photo: Chung Kui Ryan Cheng/Creative Commons
Living in Manhattan, where on a good night you only see a couple stars in the sky and it's never truly dark due to light pollution, this one particularly hits home for me: BBC News reports that a new presentation at the American Geophysical Union meeting in San Francisco shows how bright city lighting makes air pollution worse. Apparently all that urban light pollution interferes with chemical reactions that normally take place at night and helps clean the air which "uses a special form of nitrogen oxide, called the nitrate radical, the break down chemicals that would otherwise go on to form the smog and ozone that can make city air such an irritant to the chest. The cleansing normally occurs in the hours of darkness because the radical is destroyed by sunlight."
The research presented shows that even though urban light pollution is 10,000 times dimmer than the light from the Sun, it still suppresses the nitrate radical, slowing down nighttime cleaning up to 7%. That's based on measurements made over Los Angeles.
As for how to prevent this, short of entirely eliminating city lighting at night (which has obvious safety implications) or using red lights (which probably won't be embraced by anyone), using lighting that concentrates light downward so that little is reflected back up into the sky would minimize the disruption to air cleaning.
It would also allow people to see the stars again. Here's a graphic illustration of the difference just a little bit of light pollution makes:
The constellation Orion, imaged at left from dark skies, and at right from the teeming metropolis of Orem, UT at the heart of the Utah County megalopolis comprising about half a million people. (Note: The preceding is sarcasm. Orem, UT is hardly a large city. This is intended to highlight the fact that light pollution is a problem everywhere, not just in cities with tens of millions of inhabitants.) Photo and caption: Jeremy Stanley/Creative Commons.
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More on Light Pollution:
Looking At Lights From Space: A Sign of Progress or Failure?
For Many People, The Stars Don't Come Out Any More
Starry Nights are Healthier, Cheaper and Reduce CO2