DfT explained to us that the national model ‘fills in the gaps’ in data collected by its monitoring stations to create a kilometre by a kilometre map of air quality across the country, and forecasts to 2030.23 DfT also told us that the results of the national model are updated to account for local experience; for example, in Southampton the local authority found that the national model underestimated NO2 compared to the results of local monitoring, due to a different fleet mix than predicted, so the model was updated.24