Air quality measurement is vital for understanding and addressing air pollution issues. The Breathe London air quality monitoring network, established in 2020, has played a crucial role in providing accessible air quality data to local communities across London by deploying a dense network of air pollution sensors. Recently, the Greater London Authority and Imperial College London released the 2021–2022 Breathe London Network Air Quality Monitoring Report, highlighting key statistics on nitrogen dioxide (NO2) and particulate matter (PM2.5) concentrations and showcasing the impressive performance of these air quality sensors in comparison to reference monitors.

Background on the Breathe London air quality monitoring network

Starting with 136 air quality sensors, the Breathe London Network has expanded rapidly to over 400 sites, including schools, hospitals, and residential communities. Its success has led to replication efforts worldwide through Breathe Cities, aiming to combat air pollution and improve public health.

The Breathe London Network uses Node-S air quality sensors to collect high-resolution air quality measurements across the city. These compact and lightweight devices can be affixed to street furniture like lampposts and operate on solar power or mains electricity. NO2 and PM2.5 measurements are collected by these air pollution sensors at regular intervals.

A Breathe London Node-S air quality sensor being installed to measure air pollution. 

Real-time data from the Breathe London network is publicly accessible on their website, featuring a map of current air quality, site-specific graphs, and downloadable hourly data. Developers/researchers can access the data directly through an Application Programming Interface (API). Despite calibration and data scaling improving accuracy, it's important to note that sensor data is considered indicative. Additional information on network accuracy is available on the Breathe London website, along with quarterly Network Reports.

Air quality sensor collocation, calibration and data performance

Before deployment, all Breathe London sensors undergo calibration and are temporarily placed at the Honor Oak Park air quality monitoring supersite for collocation. This allows a comparison of sensor data with reference monitor data to ensure compliance with accuracy criteria and standardization.

Air quality sensors collocated with a reference air quality monitor ahead of deployment for the Breathe London air quality monitoring network.

With 19 sensors permanently collocated at London Air Quality Network reference sites, continuous comparisons between sensor and reference data are possible. A dynamic network scaling algorithm is applied to the entire network, incorporating factors like humidity, ozone, NO2, and PM2.5 readings. Developed by Imperial College London, this hybrid approach enhances sensor data accuracy.

Calibration processes and collocation with reference monitors ensure high accuracy, as reflected in the close correlation between sensor data and reference data — demonstrating that properly calibrated and operated air quality sensors can achieve near-reference level accuracy. 

Collocated air quality sensors demonstrated extremely high correlation with reference air monitoring equipment for NO2.
The collocated air quality sensors also demonstrated high accuracy for particulate matter measurements when compared to reference air quality monitors.

Nitrogen dioxide concentrations

The report reveals that annual average NO2 concentrations exceeded WHO guidelines at all sites, emphasizing the severity of air pollution in London. Out of the 284 sites with sufficient NO2 data, all sites recorded concentrations surpassing the WHO annual average NO2 guideline for both 2021 and 2022. A striking 98% of these sites also exceeded the WHO interim target of 20 µgm-3. 

The Breathe London air quality data report revealed that nitrogen dioxide pollution levels exceeded WHO recommendations for both 2021 and 2022.

Particulate matter concentrations

PM2.5 concentrations also exceeded WHO guidelines at all sites, with variations between 2021 and 2022. The data suggests a need for continuous efforts to reduce PM2.5 levels. 

In 2021 and 2022, annual average concentrations of PM2.5 ranged from 7 µgm-3 to 17 µgm-3 and 7 µgm-3 to 16 µgm-3, respectively. The findings from indicate that all monitoring sites surpassed the World Health Organization (WHO) annual average guideline for PM2.5 in both years. 

While nearly half of the sites met the WHO interim target and legal objective of 10 µgm-3 in 2021, this percentage decreased to a third as more sites were incorporated into the network in 2022. The majority of sites consistently measured concentrations between 10 and 15 µgm-3 for both years. 

Notably, one site each year, namely the Brent Ikea co-location site in 2021 and Mint Walk in Croydon in 2022, recorded averages exceeding 15 µgm-3. The Brent Ikea site showed a substantial reduction of 23%, lowering its concentration to 13.1 µgm-3 in 2022.

Particulate matter air pollution concentrations exceeded WHO recommendations for air quality measurement sites across London.

Clean air action starts with accurate air quality data

The Breathe London air quality monitoring network stands as a beacon of success in providing local, reliable, and accurate air quality data. As the network continues to grow, so does its ability to evaluate London air pollution trends at a level of granularity never before possible. 

The high accuracy reported for the Node-S air quality sensors used by the Breathe London air quality network demonstrate the viability for replication of this air quality monitoring network in other regions worldwide.

While air quality measurement is an important first step, the increasing levels of PM and NO2 air pollution revealed in the report highlight the urgency of taking further policy action to reduce the risks of air pollution in London. You can find a link to the full report on the Breathe London website here