Much of the hydroxyl radical available in the atmosphere to react with and remove pollutants may not come from nitrous acid as generally believed, according to studies of air above Italy (Science 2014, DOI: 10.1126/science.1248999). HONO is commonly thought to enter the atmosphere primarily through direct emission from combustion or from reactions on snow, soils, and leaves. HONO then photolyzes to hydroxyl radical and nitric oxide. Using a Zeppelin NT airship to profile atmospheric compounds, a group led by Xin Li and Franz Rohrer of Germany’s Jülich Research Center found unexpectedly high HONO concentrations far above possible surface sources. Known airborne sources—aerosol particles and nitrogen dioxide—also aren’t abundant enough to explain the observed HONO concentrations. Li, Rohrer, and colleagues point to gas-phase reactions of NOx and HOx species as the likely culprits. But if hydroxyl radical is consumed to make HONO, then the net yield of the radical from HONO chemistry would decrease, necessitating new thinking about the sources of hydroxyl radical as well.