The bright lights of Los Angeles are increasing pollution in that city, reported Harald Stark of the University of Colorado, Boulder, at the American Geophysical Union meeting in San Francisco this week. The light is destroying a chemical that helps to scrub the night sky of pollution, he says, reducing night cleansing by about 7%, and increasing the pool of chemicals that create smog by 5% in the mornings, he estimates.

Stark used a NOAA research plane to measure light intensity over LA, and used that to calibrate satellite measures of night illumination. They also measured levels of chemicals including NO3 radicals – a compound that helps to break down volatile organics at night, but is destroyed by sunlight in the day. He and his team then plugged all this into models of known atmospheric chemical reactions, and did lab-based tests to confirm how artificial light also serves to destroy NO3 radicals.

The effect isn’t surprising, but no one has measured it before, says Barry Lefer of the University of Houston, also at the meeting. “Most people turn off their instruments when the Sun goes down – they just assume it’s dark,” he says. Stark had to adjust his instruments’ sensitivity to get night readings, he says.

The night light in LA is 10,000 times dimmer than sunlight, Stark says, but 25 times brighter than a full moon. Any kind of visible light serves to destroy this natural chemical cleanser, he says, except red light – which isn’t a popular colour for lighting. “The only thing we could do is turn our cities red,” he says. Making sure that lights point downwards instead of up at the sky should also help.

The research is preliminary, Stark says, and it’s impossible to say yet how much this affects a city’s overall smog level – it depends on other factors like wind, temperature, and more. “I don’t think I’m yet willing to advocate for any changes in city lighting,” he says. The astronomers who campaign for dark skies, however, could always pick up the research to help their cause.