Breathe London network accuracy
This page describes how we use the London Air Quality Network reference analysers to monitor and improve the performance of the Breathe London Nodes in real-time and assess current network accuracy
Reference Vs Indicative
First, let’s have a look at what is meant by ‘reference’ air quality instruments and what is meant by ‘indicative’.
Reference
The term ‘reference’ is applied to instruments which have demonstrated compliance with Defra’s minimum performance requirements and data quality objectives.
Instrument performance is judged by reference to a standard which specifies a series of tests and requirements instruments must achieve in both field and laboratory tests.
In the UK, approval of instruments is carried our by the Environment Agency under it’s MCERTS scheme. Final decisions on instrument approval are made by a certification committee. In addition this, Defra also require particulate instruments to comply with a specific MCERTS for particulate instruments scheme.
For gaseous analysers, instruments must achieve a measurement uncertainty of +/- 15% at the relevant limit value. For particulate analysers, instruments must achieve a measurement uncertainty of +/- 25% at the relevant limit value.
As well as instrument performance, Defra also set out technical guidance on how air quality monitors should be operated to ensure high data quality. This includes guidance on where monitors should be sited, calibration process and frequency and service and audit schedules.
These standards and the certification process for reference instruments, combined with standard operating procedures, provides the public, government and researchers a high degree of confidence in the performance and comparability of air quality data on the London and national networks.
Indicative
Over the past decade there has been a rapid growth in lower-costs air quality sensors employing a range of alternative methods to measure air pollution.
These sensors range from simple single-pollutant devices available for tens of pounds up to more sophisticated multi-pollutant devices devices like the Breathe London Nodes.
Lower cost, portability and compactness mean these devices can be deployed more easily in locations where it may be difficult or impossible to locate a reference instrument. The UK’s Air Quality Expert Group (AQEG) has produced guidance on the use of small sensors.
Though this new generation of small sensors are improving, none of them have yet achieved ‘reference equivalent’ status. Instead, these sensors are assessed against a standard known as ‘indicative’.
For particulate sensors, Defra has produced an MCERTS performance standard for indicative sensors which stipulates a measurement uncertainty of +/-50% at the relevant limit value. There is no current MCERTS equivalent for gaseous sensors, however Annexe I of the EU ambient air quality directive stipulates a measurement uncertainty of +/-25% for indicative nitrogen dioxide sensors.
As the network correction develops, we will display on this page how the Breathe London Nodes are performing vs the reference instruments on the London network.
Pre-deployment co-location
Prior to deployment, every Breathe London Node is co-located at a London Air reference site for two weeks. This allows us to check how the sensors are performing vs the reference monitors.
This co-location provides us with a device-specific correction factor which is used for initial deployment into the field.
Realtime network correction
The pre-deployment co-location provides an initial correction factor, but it is necessarily limited to the pollution conditions at the time.
Pollution changes continually of course. To address this, around 20 Breathe London Nodes are permanently co-located with London Air reference sites to allow us to calculate real-time correction factors for each pollutant and to continually assess performance of the Nodes vs the reference network.
You can read more about the performance of the Breathe London network on the Network Reports page.