Environmental Chemistry • Topic 1 of 3

Atmospheric Pollution

Environmental chemistry studies the chemical changes in our surroundings — air, water and soil — and how human activity disturbs them. A pollutant is a substance present at a concentration high enough to harm living things; a contaminant is introduced by human activity and not naturally present. The atmosphere is layered into the troposphere (0–10 km, where we live) and the stratosphere (10–50 km, with the protective ozone layer); pollution is studied separately in each.

Tropospheric pollution

This arises from gases and particulates near the ground.

  • Oxides of sulphur (SO2, SO3): released when sulphur-rich fossil fuels burn; SO2 irritates the respiratory tract and damages plants.
  • Oxides of nitrogen (NO, NO2): formed when N2 and O2 combine at high engine temperatures (N2 + O2 → 2NO); they cause respiratory problems and feed smog and acid rain.
  • Oxides of carbon: CO binds haemoglobin ~300 times more strongly than O2, forming carboxyhaemoglobin and starving tissues of oxygen; CO2 is non-toxic but is the chief greenhouse gas.
  • Hydrocarbons: unburnt fuel vapours; many are carcinogenic and help form smog.
  • Particulate matter: tiny solid/liquid particles (smoke, dust, mist) that lodge in the lungs.

Photochemical smog

In warm, sunny, traffic-heavy cities, sunlight acts on NO2 and hydrocarbons to give oxidising smog rich in ozone, PAN (peroxyacetyl nitrate) and aldehydes. It irritates the eyes and throat, cracks rubber and damages plants. (Classical London smog is the opposite — reducing, from SO2 and soot.)

Acid rain

SO2 and NO2 dissolve in rain to form sulphuric and nitric acids, lowering rain pH below ~5.6. Acid rain corrodes marble (the Taj Mahal’s “marble cancer”), acidifies lakes and harms forests.

Stratospheric pollution — the ozone layer

High in the stratosphere, UV light makes and breaks ozone in a natural cycle (O2 → 2O; O + O2 → O3), shielding us from harmful UV. Chlorofluorocarbons (CFCs) such as Freons drift up and are split by UV to release chlorine radicals that destroy ozone catalytically: Cl• + O3 → ClO• + O2; ClO• + O → Cl• + O2. One Cl atom destroys thousands of O3 molecules, creating the Antarctic ozone hole. Extra UV-B raises rates of skin cancer, cataracts and crop damage.

Greenhouse effect and global warming

Greenhouse gases (CO2, CH4, water vapour, O3, CFCs) trap outgoing infrared radiation, warming the Earth; rising levels cause global warming, melting ice and shifting climate.

CFC chlorine-radical cycle depleting stratospheric ozoneCFC – Ozone Depletion CycleCFC + UV → Cl•Cl•catalystCl• + O3 → ClO• + O2ClO• + O → Cl• + O2Cl regenerated →chain repeats
Solved Examples
1
Worked Example
Explain why carbon monoxide (CO) is far more dangerous to humans than carbon dioxide (CO2) even though both are products of combustion.
Solution
  1. CO binds to haemoglobin to form a stable complex, carboxyhaemoglobin, about 300 times more strongly than oxygen.
  2. This blocks haemoglobin from carrying O2, so body tissues are starved of oxygen even at low CO levels.
  3. CO2 is non-toxic at ordinary concentrations; it is harmful mainly as a greenhouse gas, not as an immediate poison.

Answer: CO is an immediate poison because it deprives tissues of oxygen by forming carboxyhaemoglobin, whereas CO2 is not directly toxic.

2
Worked Example
Photochemical smog is described as 'oxidising' while classical London smog is 'reducing'. Account for this difference and name a key component of each.
Solution
  1. Photochemical smog forms in warm, sunny cities from NO2 and hydrocarbons acted on by sunlight, producing oxidants like ozone (O3) and PAN.
  2. Because ozone and PAN are oxidising agents, the smog is called oxidising.
  3. London smog forms in cold, humid air from SO2 and unburnt carbon (soot); SO2 is a reducing agent, so this smog is reducing.

Answer: Photochemical smog is oxidising (key component: ozone/PAN); London smog is reducing (key component: SO2/soot).

3
Worked Example
Write the chemical equations showing how acid rain is produced from oxides of sulphur and nitrogen, and name the acids formed.
Solution
  1. Sulphur dioxide is oxidised and dissolves: 2SO2 + O2 → 2SO3; SO3 + H2O → H2SO4.
  2. Nitrogen oxides dissolve: 4NO2 + O2 + 2H2O → 4HNO3.
  3. These acids lower the pH of rain below the natural value of about 5.6.

Answer: Acid rain contains sulphuric acid (H2SO4) and nitric acid (HNO3).

4
Worked Example
Show, using equations, how a single chlorine radical from a CFC can destroy many ozone molecules in the stratosphere.
Solution
  1. UV light splits a CFC: CF2Cl2 → CF2Cl• + Cl•.
  2. The chlorine radical attacks ozone: Cl• + O3 → ClO• + O2.
  3. The ClO• reacts with atomic oxygen and regenerates Cl•: ClO• + O → Cl• + O2, so the same Cl atom repeats the cycle thousands of times.

Answer: Because Cl• is regenerated each cycle, it acts as a catalyst and one atom can destroy thousands of O3 molecules.

5
Worked Example
Why is the depletion of the stratospheric ozone layer harmful, and which radiation does the ozone layer block?
Solution
  1. The ozone layer absorbs harmful ultraviolet-B (UV-B) radiation from the Sun.
  2. When ozone is depleted, more UV-B reaches the Earth’s surface.
  3. Increased UV-B causes skin cancer, cataracts and ageing of skin in humans, and damages plants, phytoplankton and the immune system.

Answer: Ozone blocks harmful UV-B radiation; its depletion raises UV-B at the surface, increasing skin cancer, cataracts and crop damage.

6
Worked Example
List the main greenhouse gases and explain in one or two sentences how they cause global warming.
Solution
  1. The chief greenhouse gases are carbon dioxide (CO2), methane (CH4), water vapour, ozone (O3) and CFCs.
  2. These gases allow short-wavelength sunlight to reach the Earth but absorb the longer-wavelength infrared (heat) that the Earth radiates back.
  3. The trapped heat warms the lower atmosphere; rising gas levels intensify this, causing global warming.

Answer: CO2, CH4, water vapour, O3 and CFCs trap outgoing infrared radiation, warming the planet (the greenhouse effect) and driving global warming.

Key Points

  • The atmosphere has the troposphere (where we live) and the stratosphere (ozone layer); tropospheric pollutants include oxides of S, N and C, hydrocarbons and particulate matter.
  • CO is a poison that forms carboxyhaemoglobin; CO2 is the chief greenhouse gas; SO2 and NO2 cause acid rain (H2SO4 and HNO3, pH below 5.6).
  • Photochemical smog (oxidising) forms from NO2 + hydrocarbons in sunlight, giving ozone and PAN; London smog (reducing) comes from SO2 and soot.
  • CFCs release Cl radicals in the stratosphere that catalytically destroy ozone (Cl• + O3 → ClO• + O2), creating the ozone hole and raising harmful UV-B.
  • Greenhouse gases (CO2, CH4, H2O vapour, O3, CFCs) trap infrared radiation, causing the greenhouse effect and global warming.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Which gas is the chief cause of the greenhouse effect contributed by human activity?
Explanation: CO2 is the main human-released greenhouse gas, trapping infrared radiation and warming the atmosphere.
Q2.Carbon monoxide is toxic to humans because it:
Explanation: CO forms carboxyhaemoglobin, binding ~300 times more strongly than O2, so tissues are starved of oxygen.
Q3.Photochemical smog is best described as:
Explanation: Photochemical smog forms in sunlight from NO2 and hydrocarbons, producing oxidants such as ozone and PAN.
Q4.Stratospheric ozone is depleted mainly by:
Explanation: UV light splits CFCs to release Cl radicals, which catalytically destroy ozone (Cl• + O3 → ClO• + O2).
Practice more MCQs → 📝 Assignment · 30 marks