Air Pollutants: Secondary Compounds And Their Sources

Air pollutants are classified as either primary or secondary. Primary air pollutants are those that are emitted directly from a source, such as carbon monoxide from vehicles. Secondary air pollutants, on the other hand, are formed when primary pollutants react with one another or with other chemicals in the atmosphere. Examples of secondary air pollutants include ozone, which is formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight, and smog, which is a combination of smoke and fog.

Smog

The term "smog" was coined by Henry Antoine Des Voeux in his 1905 paper, "Fog and Smoke," for a meeting of the Public Health Congress. However, the term appeared 25 years earlier in the Santa Cruz & Monterey Illustrated Handbook, published in 1880. London was plagued by a particular type of smog in the early 1900s, resulting from a combination of dense fog and soot from coal combustion. This type of smog, nicknamed "pea-soupers," persisted through the mid-20th century.

Photochemical smog, commonly associated with the Los Angeles Basin, is produced through a combination of vehicle exhaust, nitrogen oxides, volatile organic compounds, and sunlight. It is a byproduct of modern industrialization and is more problematic in large cities with warm, sunny, and dry climates. Photochemical smog is also referred to as oxidizing smog, as oxidation reactions play a key role in its formation. During the summer, when temperatures are warmer and sunlight is more intense, photochemical smog is the dominant type of smog formation.

In contrast, winter smog is characterized by increased coal and fossil fuel usage for heating, coupled with the lack of pollutant dispersion due to atmospheric inversions. Sulfur dioxide, a common component of London smog, is linked to respiratory irritations, including coughing, wheezing, and pharyngitis.

Ozone

Ground-level ozone, on the other hand, is considered "bad" due to its adverse health effects and contribution to smog formation. This type of ozone is not emitted directly into the air but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs). These reactions occur when pollutants from cars, power plants, industrial boilers, refineries, and other sources interact in the presence of sunlight. Ground-level ozone is particularly harmful to children, the elderly, and individuals with lung diseases such as asthma. It can irritate the eyes, nose, and throat, and can exacerbate respiratory conditions.

Primary pollutants

Particulates are tiny solid or liquid particles that can be suspended in the air. They can come from a variety of sources, including dust, smoke, and liquid droplets. Carbon monoxide is a toxic gas that is produced by the incomplete combustion of fossil fuels, such as in motor vehicles and industrial processes. Nitrogen oxides are also produced by the combustion of fossil fuels and contribute to the formation of acid rain and smog.

Sulfur oxides are released during the combustion of sulfur-containing fuels, such as coal. They can also be released during industrial processes, such as metal smelting and paper milling. These pollutants contribute to the formation of acid rain and can have harmful effects on human health and the environment.

It is important to note that some pollutants can be both primary and secondary. For example, NOx can be emitted directly from vehicles and power plants, but it can also form in the atmosphere from other chemicals. This highlights the complex nature of air pollution and the need for comprehensive strategies to address it.

Chemical reactions

Secondary air pollutants are those that are not directly emitted into the atmosphere. Instead, they are formed when primary pollutants react with each other or with other chemicals in the atmosphere. This process can occur naturally or be influenced by human activity.

One example of a secondary air pollutant is ozone, which is formed when volatile organic compounds, carbon monoxide, hydrocarbons, and other elements react in the presence of sunlight. This type of reaction is known as a photochemical reaction, and it results in the formation of tropospheric ozone, which is a harmful pollutant.

Another example is sulfuric acid (H2SO4), which is formed through chemical reactions in the atmosphere. Sulfur dioxide (SO2) is a common air pollutant emitted from sources such as coal-burning power plants, volcanoes, and industrial processes. When sulfur dioxide reacts with other chemicals in the atmosphere, it can lead to the formation of sulfuric acid, contributing to acid rain and particulate pollution.

Particulate matter, or aerosol, is also considered a secondary pollutant. It is composed of fine particles, often with an aerodynamic diameter of less than 2.5 microns, known as PM2.5. These particles are a result of combustion activities, such as motor vehicles and power plants, and certain industrial processes. The formation of particulate matter involves complex chemical reactions that transform emitted gases into solid or liquid particles, which then become suspended in the air.

Additionally, smog, a combination of smoke and fog, is another secondary air pollutant. It is composed of particulate matter, including pollen, dust, sulphur oxides, volatile organic compounds, nitrogen oxides, and ammonia gas. Smog is often associated with urban areas and is influenced by human activities such as industrial emissions and vehicle exhaust.

Secondary air pollutants pose significant challenges due to their complex formation processes and the difficulty in controlling their synthesis. Understanding the chemical reactions that give rise to these pollutants is crucial for developing strategies to mitigate their harmful impacts on human health and the environment.

Natural processes

Secondary air pollutants are those that are not directly emitted into the atmosphere. They form when primary pollutants react in the atmosphere, causing harmful effects. Natural processes that contribute to the formation of secondary air pollutants include:

Photochemical Reactions: Photochemical smog is a result of interactions between different air pollutants, primarily volatile organic compounds (VOCs) and nitrogen oxides (NOx). This type of smog has a brown haze and can cause eye and respiratory irritation. It is often seen in large cities with significant industrial activity and traffic, such as Los Angeles and Mexico City. The geography of these cities can trap the smog, preventing its dispersal by wind.

Sunlight and Combustion: The combustion of fuels, such as diesel, petrol, fuel oil, and natural gas, along with sunlight, contributes to the formation of ozone, a major secondary air pollutant. Incomplete combustion processes can also lead to the formation of aldehydes, which can cause eye irritation.

Natural Sources: Natural processes, such as volcanic eruptions and natural water springs, emit hydrogen sulfide. Additionally, natural sources of chlorine include chemical industries, water purification processes, sewage plants, and swimming pools. Chlorine-containing compounds can cause respiratory tract irritation in humans.

Dust Storms: Natural weather patterns, such as dust storms, can transport large amounts of dust and particulate matter over long distances, impacting air quality in various regions.

Radioactive Gases: Nuclear power plants and atomic bomb testing contribute to the release of radioactive gases into the atmosphere, which can lead to secondary air pollution.

It is important to note that while these natural processes contribute to secondary air pollution, human activities, such as industrial emissions, transportation, and power generation, also play a significant role in the formation of secondary air pollutants.

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