Unlocking cities’ dirty secrets

Grime is found on every surface in urban environments. Buildings, statues, street signs and pavements are all covered with a grimy layer of chemicals from cars, factories and other polluting sources.

We’ve always thought that once this layer has been deposited on a surface the complex mixture of organic and inorganic chemical pollutants contained within it are locked in, remaining in their resting place until a window cleaner or street sweeper scrapes them off.

New observations suggest this isn’t correct. James Donaldson from the University of Toronto, Canada, and his collaborators in Leipzig, Germany, now believe sunlight can cause nitrogen compounds to be re-released from the grime and recycled back into the atmosphere where these compounds once again contribute to ozone and smog.