"The third way is the one we are concerned with here, the way that involves breaking these hydrocarbons down chemically," Francisco said. "For that, the atmosphere relies on a reactive group of chemicals called OH radicals that attach themselves to hydrocarbons and rip them into inert pieces."
OH radicals arise naturally from many atmospheric constituents, and the effect they have on pollution has long been factored into models that describe the atmosphere and attempt to predict how it will react to increasing quantities of hydrocarbon pollutants, which generate smog. But these models do not always function well, Francisco said, in part because OH radicals are in some ways an unknown quantity.
"One of the biggest questions in our field concerns the amount of OH radicals the atmosphere holds," he said. "It's tough to get a handle on them because they are so reactive ?which means they vanish fast ?and also because we don't have complete knowledge of all the sources that produce them yet."
The experiments, which Sinha and Matthews performed at UCSD, used a laser technique that allowed the team to look at the OH radical-producing molecules in a new way. More precisely, it allowed the researchers to observe a portion of the spectrum that the molecules absorb ?a portion that has been something of a blind spot for scientists, who often detect chemical reactions by perceiving the telltale light frequencies that certain reactions are known to emit or absorb. Many sources of OH radicals strongly absorb UV light, making them easily detectable. However, the weak absorptions in the lower region of the ultraviolet spectrum, from wavelengths of about 360 nanometers to 630 nanometers, has been more challenging.