Any of you who've lived in a smoggy city know that the ugly haze overhead is its worst in late afternoon and relatively tolerable in early morning. One reason, of course, is that far fewer exhaust-spewing vehicles are on the roads at night.
Atmospheric chemists also realize that each night the nitrate radical (NO3), a compound destroyed by sunlight, builds up in the darkened sky. As city dwellers sleep, it neutralizes some of the nitrogen oxides (NOx) that foul the daytime air and lead to wheeze-inducing levels of ozone (O3).
But new research shows that this nightly cleansing action isn't as effective as it could be — because nitrate radicals are being destroyed by light beamed into the sky by outdoor lighting on the ground. That's the unsettling revelation announced Monday at a meeting of the American Geophysical Union.
Several months ago Harald Stark of the NOAA's Earth System Research Laboratory conducted a series of aircraft sorties over Los Angeles to measure the intensity of light pollution at altitude. He found that the urban glow is up to 25 times stronger than the full Moon's light, and he unambiguously identified the spectral emissions of high-pressure-sodium (HPS) and metal-halide (MH) fixtures — the most common types used in street and parking-lot lighting.
Remarkably, measurements of the glow's intensity remained almost constant to altitudes of 2 miles (3 km), because so much light pollution travels at shallow angles through the atmosphere, not straight up.
"My original goal was simply to quantify the intensity of city lights," Stark explains. However, he adds, after the
first measurements, it became clear that that the urban glow was affecting nighttime atmospheric chemistry.
When Stark and his team plugged all of this artificial illumination into photochemical models, they found that the city lights were destroying up to 7% of the nitrate radicals that would normally be present in the nighttime atmosphere. This draw-down in turn results in NOx levels some 5% higher than they'd otherwise be, making more of these gases available for smog- and ozone-producing reactions each morning. "Ozone production during daytime could be increased by nighttime light sources," Stark concludes.
Of course, Los Angeles is just one of hundreds of vast "light islands" visible from space around the globe — and while LA might be synonymous with smog, it's hardly the worst light polluter. (For example, the light streaming upward from Las Vegas is five times more intense.) To that end, Stark and his team can use the wasted light recorded by satellites can be used to estimate the degree to which it's disrupting atmospheric chemistry over a given city.
"Many cities are close to their limits of allowable ozone levels," explains Bob Parks, executive director of the International Dark-Sky Association, "so this news is expected to have big implications for outdoor-lighting practices and should be of special interest to the Environmental Protection Agency."
Two years ago the EPA was petitioned to assess light pollution's role in atmospheric discoloration of the night sky under the Clean Air Act, but so far the agency's officials have made no formal response. But this new scientific result will certainly get their attention. As Parks notes, "There's nothing like another federal agency validating a position to increase your credibility."