Kiera Jefferson
Smog is a form of air pollution that is common in industrial areas and big cities with a lot of traffic. It is a combination of smoke and fog, and the term was first used in the early 1900s. Smog is mainly composed of ground-level ozone and particulate matter, which are harmful to human health and can cause respiratory problems. The primary pollutants that react to form smog include nitrogen oxides, volatile organic compounds (VOCs), and hydrocarbons, which are emitted from vehicle exhausts, industrial plants, and the combustion of fossil fuels. These pollutants undergo chemical reactions in the atmosphere, fueled by sunlight, to form the harmful chemicals that make up smog.
| Characteristics | Values |
|---|---|
| Primary Pollutants | Nitrogen oxides (NO, NO2), volatile organic compounds (VOCs), hydrocarbons, carbon monoxide (CO), sulfur dioxide |
| Secondary Pollutants | Peroxyacetyl Nitrate (PAN), Tropospheric Ozone, Aldehydes, Nitric Acid, Nitrogen Dioxide (NO2) |
| Conditions Favouring Formation | Summer, sunlight, warmth, stable atmosphere, low wind, fossil fuel combustion, vehicle emissions, industrial emissions |
| Health Effects | Respiratory distress, eye irritation, premature death, pulmonary and skin diseases, aggravates asthma, damages lung tissue |
| Preventative Measures | Reduced vehicle use, improved vehicle maintenance, use of electric appliances, reduced VOC products, fuel during cooler hours |
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What You'll Learn
Nitrogen oxides, volatile organic compounds, and sunlight
Smog is a type of air pollution that reduces visibility. The term "smog" was first used in the early 1900s to describe a mix of smoke and fog, with smoke usually coming from burning coal. Smog is commonly seen in industrial areas and cities with large numbers of automobiles.
Photochemical smog, often referred to as "summer smog", is the chemical reaction of sunlight, nitrogen oxides, and volatile organic compounds (VOCs) in the atmosphere. This reaction leaves airborne particles and ground-level ozone, which is toxic to humans and can cause severe sickness, a shortened lifespan, or premature death. Nitrogen oxides are emitted into the air as pollutants from internal combustion engines, such as car exhaust, and from industrial sources like coal power plants and factory emissions. VOCs are released from gasoline, paints, and many cleaning solvents. When sunlight hits these chemicals, they form the noxious vapors, ground-level ozone, and particles that comprise smog.
VOCs play a significant role in the formation of ozone and fine particulates in the atmosphere. Under sunlight, VOCs react with nitrogen oxides emitted mainly from vehicles, power plants, and industrial activities to form ozone, which in turn helps form fine particulates. The accumulation of ozone, fine particulates, and other gaseous pollutants results in smog.
Ozone (O3) is a highly reactive gas. It causes inflammation and has large-scale temporal fluctuations. There are serious risks to health from exposure to excessive ozone, as it is a major risk factor in asthma morbidity and mortality. Other marked effects of ozone on human health include breathing problems, reduced lung function, and lung diseases.
Nitrogen oxides (NOx) are also important because of their indirect effect on climate through their role in affecting global ozone concentrations. Although NOx species are relatively short-lived, they can react chemically with hydrocarbons to produce peroxyacetyl nitrate (PAN). PAN is known to be an eye irritant, phytotoxin, and bacterial mutagen. The most serious biological effects of PAN are of a phytotoxic nature, resulting in injury to plants and vegetation.
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Fossil fuel combustion and industrial emissions
Nitrogen oxides, including nitric oxide (NO) and nitrogen dioxide (NO2), are primary pollutants that play a significant role in the formation of smog. They are released into the air primarily through the combustion of fossil fuels associated with transportation and industrial activities. These nitrogen-based compounds react with other pollutants and sunlight to form the noxious vapors, ground-level ozone, and particles that comprise smog.
Vehicular emissions from internal combustion engines are a major source of nitrogen oxides and hydrocarbons, which are key components of smog. Cities with high traffic volumes, such as Los Angeles, Beijing, Delhi, and Mexico City, often experience severe photochemical smog due to the combination of vehicle emissions and atmospheric conditions that trap pollution close to the ground.
Industrial emissions also contribute significantly to the release of nitrogen oxides and other pollutants. Fossil fuel combustion in power generation, manufacturing, and other industrial processes releases large quantities of nitrogen oxides and particulate matter into the atmosphere, exacerbating smog formation.
The health impacts of fossil fuel combustion and industrial emissions are particularly concerning for children and vulnerable populations. Exposure to air pollution from these sources has been linked to respiratory illnesses, cognitive and behavioral disorders, cardiovascular disease, and other chronic health issues. Additionally, the greenhouse gases emitted, such as carbon dioxide, contribute to climate change, creating synergistic effects that further magnify the harm to human health and the environment.
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Vehicle emissions and exhausts
Nitrogen oxides (NOx) are a significant component of vehicle emissions and a primary pollutant in the formation of photochemical smog. They include nitric oxide (NO) and nitrogen dioxide (NO2). When nitrogen oxides are emitted into the atmosphere, they can react with sunlight to form secondary pollutants like ozone (O3). This reaction involves the release of free oxygen atoms that combine with molecular oxygen (O2) to create ozone.
Carbon monoxide (CO) is a colorless, odorless, and poisonous gas emitted by vehicles. It is a smog-forming emission that contributes to the degradation of air quality and poses health risks. Particulate matter (PM), another product of vehicle emissions, consists of tiny particles of solid matter that can lodge in the lungs and deposit on buildings, further reducing air quality.
Hydrocarbons, which are the main component of petroleum fuels like gasoline and diesel, also play a role in smog formation. They react with nitrogen oxides and sunlight to produce the chemical reactions that create photochemical smog. Additionally, vehicles emit volatile organic compounds (VOCs), which include hydrocarbons and their derivatives that readily vaporize at room temperature. These compounds undergo a series of chemical reactions with sunlight, heat, ammonia, moisture, and other compounds to form the vapors, ground-level ozone, and particles that make up smog.
The proliferation of automobiles since World War II has significantly contributed to environmental pollution, particularly in urban areas with high vehicle densities. Photochemical smog, commonly associated with cities like Los Angeles, is a direct result of emissions from internal combustion engines. These emissions, along with industrial fumes, create a toxic mixture that causes a light brownish discoloration of the atmosphere, reduced visibility, plant damage, eye irritation, and respiratory distress.
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Hydrocarbons and ground-level ozone
Smog is a community-wide air pollution issue that is more common in urban areas with a high number of automobiles. Photochemical smog, also known as "summer smog", is the dominant type of smog formation during the summer season when temperatures are warmer and there is more sunlight. It is formed by the chemical reaction of sunlight with nitrogen oxides and volatile organic compounds (VOCs) in the atmosphere, which include hydrocarbons.
Hydrocarbons are the main component of petroleum fuels such as gasoline and diesel fuel. They are present in urban air due to incomplete combustion and the evaporation of solvents and liquid fuels. Hydrocarbons are also emitted from automobiles, power plants, industrial boilers, refineries, and chemical plants.
Ground-level ozone is a harmful air pollutant and the main ingredient in "smog". It is formed by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. About 10% of total ozone in the atmosphere is found at ground level, in the troposphere, which is the lowest level of the Earth's atmosphere. Ground-level ozone is considered "bad" because it can trigger a variety of health problems, particularly for children, the elderly, and people with lung diseases such as asthma. It can also cause damage to plants and ecosystems.
The formation of ground-level ozone is influenced by a range of factors, including the emission sources of its precursors (such as automobiles and industrial processes), the presence of sunlight, and the temperature. Warmer temperatures and more sunlight during the summer months contribute to increased ozone formation, leading to higher levels of pollution during this season.
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Peroxyacyl nitrates and aldehydes
Peroxyacyl nitrates, also known as acyl peroxy nitrates, APN, or PANs, are powerful respiratory and eye irritants found in photochemical smog. They are nitrates produced in the thermal equilibrium between organic peroxy radicals by the gas-phase oxidation of volatile organic compounds (VOCs) or by aldehydes and other oxygenated VOCs that oxidize in the presence of NO2.
Peroxyacyl nitrates are secondary pollutants, meaning they are not directly emitted as exhaust from power plants or internal combustion engines. Instead, they are formed from other pollutants through chemical reactions in the atmosphere. Sources of the pollutants required to create peroxyacyl nitrates include motor vehicles, tobacco smoke, and the burning of fossil fuels.
Peroxyacyl nitrates can remain in the atmosphere for about three months after formation under cold conditions of -20°C and lower. They are very stable at cold temperatures and easily decompose to release NOx at warmer temperatures. This property is important for tropospheric ozone production, as peroxyacyl nitrates can transport NOx to regions where it can more efficiently produce ozone.
Aldehydes, on the other hand, are formed as reaction products in the photooxidation of hydrocarbons. They contribute to the irritant qualities of photochemical smog, with formaldehyde and acrolein being the most significant. Aldehydes are toxic and, due to their lipophilicity, are not easily excreted. They are reactive and intensely irritating to the respiratory system.
In summary, peroxyacyl nitrates and aldehydes are both secondary pollutants that contribute to the formation of smog. Peroxyacyl nitrates are powerful respiratory and eye irritants, while aldehydes are highly reactive and irritating to the respiratory system. Both pollutants have adverse effects on human health and the environment.
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Frequently asked questions
Smog is a form of air pollution that reduces visibility and is harmful to human health. It is caused by the combustion of fossil fuels and industrial emissions, which release primary pollutants into the atmosphere.
Primary pollutants are emitted directly from a source, such as nitrogen oxides and volatile organic compounds (VOCs) from car exhausts, and sulfur dioxide from coal combustion.
Nitrogen oxides (NOx) and hydrocarbons (HCs) are the primary pollutants that react to form smog. These pollutants are emitted from vehicle exhausts, industrial plants, and fossil fuel combustion.
Nitrogen oxides and hydrocarbons react with sunlight, heat, and other compounds in the atmosphere to form the noxious vapors, ground-level ozone, and particles that comprise smog. This process is known as photochemical smog formation.
Smog is toxic to humans and can cause respiratory problems, eye irritation, and reduced lung function. It can also aggravate asthma and cause coughing and shortness of breath. Additionally, smog can damage lung tissue and is especially harmful to individuals with respiratory illnesses.
