Measuring Air Pollutants
Monitoring and measuring air quality is carried out by a variety of different organisations, for example the Government, local councils, industry, research bodies and environmental pressure groups.
Many towns and cities in the UK and around the world experience unhealthy levels of pollution from time to time. In places where there is lots of road traffic, pollution levels can increase, especially particles and nitrogen dioxide. In the UK, where levels are found to be above or close to the levels set to protect our health, local councils are required to measure a range of air pollutants.
A different type of instrument is needed to measure each pollutant. All major towns and large cities have at least one air monitoring station that contains highly specialised and delicate instruments for measuring a range of pollutants. These sites are about the size of a small caravan and are very expensive to buy and maintain.
A widely used method of measuring one of the main air pollutants from road traffic, nitrogen dioxide, uses a simple device called a diffusion tube. Most air pollution measuring equipment are called active devices because they use a pump to suck air into them.
However, diffusion tubes do not use pumps so they are called passive diffusion tubes.
Why is air pollution measured?
Every hour of every day, air quality is being measured around the country by automated continuous monitors. This is because the UK Government is legally required to measure air pollution by the European Union. The EU, through legislation called Directives, requires the countries of Europe that are members to measure and improve air quality. The EU can only assess how a country is doing in its attempts to reduce air pollution, if proper monitoring is carried out and reported back to them.
Local councils must also carry out monitoring and assess air pollution in its area. If any local council finds air pollution levels above set limits, it has to declare the area an Air Quality Management Area (AQMA). It must then put in place an Air Quality Action Plan to try and improve the situation.
Measuring air pollution provides data that are used in a variety of ways, for example to assess how the Air Quality Action Plan is progressing. It can also be used to examine the impact on air quality of changes to the road network, or of a new building, such as a supermarket, that may attract lots of vehicles. Health experts can also use the measurement information to assess how air pollution impacts on people in the area.
Getting hold of the measurements
The thousands and thousands of bits of information that air quality monitors produce every year needs to be collected and stored in such a way that it can be easily accessed and understood.
The UK Government keeps all of the data produced from its air quality monitoring on one web site www.airquality.co.uk and produces reports every year. It also operates an air pollution bulletin service that updates every day. This classifies the air pollution as Low, Moderate, High or Very High depending on the pollution forecast. If the air pollution levels go up you may sometimes hear weather forecasters refer to it in their bulletins.
Many local councils also keep their air pollution data on websites. The councils within the Care4Air area provide information on a wide range of air quality issues.
A good place to start looking for information on the air pollution levels in Barnsley is the council website
Doncaster Council has a map-based website where you can find all of the information about current air pollution levels and air pollution issues in the borough
They also have an excellent air pollution website especially for young people
The best site for local air quality information has been produced by Rotherham Council. It shows all of the background information and reports, and also gives you access to a map-based system where you can produce printouts of past and present air pollution levels. Access it here
In Sheffield the city council has produced a web site called “Airmap” www.sheffieldairmap.org this is an interactive map showing the location of all of the air pollution monitoring sites in the city together with the data that each site produces. It also shows the network of nitrogen dioxide diffusion tubes that the city council operates and the network that is run by the local community.
How are fine particles measured?
In the UK the most common way of measuring PM10 and PM2.5 automatically is to use an analyser called a TEOM (Tapered Element Oscillating Microbalance).
Air is sucked in through a sampling head, which is designed to stop large particles entering the device (for instance a PM10 sampling head will only allow particles with a diameter less than or equal to 10 microns). Some of the air then passes through the device and onto a filter that sits on a vibrating quartz rod (this is the oscillating microbalance part).
As the number of particles deposited increases, the vibration of the quartz rod decreases. There is a direct relationship between the change in the vibration and the collected particles on the filter. The TEOM measures the amount of particles in the air and can produce readings every 15 minutes; it’s very useful for tracking pollution incidents as they happen. The TEOM provides readings expressed as micrograms of particles per cubic metre of air, this is written as µg m-3.
How is nitrogen dioxide (NO2) measured?
Nitrogen dioxide is commonly measured using automatic analysers that run 24 hours a day. The main measuring method is to use a chemiluminescence continuous analyser (chemiluminescence means the emission of light as a result of a chemical reaction).
Air is drawn into the analyser where nitrogen monoxide (NO) is made to react with ozone, to give nitrogen dioxide in a reaction that generates light:
Nitrogen monoxide + ozone » nitrogen dioxide + oxygen + light emission.
This method doesn’t actually measure nitrogen dioxide but calculates the airborne levels of nitrogen monoxide and total oxides of nitrogen (NOx), and then subtracts one reading from the other. The final results are expressed in parts per billion (ppb) or micrograms per cubic metre of air, this is written as µg m-3.
Nitrogen dioxide is also measured using diffusion tubes. For more information on this please go to the diffusion tube page.
How is sulphur dioxide (SO2) measured?
Sulphur dioxide is usually measured automatically, by a continuous analyser that uses ultra-violet fluorescence. The concentration of sulphur dioxide is calculated by measuring the fluorescent radiation energy produced when the sulphur dioxide molecules are bombarded by UV radiation inside the analyser.
This then provides a concentration in air, usually expressed in parts per billion (ppb) or micrograms per cubic metre (µg m-3).
How is ozone (O3) measured?
An ozone monitor operates by pulling an air sample from the atmosphere into the machine with an air pump. Inside the analyser, specially produced light is shone through the sample inside a tube, and a measurement of how much light has been absorbed is taken at the other end. The absorption of light is proportional to the concentration of ozone.