Secondary Pollutants: A Hidden Danger Lurking In The Air?

Secondary pollutants are contaminants that are formed in the atmosphere as a result of other factors. They are not emitted directly but are instead the result of interactions between primary emissions. These pollutants are very sensitive to weather patterns and can be formed from a wide variety of compounds, making them harder to control. Some examples of secondary pollutants include ground-level ozone, peroxyacyl nitrates (PANs), nitric acid, and acid rain. These pollutants can have serious adverse effects on human health, vegetation, lakes, and fish, as well as structures.

Ground-level ozone

Ozone is a gas composed of three oxygen atoms. While stratospheric ozone occurs naturally in the upper atmosphere and shields the Earth from harmful ultraviolet radiation, ground-level ozone is considered ""bad"" as it can trigger a range of health issues. It is the primary constituent of smog, a type of photochemical fog containing harmful substances such as NOx, hydrogen peroxide, and nitric and sulfuric acids. Smog is responsible for the ""pollution beret"" often observed over cities and towns.

In addition to its impacts on human health, ground-level ozone also harms vegetation and ecosystems. It can cause mass crop die-offs, reduce plant growth, and increase disease susceptibility. This pollutant slows photosynthesis by reducing the absorption of CO2 by plants, which contributes to acidification as it lowers the pH of the soil and water.

To address the issue of ground-level ozone, the US Environmental Protection Agency (EPA) has established regulations and standards to reduce ozone levels in outdoor air. States are required to develop implementation plans (SIPs) to improve air quality in areas that do not meet the national standards, and these plans outline specific measures to reduce emissions and improve overall air quality.

Nitrogen oxides and sulphur dioxide

Nitrogen oxides (NOx) and sulphur dioxide (SO2) are both primary and secondary pollutants. They are formed by the combustion of fossil fuels for domestic heating, industries, and power generation. They can also be emitted from vehicles and industrial processes.

Nitrogen dioxide (NO2) is a reddish-brown gas that is soluble in water and a strong oxidant. It is formed by high-temperature combustion of fuels in processes such as heating, transportation, industry, and power generation. Household sources of nitrogen oxides include equipment that burns fuels, such as furnaces, fireplaces, and gas stoves and ovens. Exposure to nitrogen dioxide can irritate airways and aggravate respiratory diseases. It is also an important precursor to ground-level ozone, which is a major component of smog and is closely linked to asthma and other respiratory conditions.

Sulphur dioxide (SO2) is a colourless gas that is also soluble in water. Exposure to SO2 is associated with asthma hospital admissions and emergency room visits. Controlled human exposure studies have shown that experimental SO2 exposure causes changes in airway physiology, including increased airway resistance. It contributes to respiratory symptoms in both healthy patients and those with underlying pulmonary disease.

Both NO2 and SO2 are important ambient air pollutants that can have significant impacts on human health. They are included in the WHO global air quality guidelines, along with particulate matter, ozone, and carbon monoxide.

As secondary pollutants, NOx and SO2 are formed in the lower atmosphere by chemical reactions between primary pollutants. They can be transported long distances by wind, meaning that even rural areas with fewer emissions can still experience high levels of these pollutants. Tropospheric ozone or "bad ozone" is formed by the interaction of various precursors, including NOx, in the presence of sunlight. It is dangerous to human health and can cause respiratory problems and eye irritation. It also has detrimental effects on the environment, damaging crops and plants by reducing their ability to absorb CO2.

Smog

There are two main types of smog: photochemical smog and winter smog. Photochemical smog is the dominant type during the summer months when temperatures are warmer and there is more sunlight. It is formed by the interaction of sunlight with nitrogen oxides and hydrocarbon vapours emitted by automobiles and other sources. This type of smog does not require the presence of smoke or fog and is characterised by a light brownish coloration of the atmosphere, reduced visibility, plant damage, eye irritation, and respiratory distress.

Winter smog, on the other hand, is more common during the colder months when there is an increase in the use of coal and other fossil fuels for heating. This type of smog is characterised by the presence of smoke and fog, with coal fires contributing significantly to its formation. London, for example, was notorious for its coal-caused smogs, often referred to as "pea-soupers".

The formation of smog relies on both primary and secondary pollutants. Primary pollutants, such as sulfur dioxide from coal combustion, are emitted directly from a source. Secondary pollutants, such as ozone, are formed when primary pollutants undergo chemical reactions in the atmosphere. Tropospheric ozone, also known as "bad ozone", is a major secondary pollutant that is particularly harmful to human health and the environment. It can cause respiratory problems, eye irritation, and damage to crops and plants by inhibiting photosynthesis.

The severity of smog can be influenced by various factors such as dampness, atmospheric inversions that trap pollution, and the presence of mountains and wind patterns that can prevent the dispersal of pollutants. Smog is toxic to humans and has been linked to severe sickness, shortened lifespans, and premature death.

Particles in suspension

PM2.5, or fine inhalable particles, have diameters that are generally 2.5 micrometers and smaller. To put this into perspective, a human hair is about 70 micrometers in diameter, making it 30 times larger than the largest fine particle. These fine particles are particularly harmful as they can penetrate deep into the respiratory system and even enter the bloodstream, carrying harmful substances into the body.

The sources of these particles vary, with some emitted directly from natural or anthropogenic sources such as construction sites, unpaved roads, fields, smokestacks, fires, and combustion engines. However, most fine particles form in the atmosphere through complex reactions of chemicals, including sulfur dioxide and nitrogen oxides, which are emitted from power plants, industries, and automobiles.

The size of particles in suspension is a critical factor in determining their impact on human health. Smaller particles, such as PM2.5 and PM10, pose greater health risks as they can be inhaled and absorbed by the body. Larger particles, such as those over 10 micrometers in diameter, are less likely to be inhaled and are often regulated by environmental agencies.

The presence of particles in suspension can lead to elevated air pollution levels, particularly in urban areas near busy roads, during rush hour, and on hot, humid days with stagnant air. These particles can remain in the atmosphere for extended periods, travelling long distances and affecting air quality in regions far from their original source.

Volatile organic compounds

VOCs are monitored as part of the air toxics monitoring network in Minnesota, for example, due to their role in forming ground-level ozone, which is a significant pollutant and detrimental to human and environmental health. Tropospheric or "bad ozone" is formed by the interaction of VOCs, carbon monoxide, and nitrogen oxide in the presence of sunlight. It can cause respiratory problems and eye irritation in humans, and damage crops and plants by slowing photosynthesis and reducing the absorption of CO2 by the plant.

The health effects of VOCs vary greatly, from highly toxic to those with no known health effects. Exposure to VOC vapors can cause a range of adverse health consequences, including eye, nose, and throat irritation, headaches, nausea, dizziness, and difficulty breathing. Long-term exposure to VOCs can lead to more severe health issues, including damage to the liver, kidneys, and central nervous system, and some VOCs are even linked to cancer.

To minimize exposure to VOCs, it is important to increase ventilation when using products that emit them and properly dispose of unwanted containers through community toxic waste disposal programs. Buying limited quantities of products containing VOCs and storing them in well-ventilated areas can also help reduce exposure.

Frequently asked questions