Air quality and climate Revision Notes

Air is a mixture of gases that surround the Earth. The two most abundant gases in clean, dry air are nitrogen and oxygen. Nitrogen makes up about 78% of the air by volume, while oxygen accounts for approximately 21%. These percentages are consistent worldwide in unpolluted environments.

Nitrogen is chemically inert under normal conditions, which means it does not easily react with other substances. This stability makes nitrogen a major component of air, providing a relatively unreactive background gas.

Oxygen is essential for respiration in most living organisms and supports combustion. Its 21% presence is critical for life and many chemical processes on Earth.

• Nitrogen: ~78% by volume
• Oxygen: ~21% by volume

The remaining approximately 1% of clean, dry air consists of several trace gases. These include noble gases such as argon, neon, helium, krypton, and xenon, as well as carbon dioxide. Although these gases are present in small amounts, they play important roles in atmospheric chemistry and environmental processes.

Argon is the most abundant noble gas in air, making up about 0.93%. Carbon dioxide is present at roughly 0.04% (400 parts per million), but this concentration can vary due to natural and human activities.

Other trace gases include water vapour, which is variable and excluded from the definition of dry air, and pollutants, which are not part of clean air.

• Noble gases (mainly argon): ~0.93%
• Carbon dioxide: ~0.04%
• Other trace gases: negligible amounts

Knowing the composition of air is essential for understanding processes such as respiration, combustion, and climate change. For example, the stable nitrogen atmosphere dilutes oxygen, preventing rapid combustion of materials.

Carbon dioxide, although a minor component, is a greenhouse gas that traps heat in the atmosphere, influencing global temperatures. Changes in its concentration can have significant environmental impacts.

The noble gases, being chemically inert, serve as useful reference gases in scientific measurements and have applications in lighting and welding.

• Nitrogen provides an inert background gas
• Oxygen supports life and combustion
• Carbon dioxide influences climate as a greenhouse gas
• Noble gases are chemically inert and useful in technology

The percentage composition of air can be determined using gas collection and analysis techniques such as gas chromatography or chemical absorption methods.

For example, oxygen concentration can be measured by passing air through a solution that absorbs oxygen, and measuring the volume change.

Repeated measurements in different locations and conditions confirm the consistency of nitrogen and oxygen proportions in clean, dry air.

• Gas chromatography separates and measures gases
• Chemical absorption can quantify oxygen
• Consistent results validate standard air composition

A simple experiment to estimate oxygen content involves burning a small candle in a sealed container filled with air. As the candle burns, it consumes oxygen and produces carbon dioxide and water vapour.

By measuring the volume of air before and after combustion, and accounting for water vapour condensation, the decrease in oxygen can be estimated.

This method demonstrates that oxygen is about one-fifth of air, consistent with the accepted 21%.

• Burn candle in sealed container
• Measure air volume before and after
• Calculate oxygen consumed to estimate percentage

Carbon dioxide (CO₂) is primarily produced when carbon-containing fuels burn completely in the presence of sufficient oxygen. This process is called complete combustion and can be represented by the general equation:

\[ \text{Fuel (hydrocarbon)} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} \]

For example, burning methane (CH₄) completely produces carbon dioxide and water: \[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \]

• Complete combustion requires enough oxygen to convert all carbon to CO₂.
• Carbon dioxide is a greenhouse gas contributing to global warming.
• Common fuels include coal, petrol, diesel, and natural gas.

Incomplete combustion occurs when there is insufficient oxygen for the fuel to burn fully. This leads to the formation of carbon monoxide (CO), a toxic gas, and particulates, which are tiny solid particles such as soot.

The incomplete combustion of methane can be simplified as:

\[ 2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O} \]

• Carbon monoxide binds to haemoglobin, reducing oxygen transport in blood.
• Particulates can cause respiratory problems and increase cancer risk.
• Incomplete combustion is common in poorly maintained engines and inefficient stoves.

Methane (CH₄) is produced naturally during the anaerobic decomposition of plant material in environments like wetlands, landfills, and rice paddies. It is also emitted from the digestive systems of ruminant animals such as cows and sheep, where microbes break down food in oxygen-free conditions.

This biological methane release is significant because methane is a potent greenhouse gas, with a global warming potential much higher than carbon dioxide over a 100-year period.

• Methane arises without combustion, from biological decay and digestion.
• Livestock farming is a major human-related source of methane.
• Methane contributes strongly to climate change despite its lower atmospheric concentration.

Nitrogen oxides (NOₓ), mainly nitric oxide (NO) and nitrogen dioxide (NO₂), form when nitrogen and oxygen in the air react at the high temperatures inside vehicle engines and power stations.

The reaction can be summarised as: \[ \text{N}_2 + \text{O}_2 \rightarrow 2\text{NO} \]

NO then reacts with oxygen to form NO₂. These gases contribute to acid rain and respiratory problems.

• High combustion temperatures cause nitrogen and oxygen to combine.
• Vehicle engines and power plants are major NOₓ sources.
• NOₓ gases irritate lungs and contribute to environmental acidification.

Fossil fuels such as coal and oil often contain sulfur compounds. When these fuels are burned, sulfur reacts with oxygen to form sulfur dioxide (SO₂).

The reaction is: \[ \text{S} + \text{O}_2 \rightarrow \text{SO}_2 \]

Sulfur dioxide contributes to acid rain, which damages ecosystems and structures.

• Sulfur in fuels oxidises during combustion to produce SO₂.
• Coal and oil are common sulfur-containing fuels.
• Reducing sulfur content in fuels lowers SO₂ emissions and acid rain risk.

Carbon dioxide (CO₂) and methane (CH₄) are greenhouse gases that trap heat in the Earth's atmosphere, leading to global warming and climate change. When sunlight reaches the Earth, the surface absorbs energy and re-emits it as infrared radiation. Greenhouse gases absorb and re-emit this infrared radiation, reducing heat loss to space and causing the atmosphere to warm.

Carbon dioxide mainly originates from burning fossil fuels such as coal, oil, and natural gas. Methane is released during the decay of organic matter in wetlands, from livestock digestion (enteric fermentation), and from landfill sites.

For example, increased CO₂ levels from car exhausts and power stations enhance the greenhouse effect, raising global temperatures. Similarly, methane released by cattle farming contributes significantly to warming because methane is more effective at trapping heat than CO₂, although it is present in smaller amounts.

• CO₂ and CH₄ absorb infrared radiation, trapping heat in the atmosphere.
• Increased levels intensify the greenhouse effect, causing global warming.
• Sources include fossil fuel combustion (CO₂) and organic decay or livestock (CH₄).

Carbon monoxide (CO) is a colourless, odourless gas produced by incomplete combustion of carbon-containing fuels, such as petrol in car engines or wood in poorly ventilated fires.

CO binds strongly to haemoglobin in red blood cells, preventing oxygen transport around the body. This can cause symptoms like headaches, dizziness, and in high concentrations, death.

For example, running a car engine in a closed garage can lead to dangerous CO buildup, posing a fatal risk to anyone inside.

• CO is toxic because it binds to haemoglobin, reducing oxygen delivery.
• Produced by incomplete combustion of fuels.
• Exposure can cause serious health effects or death.

Particulates are tiny solid or liquid particles suspended in air, such as soot from diesel engines or smoke from burning coal.

When inhaled, particulates can penetrate deep into the lungs, causing inflammation and increasing the risk of respiratory diseases like asthma and bronchitis. Long-term exposure is linked to lung cancer.

For example, people living in cities with heavy traffic often experience higher rates of respiratory problems due to particulate pollution.

• Particulates are small airborne particles from combustion.
• They irritate and damage lung tissue, causing respiratory diseases.
• Long-term exposure increases cancer risk.

Nitrogen oxides (NOₓ), mainly nitrogen monoxide (NO) and nitrogen dioxide (NO₂), form during high-temperature combustion in vehicle engines and power stations.

NOₓ gases react with water vapour in the atmosphere to form nitric acid, contributing to acid rain. Acid rain damages plants, soils, aquatic ecosystems, and buildings.

Additionally, nitrogen oxides irritate the respiratory system, worsening conditions such as asthma and bronchitis. For example, urban areas with heavy traffic often have elevated NOₓ levels causing health issues.

• NOₓ gases form at high combustion temperatures.
• They cause acid rain by forming nitric acid in the atmosphere.
• NOₓ irritate lungs and worsen respiratory diseases.

Sulfur dioxide (SO₂) is produced by burning fossil fuels containing sulfur compounds, such as coal and oil.

SO₂ dissolves in atmospheric moisture to form sulfurous and sulfuric acids, leading to acid rain. Acid rain lowers soil pH, harms aquatic life, and damages buildings and monuments.

For example, industrial regions burning high-sulfur coal have historically experienced severe acid rain effects on forests and lakes.

• SO₂ originates from burning sulfur-containing fuels.
• It forms acids in the atmosphere causing acid rain.
• Acid rain harms ecosystems and man-made structures.

Trees absorb carbon dioxide (CO₂) from the atmosphere during photosynthesis, converting it into organic matter and storing carbon in their biomass. This process reduces the concentration of CO₂, a major greenhouse gas, thereby mitigating global warming.

For example, planting a forest of fast-growing trees like poplars can sequester several tonnes of CO₂ per hectare annually. This natural carbon capture helps offset emissions from human activities.

However, the effectiveness depends on tree species, growth rate, and land area available. Mature forests store more carbon, but young trees absorb CO₂ faster during growth phases. Maintaining forests prevents carbon release from deforestation.

• Photosynthesis removes CO₂ from air
• Carbon stored in tree biomass
• Large-scale planting increases carbon sequestration
• Forest conservation prevents carbon release

Livestock such as cows and sheep produce methane (CH₄) during digestion through enteric fermentation. Methane is a potent greenhouse gas with a global warming potential about 28 times greater than CO₂ over 100 years.

By reducing the number of livestock or improving farming practices (e.g., diet changes, manure management), methane emissions can be significantly decreased.

For instance, shifting some meat consumption to plant-based diets reduces demand for livestock farming, thus lowering methane output. Methane from manure can also be captured and used as biogas, turning waste into energy.

• Methane from enteric fermentation is a strong greenhouse gas
• Reducing livestock lowers methane emissions
• Diet and manure management can reduce methane production
• Biogas capture utilises methane as renewable energy

Burning fossil fuels like coal, oil, and natural gas releases large amounts of CO₂ and other pollutants. Reducing fossil fuel consumption directly lowers greenhouse gas emissions responsible for climate change.

This can be achieved by improving energy efficiency, using public transport, and adopting cleaner technologies.

For example, replacing petrol cars with electric vehicles powered by low-carbon electricity reduces CO₂ emissions from transport. Industrial processes can switch to less carbon-intensive fuels or capture emitted CO₂.

• Fossil fuel combustion emits CO₂
• Reducing use lowers greenhouse gases
• Energy efficiency and cleaner transport help
• Industrial fuel switching and carbon capture reduce emissions

Renewable energy sources such as wind turbines and solar panels generate electricity without emitting greenhouse gases during operation.

Expanding renewable energy capacity reduces reliance on fossil fuels, thus decreasing CO₂ emissions.

For example, a solar farm converts sunlight directly into electricity, providing clean energy for homes and businesses. Wind farms harness kinetic energy from air currents to produce power sustainably.

• Renewables produce electricity with no direct emissions
• Wind and solar are abundant and sustainable
• Replacing fossil fuels with renewables cuts CO₂
• Investment in renewables supports long-term climate goals

Acid rain forms when sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) are released into the atmosphere and react with water, oxygen, and other chemicals to produce acidic compounds like sulfuric acid (H₂SO₄). These acids mix with rainwater, lowering its pH below normal levels (around 5.6), causing environmental damage.

Sulfur dioxide is mainly produced by burning fossil fuels such as coal and oil that contain sulfur impurities. For example, power stations burning high-sulfur coal emit large amounts of SO₂, which then contributes to acid rain formation.

• SO₂ reacts with water in the atmosphere to form sulfuric acid.
• Acid rain damages plants, aquatic life, and buildings.
• Fossil fuels containing sulfur release SO₂ when burned.

Sulfur dioxide is a primary pollutant responsible for acid rain. Reducing SO₂ emissions directly decreases acid rain formation and its harmful effects on ecosystems and human-made structures.

For instance, lakes exposed to acid rain can become too acidic for fish to survive. Similarly, acid rain accelerates the decay of limestone buildings and statues. Therefore, controlling SO₂ emissions helps protect both natural and built environments.

• Lower SO₂ emissions mean less acid rain.
• Protects aquatic ecosystems and vegetation.
• Prevents damage to infrastructure and cultural heritage.

One effective method to reduce SO₂ emissions is to use fuels that contain less sulfur. Low-sulfur fuels release less sulfur dioxide when burned because there is less sulfur to oxidise.

For example, switching from high-sulfur coal to low-sulfur coal or natural gas reduces SO₂ emissions significantly. Natural gas contains very little sulfur, so it produces minimal SO₂ compared to coal or oil.

This approach is practical for power stations and industries that rely on fossil fuels, as it lowers pollution without requiring major changes to existing combustion technology.

• Low-sulfur fuels contain less sulfur, so emit less SO₂.
• Natural gas is a low-sulfur fuel alternative to coal and oil.
• Switching fuels reduces acid rain-causing emissions effectively.

Besides using low-sulfur fuels, other strategies include installing flue gas desulfurisation (FGD) units in power plants. These scrubbers chemically remove SO₂ from exhaust gases before release.

Regulations and emission standards also encourage industries to limit sulfur emissions by adopting cleaner technologies and fuel choices.

Promoting renewable energy sources like wind and solar reduces reliance on fossil fuels altogether, thus cutting SO₂ emissions at the source.

• Flue gas desulfurisation removes SO₂ from exhaust gases.
• Emission regulations enforce limits on sulfur emissions.
• Renewable energy reduces fossil fuel use and SO₂ emissions.

Acid rain is mainly caused by sulfur dioxide emissions from burning sulfur-containing fossil fuels. Using low-sulfur fuels reduces the amount of sulfur dioxide released, thereby lessening acid rain's environmental damage.

Power stations and industries can switch to low-sulfur coal, oil, or natural gas to achieve this reduction. Combining fuel switching with technologies like flue gas desulfurisation and adopting renewable energy sources provides a comprehensive approach to controlling acid rain.

Understanding these methods helps in making informed decisions about energy use and pollution control to protect ecosystems and infrastructure.

• Acid rain forms from SO₂ emissions.
• Low-sulfur fuels reduce SO₂ released during combustion.
• Combining fuel choice and technology controls acid rain effectively.

The Earth receives energy from the Sun primarily as visible light and ultraviolet radiation. This energy warms the Earth's surface, which then emits energy back towards space as thermal infrared radiation. Greenhouse gases in the atmosphere, such as carbon dioxide (CO2) and methane (CH4), interact with this thermal energy. Unlike the incoming solar radiation, the thermal radiation emitted by Earth is absorbed by these gases, which affects the planet's temperature.

Greenhouse gases are transparent to most incoming solar radiation but absorb and re-emit the longer wavelength thermal radiation emitted by the Earth. This selective absorption is key to their role in global warming.

• Solar radiation warms Earth's surface.
• Earth emits thermal infrared radiation.
• Greenhouse gases absorb this thermal radiation but not most incoming solar radiation.

Carbon dioxide and methane molecules absorb thermal infrared radiation emitted by the Earth's surface. When these gases absorb this energy, their molecules vibrate and gain kinetic energy. They then re-emit thermal radiation in all directions, including back towards the Earth's surface, effectively trapping heat in the lower atmosphere.

This process is not simple reflection like a mirror but involves absorption and re-emission of energy at specific wavelengths. The re-emitted energy directed back to Earth increases the temperature near the surface.

For example, methane is more effective per molecule at absorbing thermal radiation than carbon dioxide, but it is present in smaller amounts in the atmosphere.

• Greenhouse gases absorb Earth's thermal infrared radiation.
• They re-emit thermal energy in all directions, including back to Earth.
• This re-emission increases surface temperature.
• Methane absorbs thermal energy more strongly per molecule than carbon dioxide.

Normally, Earth loses thermal energy to outer space by emitting infrared radiation. Greenhouse gases reduce this loss by absorbing some of the outgoing radiation and re-emitting part of it back towards Earth. This reduces the net energy lost from the planet.

This reduction in thermal energy loss causes an energy imbalance, where more energy is retained in the Earth's atmosphere than is lost. Over time, this leads to an increase in the average global temperature, a phenomenon known as global warming.

For instance, without greenhouse gases, Earth's average temperature would be about -18°C, but with them, it is approximately 15°C, demonstrating their warming effect.

• Greenhouse gases absorb outgoing infrared radiation.
• They re-emit some energy back to Earth, reducing energy loss to space.
• This causes an energy imbalance, increasing Earth's temperature.
• Without greenhouse gases, Earth would be much colder.

Carbon dioxide is the most abundant anthropogenic greenhouse gas and contributes significantly to global warming due to its long atmospheric lifetime and ability to absorb thermal radiation.

Methane, although less abundant, is about 25 times more effective than carbon dioxide at trapping heat over a 100-year period. It originates from sources such as decaying organic matter and livestock digestion.

Both gases contribute to the enhanced greenhouse effect caused by human activities, such as burning fossil fuels and agriculture, increasing their atmospheric concentrations and intensifying global warming.

• Carbon dioxide is abundant and long-lived in the atmosphere.
• Methane is more potent but less abundant than carbon dioxide.
• Human activities increase concentrations of both gases.
• Both gases enhance the natural greenhouse effect, causing warming.

Greenhouse gases absorb and re-emit Earth's thermal energy, trapping heat in the atmosphere and reducing energy loss to space. This process raises the planet's average temperature, contributing to global warming.

Understanding the role of carbon dioxide and methane helps explain why reducing emissions of these gases is critical to mitigating climate change.

Effective climate action involves limiting the release of these gases to slow the increase in global temperatures and reduce harmful environmental impacts.

• Greenhouse gases trap heat by absorbing and re-emitting thermal energy.
• This trapping reduces Earth's thermal energy loss to space, causing warming.
• Carbon dioxide and methane are key contributors to this effect.
• Reducing emissions of these gases is essential to combat climate change.