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.
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