Properties and effects of atmospheric pollutants: Students should be able to describe and explain the problems caused by increased amounts of these pollutants in the air.

The idea

The atmosphere normally contains pollutants at low concentrations, but human activity raises levels of carbon monoxide, sulfur dioxide, nitrogen oxides and particulates. Increased carbon monoxide binds to haemoglobin, reducing oxygen transport and causing dizziness or death. Sulfur dioxide and nitrogen oxides dissolve in rainwater to form acid rain, which leaches nutrients from soil, acidifies lakes and corrodes limestone buildings. Nitrogen oxides also cause respiratory problems. Particulates from incomplete combustion scatter sunlight, causing global dimming, and can enter lungs, worsening asthma and heart disease. Describing means naming the pollutant and its effect; explaining means linking the pollutant's property to the harm caused.

What earns the marks

• Carbon monoxide is toxic because it binds to haemoglobin in red blood cells, reducing the oxygen carried around the body, which can cause fainting, coma or death.
• Sulfur dioxide dissolves in atmospheric water to form acid rain, which lowers the pH of lakes and soils, damaging plants, fish and limestone or marble buildings.
• Nitrogen oxides dissolve in rainwater to form acid rain and also cause respiratory problems in humans.
• Particulates are tiny solid particles from incomplete combustion that cause global dimming by reflecting sunlight, and can be inhaled deep into the lungs, worsening asthma and cardiovascular problems.
• Increased pollutant levels arise mainly from burning fossil fuels in power stations, factories and vehicles, so describing the problem should link the source to the effect.
• Explaining an effect requires a causal chain, for example sulfur dioxide → dissolves in rain → acid rain → soil and lake acidification → harm to organisms and buildings.

Common mistakes and how to put them right

• Confusing carbon monoxide with carbon dioxide.

  Put it right: Carbon dioxide contributes to global warming, whereas carbon monoxide is toxic because it reduces oxygen transport by haemoglobin.

• Stating that acid rain directly burns plants.

  Put it right: Acid rain damages plants indirectly by acidifying soil and releasing aluminium ions that harm roots, so the correction is to describe soil and water chemistry rather than burning.

• Treating particulates as gases.

  Put it right: Particulates are solid or liquid particles suspended in air, so they are inhaled and cause global dimming rather than dissolving to form acid rain.

Exam tips

• Use the command word: 'describe' needs a clear account of what happens, while 'explain' needs a causal link between pollutant property and harm.
• Name the specific pollutant and its source before giving the effect, so the examiner can credit both the cause and the consequence.
• For higher-tier answers, include a chain of at least two steps, such as particulates reflecting sunlight to cause global dimming.