Many city surfaces are coated with a layer of soot, pollutants, metals, organic compounds and other molecules known as “urban grime.”

Chemical reactions that occur in this complex milieu can affect air and water quality. Now, researchers reporting in ACS Earth and Space Chemistry have taken a closer look at urban grime collected from two U.S. cities, revealing for the first time that the material absorbs sunlight and therefore might participate in photochemical reactions.

Scientists have previously analyzed lab-prepared urban grime, as well as samples collected from cities, but they still don’t have a complete understanding of what’s in the material, or how it varies by location. Some components can react with other molecules in the grime or air, which could affect what gets released into the atmosphere or into the water when it rains.

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