Provisional 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.
NO2: The provisional NO₂ data is 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 goodness-of-fit parameter is sufficiently high.
For NO2 data scaled using the ‘gold pod co-location’ methods, initial estimates of 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 data scaled using the network-based method 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, the current requirement of uncertainty for reference or equivalent monitors is ±15%.
PM2.5: The provisional PM2.5 data is produced using the network-based calibration method exclusively as this method can more effectively disaggregate the contribution of background PM2.5 levels across Greater London. The network-based method yields broadly consistent results as co-locations using ‘gold pods’ (agreement of medians during reference co-location periods within 10-30%). A comparison of scaled AQMesh measurements using the network-based method (after filtering periods of high humidity or fog) and co-located reference measurements showed median-normalized root mean square errors of ~10-20% (for a median PM2.5 range of 11-30 ug/m3).
Currently, the network-based scaling method produces results relative to a select gold pod. Analysis is on-going to evaluate the sensitivity of the network-based scaling factors to different gold pods. Because measurements from reference instruments used to calibrate gold pods are provisional, the reported Breathe London measurements are also provisional and subject to revision when reference measurements are ratified (early-mid 2020).
Preliminary evaluation of AQMesh PM2.5 measurements against reference monitors suggests the possibility that high relative humidity and/or fog can lead to AQMesh measurements that are spuriously elevated. The project team is developing a method to correct for the effect of humidity/fog on measurements based on recently published work (Crilley et al. 2018). We anticipate including this correction in future data releases.
The AQMesh uses an optical particle counter (OPC) to estimate particulate matter mass emissions, and only particles larger than ~300 nm are counted. In an environment with fresh emissions of small (nano)particles, the portion of particle mass from those particles smaller than 300 nm would not be detected by an OPC. The effect of this undercounting is greatest when instrument placement is closer to roadways and other combustion sources.
The full NO2 dataset can be downloaded here.
The full PM2.5 dataset can be downloaded here.
The data files 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.
You can further explore and make customised visualizations with these datasets on the Air Quality Data Commons 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.