Air Pollution: Photochemical Oxidants And Their Harmful Effects

Photochemical oxidants are secondary air pollutants that are formed by the action of sunlight on nitrogen oxides and reactive hydrocarbons. The most common photochemical oxidant is ozone, which is formed from photochemical reactions involving nitrogen oxides and volatile organic compounds (VOCs) in the presence of sunlight. Other photochemical oxidants include peroxyacyl nitrates, aldehydes, ketones, organic and inorganic acids, aerosols, and nitrogen dioxide. These oxidants are found in photochemical smog, which first gained attention during the 1940s in Los Angeles and has since become the most important air pollutant in North America.

Ozone is a photochemical oxidant and secondary pollutant

Ozone (O3) is a photochemical oxidant and a secondary pollutant. It is formed by the action of ultraviolet (UV) light from the sun on nitrogen oxides, a process called photolysis. The presence of volatile organic compounds (VOCs) in the atmosphere plays a crucial role in the formation of ground-level ozone. VOCs, such as nitrogen oxides and reactive hydrocarbons, react with sunlight to produce ozone, which then contributes to the formation of organic aerosols and smog.

Ozone is not directly emitted into the air but is a byproduct of chemical reactions between oxides of nitrogen (NOx) and VOCs. This typically occurs when pollutants from cars, power plants, industrial boilers, refineries, and other sources react in the presence of sunlight. As a result, ozone levels tend to be highest on hot, sunny days in urban areas, although wind can carry ozone over long distances, affecting even rural regions.

The Clean Air Act Amendments of 1970 first defined the criteria pollutants, which included photochemical oxidants. In 1979, the photochemical oxidant standard was revised and restated as ozone, recognizing its significant impact on air quality. Ozone is one of the six common air pollutants identified in the Clean Air Act, and its outdoor levels are regulated by national ambient air quality standards (NAAQS).

Ozone can be categorized as "good" or "bad" depending on its location in the atmosphere. Stratospheric ozone is considered "good" because it forms a protective layer that shields living organisms from the sun's harmful ultraviolet radiation. In contrast, ground-level ozone is deemed "bad" due to its adverse effects on human health and the environment.

Ground-level ozone is a harmful air pollutant and the primary component of smog. It can trigger various health issues, particularly for children, the elderly, and individuals with lung diseases like asthma. Ozone irritates the eyes and exacerbates respiratory diseases at certain concentrations. Additionally, it has been shown to damage plants and contribute to insect-related disturbances.

VOCs and nitrogen oxides react to form ozone

Photochemical oxidants are secondary air pollutants formed by the action of sunlight on nitrogen oxides and reactive hydrocarbons. The most common photochemical oxidant is ozone, which is harmful to both humans and the ecosystem.

Ozone (O3) is produced in the troposphere as a result of a complex set of reactions involving volatile organic compounds (VOCs) and oxides of nitrogen (NOx). VOCs are anthropogenic volatile organic compounds that arise from human activities and are capable of producing photochemical oxidants such as ozone through atmospheric chemical reactions with nitrogen oxides in the presence of sunlight.

VOCs come from many sources, but internal combustion engines that burn gasoline, diesel fuel, and other mixtures of hydrocarbons are a significant source. While complete combustion results in carbon dioxide and water, incomplete combustion results in VOCs, as well as new hydrocarbons formed during the process. These compounds then react with nitrogen oxides emitted mainly from vehicles, power plants, and industrial activities to form ozone.

The formation of ozone from VOCs and nitrogen oxides is influenced by various factors, including the initial concentrations of VOCs and NOx, geographic location, and the availability of other reactants. At higher VOC/NOx ratios, typically found in urban areas, NOx control is more effective in lowering ozone concentrations. On the other hand, in rural areas with lower VOC/NOx ratios, lowering NOx concentrations at constant VOC concentrations results in lower peak ozone concentrations.

The reactions between VOCs and NOx are complex and lead to the formation of other harmful by-products, such as peroxyacetyl nitrate (PAN) and aldehydes like formaldehyde and acrolein. These by-products contribute to air pollution and have negative impacts on human health and the environment.

UV light from the sun causes photolysis

The sun emits ultraviolet (UV) radiation, which is a form of energy with shorter wavelengths than visible light. While UV radiation from the sun is essential for the production of vitamin D in our bodies, overexposure to it can be harmful. UV light from the sun can cause skin cancer and wrinkling, and it can damage the fibres in our skin called elastin, leading to sagging, stretching, and bruising.

UV light from the sun also plays a role in the formation of photochemical oxidants, which are secondary air pollutants. Photochemical oxidants are formed through a process called photolysis, which is a chemical reaction in which molecules of a chemical compound are broken down by the absorption of light or photons. Specifically, UV light from the sun can cause the photolysis of primary pollutants such as hydrocarbons and nitrogen oxides, leading to the formation of secondary pollutants.

Hydrocarbons are produced by incomplete combustion in internal combustion engines that burn gasoline, diesel fuel, and other hydrocarbon mixtures. In the presence of sunlight, these hydrocarbons react with nitrogen oxides emitted from vehicles, power plants, and industrial activities to form photochemical oxidants. This reaction is particularly complex and involves the formation of unstable intermediate free radicals that undergo a series of changes.

The most important phytotoxic component produced by these atmospheric photochemical reactions is ozone (O3), which is the pollutant most closely associated with smog. Ozone can have direct health effects on humans, as it is harmful to our respiratory system. In addition to ozone, other photochemical oxidants formed include peroxyacetyl nitrate (PAN), aldehydes, ketones, organic and inorganic acids, aerosols, and nitrogen dioxide.

Overall, while UV light from the sun is necessary for certain vital functions, it can also contribute to air pollution through the photolysis of primary pollutants and the subsequent formation of photochemical oxidants, which have negative impacts on human health and the environment.

Hydrocarbons and nitrogen oxides form photochemical smog

Photochemical smog is a brownish-grey haze caused by the action of solar ultraviolet radiation on atmospheric pollutants, including hydrocarbons and nitrogen oxides. It is a complex mixture of primary and secondary air pollutants. The largest contributor to photochemical smog is automobiles, while coal-fired power plants and other power plants also produce the necessary pollutants to facilitate its production.

Hydrocarbons come from many sources, but internal combustion engines that burn gasoline, diesel fuel, and other mixtures of hydrocarbons are a significant source. When combustion is incomplete, hydrocarbons are released into the atmosphere. Similarly, nitrogen oxides are emitted into the air as pollutants from internal combustion engines.

When exposed to ultraviolet radiation, nitrogen dioxide (NO2) undergoes a complex series of reactions with hydrocarbons to produce the components of photochemical smog. This includes ozone, nitric acid, aldehydes, peroxyacyl nitrates (PANs), and other secondary pollutants. These compounds are known as photochemical oxidants because they can react and oxidise certain compounds in the atmosphere or within a person's lungs that are not normally oxidised.

The formation of photochemical smog is influenced by various factors. Firstly, hotter days and warm temperatures contribute to the development of photochemical smog. This is particularly evident in densely populated cities with warm and dry climates, where the concentration of pollutants is higher. Secondly, the intensity of sunlight plays a role. Ozone (O3) levels tend to build up when the sun's intensity is greatest at midday, as the reactions that produce O3 are influenced by solar radiation.

Photochemical oxidants damage plants and human health

Photochemical oxidants are formed by the action of sunlight on nitrogen oxides and reactive hydrocarbons, and they have become the most important air pollutants in North America. They are also a threat to vegetation and human health in other parts of the world, including South and Central America, Asia, and Australia.

The photochemical oxidant ozone (O3) is known to damage plants. Ozone is a secondary pollutant, formed by the action of ultraviolet (UV) light from the sun on nitrogen oxides. Its production and concentration depend on the presence of primary pollutants and UV light. In the presence of volatile organic compounds (VOCs), high concentrations of ozone are formed. VOCs are an important class of air pollutants commonly found at ground level in all urban and industrial centres.

Ozone is harmful to the respiratory system, and it also damages crops. The first observations of ozone and peroxyacetyl nitrate (PAN) injury to vegetation in Europe were made by Dr Ellis F. Darley during a study visit to the Federal Republic of Germany in 1963-64. Formaldehyde and acrolein are major products of reactions involving hydrocarbons and free atomic oxygen, and they are present in urban atmospheres.

The balance of reactions involving hydrocarbons and free atomic oxygen is upset when the intensity of the sun is at its greatest at midday, leading to a build-up of O3 levels. The compounds that become free radicals, like the oxides of nitrogen, are readily available from internal combustion engines. Although nitrogen (N2) is relatively unreactive, it combines with O2 from the fuel/air mix in engines under high temperature and pressure conditions, generating oxides that can provide electrons to photochemical reactions.

The atmospheric oxidation of VOCs leads to two important by-products in the lower atmosphere: ozone and organic aerosol. These have direct health effects on humans as they are harmful to the respiratory system.

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