Air Pollutants

Common Air Pollutants

Several pollutants are released into the atmosphere from human activities and natural sources. These include:

• Carbon monoxide (CO): A colourless, odourless, and poisonous gas formed by incomplete combustion of fossil fuels (e.g. petrol in car engines). Incomplete combustion occurs when there is not enough oxygen to produce carbon dioxide.
• Sulfur dioxide (SO₂): A colourless gas with a sharp smell, produced when fuels containing sulfur (like coal and oil) are burned.
• Nitrogen oxides (NOₓ): A group of gases including nitrogen monoxide (NO) and nitrogen dioxide (NO₂), formed when nitrogen and oxygen react at high temperatures during combustion.
• Particulates: Tiny solid particles or liquid droplets suspended in the air, such as soot, dust, and smoke.

Note: Carbon dioxide (CO₂) and methane (CH₄) are greenhouse gases and are covered in other topics related to climate change. See the Greenhouse Gases section for more details.

Sources of Air Pollutants

Air pollutants come from various sources, mainly linked to human activities but also natural events:

• Combustion of fossil fuels: Burning coal, oil, petrol, and diesel in power stations, vehicles, and heating releases CO, SO₂, NOₓ, and particulates.
• Industrial processes: Factories and power plants burn fossil fuels and use chemical reactions that release sulfur dioxide and nitrogen oxides.
• Vehicle emissions: Cars, lorries, and buses produce carbon monoxide and nitrogen oxides from burning petrol and diesel.
• Natural sources: Wildfires, volcanic eruptions, and dust storms release particulates and gases like sulfur dioxide naturally.

For example, when petrol burns incompletely in a car engine, carbon monoxide is produced:

$\text{Fuel} + \text{O}_2 \rightarrow \text{CO} + \text{other products} \quad \text{(incomplete combustion)}$

Sulfur in coal reacts with oxygen during combustion to form sulfur dioxide:

$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$

Effects of Air Pollutants

Air pollutants have harmful effects on health, the environment, and visibility:

• Health impacts: Breathing in carbon monoxide reduces oxygen transport in the blood, causing dizziness and even death in high amounts. Nitrogen oxides and particulates irritate the lungs, causing asthma and bronchitis.
• Acid rain formation: Sulfur dioxide and nitrogen oxides dissolve in rainwater forming acidic solutions (sulfuric and nitric acid). Acid rain damages plants, soils, buildings, and aquatic life.
• Smog and reduced visibility: Nitrogen oxides and particulates contribute to smog, a mixture of smoke and fog that reduces air quality and visibility, especially in cities.
• Environmental damage: Acid rain lowers pH in lakes and soils, harming wildlife and reducing biodiversity.

For instance, sulfur dioxide reacts with water in the atmosphere to form sulfurous acid, which further oxidises to sulfuric acid:

$\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \quad \text{(sulfurous acid)}$

$2\text{H}_2\text{SO}_3 + \text{O}_2 \rightarrow 2\text{H}_2\text{SO}_4 \quad \text{(sulfuric acid)}$

Control and Reduction Methods

Several methods are used to reduce air pollution and protect health and the environment:

• Catalytic converters: Fitted to car exhausts, these convert carbon monoxide and nitrogen oxides into less harmful gases like carbon dioxide and nitrogen using catalysts.
• Desulfurisation of fuels: Removing sulfur from fuels before burning reduces sulfur dioxide emissions.
• Use of cleaner fuels: Switching to fuels that produce fewer pollutants, such as natural gas or electricity, helps reduce emissions.
• Legislation and regulations: Laws limit emissions from vehicles and industries, setting maximum allowed levels of pollutants.

Catalytic converters use platinum or rhodium catalysts to speed up reactions like:

$2\text{CO} + 2\text{NO} \rightarrow 2\text{CO}_2 + \text{N}_2$

Example: If a vehicle emits 1 mole of nitrogen monoxide (NO), how many moles of carbon monoxide (CO) can it react with in a catalytic converter?

Using the equation $2\text{CO} + 2\text{NO} \rightarrow 2\text{CO}_2 + \text{N}_2$, the mole ratio of CO to NO is 1:1, so 1 mole of NO reacts with 1 mole of CO.