Natural Air Pollutants: Sources And Their Impact

Natural sources of air pollution can be organic compounds from plants, sea salt, suspended soils, wind-blown dust, and dust from faraway sources like the Sahara. Other natural sources are released during events such as volcanic eruptions, forest fires, and wildfires. These events release large amounts of harmful gases and smoke that can increase background pollution levels for years, even in areas far away from the original source.

Wildfires and volcanoes

Volcanoes, on the other hand, emit ash, gases, and volcanic smog, known as "vog". Kilauea, a volcano in Hawaii, is known for emitting about 2,000 tons of sulfur dioxide (SO2) gas daily during sustained eruptions. SO2 is a poisonous gas that irritates the skin, eyes, nose, and throat. It can cause respiratory distress, especially during physical activity, as it penetrates deeply into the airway. Vog is formed when SO2 and other volcanic gases interact chemically with oxygen, moisture, dust, and sunlight in the atmosphere. It consists of gas and a mixture of tiny liquid and solid aerosol particles, which may contain toxic metals such as selenium, mercury, arsenic, and iridium.

The U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) continuously monitors the amount and composition of gas emissions from active volcanoes like Kilauea to better understand and address the hazards posed by volcanic air pollution. They work closely with communities and government officials to prepare for eruptions, enhance public safety, and minimise social and economic disruption. The HVO also provides regular updates and warnings to communicate changes in volcanic activity and identify unusual or hazardous circumstances.

While wildfires and volcanoes are natural sources of air pollution, it is worth noting that human activities, such as vehicle emissions, fuel oils, industrial processes, and power generation, remain the primary contributors to air pollution globally.

Organic compounds from plants

Air pollution is a mix of hazardous substances from both human-made and natural sources. Natural sources of air pollution include wind-blown dust, wildfires, and volcanoes. Wildfires, often caused by people, release smoke into the air, while volcanic eruptions emit ash and gases such as sulphur dioxide. Gases, like methane, are also emitted from decomposing organic matter in soils, and from livestock such as cows and sheep.

The impact of air pollutants on plants can vary substantially among different species. For example, a recent study involving hybrid poplar seedlings observed varying levels of isoprene emissions between upper and lower leaves. Another study examined the effect of O3 on isoprenoid emissions from three rose varieties, finding that floral emissions were higher and more sensitive during short-term exposure, while isoprenoid levels declined during long-term exposure due to substrate unavailability.

While the average concentrations of VOCs in the air are typically small, maximum hourly concentrations can be up to 100 times larger. This variability makes it challenging to study the long-term effects of VOCs on plants, and the potential health risks associated with exposure. However, it is clear that air pollutants can have significant impacts on vegetation and crop yields, and understanding these effects is an active area of research.

Sea salt

Furthermore, sea salt has been found to worsen air pollution in coastal areas, particularly when it combines with industrial and shipping pollution. A study by Professor Hans Osthoff of the University of Calgary revealed unexpectedly high levels of certain air pollutants where pollution from cities and ships meets salt in the ocean air along the southeast coast of the United States. This phenomenon has been shown to significantly increase ground-level ozone and other pollutant levels in coastal regions.

Suspended soils and dusts

Soil particles suspended in the air, commonly known as dust, pose risks to human health, environmental health, transportation safety, and the economy. Human exposure to dust has been associated with adverse health effects, including asthma, fungal infections, and even premature death. Dust can also cause Valley fever, an infection caused by the inhalation of soil-dwelling fungi. In addition, dust can contaminate water and food, spread pathogens, microplastics, heavy metals, and radionuclides, and reduce solar and wind power generation.

Dust particles in the atmosphere, also known as particulate matter (PM), vary in size and chemical composition depending on their source and history. Coarse particulates have a diameter greater than 2.5 μm, while fine particles have a diameter of less than 2.5 μm. These fine particles can be inhaled deeply into lung tissue and contribute to serious health problems. The rate at which particles are removed from the atmosphere depends on their size, with larger particles depositing rapidly near their source and intermediate-sized particles depositing slowly, allowing them to be transported over long distances.

Soil and dust particles can contain both water-soluble components, such as sulphate, nitrate, and ammonium, and insoluble components, such as carbon, heavy metals, and soil minerals. They can also include endogenous components such as minerogenic colloidal clays, trace elements, biogenic organic materials, and biota, which may be beneficial or detrimental to human health depending on their concentrations and exposure pathway. Soils can also contain human-produced (anthropogenic) pollutants from agricultural and industrial activities, such as dark carbon from coal, wood, and dung fires. These fires contribute significantly to the atmospheric mix of deflated soil materials and microorganisms.

Gases from decomposing organic matter

Gases released from decomposing organic matter are a significant natural source of air pollution. This process, known as anaerobic composting, occurs in the absence or limited supply of oxygen. It results in the production of gases like methane, hydrogen sulfide, organic acids, and other compounds. These gases contribute to air pollution and have an impact on both climate change and air quality.

The decomposition of organic matter is facilitated by microorganisms, primarily bacteria, in the soil. This process is influenced by various factors, including climatic conditions such as temperature, humidity, and precipitation. The type of soil and its properties, such as clay content and water-holding capacity, also play a crucial role. Additionally, the quality and quantity of the organic matter itself affect the decomposition process.

One notable example of gases from decomposing organic matter is the methane produced by livestock, particularly cows and sheep, through belching and flatulence. Livestock is the largest source of methane globally, and methane is a potent greenhouse gas that contributes to climate change.

Another source of these gases is the anaerobic decomposition of solid waste in landfills and manure storage lagoons. Composting is often employed to manage organic waste and reduce methane emissions from landfills. However, the composting process itself releases greenhouse gases and air pollutants, and the extent of these emissions is not yet fully understood.

The gases released from decomposing organic matter can have significant impacts on the environment and human health. Methane, for instance, is a powerful greenhouse gas that contributes to global warming and climate change. Additionally, the release of gases from organic matter decomposition can affect air quality, leading to potential respiratory issues and other health problems for humans and other living organisms.

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