One of the chemical agents responsible for smog has a sleeping spot — and it is right beneath your feet. Nitrous acid (HNO2), which quickly degrades in sunlight to form the hydroxyl radical OH, appears to spend the night tucked in the ground so that it can be steadily released under the right atmospheric conditions.

“Oxidation of volatile organic compounds initiated by the hydroxyl radical, then catalyzed by nitrogen oxide — that’s what makes ozone,” says Jennifer Murphy, a chemistry professor at the University of Toronto. In the summer, what most of us would consider to be a smog episode is actually higher amounts of ground-level ozone, says Murphy. During such episodes, she adds, there seems to be an ongoing supply of nitrous acid to maintain OH levels, which raises the question of where that supply is stored. “If it’s destroyed really quickly, but it’s still around all the time, it must be made really quickly,” she says. “If we want to understand the budget of the hydroxyl radical that’s kicking off all the smog production, we need to understand nitrous acid.”  

Murphy acknowledges the inherent difficulty of teasing apart the intricate dynamics of atmospheric chemistry. Any model being tested under controlled laboratory conditions is invariably a simplification of field conditions, so the two seldom provide matching measurements. Nevertheless, she and her colleagues were able to obtain just such a match by looking down, not up. They speculated that nitrous acid may be attaching itself to elements found in the ground, only to be released later on. “We saw nitrite, the base of nitrous acid, in particles associated with mineral cations like calcium and sodium,” Murphy says. “That was a hint to us that maybe it’s making some sort of salt in particles that are more or less representative of the soil on the ground.”

In fact, nitrous acid could take advantage of different kinds of ground cover for this purpose. Murphy speculates that other substrates — such as carbonates found in concrete or even simple dust particles — could likewise provide suitable storage. After taking field measurements in one of California’s smoggier corners, the researchers designed an experiment to test this mechanism, subsequently publishing the findings late last year in Nature Geoscience. “It was one of the more satisfying projects that we’ve done,” says Murphy, “in terms of being able to go back and forth and take the lab and the field and get that complementary information.”