Fresh soot may be the root of urban smog. The finding, published in today's Nature, could solve a long-standing mystery about what triggers smog formation, but it probably won't make the air above Los Angeles any cleaner.
Smog is not spewed wholesale from tailpipes and smokestacks. Rather, it's composed of many different particles that react in complex ways with sunlight and atmospheric compounds. Nitrous acid, for example, has long been blamed for instigating a chain of reactions that leads to smog. After the gas builds up in the atmosphere each night, sunlight quickly transforms it into reactive molecules called free radicals. Scientists knew that these radicals jump-start the production of other smog particles during the day. The source of nitrous acid, however, remained unknown.
Aerosol chemist Markus Ammann of the Paul Scherrer Institute in Villigen, Switzerland, and his colleagues, suspected that soot particles--spewed when fossil fuels are burned--might have a hand in creating nitrous acid. To find out, they burned gasoline and let the soot react with nitrogen dioxide, a by-product of hydrocarbon fuel combustion and the precursor of nitrous acid. This created nitrous acid up to 10 million times faster than had ever been observed before.
"Results like this are tantalizing," says atmospheric chemist Daniel Jacob of Harvard University. But he notes that fresh soot--the kind used in the experiment--is bound to react far differently than the stuff in the air around us. Older soot, Jacob notes, has reacted with other airborne compounds and thus has a different chemical composition than fresh soot.