Environmental & Industrial Chemistry

Air & Atmosphere

5th Year · 6th Year (Leaving Cert)

✓By the end of this lesson students will be able to state the main components of dry air and their approximate percentages.
✓By the end of this lesson students will be able to describe the formation and importance of the ozone layer and explain its depletion.
✓By the end of this lesson students will be able to explain the greenhouse effect and identify the main greenhouse gases.
✓By the end of this lesson students will be able to discuss the causes and consequences of global warming.
✓By the end of this lesson students will be able to outline measures taken to address ozone depletion and global warming.

Key concepts

Composition of Air

Air is a mixture of gases, primarily nitrogen and oxygen, with smaller amounts of argon, carbon dioxide, and other trace gases. The composition of dry air is relatively constant, but the amount of water vapour varies significantly depending on location and weather conditions. Water vapour is a crucial component of the atmosphere but is often excluded when discussing 'dry air' composition.

Ozone Layer

The ozone layer is a region within the Earth's stratosphere (approximately 15-30 km above the surface) that contains a relatively high concentration of ozone (O₃). It plays a vital role in absorbing most of the Sun's harmful ultraviolet (UV) radiation, particularly UV-B and UV-C, preventing it from reaching the Earth's surface where it can cause skin cancer, cataracts, and damage to ecosystems. Ozone is continually formed and broken down in the stratosphere through natural processes involving oxygen molecules and UV radiation.

Formation: O₂(g) + UV radiation → 2O(g) O(g) + O₂(g) → O₃(g)

Ozone Depletion

Ozone depletion refers to the thinning of the ozone layer, primarily caused by anthropogenic emissions of ozone-depleting substances (ODS), such as chlorofluorocarbons (CFCs), halons, and carbon tetrachloride. These stable compounds migrate to the stratosphere, where UV radiation breaks them down, releasing highly reactive free radicals (e.g., chlorine atoms). These radicals then catalyse the destruction of ozone molecules in a chain reaction, significantly reducing the ozone concentration. The Montreal Protocol (1987) is an international treaty designed to phase out the production of ODS.

Catalytic destruction by chlorine radical (Cl•): Cl• + O₃ → ClO• + O₂ ClO• + O → Cl• + O₂ Net reaction: O₃ + O → 2O₂

Greenhouse Effect

The greenhouse effect is a natural process by which certain gases in the Earth's atmosphere trap heat, warming the planet to a temperature suitable for life. Solar radiation (short-wavelength) passes through the atmosphere and is absorbed by the Earth's surface, which then re-radiates energy as infrared radiation (long-wavelength heat). Greenhouse gases (GHGs) absorb this outgoing infrared radiation, preventing it from escaping directly into space and re-radiating some of it back towards the Earth's surface, thus warming the lower atmosphere.

Global Warming

Global warming refers to the observed increase in the average temperature of the Earth's atmosphere and oceans over recent decades. This is primarily attributed to an enhanced greenhouse effect caused by increased concentrations of anthropogenic greenhouse gases (e.g., carbon dioxide, methane, nitrous oxide) in the atmosphere. These increased concentrations are largely due to human activities such as the burning of fossil fuels, deforestation, and industrial processes. Consequences include rising sea levels, extreme weather events, and disruptions to ecosystems.

Key facts to remember

1Dry air is composed of approximately 78% nitrogen, 21% oxygen, 0.9% argon, and 0.04% carbon dioxide.
2The ozone layer, located in the stratosphere, absorbs harmful UV radiation, protecting life on Earth.
3Ozone (O₃) is formed naturally from oxygen (O₂) and oxygen atoms (O) under the influence of UV radiation.
4Chlorofluorocarbons (CFCs) release chlorine radicals (Cl•) in the stratosphere, which catalytically destroy ozone molecules.
5The Montreal Protocol is an international agreement to phase out ozone-depleting substances.
6The greenhouse effect is a natural process where atmospheric gases trap heat, warming the Earth.
7Major greenhouse gases include water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O).
8Global warming is the observed increase in Earth's average temperature, largely due to enhanced greenhouse effect from human activities.

Worked examples

Example 1

A sample of dry air has a volume of 2.5 L. Calculate the approximate volume of nitrogen and oxygen present in this sample.

IStep 1: Recall the approximate percentages of nitrogen and oxygen in dry air.

IIDry air is approximately 78% nitrogen (N₂) and 21% oxygen (O₂).

IIIStep 2: Calculate the volume of nitrogen.

IVVolume of N₂ = 78% of 2.5 L = (78/100) × 2.5 L = 0.78 × 2.5 L = 1.95 L

VStep 3: Calculate the volume of oxygen.

VIVolume of O₂ = 21% of 2.5 L = (21/100) × 2.5 L = 0.21 × 2.5 L = 0.525 L

Answer

The approximate volume of nitrogen is 1.95 L, and the approximate volume of oxygen is 0.525 L.

The remaining volume would be trace gases like argon and carbon dioxide.

Example 2

Explain the mechanism by which chlorofluorocarbons (CFCs) lead to the depletion of the ozone layer, including relevant chemical equations.

IStep 1: Describe the stability and transport of CFCs.

IICFCs (e.g., CCl₃F, CCl₂F₂) are very stable compounds at ground level and do not react in the troposphere. They slowly diffuse upwards into the stratosphere.

IIIStep 2: Explain the breakdown of CFCs in the stratosphere.

IVIn the stratosphere, intense UV radiation provides enough energy to break the C-Cl bond in CFC molecules, releasing highly reactive chlorine free radicals (Cl•).

VExample: CCl₃F(g) + UV radiation → Cl•(g) + CCl₂F(g)

VIStep 3: Describe the catalytic destruction of ozone by chlorine radicals.

VIIThe chlorine radical then reacts with an ozone molecule (O₃), destroying it and forming chlorine monoxide (ClO•) and an oxygen molecule (O₂).

VIIICl•(g) + O₃(g) → ClO•(g) + O₂(g)

9Step 4: Explain the regeneration of the chlorine radical.

10The chlorine monoxide radical (ClO•) then reacts with a free oxygen atom (O), which is naturally present in the stratosphere, regenerating the chlorine radical (Cl•) and forming another oxygen molecule (O₂).

11ClO•(g) + O(g) → Cl•(g) + O₂(g)

12Step 5: Summarise the catalytic cycle.

13The regenerated chlorine radical can then go on to destroy many more ozone molecules, making it a catalytic process. The net effect of these two steps is the destruction of an ozone molecule and an oxygen atom to form two oxygen molecules, with the chlorine radical acting as a catalyst.

14Overall reaction: O₃(g) + O(g) → 2O₂(g) (catalysed by Cl•)

Answer

CFCs release chlorine radicals in the stratosphere upon exposure to UV radiation. These chlorine radicals then catalytically destroy ozone molecules through a two-step cycle: Cl• + O₃ → ClO• + O₂ and ClO• + O → Cl• + O₂. The chlorine radical is regenerated, allowing it to destroy thousands of ozone molecules, leading to significant ozone depletion.

A single chlorine radical can destroy tens of thousands of ozone molecules before it is eventually removed from the stratosphere.

Example 3

Identify two major greenhouse gases (excluding water vapour) and explain how human activities contribute to their increased concentration in the atmosphere.

IStep 1: Identify the first major greenhouse gas and its anthropogenic sources.

IIGreenhouse Gas 1: Carbon Dioxide (CO₂)

IIIHuman activities contributing to increased CO₂: The primary source is the burning of fossil fuels (coal, oil, natural gas) for energy production, transport, and industrial processes. Deforestation also contributes, as trees absorb CO₂ through photosynthesis, and their removal or burning releases stored carbon.

IVStep 2: Identify the second major greenhouse gas and its anthropogenic sources.

VGreenhouse Gas 2: Methane (CH₄)

VIHuman activities contributing to increased CH₄: Methane is produced from anaerobic decomposition. Major anthropogenic sources include agriculture (e.g., livestock digestion, rice cultivation), landfill sites (decomposition of organic waste), and the production and transport of fossil fuels (e.g., natural gas leaks).

Answer

Two major greenhouse gases are carbon dioxide (CO₂) and methane (CH₄). Human activities increase CO₂ primarily through the burning of fossil fuels and deforestation. Methane concentrations are increased by agricultural practices (like livestock farming and rice paddies) and waste decomposition in landfills.

Nitrous oxide (N₂O) and fluorinated gases are also significant anthropogenic greenhouse gases.

Common mistakes

✗Confusing the ozone layer (protection from UV) with the greenhouse effect (trapping heat).
✗Incorrectly stating the main components or their percentages in dry air (e.g., thinking CO₂ is a major component).
✗Not understanding that the greenhouse effect is a natural and essential process; only the *enhanced* greenhouse effect is problematic.
✗Forgetting that ozone depletion and global warming are distinct environmental issues, though both involve atmospheric chemistry.
✗Failing to include chemical equations when asked to explain ozone depletion mechanisms.

Exam tips

★Memorise the approximate percentages of the main gases in dry air (N₂, O₂, Ar, CO₂).
★Be able to clearly distinguish between the formation of ozone and its destruction, including the relevant chemical equations.
★Understand the catalytic nature of ozone depletion by radicals (e.g., chlorine) and be able to write the full reaction cycle.
★Clearly differentiate between the natural greenhouse effect and the enhanced greenhouse effect leading to global warming.
★Provide specific examples of human activities that contribute to the emissions of major greenhouse gases.

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