Aerosols, N₂O₅ play key role in ozone cycle

In work that elucidates themechanism of regional ozone pollution, the efficiency of dinitrogen pentoxide (N₂O₅) breakdown during nighttime reactions on aerosol particles has been found to depend on aerosol composition to a previously unrecognized degree (Science 2006, 311, 67). In sunlight, nitrogen oxides catalyze production of ozone, a major lower atmosphere air pollutant and health hazard. But in the absence of sunlight, nitrogen oxides and ozone are depleted by a series of reactions that includes the aerosol-mediated hydrolysis of N₂O₅ to form nitric acid. Little has been known about the efficiency of N₂O₅ hydrolysis, however. Steven S. Brown of the National Oceanic & Atmospheric Administration, Boulder, Colo., and coworkers have now analyzed levels of nitrogen oxides and related compounds measured from an aircraft. The analysis reveals that N₂O₅ reactivity is highly variable and strongly dependent on aerosol composition, particularly aerosol sulfate content. The data suggest that increases in SO₂ emissions, and consequent increases in sulfate aerosols, may decrease regional levels of nitrogen oxides and possibly ozone more dramatically than was previously realized.