What is particulate matter?

Particulate matter (PM) is a term used to describe the mixture of solid particles and liquid droplets in the air.  It can be either human-made or naturally occurring. Some examples include dust, ash and sea-spray. Particulate matter (including soot) is emitted during the combustion of solid and liquid fuels, such as for power generation, domestic heating and in vehicle engines. Their potential for causing health problems is directly linked to the size of the particles.

Airborne particulate matter is made up of a collection of solid and/or liquid materials of various sizes that range from a few nanometres in diameter (about the size of a virus) to around 100 micrometres (100 µm, about the thickness of a human hair). It consists of both primary components, which are released directly from the source into the atmosphere, and secondary components, which are formed in the atmosphere by chemical reactions. It contains a range of chemical compounds and the identity of these compounds provides clues to its origin. Particulate matter is classified according to its size and this classification is used in concentration measurements. For example, PM10 is – to a good approximation – the concentration of particles that are less than or equal to 10 µm in diameter similarly, PM2.5 describes the concentration of particles that are less than or equal to 2.5 µm in diameter.

Research has pointed towards the smaller particles, in particular PM less than 2.5 micrometres in diameter (PM2.5), as a metric more closely associated with adverse health effects than other metrics such as PM10 (particles with a diameter less than 10 micrometres).

Particulate matter (PM) In the UK

Emissions of air quality pollutants in the UK contribute to both local and transboundary air pollution. Particulate matter (PM) is also formed from the long-range transport of precursor gases forming PM through atmospheric processes.

Concentrations of particulate matter (PM) in the atmosphere arise from direct emissions of PM from a wide range of sources such as fuel combustion, surface erosion and windblown dusts and mechanical break-up processes, for example, in quarrying and construction sites. Further concentrations of PM are produced from the interaction of NOx, SO2, NH3and NMVOCs in the atmosphere.  These are important precursors to secondary PM.

In the UK, residential combustion becomes the most significant source of primary PM10 in recent years due to increase of wood burning.  Road transport, production processes and residential combustion are other important sources of primary PM10.

PM_1.0 and PM_0.1 (Finer particulates)

Particulate matter with an aerodynamic diameter of less than 1.0 and 0.1 micrometres (µm) is referred to as PM1.0 and PM0.1 respectively.

Between 1990 and 2014, UK emissions of PM1.0 fell by 56%. In 2014, residential stationary combustion accounted for 34% of PM1.0emissions, of which 75% of emissions came from the combustion of wood.

Between 1990 and 2014, UK emissions of PM0.1 fell by 54%. In the past, road transport became an increasingly important sector as the particle size decreased, for example, in 1990, road transport accounted for 12% of PM10 emissions, but 31% of PM0.1 emissions. This trend is not as visible anymore due to tighter regulations on PM emissions from new vehicles. In 2014, road transport was the most important sector, accounting for 25% of PM0.1 emissions.

PM2.5 (Fine particulate matter)

PM2.5means the mass per cubic metre of air of particles with a size (diameter) generally less than 2.5 micrometres (µm). PM2.5 is also known as fine particulate matter  (2.5 micrometres is one 400th of a millimetre).

Health Effects of PM: Inhalation of particulate pollution can have adverse health impacts.

The biggest impact of particulate air pollution on public health is from long-term exposure to PM2.5, which increases the age-specific mortality risk, particularly from cardiovascular causes. Exposure to high concentrations of PM (e.g. during short-term pollution episodes) can also exacerbate lung and heart conditions, significantly affecting quality of life, and increase deaths and hospital admissions. Children, the elderly and those with predisposed respiratory and cardiovascular disease, are known to be more susceptible to the health impacts from air pollution.

Sources of PM_2.5: Human-made sources of PM2.5 are more important than natural sources, which make only a small contribution to the total concentration. Within UK towns and cities, emissions of PM2.5 from road vehicles are an important source. Consequently, levels of PM2.5 (and population exposure) close to roadsides are often much higher than those in background locations. In some places, industrial emissions can also be important, as can the use of non-smokeless fuels for heating and other domestic sources of smoke such as bonfires. Under some meteorological conditions, air polluted with PM2.5 from the continent may circulate over the UK – a condition known as the long range transportation of air pollution. Long range transport, together with pollution from local sources, can result in short term episodes of high pollution which might have an impact on the health on those sensitive to high pollution. In addition to these direct (i.e. primary) emissions of particles, PM2.5 can also be formed from the chemical reactions of gases such as sulphur dioxide (SO2) and nitrogen oxides (NOx: nitric oxide, NO plus nitrogen dioxide, NO2) these are called secondary particles. Measures to reduce the emissions of these precursor gases are therefore often beneficial in reducing overall levels of PM2.5.

Primary emissions of PM, the formation of secondary PM within the UK and long range transport of pollution from outside the UK all contribute to regional PM levels across the UK.

Distribution of air pollution:  With the exception of ozone, concentrations of air pollutants are generally higher in urban than rural areas.  For PM2.5, there is a gradient in concentration across the country with higher concentrations found in the South East than other areas.  Within cities, air quality (particularly in relation to concentrations of PM10and NO2) tends to be worse close to busy roads, where poorer communities often live.

UK Air Quality Strategy

The Air Quality Expert Group (AQEG) was set up in 2002, following the publication of the 'Air Quality Strategy for England, Scotland, Wales and Northern Ireland', which describes the plans drawn up by the Government and Devolved Administrations to protect and improve air quality in the United Kingdom (UK). AQEG aims to help the UK Government tackle air pollution by examining where pollutants come from and what their concentrations are in the UK.

To protect our health, Defra and the Devolved Administrations have set two air quality objectives for PM10 in their Air Quality Strategy:

1. The 24-hourly objective, which is the concentration of PM10 in the air averaged over 24 hours, is designed to make sure that we are not exposed to high concentrations of PM10 for short periods of time. High concentrations can arise during pollution episodes, which are short periods of high levels of pollution that are usually associated with particular weather conditions.

2. The annual objective, which is the concentration of PM10 in the air averaged over one year, aims to protect us from being exposed to PM10 over a long time period.

Networks of monitoring sites for particulate matter have been established throughout the UK. The majority of sites use a TEOM analyser, but a few use the gravimetric and other methods. Most sites monitor PM10, but a few also measure PM2.5.

In recent years, there has been reductions in particulate matter emissions in the UK.  The fall in total emissions has been dominated by reductions in emissions from domestic heating, energy production and from industrial combustion largely as a result of a decrease in the use of coal as a fuel. There have also been reductions in the emissions of those pollutants that lead to the formation of secondary particulate matter.

Sources:

National Atmospheric Emissions Inventory (NAEI)

Department for Environment, Food & Rural Affairs

https://www.gov.uk/government/publications/fine-particulate-matter-pm2-5-in-the-uk

http://naei.defra.gov.uk/data/

https://uk-air.defra.gov.uk/assets/documents/reports/aqeg/pm-summary.pdf

https://laqm.defra.gov.uk/public-health/pm25.html

Report by Air Quality Expert group(UK)