NO2 (top) and PM10 (bottom) concentration trends at urban background (left) and traffic (right) locations

Top: NO2 concentration in selected cities from 2002 to 2009. Bottom: PM10 concentrations in selected cities from 1999 to 2009. Values are presented for single monitoring stations that provide reliable time series data for the last years. Selected cities have at least one background and one traffic station that provide such reliability and can therefore be compared for analysis. Therefore, this figure does not represent air quality results citywide, but rather serves as a snapshot of the different trends in background and traffic stations wherever comparable long-term time-series data are available. Because the different lines represent individual measurement points, there can be a significant effect from local changes in traffic flows. According to sources, this is part of the background for the large change in the NO2 levels in London.