ESRL Quarterly Newsletter - Winter 2011

Night Lights

ESRL researchers find small but significant effect of nighttime city lighting on air pollution

Every night, the cities of the Los Angeles Basin throw a dome of light into the dark sky, cast by millions of street lamps and other outdoor lights. That glow is 10,000 times dimmer than sunlight, but still powerful enough to influence chemistry affecting air pollution, ESRL scientists discovered. 

In a press conference and poster session at the American Geophysical Union annual fall meeting, Harald Stark (Chemical Sciences Division, CSD) and colleagues presented measurements made during several nighttime flights in California last summer, part of the CalNex field campaign in California, to study the nexus of air quality and climate change.

“We showed that city lights diminish the nighttime cleansing of the atmosphere,” Stark said, “and that could have an influence on what happens the next day,” in terms of air pollution.

Los Angeles at night.

Stark’s coauthors are Steve Brown, William Dubé, Nicholas Wagner, Thomas Ryerson, Illana Pollack, and David Parrish, all of CSD.

In a typical air quality field campaign, scientists make masses of measurements during the day, when sunlight triggers interesting chemistry. Several of CSD’s recent field experiments have looked at new dimensions of the air quality issue by studying nighttime chemistry. CalNex involved several nighttime flights, although Stark wasn’t originally planning to participate.

At Brown’s urging, Stark decided to pull a few all-nighters, re-calibrating a key instrument – designed to measure sunlight during the day – to pick up the faint intensity of city lights at night.

He and his colleagues also used controlled ground-based measurements and chemical models to further understand the chemistry taking place in the Los Angeles (LA) Basin.

“We measured light intensities 10,000 times dimmer than the Sun…but 25 times brighter than the full moon,” Stark said. His team also found evidence of the breakdown of nitrate radicals (NO3). Nitrate typically helps cleanse the nighttime atmosphere by breaking down certain air pollutants.

Sunlight quickly destroys NO3, which is consequently at extremely low concentrations during the day. Stark and his colleagues determined that the nighttime glow of the LA Basin was intense enough to do some damage, reducing the nighttime cleansing activity of NO3 by as much as 7 percent.

That, in turn, could leave more pollutants in the air overnight, the team calculated – as much as 5 percent more nitrogen dioxide, NO2, in particular. NO2 is a key ingredient in the daytime formation of ozone, a regulated pollutant that can harm people’s lungs as well as crops and ecosystems.

“Many cities are really close to their regulatory limits for ozone… so even a small effect like this could be important for those regulations,” Stark said.

Many media outlets covered the story following the AGU press conference, from the BBC to Nature and the Economist.