According to a recent NASA study, the millions of people who work in New York City not only alter the population density each work day, they add to the density of tiny particles in the air called aerosols. Researchers have discovered for the first time in an American city a workweek pattern of aerosols clouding the air, believed to be created by the comings and goings of people working in the city. The study also finds these urban aerosols, thickest on Wednesdays and lightest on weekends, can affect air temperatures and clouds in big cities...

To read more: http://www.nasa.gov/vision/earth/environment/ny_air.html

For the full text of study: http://climate.gsfc.nasa.gov/publications/fulltext/Jin_et_al._(2005).pdf