All hourly values are localized to their local timezone, before being calculated into monthly/annual values.

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.