Indexed trends in air quality

"Annual mean concentrations of air pollutants in urban areas"
Indexed trend in annual mean urban air quality over the period 1997-2008 (reference year 1997). All urban and suburban background stations operational (that is at least reporting over 9 years with a yearly coverage of at least 274 days) during the period 1997-2008 are included. A general, Europe-wide averaged picture is shown, with a bias towards regions with high station density.