To cut daytime smog, attack it at night

The battle for smog-free air seems to be entering a new phase. As standards tighten, air-quality scientists are working for a deeper understanding of what is involved. And, quixotic as it may sound, there's hope that some aspects of our man-made environment may help give us cleaner air.

Already, scientists are looking into processes atmospheric chemists have neglected. What's going on at night, for example, can surprise them, as a National Oceanic and Atmospheric Administration (NOAA) research team has discovered.

The recipe for an ozone "event" seems simple. Mix together nitrogen oxides and hydrocarbons that cars, factories, and some natural processes emit. Add sunshine. Voilà! Ozone smog. Like a vaguely stated cake recipe, apparent simplicity hides underlying complexity. It takes a lot of preparation to set things up so the ingredients mix to make major smog.

While smogmaking stops when the sun goes down, the atmospheric cauldron keeps on bubbling. Its action helps determine what happens after sunrise the next day. Steven Brown at NOAA's Aeronomy Laboratory in Boulder, Colo., and his colleagues have dis-covered an effect that makes this point. They find that the atmosphere in the marine environment of coastal New England can do a trick that "will short-circuit some of the ozone production that would have occurred the next day," Dr. Brown says.

During the night, ozone-forming nitrogen oxides undergo chemical reactions that transform them into nitric acid gas. This gas rapidly deposits on the surface, partially cleansing the air of key smogmaking ingredients. As Brown's colleague A.R. Ravishankara notes, this glimpse of a previously unknown process shows that "the nighttime chemistry is a new piece of the air-quality puzzle.... We need to find out more about when and where it is important." For example: Are there circumstances where it enhances smogmaking rather than moderating it? And what else is going on?

This research is part of an ongoing comprehensive study of New England air quality. Some of the results will be specific to this region. Other findings may have wider relevance. It is already clear that scientists everywhere need to understand what's going on in the atmosphere 24 hours a day.

They may need new tools to help them. The discovery Brown and his colleagues published in Geophysical Research Letters was made possible by new instruments that measured concentrations of key chemicals that had been nearly impossible to assess before.

Meanwhile, some air-quality engineers are experimenting with ways to clean up local air. The European Union is funding tests of a paint that contains tiny particles of titanium dioxide and calcium carbonate. That combination transforms polluting nitrogen oxides (NOX) into calcium nitrate with water and carbon dioxide as harmless byproducts.

Also, tests in Italy showed that a concrete made with NOX-scrubbing cement reduced street-level concentration of the oxides by up to 60 percent, according to a report in New Scientist magazine.

By getting a better handle on the complex chemistry of smog, scientists may make it easier to fight air pollution.

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