Greta Jimenez
Photochemical pollution, also known as photochemical smog, is a type of air pollution that occurs when primary pollutants such as nitrogen oxides and hydrocarbon vapours emitted by automobiles react with sunlight to form secondary pollutants like ozone, nitric acid, and other toxic chemicals. This process results in a mixture of airborne particles and ground-level ozone, which can have adverse effects on human health, causing irritation to the eyes and respiratory system, as well as damaging crops and trees. Photochemical smog is typically more prevalent during the summer months when temperatures are warmer and there is ample sunlight. It is often associated with densely populated cities like Los Angeles, where high emissions from vehicles and industrial activity contribute to its formation.
| Characteristics | Values |
|---|---|
| Cause | Nitrogen oxides, carbon monoxide, volatile organic compounds, hydrocarbons, and other primary pollutants emitted from vehicles, industrial fumes, and power plants |
| Formation | Chemical reaction of sunlight with nitrogen oxides and volatile organic compounds, leading to the formation of secondary pollutants like ozone, nitric acid, and aldehydes |
| Impact | Adverse health effects on humans, including irritation to eyes and respiratory distress; damage to crops, trees, and agricultural plants |
| Prevalence | More common during warmer seasons and in densely populated cities with high vehicular emissions, such as Los Angeles and other urban areas |
| Measurement | Air quality monitoring measures pollutant levels, with indoor air often containing higher pollutant concentrations than outdoor air |
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What You'll Learn
Photochemical smog
The formation of photochemical smog requires specific conditions, including warmth, ample sunlight, and relatively low air movement to prevent the dilution of reactants. It occurs when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react with sunlight, heat, ammonia, moisture, and other compounds. This reaction results in the creation of noxious vapours, ground-level ozone, and particles that comprise smog. The ozone formed in this process is considered the most toxic constituent of photochemical smog, causing eye irritation, respiratory distress, and damage to agricultural plants and native flora.
Nitrogen oxides (NOx), specifically nitric oxide (NO) and nitrogen dioxide (NO2), are emitted from the combustion of fossil fuels, as well as natural sources such as volcanoes and forest fires. These nitrogen oxides react with sunlight to form singular oxygen atoms, which then combine with molecular oxygen (O2) to produce ozone (O3). This ozone, along with nitric acid and aldehydes, are considered secondary pollutants that contribute to the formation of photochemical smog.
The adverse health effects of photochemical smog are significant, particularly in cities where many people encounter the polluted air. Humans are sensitive to ozone, which can irritate and damage the membranes of the respiratory system and eyes. Additionally, the high levels of atmospheric nitric oxide resulting from industrial activity have contributed to gradual depletion and environmental concerns.
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Primary and secondary pollutants
Photochemical smog, also known as "summer smog", is a type of air pollution caused by the chemical reaction of sunlight with primary pollutants, leading to the formation of secondary pollutants.
Primary pollutants are emitted directly from a source. Examples of primary pollutants include:
- Nitrogen oxides, particularly nitric oxide (NO) and nitrogen dioxide (NO2)
- Volatile organic compounds (VOCs), such as unburned hydrocarbons that are present in urban air due to uncompleted combustion and the evaporation of solvents and liquid fuels
- Carbon monoxide (CO), a colourless and odourless gas that is harmful to human health and can cause carbon monoxide poisoning
- Carbon dioxide (CO2), a greenhouse gas that contributes to global warming and climate change
- Sulfur dioxide (SO2) from coal combustion
Secondary pollutants are formed when primary pollutants undergo chemical reactions in the atmosphere. Some important examples of secondary pollutants include:
- Tropospheric ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NOx) combine in the presence of sunlight. Ozone is the most toxic constituent of photochemical smog and is harmful to both human health and agriculture.
- Nitrogen dioxide (NO2), which is formed when nitric oxide (NO) combines with oxygen (O2) in the air
- Peroxylacyl nitrates (PAN)
- Aldehydes
- Nitric acid (HNO3) and sulfuric acid, which, when mixed with water, form acid rain
Photochemical smog is primarily associated with vehicular emissions from internal combustion engines, industrial fumes, and agricultural activities. It is particularly severe in cities with high traffic density and during seasons with warmer temperatures and more sunlight, such as summer.
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Nitrogen oxides and volatile organic compounds
VOCs, on the other hand, are emitted as gases from certain solids or liquids and include a variety of chemicals. They are found in products such as solvent-based paints, printing inks, consumer products, organic solvents, and petroleum products. Motor vehicles, power plants, and industrial activities are significant sources of VOC emissions. VOCs can react with nitrogen oxides in the presence of sunlight, contributing to the formation of ozone.
The accumulation of ozone, fine particulates, and other gaseous pollutants results in smog, which reduces visibility and has negative impacts on human health and the environment. Ground-level ozone is a major component of photochemical smog and is considered the most toxic constituent. It can irritate and damage the membranes of the respiratory system and eyes, and it is a significant risk factor for asthma and other respiratory issues.
VOCs can also have adverse health effects, with potential short-term and long-term impacts. Regulations have been implemented to control VOC emissions and reduce their presence in various products to mitigate their environmental and health effects.
Overall, the interaction of nitrogen oxides and VOCs with sunlight plays a crucial role in the formation of photochemical smog, leading to air pollution and associated health and environmental consequences.
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The role of sunlight
Nitrogen oxides (NOx) and volatile organic compounds (VOCs) are emitted from vehicles and industrial sources. When these primary pollutants are exposed to sunlight, they undergo a series of chemical reactions. Sunlight, in particular, its ultraviolet radiation, interacts with nitrogen dioxide (NO2) to produce highly reactive species. These reactive species then participate in further reactions with other molecules in the atmosphere, leading to the formation of secondary pollutants.
One of the critical reactions is the formation of ozone (O3). Sunlight helps break down nitrogen dioxide (NO2) into nitric oxide (NO) and free oxygen atoms. These free oxygen atoms then combine with molecular oxygen (O2) to create ozone. Ozone is a major component of photochemical smog and is considered the most toxic constituent. It is harmful not only to humans but also to agricultural plants, causing considerable damage.
Additionally, sunlight plays a role in the formation of nitric acid, another component of photochemical smog. Nitrogen oxides (NOx) emitted from vehicles can react with sunlight to produce oxygen atoms. These oxygen atoms may then combine with water vapour to form nitric acid. Nitric acid contributes to acid rain, which has adverse environmental effects.
The presence of ample sunlight, along with other factors such as warmth and relatively calm air, creates favourable conditions for the formation of photochemical smog. These conditions allow the chemical reactions to occur more readily and facilitate the accumulation of pollutants in the atmosphere. Therefore, sunlight is a crucial factor in the development and severity of photochemical smog.
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Impact on human health
Photochemical smog, or smoke fog, is a type of intense air pollution. It is a brownish-grey haze caused by the action of solar ultraviolet radiation on an atmosphere polluted with hydrocarbons and oxides of nitrogen. The primary sources of these pollutants are automobiles, coal-fired power plants, and other power plants.
Photochemical smog has several adverse impacts on human health. Firstly, it can cause eye irritation when combined with hydrocarbons. The chemicals formed from this combination can irritate the membranes of the eyes and the respiratory system. Even small traces of these chemicals can affect the respiratory tract of humans and animals. Prolonged exposure to photochemical smog can lead to reduced lung function, respiratory distress, and severe lung damage, causing discomfort, edema, and even death by asphyxiation.
The health effects of photochemical smog are particularly prominent in densely populated cities, where the concentration of pollutants is higher. The severity of smog in certain cities, such as Delhi, is often exacerbated by stubble burning in neighbouring agricultural areas. The atmospheric pollution levels of major cities like Los Angeles, Beijing, Delhi, Mexico City, and Tehran, are often increased due to an inversion that traps pollution close to the ground. This results in the smog lingering over these cities, subjecting residents to its adverse health effects.
Photochemical smog is also associated with premature ageing and an increased risk of heart attacks, irregular heartbeats, and premature death, especially in individuals with pre-existing heart and lung diseases. The World Health Organization (WHO) has estimated that air pollution, including photochemical smog, is responsible for approximately 6.7 million premature deaths annually.
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Frequently asked questions
Photochemical pollution is air pollution derived from vehicular emissions from internal combustion engines and industrial fumes.
Photochemical pollution can cause severe sickness, a shortened life span, immature death, irritation and damage to the membranes of the respiratory system and eyes.
Los Angeles, Beijing, Delhi, Lahore, Mexico City, and Tehran are some of the cities that experience high levels of atmospheric pollution and are often affected by photochemical smog.
