The glow of city lights obliterates light-sensitive molecules that scrub the atmosphere by night, adding to the problem of air pollution, a new study shows.
Artificial lighting over the Los Angeles area is 10,000 times dimmer than sunlight, but it still cuts night-time atmospheric cleansing by up to 7 percent, say researchers with the National Oceanic and Atmospheric Administration, which flew aircraft over the region four times this summer to measure city light intensity.
The lights prevent production of a particular type of nitrogen-oxide molecule known as a nitrate radical, which forms from reactions of nitrogen dioxide with ozone. It bonds well with pollutants in the atmosphere, but breaks down quickly in the presence of light.
"It doesn't exist during the day," said Stark. "At night, it shows up. It reacts with pollutants and cleans them up."
Researchers correlated the measurements of Los Angeles' city lights, which are 25 times brighter than the full moon, with satellite data, then produced computer models for assessing the impact of night-time lighting for other areas.
In addition to slowing the night-time cleansing of the atmosphere, the city lights leave more polluting molecules in the skies for the next day's chemistry to start with, exacerbating the problem, Stark said.
In Los Angeles, for example, the study found the city's lights, which primarily are high-pressure sodium and metal halide lights, hiked the chemistry for day-time ozone pollution by up to 5 percent.
The pollutants people care about the most are particulates and surface-level ozone, which is produced from other chemicals, said NASA atmospheric scientist Anne Douglass, with the Goddard Space Flight Center in Maryland.
"You need sunlight to make the reactions happen, so typically things turn off at night," Douglass told Discovery News.
"This sounds like it adds in a new factor," she added, referring to the research about the impact of city lights on atmospheric cleansing. "I hadn't heard about it before."
Changing the type of lighting wouldn't have much impact, the scientists suspect, unless cities would use red lights, which don't interfere with nitrate radical formation.
"I don't think from a policy perspective that would be a reasonable," Stark said.
The research was presented at the American Geophysical Union in San Francisco this week.