more needed," June 27, 2014
23. ↑ Washington Examiner, "EPA tries to appease green groups mad about ozone rules," October 1, 2015
24. ↑ Texas Commission on Environmental Quality, "Will EPA’s Proposed New Ozone Standards Provide Measurable Health Benefits?" October 9, 2014
25. ↑ American Lung Association, "State of the Air 2015," accessed September 16, 2016
26. ↑ American Lung Association, "State of the Air 2016," accessed September 16, 2016
27. ↑ American Enterprise Institute, "American Lung Association’s 2007 Report Distorts Air Quality Facts," July 1, 2007
28. ↑ Air & Waste Management Association, "Fact Sheet: Air Pollution Emission Control Devices for Stationary Sources," accessed June 13, 2017
29. ↑ Energy Projects Limited, "Air Pollution Control Equipment," accessed June 13, 2017
30. ↑ 30.0 30.1 30.2 30.3 30.4 30.5 30.6 U.S. Department of Transportation, "An overview of federal air quality legislation as related to federally-assisted highway and transit programs," accessed September 16, 2016
31. ↑ Southwest Ohio Air Quality Agency, "The History of Air Pollution Control in Cincinnati, Ohio," accessed October 24, 2016
32. ↑ Journal of the Air Pollution Control Association, "History of Air Pollution Control Association," accessed October 24, 2016
33. ↑ Energy.gov, "The National Environmental Policy Act of 1969, as amended," accessed August 14, 2014