Defra predictions are based on outputs from their Pollution Climate Mapping (PCM) model which is used to report legal compliance to the EU. The model is calibrated against measured concentrations from the national monitoring network (Defra’s Automatic Urban and Rural Network – AURN), but does not take into consideration all data available from Local Authority monitoring networks. The verification process for the model is therefore reliant on fewer monitoring locations than local models, which can result in over or under estimations in the PCM model. Defra estimates this overarching uncertainty to be in region of +/-29% (Defra AQ Plan – Technical Report, pg 16). This varies by city. Only one monitoring station in Bristol is used for verification of the PCM modelling.
Conversely, local predictions use data from 4 automatic monitoring stations and 105 NO2 diffusion tube monitoring sites across Bristol. The difference between the number of monitoring stations used by each model is the driving factor for the difference in air quality predictions. Defra acknowledge that BCC data more accurately reflects the local circumstances, and this data will be used to determine the most appropriate course of action.