Carbon Monoxide (CO), Carbon Dioxide (CO2), Ozone (O3), Nitrogen Oxides(NOx), Sulphur dioxide (SO2), Polyaromatic Hydrocarbons (PAHs), Volatile organic compounds (VOCs), Benzene – UPSC Environment Notes

Carbon dioxide, carbon monoxide, ozone, nitrogen oxide, and sulfur dioxide are quintessential atmospheric components that play pivotal roles in Earth’s environmental equilibrium. These compounds, collectively referred to as air pollutants, wield substantial influence on atmospheric chemistry, climate patterns, and public health. While some are naturally occurring, anthropogenic activities have significantly augmented their presence in the atmosphere, leading to a plethora of environmental and health concerns. Understanding the characteristics, sources, and impacts of these compounds is paramount for devising effective strategies to mitigate their adverse effects and safeguarding the well-being of both ecosystems and human populations.

CARBON DIOXIDE

• A colorless and odorless gas, carbon dioxide (CO2) possesses greater density than air. Its natural sources encompass volcanic emissions, hot springs, geysers, and the liberation from carbonate rocks through dissolution in water and acids. 
• Given its solubility in water, carbon dioxide occurs naturally in groundwater, rivers, lakes, ice caps, glaciers, and seawater.

Health Impacts:

• Carbon dioxide functions as an asphyxiant gas, contributing to asphyxia—a state arising from oxygen deprivation, leading to unconsciousness or fatality. 
• Concentrations as low as 7% can induce suffocation, even in the presence of ample oxygen, resulting in symptoms like dizziness, headaches, and loss of consciousness.

Environmental Effects:

• Carbon dioxide holds significance as a greenhouse gas. 
• The combustion of carbon-based fuels since the industrial revolution has contributed to global warming. 
• Additionally, carbon dioxide is a primary driver of ocean acidification as it dissolves in water, forming carbonic acid.

CARBON MONOXIDE

Carbon monoxide (CO) is a colorless, odorless, tasteless, and highly toxic gas with slightly less density than air, and it has a short atmospheric lifespan, lasting only a few months.

Sources of Carbon Monoxide:

• Carbon monoxide is generated from internal combustion engine exhaust and incomplete combustion of various fuels. 
• Iron smelting is another source of carbon monoxide as a by-product. It forms in conditions where there is insufficient oxygen to produce carbon dioxide (CO2). 
• In the presence of oxygen, carbon monoxide burns with a blue flame, producing carbon dioxide. 
• The largest global source of carbon monoxide is natural, resulting from photochemical reactions in the troposphere. 
• Other natural sources include volcanoes, forest fires, and various combustion processes.

Health Impacts:

• Carbon monoxide poisoning, often stemming from poor ventilation and heat management in electronic devices like laptops, is the most common type of fatal air poisoning. 
• Concentrations above approximately 35 parts per million (ppm) are toxic to hemoglobin animals, including humans. 
• It forms carboxyhemoglobin when combining with hemoglobin, displacing the oxygen-carrying function of hemoglobin.

Environmental Consequences:

• While carbon monoxide itself is not classified as a direct greenhouse gas (GHG), it exhibits spatial variability and a short lifespan in the atmosphere. 
• It plays a role in the formation of ground-level ozone (tropospheric ozone) and can elevate methane concentrations, a potent GHG. 
• Carbon monoxide reacts with hydroxyl radicals (-OH) to produce peroxy radicals, which further react with nitrogen oxide (NO) to form nitrogen dioxide (NO2) and hydroxyl radicals. 
• The subsequent photolysis of NO2 results in the production of ozone (O3).

NITROGEN OXIDE

• NOx, a generic term encompassing various nitrogen oxides produced during combustion, is primarily generated in internal combustion engines and coal-burning power plants. 
• Additionally, natural occurrences like lightning contribute to their production. 
• While oxygen and nitrogen remain unreactive at ambient temperatures, high temperatures within internal combustion engines or power station boilers lead to the production of nitrogen oxides.
• Agricultural practices, including fertilization and the use of nitrogen-fixing plants, further contribute to atmospheric NOx levels by enhancing nitrogen fixation by microorganisms. 
• It’s essential to distinguish between NO and NO2, which contribute to global cooling, and nitrous oxide (N2O), a greenhouse gas with various applications as an oxidizer.

Health and Environmental Impacts:

• NOx is associated with the exacerbation of asthmatic conditions and the onset of respiratory health issues. 
• The reduction of NOx emissions stands as a significant technical challenge, especially in biodiesel technology.
• NOx gases play a central role in the formation of smog and acid rain, contributing to the creation of tropospheric ozone. 
• When NOx reacts with volatile organic compounds (VOCs) in the presence of sunlight, it forms photochemical smog. 
• Mono-nitrogen oxides eventually transform into nitric acid when dissolved in atmospheric moisture, becoming a component of acid rain.

Notably, NO and NO2 emissions contribute to global cooling by initiating the formation of -OH radicals, which, in turn, destroy methane molecules. This counteracts the effect of greenhouse gases, demonstrating the intricate role of NOx in atmospheric dynamics.

SULPHUR DIOXIDE

• Sulfur dioxide is a noxious gas characterized by a pungent, irritating smell and is notorious for its contribution to acid rain. 
• While it is naturally released by volcanic activity and is prevalent in the atmosphere of Venus, sulfur dioxide is primarily manufactured for the production of sulfuric acid.
• Inhaling sulfur dioxide is linked to an increase in respiratory symptoms and premature death. 
• Additionally, exposure to this gas can weaken the functioning of specific nerves. 
• Human activities contribute significantly to sulfur dioxide emissions, including the burning of coal in thermal power plants and diesel fuels. 
• Industrial processes, such as paper production and metal smelting, also generate sulfur dioxide. 
• Furthermore, reactions involving Hydrogen Sulphide (H2S) and oxygen, as well as the roasting of sulfide ores like pyrite, sphalerite, and cinnabar (mercury sulfide), contribute to the release of sulfur dioxide into the environment.

OZONE

• Ozone naturally occurs in the stratosphere, where it serves a vital role by absorbing harmful ultraviolet rays from the sun. 
• However, at ground level, ozone transforms into a pollutant, acting as a greenhouse gas with highly toxic effects. 
• Major contributors to ground-level ozone emissions are vehicles and industrial activities.
• Carbon monoxide and nitrogen dioxide play significant roles in the conversion of oxygen (O2) to ozone (O3). 
• Ground-level ozone has adverse effects on human health, causing itchy and watery eyes, as well as lowering resistance to cold and pneumonia.

VOLATILE ORGANIC COMPOUNDS

• Volatile Organic Compounds (VOCs) constitute a broad category of carbon-based chemicals known for their ability to readily evaporate at room temperature. 
• An illustrative example is formaldehyde, which has a boiling point as low as -19 °C. 
• Formaldehyde, commonly emitted from paints, can lead to eye and nose irritation as well as allergic reactions.
• Indoor environments witness significant VOC emissions, with main sources including perfumes, hair sprays, furniture polish, adhesives, air fresheners, moth repellents, wood preservatives, and various other products. 
• Exposure to VOCs can result in immediate health effects such as irritation of the eyes, nose, and throat, along with symptoms like headaches, nausea, and loss of coordination.
• Furthermore, long-term exposure to VOCs is suspected to have adverse effects on health, potentially causing damage to the liver and other organs in the body.

BENZENE

• Benzene occurs naturally as a component of crude oil and is classified as one of the fundamental petrochemicals. 
• Its high octane number makes it a significant ingredient in gasoline (petrol), contributing to the fuel’s performance.
• However, despite its industrial applications, benzene poses serious health risks. 
• Prolonged exposure to benzene is known to increase the risk of cancer and other illnesses. 
• Notably, benzene is infamous for being a causative factor in bone marrow failure, highlighting the severe health consequences associated with its exposure.

FAQs on Air Pollutants: Carbon Dioxide, Carbon Monoxide, Ozone, Nitrogen Oxide, Sulphur Dioxide, VOCs, and Benzene

Q1: What is carbon dioxide, and where does it come from?

A1: Carbon dioxide (CO2) is a colorless, odorless gas that occurs naturally from volcanic emissions, hot springs, geysers, and the dissolution of carbonate rocks in water and acids. Human activities, particularly the combustion of carbon-based fuels, contribute to elevated CO2 levels.

Q2: What health impacts are associated with carbon dioxide exposure?

A2: Carbon dioxide functions as an asphyxiant gas, causing suffocation at concentrations as low as 7%. Symptoms include dizziness, headaches, and unconsciousness.

Q3: How does carbon dioxide affect the environment?

A3: Carbon dioxide is a greenhouse gas, contributing to global warming. It also plays a role in ocean acidification when dissolved in water, forming carbonic acid.

Q4: What is carbon monoxide, and where does it come from?

A4: Carbon monoxide (CO) is a colorless, odorless, and highly toxic gas produced from internal combustion engine exhaust, incomplete combustion of fuels, and iron smelting. Natural sources include volcanic activity, forest fires, and various combustion processes.

Q5: What are the health impacts of carbon monoxide exposure?

A5: Carbon monoxide poisoning, a result of inadequate ventilation, can be fatal. Concentrations above 35 ppm are toxic, forming carboxyhemoglobin and displacing oxygen-carrying functions in hemoglobin.

Q6: How does carbon monoxide contribute to the environment?

A6: While not a direct greenhouse gas, carbon monoxide is involved in the formation of ground-level ozone and can elevate methane concentrations, impacting atmospheric dynamics.

Q7: What does NOx refer to, and where is it produced?

A7: NOx is a term for nitrogen oxides produced during combustion, prevalent in internal combustion engines, coal-burning power plants, and naturally from lightning. Agricultural practices also contribute.

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