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The International Dark-Sky Association (IDA) has worked tirelessly to address the problem of light pollution that ruins the view of the night sky over many regions. Besides blocking casual views of the stars and planets, light pollution is a serious problem for both amateur and professional astronomers.

The principal goal of IDA is to reduce light pollution, and save energy in the process, by substantially reducing the amount of artificial light that is wasted by being directed upward into the sky.

Light pollution is easy to find. Any light source that sends light upward is a potential source of light pollution. This includes everything from security lights to the brilliant lights of outdoor sports stadiums. Parking lot lights, street lights and even automobile headlights can also cause light pollution.

But light is only part of the problem. That's because light pollution cannot occur unless the upward rays of light are scattered back toward the ground.

On this moderately clear day at the University of New Hampshire, the solar aureole from scattered sunlight is relatively small. AOT is the aerosol optical thickness measured by a Microtops Sun Photometer. Click image to enlarge.

On this very hazy day at the University of New Hampshire, the solar aureole is very large. The most likely source of the haze is sulfate particles from coal-burning power plants. Note the significant increase in AOT. Click image to enlarge.

A classic example of Rayleigh scattering is shown on my home page (www.forrestmims.org). The image there shows the brilliant green beam from Dr. John Barnes' lidar at Hawaii's Mauna Loa Observatory.

The sky is rarely free of aerosols such as those listed above. When they are present, they can scatter considerable light back toward the ground. This phenomenon is known as Mie scattering.

This brings us to the dark-sky conundrum, for reducing the amount of light sent skyward will never return fully dark skies to urbanized regions. There are just too many sources of light. Thus, it is crucial to also reduce the aerosols that are present in the sky.

The principle source of light-scattering aerosols in the skies over the Eastern United States, Europe, South Africa and Asia is the burning of coal that contains sulfur. The sulfur dioxide released through the burning of coal becomes sulfate particles. These particles grow in size as liquid water is condensed on them and form thick blankets of whitish haze.

The two photographs above illustrate the light scattering problem caused by aerosols. The photo on the left shows a relatively clear sky, and the one of the right shows a hazy sky.

The photo on the right does not show the full magnitude of the problem, for the camera automatically compensated for the brilliance of the solar aureole. This is apparent by the underexposed building in the photo.

In short, the massive problem of sulfate aerosols is as much a villain as light sources. Besides the light scattering they cause, the optical depth of the sulfate aerosols can also be a very serious problem.

For example, in the left photo above, the AOT at 440 nm is 0.073. This means that 93 percent of the blue wavelengths of sunlight were transmitted through the atmosphere to the Microtops Sun photometer.

In the right photo above, the AOT at 440 nm is 0.886. This means that only 41 percent of the blue wavelengths of sunlight were transmitted through the atmosphere. Thus, the combination of light absorption and scattering by the haze layer is very significant.

(AOT is a dimensionless measure of light extinction. To convert AOT to the percent of light transmission, enter the AOT into a calculator. Press the +/- key to change the sign. Then press the e^x key.)

These two daytime photographs illustrate why the night sky over sulfate polluted regions is often devoid of stars. Sulfate pollution can be much worse than that shown in the right photograph above.

Air quality will improve dramatically when coal-burning power plants are modernized. This was proved during the August 2003 electrical blackout across the North Eastern United States, when scientists from the University of Maryland found that visibility increased by more than 40 kilometers downwind from coal-burning power plants that were switched off during the blackout.

Dr. Lackson Marufu and his colleagues at the University of Maryland made important measurements during the blackout and concluded that "... emissions from power plants hundreds of kilometers upwind play a dominant role in regional haze and ozone production." (See Lackson T. Marufu, et al., in The 2003 North American electrical blackout: An accidental experiment in atmospheric chemistry, Geophysical Research Letters 31, L13106, doi:10.1029/2004GL019771,15 July 2004.)

The light-reduction activities of the International Dark-Sky Association are commendable. They can also save energy.

Yet sulfate haze is a major part of the problem. Based on night satellite imagery of regions in the Northeastern U.S. during and after the blackout, it seems obvious that many of the people most affected by light pollution derive their electricity from the coal-burning power plants that obscure their view of the night sky.

Coal burning pollutes the air, but coal is cheap and plentiful. The power plants that burn it are up and running, while some new natural gas plants have been idled or even mothballed due to the high price of their much cleaner fuel.

Because light pollution greatly concerns many amateur astronomers, The Citizen Scientist will be very interested in receiving comments for Backscatter about this editorial. We will assume that those sending e-mails will not object to their comments being published.