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Blue skies are the backdrop to the Bellas Artes Palace in downtown Mexico City, March 24, 2010. (Tim Johnson/MCT)
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On a November 26, 2010 afternoon, a light haze reduced visibilityin Mexico City -- but nothing like the choking smog of the 1990s. (Tim Johnson/MCT)
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A technician from the Mexico City atmospheric monitoring center checks a rooftop air quality measuring device November 26, 2010. (Tim Johnson/MCT)
MCT
Since 2005, Mexico City has had a network of low-emission high-speed buses that travel along special lanes. By the end of 2012, six lines should be operating. The high-speed system is considered a resounding success. (Tim Johnson/MCT)
MCT
Mexico City's bicycles-for-hire plan began in February 2010. Some 85 bike stations like this one, seen April 17, 2010, are scattered in the central part of the city. (Tim Johnson/MCT)
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Mexico City air quality official Alfonso Lopez Medina holds two air filters that have turned gray after two weeks exposure to the capital's air, November 26, 2010. (Tim Johnson/MCT)
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An air quality worker stands by a rooftop box in Mexico City that measures particulates in the atmosphere, November 26, 2010. (Tim Johnson/MCT)
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