Has there ever been a big bonfire near your home? If so, the air probably smelt smoky and may have even looked a little grey. That's because the bonfire has released particulate matter into the air. Particulate matter is a form of air pollution.

We discuss the definition of air pollutants, their causes and sources, the different types that exist, the difference between primary and secondary pollutants, and some examples. Take a deep breath and get ready to learn about air pollutants!

The Definition of Air Pollutants

What exactly is the definition of air pollutants? Air pollutants can either be natural or artificial substances introduced into the environment in a concentration sufficient to have some sort of measurable effect on living things and the planet as a whole.

Air pollutants can be solids, liquids, or gases. They can come from natural sources or originate from human activities like burning fossil fuels.

Sources of Air Pollutants

Among the criteria for classifying air pollutants is the nature of their source. Sources of air pollutants are either natural or anthropogenic (human).

Natural sources of air pollutants include:

• Volcanic eruptions

• Wildfires

• Dust storms

• Sea salt spray

• Plants

Human Causes of Air Pollutants

One of the main causes of air pollutants is human activity. A major source of atmospheric pollutants is the combustion of fossil fuels.

Fossil fuels are burned to release energy. However, they also release carbon dioxide (a greenhouse gas) and harmful atmospheric pollutants.

Human sources of air pollutants can be either:

• Static: such as power stations

• Mobile: such as vehicles

Figure 1: Car engines are a major source of air pollutants. Source: unsplash.com

Types of Air Pollutants

Now, we're going to look at four common types of air pollutants in detail: carbon monoxide, sulphur dioxide, oxides of nitrogen, and particulates.

Carbon Monoxide

Sources of Carbon Monoxide

When hydrocarbon fuels are burned in the air, they produce carbon dioxide and water. For example, let's look at the combustion of propane:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

However, incomplete combustion occurs when the supply of oxygen is poor. Carbon monoxide is produced alongside carbon dioxide:

C₃H₈ + 4O₂ → CO₂ + 2CO + 4H₂O

Incomplete combustion is common in vehicle engines, which can be poorly ventilated.

Effects of Carbon Monoxide

Red blood cells contain a protein called haemoglobin – a red protein responsible for transporting oxygen around the bloodstream. If carbon monoxide is inhaled, it bonds with haemoglobin molecules in place of oxygen, reducing the capacity of the blood to transport oxygen.

Symptoms of carbon monoxide poisoning include nausea, headaches, dizziness, and loss of consciousness. High doses can lead to oxygen starvation and even death.

Sulphur Dioxide

Sources of Sulphur Dioxide

Burning fuels containing sulphur produces sulphur dioxide: S + O₂ → SO₂

Coal burned in power stations and diesel burned in ships often contain sulphur.

Effects of Sulphur Dioxide

In moist air, sulphur dioxide reacts with water vapour to form sulphuric acid (H₂SO₄), producing acid rain. Consequences of acid rain include:

• Soil erosion, affecting nutrient availability

• Plant death

• Acidification of aquatic environments, harming living organisms

• Damage to buildings and infrastructure

Figure 2: A forest on the German-Czech border devastated by acid rain. Source: Wikimedia Commons

Oxides of Nitrogen

Sources of Oxides of Nitrogen

Oxides of nitrogen are indirectly produced by burning hydrocarbons. Vehicle engines typically burn hydrocarbons between 75 and 105ºC. In these hot conditions, nitrogen and oxygen molecules combine to form nitrogen monoxide.

N₂ + O₂ → 2NO

When nitrogen monoxide is released from vehicle exhausts, it reacts with oxygen in the air to form nitrogen dioxide.

2NO + O₂ → 2NO₂

Oxides of Nitrogen Effects

Like sulphur dioxide, oxides of nitrogen can contribute to acid rain. When NO_x react with water and air, they produce nitric acid (HNO₃).

Particulates

Sources of Particulates

The predominant source of particulate pollution is the incomplete combustion of hydrocarbon fuels in diesel engines producing carbon (soot).

• Dust from roads and industry

• Landfill sites

• Agriculture

Effects of Particulates

Inhaling particulate pollution is associated with respiratory diseases. Small particles can penetrate deep into people's lungs, leading to heart disease and lung cancer.

High levels of particulates can worsen smogs and modify albedo.

Figure 3: Nelson's Column during the Great Smog of London, 1952. High levels of particulate pollution during a temperature inversion led to this unusual weather event. Approximately 12,000 people died as a result. Source: Wikimedia Commons

Particulate matter with a high albedo reflects solar energy away from Earth, causing a decline in temperature and light availability.

Alternatively, some particulate matter has a low albedo and thus absorbs solar energy. If this particulate matter lands on ice or snow, it enhances melting and increases sea level.

Secondary Air Pollutants

What's the difference between primary pollutants and secondary air pollutants? Well, primary pollutants are emitted directly from a source. In contrast, secondary pollutants are formed in the atmosphere. Secondary pollutants are much harder to control, as they can be synthesised in many ways.

Air pollutants are particles or chemicals in the air that are harmful to human health and the environment. Some pollutants have natural causes, but many originate from the combustion of fossil fuels. Major air pollutants include carbon monoxide, sulphur dioxide, nitrogen oxides, and particulates.

^{1. Alessandra Potenza, In 1952 London, 12,000 people died from smog — here's why that matters now, The Verge, 2017}

^{2. AutoDoc, Causes of overheating, 2022}

^{3. Linda C. Brinson, How Much Air Pollution Comes From Cars?, HowStuffWorks, 2021}