Key information
Request reference number: MGLA200924-1384
Date of response:
Summary of request
Request:
Please itemize the specific type of equipment that was used at the specific air quality measurement sites.
What calibration and accuracy checks were carried out on this equipment?
What sampling methods were used and what weighting/adjustment was made to the measurements?
What criteria were used to select the specific air quality measurement sites, and why was such a small population used for each of the four categories?
What cross-referencing was made to borough measurements during the period of analysis?
What statistical confidence does the GLA group have in its actual measurements?
Please find below the information we hold within the scope of your request.
To address the first three requests:
All information about the air quality monitoring stations used in the analysis is available publicly online. Please refer to the supplementary data sheet provided here: London-wide ULEZ Six Month Report | London City Hall. This outlines the air quality monitoring stations used in the analysis for the London-wide ULEZ Six Month Report, and the relevant website for each air quality monitoring network holds further information about each individual air quality monitoring station. These monitoring stations are not operated by the Greater London Authority.
Criteria to select the sites:
This information is available publicly in the published report: London-wide ULEZ Six Month Report | London City Hall. Please refer to Appendix 3 – Air quality concentrations methodology. The section entitled “Air pollution monitoring concentrations” provides the methodology for air quality monitoring station selection. In order to remove potential outliers and reduce noise in the averaging and smoothing of the data, a number or rules have been applied to the dataset, which are provided on page 106.
Borough measurements:
Air quality monitoring stations are run and managed by a variety of different organisations including national and locally-funded sites. Therefore the air quality monitoring sites used in the analysis include sites funded and operated by the London boroughs.
Statistical confidence:
The full methodology for analysis of air quality concentration impact analysis is provided in Appendix 3 of the published report: London-wide ULEZ Six Month Report | London City Hall. The method used for analysing the preliminary impact on concentrations builds upon a previously peer reviewed method. The Six Month Report provides an extensive analysis of the impacts of the first six months of the London-wide ULEZ and clearly notes the limitations of the data, as well as providing detailed summary tables of the data for readers. We have published the air quality data in the report, in line with best practice so that people can review our analysis.