Air Pollutants: Correctly Identifying Pairs

Air pollution is a pressing issue that poses a significant threat to human health, particularly to children. The main sources of air pollution can be attributed to human activities, primarily from industries, transportation, agriculture, and residential incineration. Among the various air pollutants, some are correctly paired with their major effects. For instance, nitrogen oxides are known for blocking UV radiation, while sulfur oxides contribute to acid precipitation. Understanding these pairings is crucial for implementing effective measures to combat air pollution and mitigate its detrimental impact on our environment and health.

Carbon oxides cause corrosion of metal

Carbon oxides, including carbon dioxide (CO2) and carbon monoxide (CO), are known to cause corrosion in metals, particularly steel and carbon steel. This corrosion occurs through various mechanisms, and several factors influence the rate and extent of corrosion.

One of the primary mechanisms of carbon oxide-induced corrosion is the formation of acidic compounds. Carbon dioxide, in the presence of water vapour and sulphur-containing compounds, can lead to the formation of diverse acids. These acids aggressively attack steel structures, causing corrosion and posing environmental and safety risks. This is particularly relevant in cargo tanks, where the presence of inert gas and water vapour can result in the formation of corrosive acids.

Additionally, carbon oxides can cause corrosion through hydrogen embrittlement cracking. In the presence of carbon monoxide, steel exhibits enhanced absorption of hydrogen, which can lead to cracking in high-stress areas. This type of corrosion is influenced by temperature, with lower temperatures favouring the formation of non-protective FeCO3 and increasing the corrosion rate.

The presence of certain ions, such as Cl−, can also affect carbon oxide-induced corrosion. Chloride ions can penetrate the metal surface through defects in the corrosion layer and accumulate in the pits of the iron oxide layer. This accumulation intensifies the corrosion process and can lead to stress corrosion cracking in stainless steel.

Furthermore, carbon dioxide corrosion can result in local pitting corrosion, moss-like corrosion, and mesalike pitting corrosion. Mese-like pitting corrosion is particularly severe and can cause significant economic losses. It occurs when the protective film formed during the corrosion reaction is non-uniform or fails to form correctly.

To mitigate carbon oxide-induced corrosion, various methods can be employed. One approach is to increase the pH, which inhibits the cathodic reaction of H+ depolarization and promotes the formation of a protective oxide film on the metal surface, reducing the corrosion rate. Additionally, alloys with excellent corrosion resistance, such as the Ni-based alloy Haynes 230, can be utilised, although their high cost and complex processing may be limiting factors.

Nitrogen oxides block UV radiation

Nitrogen oxides (NOx) are a group of air pollutants that can be removed using TiO2 photocatalysts. These catalysts work by absorbing UV energy from sunlight to generate holes and electrons, which then react with H2O and O2 in the atmosphere to produce reactive oxygen species. This process decomposes NOx, a representative precursor, and improves indoor air quality.

While nitrogen oxides are air pollutants, nitric oxide (NO) is a vasodilator that is generated in the body by the enzyme nitric oxide synthase (NOS). Exposure to UV-A radiation from sunlight can also liberate nitric oxide from skin cells, eliciting vasodilation in vivo. This effect has been observed in keratinocytes, fibroblasts, and endothelial cells, with the greatest NO release following biologically relevant doses of UV-A.

The duration of nitric oxide release in skin cells after UV exposure is not well studied, but recent evidence suggests that UV-A exposure upregulates cNOS alongside iNOS. This results in the persistence of nitric oxide release in a dark reaction, peaking around 48 hours post-exposure. This effect may be especially beneficial for the elderly, hypertensive patients, or those with impaired nitric oxide function.

While UV-A radiation has some positive effects, it is still responsible for undesirable outcomes, including oxidative damage, skin photo-aging, and immunosuppressive effects. Therefore, public health advice regarding sunlight exposure must constantly be reassessed in light of new evidence about the effects of UV radiation, whether beneficial or detrimental.

Hydrocarbons reduce visibility

Hydrocarbons are organic compounds composed entirely of hydrogen and carbon. They can be gases (e.g. methane, propane), liquids (e.g. hexane, benzene), low-melting solids (e.g. paraffin wax, naphthalene), or polymers (e.g. polyethylene, polystyrene). Hydrocarbons are generally colourless and hydrophobic, with a faint odour that may resemble gasoline or lighter fluid. They occur in diverse molecular structures and phases, and their combustion is the primary energy source for humanity.

Hydrocarbons are a significant contributor to air pollution, with industrial sources such as power generation, vehicle emissions, and manufacturing activities being major culprits. Polycyclic Aromatic Hydrocarbons (PAHs), for example, are toxic and found in coal-tar, a common pavement sealant ingredient. PAHs have been linked to adverse health effects, including cancer and genetic mutations, and they accumulate in aquatic life, potentially impacting ecosystems.

Hydrocarbons are known to reduce visibility, particularly in humid environments. Organic carbon particles, which are a product of hydrocarbon reactions, scatter light and contribute to haze formation. This phenomenon is exacerbated in regions with a high number of coal-fired power plants, such as the eastern United States, where sulphates are the largest contributor to haze.

To address visibility impairment caused by hydrocarbons and other pollutants, regulatory efforts have been implemented. The United States Environmental Protection Agency (EPA), for instance, issued regulations in 1999 to reduce haze and protect visibility across the nation. Additionally, advancements in technology and the implementation of programs have led to cleaner cars, trucks, and industries, although these measures may not be sufficient to restore visibility to its natural state in certain protected areas.

In summary, hydrocarbons are a significant contributor to air pollution, with industrial sources being a major concern. Their presence in the atmosphere leads to reduced visibility, particularly in humid regions, due to the formation of haze-causing organic carbon particles. Regulatory efforts and technological advancements aim to mitigate the impact of hydrocarbon pollution on visibility and the environment.

Particulate matter creates photochemical smog

Particulate matter is a major component of photochemical smog, which is the brown industrial haze that sits over cities like Los Angeles. Photochemical smog is a mix of pollutants, with the most concerning being ozone and nitrogen oxides, according to the Environmental Protection Agency (EPA). Ground-level ozone is a primary ingredient of smog and is formed by the photochemical reaction of air pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) with ultraviolet light (sunlight).

Particulate matter is made up of tiny liquid droplets and solid particles. While some particulate matter is large enough to be visible to the naked eye, much of it is microscopic. It is directly emitted during combustion, for example from fires or diesel engines, and forms in the atmosphere as a result of chemical reactions involving VOCs.

Particulate matter is a concern because it is harmful to human health. Long-term exposure to particulate matter in photochemical smog can lead to reduced lung function, heart attacks, irregular heartbeat, and possible premature death for people with heart and lung disease. It is also damaging to the environment, as it can reduce photosynthesis in sensitive plants and slow their growth, which can affect entire ecosystems by altering biodiversity.

Particulate matter also reduces visibility and is the reason smog appears brown. Atmospheric particulate matter scatters sunlight back into space, which has a cooling effect on Earth. However, heavy pollution can suppress cloud formation.

To reduce the formation of photochemical smog, state and federal regulatory agencies have set air quality standards for ozone and other harmful air pollutants. Agencies also regulate vehicle and factory emissions and fuel production.

Sulfur oxides cause acid precipitation

Acid rain is a term used to describe any form of precipitation that contains acidic components. This can include rain, snow, fog, hail, or even dust that is acidic. Acid rain is caused by the emission of sulfur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere, which are then transported by wind and air currents.

SO2 and NOx react with water, oxygen, and other chemicals to form sulfuric and nitric acids, which are the main components of acid rain. The major sources of these emissions are the burning of fossil fuels, such as coal, to generate electricity, as well as vehicles, heavy equipment, manufacturing, and oil refineries.

The effects of acid rain are widespread and detrimental. It can cause chemical changes in soil, making it more acidic and stripping it of essential nutrients such as calcium and magnesium, which are crucial for plant growth and maintaining healthy soil. Acid rain has also been linked to the corrosion of steel structures, the weathering of stone buildings and statues, and negative impacts on human health.

In natural bodies of water, acid rain can cause lakes and streams to turn acidic, harming aquatic ecosystems and making them uninhabitable for certain species. For example, in New England, it was reported that 5% of the lakes had become acidic, with sulfates being the predominant issue. This resulted in a decline in the survival of various fish species, such as brook trout and minnow.

The release of sulfur oxides into the atmosphere, which leads to the formation of sulfuric acid and subsequent acid rain, has detrimental effects on the environment and infrastructure. As a result, governments have implemented regulations to reduce these emissions and mitigate their harmful consequences.

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