Brian Barton
Photochemical oxidants are a major air pollutant, threatening vegetation and human health worldwide. The Clean Air Act Amendments of 1970 defined photochemical oxidants as one of the first six criteria pollutants, alongside carbon monoxide, nitrogen dioxide, and sulfur dioxide. Photochemical oxidants are formed by the action of ultraviolet (UV) light from the sun on nitrogen oxides, and their production is dependent on the presence of primary pollutants and UV light. The most well-known photochemical oxidant is ozone, which is a secondary pollutant that does not come from direct emissions but is instead formed in the atmosphere through complex reactions involving hydrocarbons and nitrogen oxides.
| Characteristics | Values |
|---|---|
| Definition | A photochemical oxidant refers to substances like ozone, formed in reactions involving pollutants like nitrogen oxides and hydrocarbons when exposed to sunlight. |
| Other Names | Photochemical oxidants are also referred to as photochemical smog. |
| Types | Common photochemical oxidants include ozone (O3), hydrogen peroxide (H2O2), and peroxyacetyle nitrate (PAN). |
| Health Impact | Photochemical oxidants can exacerbate respiratory diseases, irritate the eyes, and have negative effects on human, plant, and animal health. |
| Environmental Impact | Photochemical oxidants can damage plants and contribute to climate change. |
| Sources | Photochemical oxidants are formed by the action of sunlight on nitrogen oxides and reactive hydrocarbons. They are also associated with smog and can be produced by internal combustion engines burning gasoline, diesel fuel, or other hydrocarbon mixtures. |
| Control Measures | Efforts to control photochemical oxidant pollution include the use of catalytic converters, vehicle emission controls, and the formation of air quality management districts. |
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What You'll Learn
The Clean Air Act Amendments of 1970
The Clean Air Act (CAA) is a comprehensive federal law that regulates air emissions from stationary and mobile sources. The Act was amended in 1970, 1977, and 1990, with the 1970 amendments being particularly significant.
The 1970 Clean Air Act Amendments were passed unanimously in the U.S. Senate and by a wide margin in the House of Representatives before being signed into law by President Richard Nixon on December 31, 1970. Nixon had established the Environmental Protection Agency (EPA) just under a month prior, on December 2, and the EPA was tasked with overseeing the implementation of the Clean Air Act.
The 1970 amendments required the EPA to determine which air pollutants posed the greatest threat to public health and welfare and to promulgate National Ambient Air Quality Standards (NAAQS) and air quality criteria for them. The health-based standards were called "primary" NAAQS, while standards set to protect public welfare other than health (e.g., agricultural values) were called "secondary" NAAQS. The Clean Air Act required the EPA to set national health-based standards for air pollution and also required the government to review, update, and enforce these standards. The first six criteria pollutants were defined as carbon monoxide, nitrogen dioxide, sulfur dioxide, total particulate matter, hydrocarbons, and photochemical oxidants. Photochemical oxidants, such as ozone, are produced by atmospheric chemical reactions between anthropogenic volatile organic compounds (VOCs) and nitrogen oxides in the presence of sunlight.
The 1970 amendments also marked a shift in responsibility for developing air quality standards from the states to the federal government. While the law recognizes that states should lead in carrying out the Clean Air Act, it requires states to develop State Implementation Plans (SIPs) for how they will meet new national ambient air quality standards. The EPA must approve each SIP, and if a state fails to meet the standards, the EPA can retain CAA enforcement in that state.
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Photochemical smog
The formation of photochemical smog is driven by electromagnetic radiation from sunlight with a wavelength of around 400 nm or less, which falls within the ultraviolet region. This radiation initiates a series of chain reactions, causing the formation of active species and subsequent photochemical reactions. The primary pollutants from automobiles and industrial activities react with normal atmospheric compounds, leading to the creation of harmful chemicals.
Nitrogen oxides (NOx), produced by the combustion of fossil fuels and emitted from vehicle engines, play a significant role in the development of photochemical smog. When exposed to sunlight, nitrogen dioxide (NO2) undergoes reactions with hydrocarbons and other compounds, resulting in the production of ozone (O3), nitric acid, aldehydes, peroxyacyl nitrates (PANs), and other secondary pollutants. These chemicals can have adverse effects on human health, particularly the respiratory and ocular systems, as well as causing damage to crops and trees.
The accumulation of ozone, a highly reactive species, is a critical aspect of photochemical smog. VOCs, such as hydrocarbons, react with nitrogen oxides in the presence of sunlight to generate ozone. This buildup of ozone, along with other pollutants, contributes to the formation of the brown haze characteristic of photochemical smog. VOCs are anthropogenic compounds arising from human activities, and they play a significant role in the formation of photochemical oxidants like ozone.
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Tropospheric ozone
The concentration of tropospheric ozone varies with height, increasing as altitude above sea level increases, with a maximum concentration at the tropopause. Its levels are typically higher in urban areas and on hot summer days, and they have been rising in several parts of the northern hemisphere. Tropospheric ozone levels have increased significantly since the industrial revolution due to the increased presence of NOx gases and VOCs, which are byproducts of combustion.
Strategies to reduce tropospheric ozone levels focus on methane reductions and cutting atmospheric pollution from vehicles, power plants, and other sources. Efforts in cities like Los Angeles, Mexico City, Beijing, and across Europe have successfully lowered tropospheric ozone levels through targeted pollution control measures. Reducing tropospheric ozone can help mitigate climate change, improve air quality, and support food security by enhancing agricultural yields for staple crops.
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Hydrocarbons and combustion
Photochemical oxidants are formed through the concentration of reactive gases in the atmosphere. These gases react with sunlight to produce secondary pollutants, such as ozone (O3), nitrogen dioxide, and peroxyacetyl nitrate (PAN). While O3 is well known for its protective layer against ultraviolet radiation, ground-level O3 is a harmful pollutant and a major component of smog.
Hydrocarbons are organic compounds that are a key component of smog and a precursor to photochemical oxidants. They are found in many sources, including internal combustion engines that burn gasoline, diesel fuel, and other hydrocarbon mixtures. When hydrocarbons are burned, they ideally react with oxygen to produce carbon dioxide and water. However, in cases of incomplete combustion, hydrocarbons can react to form carbon monoxide, a poisonous gas. In addition, the high temperatures and pressures in internal combustion engines can cause nitrogen to react with oxygen and form nitrogen oxides, which are another precursor to photochemical oxidants.
The combustion process involves the reaction between a fuel source and an oxidant, typically atmospheric oxygen. This reaction produces oxidized products, often in the form of smoke. While combustion does not always result in fire, a flame indicates that the substances undergoing combustion have vaporized. The study of combustion, or burning, is known as combustion science.
The oxidants used in combustion have a high oxidation potential and can include atmospheric oxygen, chlorine, fluorine, chlorine trifluoride, nitrous oxide, and nitric acid. Platinum and vanadium can catalyze combustion, as seen in the contact process. Incomplete combustion of hydrocarbons can lead to the formation of carbon monoxide, which is hazardous to human health.
To address the issue of photochemical oxidants, the Clean Air Act Amendments of 1970 defined the first six criteria pollutants, which included hydrocarbons and photochemical oxidants. The list has since been revised, with lead added in 1976 and the photochemical oxidant standard revised to focus on ozone in 1979. The EPA's Office of Research and Development plays a crucial role in developing criteria documents that assess the health and environmental impacts of air pollutants, including photochemical oxidants.
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Health and welfare effects
Photochemical oxidants are formed by the action of sunlight on nitrogen oxides and reactive hydrocarbons. They are found in photochemical smog, a mixture of primary and secondary air pollutants. The most important phytotoxic components produced by these atmospheric photochemical reactions are ozone and peroxyacetyl nitrate.
Ozone, a potent photochemical oxidant, is a major health concern in urban and rural communities. It is harmful to the respiratory system and can also affect vegetation and decrease crop productivity. The Clean Air Act Amendments of 1970 defined the first six criteria pollutants, which included photochemical oxidants. The primary standards aim for immediate protection of public health, including sensitive populations such as asthmatics, children, and the elderly.
The presence of VOCs (volatile organic compounds) in the atmosphere plays a crucial role in the formation of ground-level ozone and photochemical oxidants. VOCs are air pollutants commonly found at ground level in urban and industrial centres. Some VOCs have a high propensity to form oxidants, while others are largely unreactive. The atmospheric oxidation of VOCs leads to the formation of ozone and organic aerosol, which have direct health effects on humans and impact air quality and climate.
To protect humans, animals, plants, and materials from photochemical oxidant injury, it is necessary to reduce the ambient concentration of the particular air pollutant. The Air Quality Act of 1967 and its subsequent amendments addressed the need for air quality criteria (AQC) to establish the relationship between exposure to pollutants and their short- and long-term effects on health and welfare.
The EPA's Office of Research and Development develops criteria documents that compile and evaluate the latest scientific knowledge to assess the health and welfare effects of air pollutants, including photochemical oxidants. These documents are periodically reviewed to ensure that the National Ambient Air Quality Standards (NAAQS) are up-to-date with the current scientific understanding of the health and environmental impacts of these pollutants.
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Frequently asked questions
A photochemical oxidant is a type of air pollutant that is formed when nitrogen oxides and hydrocarbons react with each other in the presence of sunlight.
Ozone (O3), peroxyacetyl nitrate (PAN), formaldehyde, and acrolein are all examples of photochemical oxidants.
Photochemical oxidants are formed through a process called photolysis, where ultraviolet (UV) light from the sun interacts with nitrogen oxides and other pollutants.
Photochemical oxidants can have negative impacts on both human health and the environment. They can damage plants and vegetation, contribute to smog formation, and cause air pollution, particularly in urban areas.
