Natural Air Pollutants: Examples From Our Environment

Natural sources of air pollution include organic compounds from plants, sea salt, suspended soils, and dusts (e.g. from the Sahara). Other natural sources are released during events such as volcanic eruptions, forest fires, and wildfires, which release large amounts of harmful gases and smoke that can increase background pollution levels for years, even in areas far from the original source. For example, volcanic eruptions can spew massive amounts of sulphur dioxide into the atmosphere, and wildfires can add carbon monoxide and particulate matter containing organic contaminants such as PAHs.

Volcanic eruptions and forest fires

Volcanic eruptions, for instance, can inject massive amounts of volcanic gas, aerosol droplets, and ash into the stratosphere. While the injected ash generally falls back to the Earth within a matter of days or weeks and has minimal impact on long-term climate change, the volcanic gases released can have a more lasting effect. Gases like sulfur dioxide (SO2) have been known to cause global cooling, while carbon dioxide (CO2), a greenhouse gas, has the potential to contribute to global warming. The 1783-1784 Laki fissure eruption in Iceland is a notable example, as it released approximately 120 million tons of SO2, leading to the regional cooling of Europe and North America.

Forest fires, on the other hand, are also responsible for releasing a significant amount of particulate matter pollution into the atmosphere. Regions affected by these fires often experience a decline in air quality. Indonesia, for instance, faces the challenge of improving its air quality while also dealing with frequent forest fires.

The impact of volcanic eruptions and forest fires on air pollution can be long-lasting and far-reaching. The release of harmful substances into the atmosphere can have both global and regional effects, affecting climate patterns and air quality for extended periods. Therefore, understanding and mitigating the impact of these natural events on air pollution is crucial for maintaining the health and well-being of ecosystems and human populations alike.

Organic compounds from plants

Natural sources of air pollution include organic compounds from plants, sea salt, suspended soils, and dust. Organic compounds emitted by plants, known as biogenic volatile organic compounds (BVOCs), can interact with other pollutants in the atmosphere, such as nitrogen oxides (NOx) and ozone (O3), leading to complex effects on air quality.

BVOCs comprise a diverse group of compounds, including isoprene, monoterpenes, and sesquiterpenes, which are released by plants in response to various environmental factors, including elevated ozone levels. These compounds have been observed to accumulate in leaf organs, causing synergistic or indirect effects that can be more detrimental than direct exposure.

The impact of BVOCs on plant health is complex and varies depending on the plant species and the specific BVOCs involved. While some BVOCs may have negative consequences, others can play a beneficial role in promoting plant growth and disease resistance. For example, isoprene emissions from plants can act as a protective mechanism against the harmful effects of ozone.

Long-term exposure to industrial emissions containing volatile organic compounds (VOCs) poses a significant risk to vegetation. Atmospheric concentrations of certain VOCs can harm sensitive tree species, and the complex interactions between VOCs, NOx, and O3 make it challenging to study the full extent of their effects on urban green spaces.

In addition to BVOCs, plants also release other organic compounds that can contribute to air pollution. For example, methane (CH4) is produced by livestock during digestion and is a significant greenhouse gas that contributes to climate change. While not directly emitted by plants, methane emissions are a consequence of the agricultural industry and the presence of methane-producing bacteria in the digestive systems of ruminant animals.

Nitrogen oxides from vehicles

Natural sources of air pollution include organic compounds from plants, sea salt, suspended soils, and dust. Other natural sources are released during events such as volcanic eruptions and forest fires, which can increase background pollution levels for years.

One significant source of air pollution is nitrogen oxides from vehicles. Nitrogen oxides (NOx) are formed when nitrogen and oxygen, present in the ambient air, combine during the combustion of fuel in gasoline and diesel engines. This process occurs in cars, vans, buses, and lorries, with larger vehicles producing more pollution. The amount of NOx formed is directly related to the combustion temperature, which increases as the load on the engine increases, such as during rapid acceleration or driving uphill.

The switch from petrol to diesel vehicles has exacerbated the issue, as diesel combustion produces higher levels of NOx overall, with a significantly larger proportion of nitrogen dioxide (NO2) compared to petrol combustion. NO2 is a harmful component of NOx, with studies linking long-term exposure to respiratory and cardiovascular issues, as well as children's respiratory symptoms and reduced lung function.

To address this problem, several methods have been developed to control and reduce NOx emissions. One technique is Exhaust Gas Recirculation (EGR), which redirects the exhaust gas back into the engine cylinder, reducing the oxygen and temperature levels, and subsequently lowering NOx production. Another approach is the use of catalytic converters, which can effectively reduce NOx through chemical reactions, converting it into nitrogen, water, and/or CO2.

Regulations and standards, such as the Euro emissions standards, have been implemented to limit the maximum emissions allowed in vehicles. These regulations have played a crucial role in reducing emissions, with an 84% decrease in nitrogen oxide emissions between 2001 and 2017. However, it was discovered in 2015 that some manufacturers had manipulated emissions control systems to circumvent vehicle tests, bringing the issue of NOx emissions into the spotlight and prompting further actions to address this serious health and environmental concern.

Sulphur dioxide from industrial processes

Sulphur dioxide (or sulfur dioxide) is a natural air pollutant that is released into the atmosphere through various human activities and natural processes. While it is only mildly toxic relative to other chemicals, it is a major air pollutant due to its ubiquity and significant impacts on human health and the environment. Sulphur dioxide is a colourless gas with a pungent smell that is responsible for the odour of burnt matches.

One of the primary sources of sulphur dioxide in the atmosphere is industrial processes, particularly the burning of fossil fuels by power plants and other industrial facilities. This includes the combustion of coal and other sulfur-containing fuels, such as bunker fuel. The industrial processes that release the largest amounts of sulphur dioxide are typically power plants, followed by other industrial facilities such as metal refineries. Smaller sources of sulphur dioxide emissions from industrial processes include extracting metal from ore and locomotives, ships, and other vehicles and heavy equipment that burn fuel with a high sulfur content.

The release of sulphur dioxide into the atmosphere through industrial processes has significant impacts on human health and the environment. Short-term exposures to high concentrations of sulphur dioxide can harm the human respiratory system and make breathing difficult, particularly for people with asthma. The formation of other sulfur oxides (SOx) can also occur due to high concentrations of sulphur dioxide in the air.

To mitigate the health risks associated with sulphur dioxide emissions from industrial processes, various organizations have set exposure limits and standards. For example, in 2008, the American Conference of Governmental Industrial Hygienists reduced the short-term exposure limit to 0.25 parts per million (ppm). Additionally, the EPA in the US has established a one-hour standard at a level of 75 parts per billion (ppb) to protect public health.

It is important to note that while natural sources of sulphur dioxide, such as volcanic activity, also contribute to air pollution, human activities, such as industrial processes, have become the dominant source of sulphur dioxide emissions in industrialized regions.

Particulate matter from wood fires

Natural sources of air pollution include organic compounds from plants, sea salt, suspended soils, dust, and large amounts of harmful gases and smoke released during catastrophes like volcanic eruptions and forest fires. One example of natural air pollution is particulate matter from wood fires.

Wood fires, including residential wood-burning stoves and fireplaces, as well as outdoor fire pits and boilers, contribute to air pollution by releasing smoke and particulate matter into the atmosphere. This wood smoke, or wood smoke, is a waste product that can stick to chimneys as creosote or be released as air pollution. It contains a mixture of solid particles and liquid droplets, including larger particles such as dust, dirt, soot, or smoke, and smaller "fine particles" with diameters of 2.5 micrometres or less, known as PM2.5. These fine particles are particularly concerning as they are small enough to enter the bloodstream through the lungs and potentially penetrate the brain.

The type of wood burned and the burning device used can impact the amount of particulate matter produced. For example, cordwood, especially when damp, releases more smoke than wood pellets, and old or poorly installed wood-burning stoves can also result in higher pollution levels. The EPA (United States Environmental Protection Agency) has been certifying wood stoves for emissions levels since 1990, and the use of EPA-certified devices can help reduce smoke and pollution levels.

The health effects of exposure to particulate matter from wood fires can be serious. Wood smoke can irritate the lungs, cause inflammation, affect the immune system, and increase the risk of lung infections, including COVID-19. Long-term exposure to the chemicals in wood smoke, such as carbon monoxide, nitrogen oxides, polycyclic aromatic hydrocarbons (PAHs), and volatile organic compounds (VOCs), has been linked to an increased risk of asthma, lung disease, and chronic bronchitis. Additionally, the compounds in wood smoke have been associated with an increased risk of certain types of cancer, including lung and breast cancer.

To reduce exposure to particulate matter from wood fires, individuals can take several measures. These include limiting strenuous outdoor activity when particle pollution levels are high, keeping windows and doors shut, and using air conditioners set to the "recycle" setting. It is also recommended to refrain from using wood-burning stoves, fireplaces, candles, or engaging in activities that can stir up dust, such as vacuuming or dusting, when particulate levels outdoors are high.

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