December 14, 2010

City Lights Contribute To Air Pollution

According to a study by U.S. scientists, bright city lights exacerbate air pollution.

Their research indicates that the glare thrown up into the sky interferes with chemical reactions.

These reactions would normally help clean the air during the night of the fumes emitted by vehicles and factories during the day.

The researchers presented their findings at the American Geophysical Union meeting in San Francisco.

Nature uses a special form of nitrogen oxide, known as nitrate radical, to break down chemicals that would otherwise go on to form the smog and ozone that can make city air an irritant on the chest.

This cleansing normally takes place at night because the radical is destroyed by sunlight.

However, new measurements taken from aircraft flying over Los Angeles revealed that the energy from all the nighttime light thrown out by this huge urban center is also suppressing the radical.  The lights may be 10,000 times dimmer than the Sun, but the effect is still significant.

"Our first results indicate that city lights can slow down the night-time cleansing by up to 7% and they can also increase the starting chemicals for ozone pollution the next day by up to 5%," Harald Stark from the US National Oceanic and Atmospheric Administration (NOAA) told BBC News.

"More work needs to be done to really quantify the next step which would be how much ozone could we actually have the next day. This work would be important to undertake because many cities are close to their regulatory limits in terms of ozone levels, so even a small effect such as this could be important."

Los Angeles' main type of lighting is high-pressure sodium lights and metal halide lights. 

However, Stark believes that by switching to different types of lighting then it would have limited effect.  She said that red lights would help, but doubts authorities would want to make the cityscape red.

Another approach that could help would be to follow the guidelines advocated by "dark skies" campaigners who want lights pointed to the ground to stop the glare smearing out stars.

"[This effect] is more important up in the air than it is directly on the ground so if you manage to keep the light pointing downward and not reflected back up into sky, into the higher parts of the air, then you would certainly have a much smaller effect of this," Stark told BBC News.


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