SAN FRANCISCO—City lights may do more than hide a starry sky: they could indirectly worsen daytime smog. Measurements of light pollution over Los Angeles have revealed a brighter glow than chemists expected—bright enough to destroy chemicals that would otherwise help cleanse the air during a dark night.

“Light pollution can slow down nighttime cleansing,” said Harald Stark of the National Oceanic and Atmospheric Administration. He is based in Boulder, Colo., and reported the findings December 14 here at a meeting of the American Geophysical Union.

“I don’t think anybody had thought about it,” says Jochen Stotz, an atmospheric chemist at the University of California, Los Angeles. “It shows that what happens at night is really important” for air quality, adds Stotz, who was not involved in the NOAA study. “We’ll have to see how important.”

The surprising hypothesis is for now just that—a hypothesis, Stark warned during a press conference. The chemistry of smog formation is very complex, and the effects of city lights are hard to quantify without further research, he said.

During several flights of a NOAA airplane this summer, Stark and his team found that the glow of Los Angeles was 10,000 times weaker than sunlight, but still 25 times brighter than a full moon. The researchers calculated that such an amount of light would be enough to break down nitrite in the air. Nitrite (NO3) indirectly reduces the daytime ozone levels because it reacts with other nitrogen compounds that are involved in ozone-forming reactions. So whereas in darker regions nitrite acts as a night cleanser, over L.A. it could be reduced, with the effect of exacerbating smog.

The impacts could be worse over more northern cities in winter, Stark said, because snow cover and clouds have a multiplying effect on light pollution.

Stotz says he and his collaborators now plan to include the effects of light pollution in their models of atmospheric chemistry to try to quantify the NOAA findings. Even if the result is an increase of a few percentage points in ozone levels, it may make an important difference for communities that are already struggling to stay within the limits imposed by the U.S. Environmental Protection Agency, Stark points out.

Cities may discover that controlling outdoors lighting may have more benefits than saving energy and keeping the stars shining.

Image: Typical western U.S. cities, such as Los Angeles, are defined by yellow-orange sodium vapor–lit streets in grids. Airport runways stand out as dark lines where, surprisingly, it is better to land an airplane on a dark runway than a well-lit one. At the edge of town, the lights abruptly fade into the surrounding desert. Credit: NASA