The black mark in the left image shows the location of the Maracanã Stadium. The right image shows the same picture overlaid with air particulate data. The skies over Rio have about the same air pollution seen in cities when there is a light haze in the air.
To be clear, the pollution imaged by MISR in Rio is nowhere near the level of smog in China where air pollution can reach dangerous levels in cities like Beijing. It's more of a light haze than anything else. But the image does show off the capabilities of MISR. Researchers plan on making images like this one, taken from the instrument available to the public in the near future for locations all over the world.