This is going to be interesting for many cities, but I can speak to the case of Madrid.<p>The Partido Popular government fixed the pollution problem by moving the sensors out of busy streets and into big parks that happen to be within the city limits[1,2].<p>The historical continuity of measurements has been lost, but Google will now provide data with more continuity. Or at least I hope it is provided.<p>[1] <a href="http://elpais.com/diario/2009/05/18/madrid/1242645857_850215.html" rel="nofollow">http://elpais.com/diario/2009/05/18/madrid/1242645857_850215...</a>
[2] <a href="http://www.ft.com/cms/s/0/022cf59c-3091-11e0-9de3-00144feabdc0.html" rel="nofollow">http://www.ft.com/cms/s/0/022cf59c-3091-11e0-9de3-00144feabd...</a>