Background

The Breathe London project uses cutting-edge air pollution sensors and models to test new ways of understanding air quality in Greater London. Data from the Breathe London project is intended to provide “hyperlocal” insights about the varying levels of pollution across the city, from street to street and neighbourhood to neighbourhood.

Operating from late 2018 through late 2019, Breathe London aims to provide data and analysis to support policy making, policy evaluation and increased citizen engagement. The project advances existing monitoring methods, tests new lower-cost sensors and evaluates new ways to visualise and present data. Air pollution measurements from the project also can further validate and improve the models currently used to assess and forecast air quality in London.

The technical team is composed of Air Monitors Ltd., Cambridge Environmental Research Consultants, Environmental Defense Fund Europe, Google Earth Outreach, National Physical Laboratory, University of Cambridge and King’s College London.

Image via Flickr - stignygaard
Air quality monitor outside Madam Tussauds

Stationary monitoring

The Breathe London stationary network is made up of 100 AQMesh pods, each containing a collection of small sensor-based air quality monitors that offer near real-time localised air quality information. They measure nitrogen dioxide (NO₂) and nitric oxide (NO) using electrochemical sensors; particulate matter (PM) in various size cuts (PM₂.₅ and PM10 are reported) using a light-scattering optical particle counter; and carbon dioxide (CO₂) using a non-dispersive infrared absorbance sensor. In some locations, the pods measure ozone (O3), also using electrochemical sensors. The pods measure temperature, humidity and air pressure for the purposes of correction for environmental conditions. Each sensor pod is set up to collect data continuously for 10-second intervals and to create an average every 1-15 minutes, synchronised to the top of each hour. Data presented on the Breathe London website is shown as hourly averages, with a two to three hour lag from real time before they appear online.

Site selection: The 100 pods are located across Greater London, seeking to achieve on a number of criteria, developed in consultation with the Greater London Authority:

  • Coverage in all 32 London boroughs plus the City of London.
  • Filling gaps in the existing network of government air quality monitors.
  • Placing priorities on “sensitive” locations, such as primary schools and medical facilities.
  • Supporting assessments of the impact of new policies designed to reduce air pollution, such as the Ultra-Low Emission Zone (ULEZ), the Expanded ULEZ and the Low-Emission Bus Zones (LEBZ).
  • Distribution across a mix of traffic levels and varying distances from major roads and intersections, parks, residential areas, high-traffic streets and other commercial areas.
  • Reserving 5 of the pods (termed “gold pods”) for performance evaluation over the long-term using periodic co-location studies alongside reference instruments.

Data verification and quality assurance

These devices are not intended to provide equivalent accuracy to conventional (i.e., reference) monitoring methods, but rather to provide information across a wide area in many locations at a much lower cost. As such, these devices are not “calibrated” in the normal manner with known standard materials; their accuracy is defined by periodic co-location with conventional monitors and comparison with each other. Various Quality Assurance/Quality Control (QA/QC) checks are carried out from factory to first installation and throughout the deployment. Data is evaluated in ‘stages’ with each stage adding one or more quality assurance steps to the previous stage.

Stage 0 (Factory Settings)

Data at this stage is in the form that Air Monitors receives from the pod manufacturer, Environmental Instruments Ltd (EI Ltd), after application of factory QA/QC. Prior to shipping to the customer, each individual gas or PM sensor is characterised by EI Ltd. in terms of sensitivity and offset. This data is unique to each sensor and are used by the AQMesh processing algorithm to apply corrections for cross gas interferences and environmental conditions.

When the individual sensors are combined into a “pod” they are subjected to a minimum co-location period of seven days at the factory in Stratford-upon-Avon, where they are compared with a range of reference-grade monitors. This process determines a scaling factor, or default slope and offset, for each sensor – which, in effect, “calibrates” them against the reference instruments. Data at this stage is available either with the calculated slopes and offsets applied, or without (also known as ‘pre-scaled’ data). Each data point generated by the pods at this stage are accompanied by a timestamp and single status code, determined by EI Ltd.

Stage 1 (Empirically Verified)

Concentrations of certain pollutants at the pod’s final field location may be quite different from those experienced during factory co-location. In order to ensure that the pods are correctly calibrated for the range of environmental conditions present at their field location, Stage 0 data is then adjusted with scaling factors determined through one of three methods:

  • Pre-deployment reference site co-location: Prior to initial field placement about half of the project pods were co-located with a reference monitor in Greater London for approximately 3 – 7 days. After this period, linear regressions were performed to determine slopes and offsets between the reference site and pod data.
  • “Gold” pod co-location: “Gold” pods are standard AQMesh pods which have been co-located at one or more reference monitoring locations, providing traceable evidence of the gold pod’s performance in different weather and pollution conditions. After the pod has been characterized, it is moved adjacent to a “candidate” pod located in the network for a period of approximately 7-14 days. After this period, a linear regression is performed to determine the slope and offset between candidate and gold pod. These scaling factors are then applied to the pre-scaled data if the slope and offset are statistically different (at a 95% confidence interval) than 1 and 0, respectively. If differences do not meet these thresholds then Stage 0 data becomes Stage 1 data without further adjustment.
  • Experimental network-based calibration method: To maximize the number of sites for which we can publish preliminary data, we are also investigating the use of an experimental calibration method being developed by the Cambridge group, which aims to scale the entire network without the need for gold pods or co-location. The separation of local sources immediately adjacent to a pod site from the non-local background pollutant levels, which are often consistent over substantial distances (10s to 50s of kilometres), allows scaling of pods across the network. The experimental approach involves selecting periods when non-local pollutant levels are likely to be relatively stable over the study area to determine relative pod sensitivities. The entire network is then scaled using co-located AQMesh pod and reference monitoring instruments at one or more sites. The scale separation methodology has been previously demonstrated by Heimann et al. (2015) and Popoola et al. (2018). Data from pods for which the experimental scaling method was used will be replaced with scaling factors based on gold pod studies as they are completed.  The method will continue to be evaluated throughout the project by comparisons with slopes and offsets derived from direct co-locations.

All three methods in this stage rely on comparisons of pod data to reference site data. Since empirical scaling factors may be determined prior to subsequent data ratification of the reference network, there may be errors in the reference data that subsequently necessitate correction of sensor scaling factors. This is considered in Stage 5. Any other measurement artefacts during a field co-location would be reviewed once the ratified reference data were available.

Stage 2 (Manual QA/QC)

Air Monitors’ technical staff conducts a manual quality review of the data each week and, as needed, flags any suspect data for review in later stages. For initial publication, all data flagged through this process will be not be published, though may be provided if determined valid at a later date.

Stage 3 (Automated QA/QC)

In Stage 3, scaled data from Stage 2 are automatically reviewed against high and low limits. Additional flags are added at this point if data exceeds any pre-set concentration limits or if the PM mass fractions are not credible (e.g. PM₂.₅ > PM10). Negative ozone measurements occurring at times when NO levels are greater than 5 ppb are also assigned a value of zero at this stage.

Stage 4 (Special Issues)

This stage captures specific issues encountered throughout the deployment that the project team envisions taking actions on prior to finalising data, but not prior to initial publication of data. Examples include accounting for changes in provisional data applied during ratification from reference network monitors and correcting PM data for effects of relative humidity.

University of Cambridge and National Physical Laboratory monitor and advise on the quality assurance process.

Photo source: Air Monitors Ltd

Data considerations

Initial data publication:

In order to display information in near-real time, data shown on the project website prior to completion of the project are provisional and subject to change as the data undergoes additional quality assurance checks. The initial data release in July 2019 includes NO₂ data verified through gold pod co-location or initial reference site co-location. For sites where neither of these scaling factors are available, data shown has been adjusted using the network-based calibration method if the associated R2 parameter is sufficiently high. As additional gold pod co-locations are completed, scaling factors from those co-locations will be used in lieu of the factors determined through the network-based method.

To see which method has been used at an individual site, download the site information file.

The full dataset, including all pods, can be downloaded here. The data file and accompanying site information file will be updated monthly. Because data is subject to change with additional QA/QC, it is recommended that users download a new data file each time they wish to work with the data, rather than using a previously downloaded file.

Breathe London data is licensed under the Open Government Licence.

Uncertainty of initial data:

For data scaled using the ‘gold pod co-location’ methods, initial estimates of NO₂ uncertainty at the EU limit value range between ± 10-20% for most pods. A minority of pods have estimated uncertainties of up to ± 40%. Uncertainty is generally lower at higher pollutant concentrations. The project team will be reviewing and updating these uncertainty estimates over the course of the project. The range of uncertainties may be attenuated as the project undergoes additional QA/QC.

Uncertainty in the network-based method data is estimated to be ± 25% at this stage of the project. Improved uncertainty estimates will be available as the project progresses.

For comparison, final ratified data from reference instruments in the London Air Quality Network have an estimated uncertainty for NO₂ measurements of ± 10% at the EU limit value.

Data platform:

The Breathe London data platform provides the data and visualisations for the Breathe London website. The platform is based on the Google Cloud, which enables user-friendly performance when querying these large datasets to provide graphs and visualisations, and ensures the replicability and scalability of the platform to other cities around the world. The platform is open-source and is capable of ingesting data automatically from AQMesh pods and also other monitor networks such as the London Air Quality Network and Defra’s Automatic Urban and Rural Network. The platform stores Stage 0 data and calibration factors separately, and supports the QA/QC process by allowing the technical team to modify calibrations and redact suspect data. Third-party platforms and apps can connect to the platform through standardized APIs. The Breathe London platform is developed and maintained by Cambridge Environmental Research Consultants.

Mobile monitoring

Two Google Street View cars, equipped with reference-type air quality monitors are measuring air pollution over approximately 600 eight-hour shifts between autumn 2018 and autumn 2019. The cars use laboratory-grade, fast-response, research-grade instruments to precisely measure pollution concentrations approximately every 1-10 seconds. Pollutants measured include black carbon (BC), CO₂, NO, NO₂, O3, PM₂.₅ (and other PM size cuts), and ultrafine particles (UFP) on a variety of London roadways. The National Physical Laboratory is responsible for regular checks of instrument performance and periodic calibrations.

The cars are collecting data from early morning to late evening, Monday to Friday – providing a representative view of on-road air pollution during these hours. The mobile monitoring routes are sampled at different times of day, days of week and time of year – with a target of achieving a minimum of approximately 15 passes of each route over the course of the study (however, traffic congestion and other factors may reduce this number).

The initial sampling plan was re-configured in early 2019 in order to capture data needed to characterise the baseline of air pollution in Central London prior to implementation of the Ultra-Low Emission Zone (ULEZ). This zone will be sampled more intensively, both before and after its 8 April 2019 introduction.

Outside the ULEZ, targeted driving routes, known as “transects” and “polygons,” were identified. The project team selected these routes based on predicted high and low NO₂ concentrations, using CERC’s ADMS-Urban 2012 model for NO₂ (summarized at the postcode district level), as well as randomly selected areas of high and low Index of Multiple Deprivation score (see Index of Multiple Deprivation 2015).

At the end of the project, air pollution maps will reveal the spatial variations in air pollution at the level of city blocks in the polygons where repeated sampling targets were achieved. The project team periodically reviews the progress toward data collection targets and may make changes to the sampling areas as needed to achieve project goals and objectives.

Please check the Breathe London website periodically for updates on the mobile data. Data will also soon be publicly available for download on the Air Quality Data Commons.

Image via Nick Martin, NPL

Complete technical documentation will be available at the end of the project. For additional information, contact hello@breathelondon.org.

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