Natural Air Pollutants: Two Unseen Sources Revealed

Air pollution is the presence of harmful substances in the air, which can be gases, solid particles, or liquid droplets. While most air pollution is caused by human activity, natural sources can also be significant. Two examples of natural sources of air pollution are wildfires and volcanic eruptions, which release harmful substances such as carbon monoxide, nitrogen oxides, and particulate matter into the atmosphere. Wildfires are a significant source of outdoor carbon monoxide, while volcanic eruptions produce a range of pollutants, including steam, carbon dioxide, sulfur dioxide, and ash particulates. These natural sources can contribute to air quality issues and have impacts on human health and the environment.

Wildfires, which also generate high levels of PM pollution, CO, and NOx

Wildfires are a natural source of air pollution, and they generate high levels of PM pollution, CO, and NOx. As wildfires become more frequent and intense due to climate change, fire smoke has significantly worsened air quality, posing greater health risks.

PM pollution, or particle pollution, includes all airborne substances that are not gases. It is a mix of microscopic solid particles or liquid droplets suspended in a gas. Wildfire smoke contains large quantities of fine particulate matter (PM2.5), with diameters smaller than 2.5 μm. These particles can adversely affect regional air quality in downwind communities that are tens to hundreds of kilometres away. Studies have shown that wildfire smoke has the most significant impact on air quality in the West Coast of the United States, followed by the Southeastern region.

Carbon monoxide (CO) is a toxic, colorless, and odorless gas produced by the combustion of fuels such as natural gas, coal, or wood. Wildfires and bonfires are now the main sources of outdoor CO, as emissions from vehicles have decreased with technological advancements. Once emitted, CO from wildfires can undergo chemical transformations in the atmosphere, generating secondary pollutants such as ozone (O3) and secondary organic aerosol (SOA).

Nitrogen oxides (NOx) are gaseous compounds that include nitrous oxide (NO) and nitrogen dioxide (NO2). NOx is produced during the burning of fossil fuels and biomass, as well as by lightning. Wildfires emit NOx, which can react with other compounds in the atmosphere to form acid rain, haze, and nutrient pollution in water. Similar to CO, NOx can also contribute to the formation of O3 and SOA through complex chemical reactions.

The impact of wildfires on air quality is complex and challenging to understand due to the variability in the quantity and composition of emissions. Factors such as the amount and type of fuel, meteorology, and burning conditions all play a role in determining the emissions from wildfires. The spatial patterns of smoke PM2.5 from wildfires can be difficult to characterize, and ground observations alone may not be sufficient to distinguish it from other sources. However, chemical transport models (CTMs) have been developed to improve the understanding of wildfire smoke's impact on air quality.

Volcanic eruptions, which release NH3 and SO2, forming secondary PM when combined with other pollutants

Volcanic eruptions are a natural source of air pollution. They release various gases and particles into the atmosphere, including steam, carbon dioxide (CO2), sulfur dioxide (SO2), ash, and other pollutants. While steam accounts for the majority of emissions during volcanic eruptions, the release of certain gases, such as SO2 and NH3, can have significant impacts on air quality and climate.

Sulfur dioxide (SO2) is a prominent gas emitted during volcanic eruptions. SO2 can react with other atmospheric components, such as sunlight, gases, and aerosols, to form fine particles. These particles contribute to the formation of volcanic smog, also known as "vog," which poses recognized health hazards. Additionally, the conversion of SO2 to sulfuric acid (H2SO4) in the atmosphere has been linked to global cooling episodes. The 1991 eruption of Mount Pinatubo, a supervolcano, injected a massive sulfur dioxide cloud into the stratosphere, leading to a significant decline in average temperatures for several years.

Volcanic eruptions also release carbon dioxide (CO2), a greenhouse gas associated with climate change. While human activities contribute significantly more CO2 emissions, very large volcanic eruptions can inject substantial amounts of CO2 into the atmosphere. For example, the 1980 eruption of Mount St. Helens released approximately 10 million tons of CO2 in just 9 hours.

Another gas released during volcanic eruptions is hydrogen fluoride (HF), which can have detrimental effects on the respiratory systems of humans and animals. Additionally, the presence of SO2 and HF in the atmosphere can lead to the formation of acid rain, impacting vegetation and ecosystems.

The impact of volcanic eruptions on air pollution can vary depending on factors such as the size and frequency of the eruptions. While large explosive eruptions are relatively rare, occurring approximately once a decade, they can have significant short-term and long-term effects on air quality and climate. The continuous eruption of the Kilauea volcano in Hawaii since 1983 has been a significant source of air pollution, releasing about 2,000 tonnes of SO2 into the lower troposphere daily.

Vegetation, which emits VOCs on warmer days, reacting with human pollution to create a seasonal haze

Natural sources of air pollution include wind-blown dust, wildfires, and volcanoes. Vegetation is also a natural source of air pollution, as it emits VOCs (volatile organic compounds) on warmer days. VOCs react with human-made pollution sources like NOx, SO2, and organic carbon to create a seasonal haze of secondary pollutants. This haze is a result of the chemical reaction between the VOCs and human-made pollutants, which can have harmful effects on both human health and the environment.

VOCs are emitted by certain types of vegetation, including black gum, poplar, oak, and willow. These species can produce abundant VOCs, leading to ozone levels up to eight times higher than those produced by low-impact tree species. While vegetation can contribute to air pollution through the emission of VOCs, it is important to note that it also plays a crucial role in reducing air pollution levels. Urban vegetation and green spaces can effectively reduce airborne pollutant concentrations, particularly particulate matter (PM).

Particulate matter refers to airborne substances that are not gases, including microscopic solid particles or liquid droplets suspended in a gas. These particles can vary in size, with smaller particles posing a greater risk to health as they can reach the bloodstream. The design and choice of urban vegetation are critical when utilizing it as an ecosystem service to improve air quality. Different types of vegetation can catch different particle sizes, and the placement of vegetation in relation to pollution sources is essential for effective filtration.

To maximize the air-purifying potential of vegetation, it is necessary to consider factors such as plant selection, spatial setup, ventilation, and maintenance. Additionally, the presence of green spaces can influence the deposition and dispersion of pollutants. Low vegetation placed close to pollution sources can enhance air quality by increasing deposition, while tall or dense vegetation may deflect the air stream above instead of filtering it.

While vegetation can contribute to air pollution through VOC emissions, it also plays a significant role in mitigating air pollution levels. The reductive effects of vegetation and green spaces on airborne pollutant concentrations are well-recognized, and their presence can help improve air quality, particularly in urban areas. However, it is important to note that the overall impact of vegetation on air pollution is complex and influenced by various factors, including vegetation type, density, and proximity to pollution sources.

Sand and dust storms, which are responsible for PM2.5 pollution due to the size of the grains spread

Natural sources of air pollution include wind-blown dust, wildfires, and volcanoes. Two examples of natural sources of air pollution are wildfires and dust storms. Dust storms, or sandstorms, are a natural meteorological phenomenon that frequently occurs in many arid and semi-arid regions of the world.

Sand and dust storms are responsible for PM2.5 pollution due to the size of the grains spread. PM2.5 refers to particulate matter that is 2.5 micrometres or smaller in diameter. These fine particles can be suspended in the air and inhaled, leading to adverse health effects. During a sandstorm, the dust contains environmental pollutants, microorganisms, and bacteria that can be harmful to humans and the environment.

Studies have shown that sandstorms significantly increase the levels of PM2.5 in the air. For example, a sandstorm in Riyadh, Saudi Arabia, led to a 76.71% increase in PM2.5 levels. Similarly, an Asian dust storm (ADS) in Taiwan resulted in PM2.5 levels of 61.93 μg/m3, much higher than the typical level of 48.87 μg/m3. These increased levels of PM2.5 have been linked to a rise in emergency room visits for cardiovascular and respiratory issues, especially in infants, children, and older adults with pre-existing heart or lung conditions.

The grains of dust and sand in a sandstorm are small enough to be considered PM2.5, and when they are spread over long distances, they contribute to the overall level of harmful particulate matter in the atmosphere. This, in turn, can lead to a range of health issues, particularly in vulnerable individuals. Therefore, it is important for health authorities to monitor and provide information to the public about sandstorms and their associated health risks.

Indoor sources, such as cooking and heating with biomass (e.g. wood), which can cause indoor air pollution

Indoor air pollution is often caused by the use of biomass, such as wood, for cooking and heating. About one-third of the world's population burns organic materials (biomass fuel) including wood, dung, or charcoal for cooking, heating, and lighting. This form of energy usage is associated with high levels of indoor air pollution and an increase in the incidence of respiratory infections, including pneumonia, tuberculosis, and chronic obstructive pulmonary disease, as well as low birth weight, cataracts, cardiovascular events, and all-cause mortality in both adults and children.

Biomass fuel smoke is a major health concern, especially in the developing world. Studies have shown a link between long-term exposure to biomass fuel smoke from cooking and the development of lung cancer. The International Agency for Research on Cancer (IARC) has termed biomass smoke a "probable carcinogen." In addition, particulate air pollution has been linked to an increased risk of cardiovascular disease.

The use of solid biomass combustion for heating can also emit various air pollutants that negatively affect human health. The most relevant pollutants from solid biomass combustion that affect air quality and pose health risks are products from incomplete combustion (PIC), such as volatile organic compounds (VOCs) and solid particulate matter (biomass char, soot, and tars). Incomplete combustion also emits carbon monoxide (CO), a toxic gas that can cause death at high concentrations. The type and level of emissions from biomass heating depend on whether complete or incomplete combustion occurs in the heating system.

To reduce indoor air pollution from cooking and heating with biomass, improved biomass stoves can be used. These stoves reduce emissions by 50-80%, while clean-fuel stoves can reduce emissions by more than 95%. Additionally, stoves that use cleaner-burning fuels such as biogas, electricity, ethanol, and liquefied petroleum can also help improve indoor air quality.

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