Air pollution is the introduction of substances into the atmosphere that are harmful to human health and the environment. These substances can exist as tiny solid or liquid particles (aerosols) or as various gases. Most air contamination today results directly from human (anthropogenic) activities that release these materials into the air. Pollutants are generally categorized as primary or secondary forms based on how they enter the atmosphere.
Primary pollutants are substances directly emitted from a source, retaining their chemical form upon release. Secondary pollutants are not emitted directly but form in the air when primary pollutants react with each other or with other atmospheric components, often in the presence of sunlight. Understanding the origin of these human contributions is important for developing strategies to manage air quality.
Energy Generation
The combustion of fossil fuels to produce electricity and heat is one of the world’s most significant sources of atmospheric contamination. Large-scale power plants, particularly those relying on coal and natural gas, operate as massive stationary combustion sources, releasing various contaminants into the atmosphere via smokestacks.
A major pollutant is sulfur dioxide (\(\text{SO}_2\)), which forms when sulfur impurities in coal and oil are oxidized during combustion. \(\text{SO}_2\) is a precursor to acid rain and reacts in the air to form fine particulate matter. Nitrogen oxides (\(\text{NO}_{\text{x}}\)) also form at high temperatures when atmospheric nitrogen and oxygen react. \(\text{NO}_{\text{x}}\) compounds contribute to the formation of ground-level ozone and smog.
Energy generation also releases significant amounts of particulate matter (\(\text{PM}\)), which includes fine particles of soot and ash suspended in the air. \(\text{PM}_{2.5}\) is concerning because its small diameter allows it to penetrate deep into the human respiratory system. Residential and commercial heating systems also contribute, especially where wood, coal, or oil are burned for warmth. In densely populated areas, the collective emissions from these smaller sources significantly contribute to poor local air quality.
Mobile Sources
Transportation is a major source of air contamination, primarily from internal combustion engines. This includes on-road vehicles (cars and trucks) and non-road sources (marine vessels, locomotives, and aircraft). The combustion of gasoline and diesel fuel releases a complex mixture of gases and particles directly into the atmosphere.
Road vehicles are significant emitters of carbon monoxide (\(\text{CO}\)), a colorless, odorless gas that forms when fuel carbon is not completely oxidized during the combustion process. Incomplete burning also releases unburned hydrocarbons, classified as volatile organic compounds (VOCs). VOCs, combined with \(\text{NO}_{\text{x}}\) from high-temperature engine operation, react in sunlight to form ground-level ozone, the main component of photochemical smog.
Emissions are highly concentrated in urban corridors, intensifying local air quality problems. Although individual vehicles contribute small amounts, the sheer number of engines creates a substantial collective impact. Non-road sources, such as large container ships or jet aircraft, also release large volumes of \(\text{SO}_2\), \(\text{NO}_{\text{x}}\), and particulate matter, affecting areas near ports and airports. Mobile sources are regulated through fuel standards and emission control technologies, like catalytic converters, which convert harmful compounds into less harmful substances.
Industrial and Manufacturing Emissions
Industrial and manufacturing facilities contribute to air contamination through the specific processes used to create products, distinct from the energy required to power them. These process emissions involve the physical or chemical transformation of raw materials, releasing unique pollutants into the air.
Manufacturing operations, such as chemical production, metal smelting, and petroleum refining, release toxic air pollutants and heavy metals. Metal smelting processes can release contaminants like lead and arsenic, while chemical plants may emit chlorine compounds. Volatile organic compounds (VOCs) are also frequently emitted from industrial sources, particularly from the use of solvents, paints, coatings, and adhesives.
Construction and demolition activities also release substantial amounts of fugitive dust and coarse particulate matter (\(\text{PM}_{10}\)). The impact of industrial sources is often highly localized, creating concentrated contamination near the facility. Because these emissions can be hazardous to human health, specialized control measures are required to limit their release.
Agricultural and Land Management Activities
Farming and land management activities are significant contributors to air contamination. These non-industrial sources primarily involve the use of chemicals and the management of livestock. The application of synthetic fertilizers and the decomposition of animal waste are major sources of atmospheric ammonia (\(\text{NH}_3\)).
Ammonia reacts in the atmosphere to form fine secondary particulate matter, contributing to regional haze and respiratory issues. Livestock, particularly cattle, are a large source of methane (\(\text{CH}_4\)). Methane, a powerful greenhouse gas, is produced as a byproduct of enteric fermentation in the animals’ digestive systems.
Land tilling and harvesting practices generate considerable amounts of dust and particulate matter, especially in arid or semi-arid regions. Intentionally burning agricultural fields or crop residues releases smoke, soot, and unburned hydrocarbons. These activities introduce a mix of gases and particles that impact both local air quality and broader atmospheric chemistry.