Does light 'pollution' contribute to metropolitan smog?

New study claims to find link between excess urban lighting and metropolitan air pollution.

Some of our friends bought a house about a year ago near Lake Winnipesaukee in New Hampshire, which we have been lucky enough to visit both at the height of summer and in the dead of winter. Both times I was struck by how dark the sky was at night, showing off constellations I almost never seen here in my North New Jersey suburb. I am sure you have had similar experiences, if you've visited some remote or rural place. It's the ultimate evidence of the "light pollution" that occurs near big cities.

For this reason, I was intrigued by a press release I just received from the International Dark-Sky Association, citing the results of a study linking light "pollution" to excess smog and ozone near cities.

The research, conducted by the National Oceanic and Atmospheric Administration (NOAA) and the Cooperative Institute for Research in Environment Sciences (CIRES) at the University of Colorado, found that outdoor lighting in cities interferes with a chemical reaction that works overnight to break down chemicals from vehicle exhaust and other non-natural sources. The chemical that makes this reaction happen, called nitrate radical, is broken down when sunlight present. So the reaction doesn't happen in the daylight hours, only at night. The NOAA/CIRES research, which was presented earlier this month in San Francisco, suggests that excess outdoor lighting slows down the cleansing process that usually happens overnight by up to 7 percent. By association, that means not as much ozone can be broken down.

The simple implication: By addressing outdoor lighting efficiency, cities might be able to address excess smog and ozone pollution. This is something that happens, by the way, on a daily basis.

One of the NOAA investigators, Harald Stark, told BBC News in presenting these finding:

"[This effect] is more important up in the air than it is directly on the ground so if you manage to keep the light pointing downward and not reflected back up into the sky, into the higher parts of the air, then you would certainly have a much smaller effect of this."

This data might act as more impetus for communities that are mulling energy efficiency and outdoor lighting upgrades, providing another case for adaptive lighting technology that not only turns lights off when they are not needed but that directs light more specifically to the locations that require illumination.

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