All hourly values are
localized to their local timezone, before being calculated into monthly/annual
Data is collected from individual monitoring stations and then grouped
under a city. For cities with more than one monitoring station, city data is
averaged by calculating the hourly median between stations in the same city.
These hourly median values are then used to calculate both the city’s monthly
and annual mean values.
For some gaps in certain European countries and cities,
historical data from European Environment Agency
(EEA) database are used to fill the gaps. To combine the European data
records between the real-time aggregated history, and the EEA historical
record, these are combined on a city-by-city basis. For any city with a data
record provided both by the real-time aggregated history and the EEA historical
data, the record which offers (primarily) the highest level of data
availability, and (secondarily) higher number of stations providing
measurements, was selected to represent that city.