Air Pollution: The Secondary Sources Revealed

Air pollution is caused by excess concentrations of foreign substances entering the Earth's atmosphere. These pollutants can be classified into three groups: primary, secondary, and re-emission sources. While primary sources are direct emissions from an air pollution source, secondary sources are formed when chemical reactions in the atmosphere alter primary pollutants. This process, which is harder to control and not well understood, creates pollutants such as ozone, haze, and carbon dioxide. These secondary pollutants are responsible for issues like photochemical smog, reduced visibility, and elevated ozone concentrations. They are influenced by factors such as climate, meteorology, and topography, which determine how much pollution reaches a given area.

Vehicle emissions

Carbon monoxide is a well-known toxic gas that is harmful to human health and a significant contributor to air pollution. Nitrogen oxides, formed through the burning of nitrogen in the air, also play a significant role in air pollution, particularly in the formation of smog. Diesel vehicles are major contributors to nitrogen oxide emissions, with the transportation sector accounting for a large portion of these emissions.

Sulfur dioxide is another pollutant produced by vehicles burning sulfur-containing fuels, especially diesel and coal. It can react in the atmosphere to form fine particles, posing health risks, especially to children and asthmatics. Particulate matter, including soot, is another type of pollutant emitted by vehicles, with fine particles penetrating deep into the lungs and causing serious health issues.

Volatile organic compounds (VOCs) are also released from vehicle emissions. These compounds react with nitrogen oxides in the presence of sunlight to form ground-level ozone, a key ingredient in smog. While ozone is beneficial in the upper atmosphere, ground-level ozone irritates the respiratory system, causing coughing, choking, and reduced lung capacity.

While newer vehicles tend to emit less pollution due to improved fuel efficiency and stronger fuel economy standards, the growing popularity of less fuel-efficient SUVs and pickup trucks offsets some of this progress. Heavy-duty vehicles, despite comprising only about 10% of all vehicles on the road, contribute significantly to global warming emissions and air pollution.

To address these issues, organizations like the EPA have implemented standards and programs to reduce emissions from transportation sources, leading to improvements in air quality and public health. The development and adoption of zero-emission heavy-duty vehicles and electric buses, trucks, and other heavy-duty vehicles are also crucial steps towards mitigating the impact of vehicle emissions on air pollution.

Industrial processes

Industrial air pollution is caused by the introduction of dust particles, gases, and smoke into the atmosphere, which often exceeds air quality levels. These pollutants include vapours, aerosols, solid particles, toxic gases, and smoke, which are emitted from industrial processes.

Some specific examples of industrial sources of air pollution include fuel-fired boilers, internal combustion gas boilers, and gas stoves, which are responsible for emitting carbon monoxide. Carbon dioxide is also a major greenhouse gas emitted from industrial combustion systems.

In addition to carbon monoxide and carbon dioxide, industrial processes emit other harmful pollutants. For instance, industrial clusters in Tehran, Iran, were found to emit high concentrations of CO, NO2, SO2, and O3. These pollutants are attributed to the increased combustion of fossil fuels during cooler months and intensified photochemical processes during warmer months.

Furthermore, industrial processes contribute significantly to the emission of coarse particulate matter (PM) with aerodynamic diameters between 2.5 μm and 10 μm. These particles are generated by physical processes such as resuspension of soil and road dust, as well as industrial activities.

To reduce industrial air pollution, several mitigation strategies are necessary. These include industrial processes upgradation, energy efficiency improvements, and the implementation of applicable technologies such as CO2 sequestering and combustion process enhancements.

Residential heating and cooking

Indoor air pollution also contributes significantly to outdoor air pollution, including toxic gases, particulate matter, and black carbon. The use of solid fuels for cooking and heating has several negative consequences, including increased environmental health risks, climate change, local environmental degradation, and poverty in low-income communities. It is estimated that exposure to emissions from solid fuels causes approximately 3.2 million premature deaths worldwide annually, with approximately 237,000 of these deaths being children under the age of five.

Household energy use, including cooking and heating, is a critical factor in daily life. The WHO has issued guidelines for indoor air quality, recommending against the use of kerosene and unprocessed coal and emphasizing the adoption of clean fuels and technologies. Clean fuels are defined as solar, electricity, biogas, liquefied petroleum gas (LPG), natural gas, alcohol fuels, and biomass stoves that meet emission targets.

To address the issue of household air pollution, the United States Environmental Protection Agency (EPA) works with other government agencies and partner organizations to promote the use of cleaner home cooking technologies and fuels that are affordable, reliable, efficient, and safer. Similarly, the U.S. Agency for International Development's Clean Air Catalyst program aims to combat air pollution by working with communities to implement locally tailored solutions to reduce pollution from household energy use.

In conclusion, residential heating and cooking practices significantly impact air pollution levels, particularly when solid fuels are burned indoors. The transition to cleaner fuels and technologies is essential to improving air quality, public health, and environmental sustainability.

Natural processes

Volcanoes emit harmful gases such as sulfur and chlorine, as well as particles of volcanic matter, which contribute to air pollution. Volcanic activity releases sulfur dioxide (SO2), which forms sulfuric acid when combined with water droplets in the air, leading to acid rain. Additionally, volcanoes emit hydrogen sulfide and ammonia (NH3), which can form secondary particulate matter when combined with other pollutants.

Wildfires and forest fires produce smoke and ash, and high levels of particulate matter (PM) pollution, along with carbon monoxide (CO) and nitrogen oxides (NOx). These fires can reduce visibility and contribute to the haze and smog that affect air quality.

Dust storms, driven by natural weather patterns, can transport large amounts of dust and particulates over long distances, impacting air quality across various regions.

Biological decay processes, including anaerobic decomposition on land and in swamps and oceans, release methane (CH4) and contribute to the atmospheric oxidation of sulfur compounds, leading to sulfur dioxide emissions.

Natural water springs are another source of hydrogen sulfide, and lightning strikes produce nitrogen dioxide (NO2), contributing to the reddish-brown colour of smog.

While natural processes are significant contributors to air pollution, it is important to recognise that human activities, such as industrial processes, vehicular emissions, and agriculture, also play a major role in polluting the air we breathe.

Secondary organic aerosol (haze)

Secondary organic aerosols (SOAs) are a type of secondary air pollutant. Secondary air pollutants are formed in the lower atmosphere through chemical reactions. Other examples of secondary pollutants include ozone and photochemical smog.

SOAs are formed from anthropogenic emissions of volatile organic compounds (VOCs). VOCs are emitted through the combustion of fossil fuels, such as coal, and biomass burning. These emissions react with sunlight and water vapour to form SOAs.

SOAs are a major contributor to particulate pollution, which is a serious environmental problem in China. In January 2013, a severe haze pollution event affected about 800 million people in China. It was found that this event was driven to a large extent by secondary aerosol formation, which contributed 30-77% of PM2.5 (particulate matter with an aerodynamic diameter of less than 2.5 micrometres) and 44-71% of organic aerosol.

SOAs have serious impacts on air quality, regional and global climates, and human health. They contribute substantially to air pollution mortality. As such, it is important to reduce the emissions of SOA precursors, such as those from fossil fuel combustion and biomass burning, in order to control PM2.5 levels and reduce their environmental, economic, and health impacts.

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