PM10

Do you suffer from hayfever? If you do, you'll know that it's caused by an allergic reaction to pollen.

During spring, plants release pollen. It's dispersed by the wind, reaching other plants, and unfortunately, our noses.

Pollen contributes to airborne particulate matter (known as PM). PM is not one single pollutant. It's a mixture of many compounds – ions, metals, organic compounds, and more. Because the elemental composition of airborne particulate matter varies so much, it's classified according to its diameter.

PM₁₀: Meaning

Let's begin with a definition.

A micrometre is one-millionth of a metre (0.000001 metres). To compare, a single human hair is about 70 micrometres across. PM₁₀ particles are too small to see with the naked eye.

Sources of PM₁₀ Pollution

When a fuel burns in plenty of air, it undergoes complete combustion. All elements in the fuel fully react with the oxygen in the air:

hydrocarbon + oxygen → carbon dioxide + water

Incomplete combustion occurs when the supply of air is poor. The elements in the fuel cannot fully react with the oxygen in the air. So, this reaction takes place instead:

hydrocarbon + oxygen → carbon monoxide + carbon + water

A major source of PM₁₀ pollution is incomplete combustion of hydrocarbons. Sources of hydrocarbons include:

• Coal

• Wood

• Diesel

• Crop waste

PM₁₀ doesn't only come from incomplete combustion. Other sources include:

• Dust from construction sites

• Landfill

• Agriculture

• Wildfires

• Industrial sources

• Pollen

• Fragments of bacteria

PM₁₀: Impacts and Examples

High levels of PM₁₀ pollution in the atmosphere can have a range of negative effects.

Respiratory Disease

Short-term exposure to PM₁₀ pollution has been associated with flare-ups of pre-existing respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD). This can lead to hospitalisation.

Long-term exposure is associated with mortality from respiratory diseases, and lung cancer.

Changing Atmospheric Albedo

Albedo is measured on a scale from 0 to 1. The higher the albedo, the more reflective a surface.

Particulate Matter and Ice

In the atmosphere, particulate matter has a high albedo. But when dark-coloured particulate matter lands on ice, the albedo of the ice decreases. A lower albedo enhances melting, contributing to sea level rise.

Smogs

The term was first coined in the 1900s, combining 'smoke' and 'fog'. Incomplete combustion of coal lead to smogs in cities.

Modern-day smogs happen in winter during temperature inversions. Usually, temperature declines with altitude. But during a temperature inversion, cold air is trapped at the ground underneath a layer of warmer air. The warm air acts like a lid, trapping particulate matter near the ground. Winter smogs are further exacerbated by households burning wood and coal for extra heat during these cold conditions.

Monitoring PM₁₀ Levels

High levels of PM₁₀ pose a threat to health, especially for elderly or vulnerable individuals. So, it's important to regularly monitor PM₁₀ and share related health guidance.

How Are PM₁₀ Levels Measured?

Particulate matter can be measured using a gravimetric method.

Particles are collected by drawing air onto a filter. Scientists weigh the filter, and compare it to its original mass.

This method is highly accurate (it's used by regulatory bodies) and it allows the particles to be analysed to learn about their chemical composition. On the other hand, it doesn't produce real-time data, and it requires trained staff to operate.

Alternatively, particulate matter can be measured using an optical particle counter. It uses a laser diode to illuminate particles, and a photodetector to measure how the light is scattered by these particles. Optical particle counters can be calibrated against gravimetric instruments to increase their accuracy.

Fig. 3 - Labelled optical particle counter

Safe PM₁₀ Levels

In the UK, PM₁₀ concentrations must not exceed:

• An annual average of 40 µg/m³

• A 24-hour average of 50 µg/m³ more than 35 times in a year

The Daily Air Quality Index functions like a weather forecast, informing the population of the predicted air pollution levels. It categorises pollution concentrations into bands, which have associated health advice.

Controlling PM₁₀ Pollution

In the UK, urban and roadside levels of PM₁₀ have been declining since the 1990s. How has PM₁₀ pollution been limited?

• The Clean Air Act (1956): after the Great Smog of London, the Clean Air Act set out to control smoke pollution. The Act created Control Orders, which gave local authorities the power to limit emissions. In large urban areas, Smoke Zones were established. In these areas, emissions of smoke, grit, dust, or fumes could be banned. Following the Act, pollution-related death rates declined significantly.

• Coal Treatment: tar can be removed from coal before burning using an emulsifying agent. The liquid product can be used for cleaning tanks, heat exchangers, and air coolers.

• Electrostatic Precipitators: suspended particles are removed from gas streams using electrical energy. The energy charges the particles either positively or negatively. They are then attracted to wires or collector plates carrying the opposite charge.

• Cyclone Separators: particulate matter is removed from air using centrifuge-like equipment. Dirty air is fed into a chamber. The inside of the chamber creates a spiral vortex, similar to a tornado. Lighter components of the gas have less inertia, so are influenced by the vortex and travel up it. Heavier particulate matter has more inertia and is not influenced by the vortex.

• Bag Filters: as these bags fill up, water passes through the tiny holes in the material and leaves behind unwanted particles.

I hope that this article has explained PM₁₀ pollution for you. PM₁₀ refers to inhalable particulate matter with a diameter of 10 micrometres and below. High levels of particulate pollution can cause respiratory diseases, affect atmospheric albedo, and cause smogs. Therefore, it's important to monitor and limit levels of PM₁₀.

^{1. Ann Gibbons, Why 536 was ‘the worst year to be alive', Science, 2018}

^{2. Department for Environment Food & Rural Affairs, What is the Daily Air Quality Index?, UK Air, 2022}

^{3. Emma Reynolds, Air pollution a cause of UK girl's death, finds global landmark ruling, CNN, 2020}

^{4. Jennifer McAlpine, How Big is a Micron?, Benchmark, 2020}

^{5. The Meteorological Office, The Great Smog of 1952, 2022}

^{6. National Statistics, Concentrations of particulate matter (PM10 and PM2.5), 2022}