Air Quality Monitoring

Air Pollution Monitoring

The City of London Corporation has been monitoring air quality in the City of London since the 1960s. Initially the focus was on sulphur dioxide and black smoke after the enactment of the  Clean Air Act 1956.

The monitoring network has continually evolved in response to relevant legislation and the statutory requirements of London Local Air Quality Management to monitor, assess and take action to improve air quality.

Automatic Continuous Monitoring

There are five locations within the City of London where sophisticated equipment is located to monitor air pollution. In addition to monitoring NO₂ and PM₁₀, monitoring of fine particulates (PM_2.5) and ozone (O₃) is also completed. The equipment monitors air pollution continuously, with a concentration provided every hour.

Two locations are 'urban background' sites. This allows the determination of long term trends in ambient air pollution as there is no immediate pollution source nearby. The remaining three other monitoring sites are 'roadside' sites. 

• The Aldgate School, an urban background site (NO₂, PM₁₀ and PM_2.5)
• Guildhall, an urban background site (O₃)
• Beech Street, a roadside site (NO₂ and PM₁₀)
• Bell Wharf Lane, a roadside site (NO₂ and PM₁₀)
• Farringdon Street, a roadside site (PM_2.5)

Details of the monitoring sites, including real-time and historic monitoring data, are available through the Air Quality in England website. Provisional data is available within an hour of a  measurement being taken. Data that has been ratified and subject to a rigorous system of auditing data is available within the City Corporation air quality Annual Status Reports.

Passive NO₂ Diffusion Tube Monitoring

Diffusion tubes are deployed at a number of locations across the City of London. They are small plastic tubes which contain a chemical reagent that absorbs NO₂ from the air. The tubes are exposed a four/five week period before being changed, and provide an indicative monthly average level of NO₂ at each location.

The monthly monitoring data is processed to assess the annual mean concentration of NO₂, and these are included in the Annual Status Reports. The location of current diffusion tube monitoring and relevant concentrations are presented through the City Corporation Interactive Mapping resource.

Air Pollution Alerts

The monitoring data collected by the City Corporation is sent to Imperial College London which uses it, along with data collected by London Boroughs, to gauge levels of pollution across the whole of Greater London.

For guidance on the type of action you may need to take when air pollution levels are high visit the Government’s Daily Air Quality Index.

The amount of pollution in the City of London varies from day to day as it is strongly linked to weather conditions. Information about daily levels of air pollution is available from the following sources:

• The free CityAir app enables users to receive pollution alerts and to find low pollution routes around London.
• The London Air Quality Network provides daily air pollution forecasts and you can sign up for pollution alerts.
• The City Corporation is also partnered with AirText. This is another network which provides air pollution alerts for Greater London. By signing up you can receive air pollution alerts either by SMS (text message), email or smartphone.