Babbage

THE term “light pollution” is not, it seems, a metaphor. The light that emanates from cities all over the world not only deprives their citizens of the pleasure of seeing the Milky Way on a moonless night, it also diminishes the freshness of the air they breathe at dawn. It interferes, says Harald Stark of American’s National Oceanic and Atmospheric Administration (NOAA), with the chemicals that mop up nasty molecules that are the raw materials of smog.

At the Autumn Meeting of the American Geophysical Union, in San Francisco, Dr Stark presented his conclusions from detailed measurements of the composition of the air over Los Angeles and its surroundings. On four days and nights a NOAA plane crisscrossed the metropolis. During the nights, not only were the concentrations of ozone, nitrogen oxides and other gases measured by this plane, but also, almost as an afterthought, the intensity of the light from below.

That there is too much of this is well known to astronomers—it is reflected back down from dust in the atmosphere and, as a result, many stars are drowned in the glow. It also makes for spectacular photographs of the Earth at night, showing clearly where people prefer to live. But even the brightest city in America, Las Vegas, emits only about a ten-thousandth as much light as the sun shines upon the same area. Inconsequential, surely, as far as the goings-on in the atmosphere are concerned?

Not at all, says Dr Stark. To provide another perspective, Los Angeles seen from above is 25 times brighter than the full moon. And the city's lights come on at a time that the atmosphere could very well do without them: when it is cleansing itself of the pollution of the day.

At night ozone, a molecule composed of three oxygen atoms, reacts with oxides of nitrogen. An especially happy outcome is when a molecule of nitrogen dioxide (one nitrogen and two oxygens) and one of ozone—both constituents of daytime smog—turn into molecule of regular oxygen (the sort that has two atoms per molecule) and nitrogen trioxide. The oxygen will bother no one, and the nitrogen trioxide soon meets its end destroying one of a range of volatile organic compounds that are another constituent of daytime smog.

The size and electrical properties of nitrogen trioxide, though, are such that it is easily broken up by light into nitrogen and ozone. That is why, by day, ozone is hard to get rid of. By night, however, there should be no stopping nitrogen trioxide. The chemical processes in which it is involved should work without hindrance, and ozone levels will drop.  

That is why shining a light into the night sky is not such a good idea. In the case of Los Angeles Dr Stark estimates that, by dawn, the amount of pollutants left over to make smog is about 5% higher than it otherwise would have been. Not huge, but still significant. Yet another reason, then, to switch that light off.