Unit 14: Environmental Chemistry (Air)
Understanding the composition of our atmosphere and the chemical challenges of pollution, climate change, and ozone depletion.
14.1 The Layers of the Atmosphere
The Earth's atmosphere is a complex mixture of gases divided into several distinct layers, each with unique characteristics.
- Troposphere (0-10 km): The lowest layer, containing about 75% of the atmosphere's mass. It's where we live and where all weather occurs. Temperature and pressure decrease with altitude.
- Stratosphere (10-50 km): This layer contains the vital ozone layer, which absorbs harmful ultraviolet (UV) radiation from the sun. Temperature increases with altitude here due to this absorption of UV radiation.
- Mesosphere (50-90 km): The layer where most meteors burn up. The temperature decreases with altitude, reaching the coldest temperatures in the atmosphere.
- Thermosphere (90-500+ km): The outermost layer, where the air is extremely thin. The temperature can be very high due to absorption of solar radiation, but it would feel cold because the molecules are so far apart.
Solved Examples:
-
In which layer of the atmosphere does all weather occur?
Solution: The troposphere. -
What is the function of the ozone layer and where is it
located?
Solution: The ozone layer absorbs most of the sun's harmful ultraviolet (UV) radiation. It is located in the stratosphere.
14.2 The Greenhouse Effect and Global Warming
The greenhouse effect is a natural process that warms the Earth's surface. Certain gases in the atmosphere, known as greenhouse gases, trap some of the outgoing heat (infrared radiation) from the Earth, preventing it from escaping into space.
The main greenhouse gases are:
- Carbon Dioxide ($CO_2$): Released from burning fossil fuels, deforestation, and industrial processes.
- Methane ($CH_4$): Released from agriculture (livestock), landfills, and natural gas production.
- Water Vapour ($H_2O$): The most abundant greenhouse gas, part of the natural water cycle.
Human activities, particularly the burning of fossil fuels, have significantly increased the concentration of greenhouse gases like $CO_2$. This enhancement of the natural greenhouse effect leads to global warming, the long-term heating of Earth’s climate system, which in turn causes climate change (long-term shifts in temperatures and weather patterns).
Solved Examples:
-
What is the difference between the greenhouse effect and global
warming?
Solution: The greenhouse effect is a natural and essential process that keeps the Earth warm enough for life. Global warming is the enhancement of this effect due to increased concentrations of man-made greenhouse gases, leading to an overall rise in global temperatures. -
Name two human activities that contribute to increased carbon dioxide in the
atmosphere.
Solution: The burning of fossil fuels (for electricity, industry, and transport) and deforestation (which reduces the number of trees available to absorb $CO_2$). -
Is the greenhouse effect entirely bad?
Solution: No. The natural greenhouse effect is essential for life on Earth. Without it, the planet's average temperature would be about -18°C, and it would be too cold to support life as we know it. The problem is the *enhanced* greenhouse effect caused by human activities.
14.3 The Ozone Layer and its Depletion
The ozone layer in the stratosphere protects life on Earth by absorbing harmful UV radiation. Ozone ($O_3$) is constantly being formed and broken down in a natural cycle.
However, certain man-made pollutants can disrupt this cycle, leading to ozone depletion. The main culprits are chlorofluorocarbons (CFCs), which were once widely used in refrigerators, air conditioners, and aerosol sprays.
In the stratosphere, UV radiation breaks down CFC molecules, releasing highly reactive chlorine atoms. A single chlorine atom can act as a catalyst to destroy thousands of ozone molecules in a chain reaction:
- $Cl \cdot + O_3 \rightarrow ClO \cdot + O_2$
- $ClO \cdot + O \rightarrow Cl \cdot + O_2$
The chlorine radical ($Cl \cdot$) is regenerated in the second step, allowing it to destroy another ozone molecule. This depletion allows more harmful UV radiation to reach the Earth's surface, increasing the risk of skin cancer and cataracts.
Solved Examples:
-
What is the main class of chemicals responsible for ozone
depletion?
Solution: Chlorofluorocarbons (CFCs). -
Why is a single chlorine atom so destructive to the ozone
layer?
Solution: Because it acts as a catalyst. It is regenerated at the end of the reaction cycle, allowing it to go on and destroy many more ozone molecules.
14.4 Acid Rain
Acid rain is rain that has been made acidic by certain atmospheric pollutants. It is formed when oxides of sulfur and nitrogen react with water in the atmosphere to produce strong acids.
- Sulfur Dioxide ($SO_2$): Released from burning fossil fuels (especially coal) that contain sulfur impurities. It reacts with water and oxygen in the atmosphere to form sulfuric acid ($H_2SO_4$).
- Nitrogen Oxides ($NO_x$): Formed at the high temperatures inside car engines and power plants. They react with water to form nitric acid ($HNO_3$).
Effects of Acid Rain:
- Damages forests by removing essential nutrients from the soil.
- Lowers the pH of lakes and rivers, harming or killing aquatic life.
- Corrodes buildings and statues made of limestone or marble (calcium carbonate).
- Accelerates the corrosion of metal structures.
Solved Examples:
-
What are the two main pollutants that cause acid rain?
Solution: Sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$). -
Write an equation showing how acid rain damages a limestone
building.
Solution: Limestone is calcium carbonate. The sulfuric acid in acid rain reacts with it:
$CaCO_3(s) + H_2SO_4(aq) \rightarrow CaSO_4(aq) + H_2O(l) + CO_2(g)$ -
What is the main source of nitrogen oxides in the atmosphere?
Solution: The high-temperature combustion of fuel in car engines and power stations.