INDOOR OZONE AND NITROGEN-DIOXIDE - A POTENTIAL PATHWAY TO THE GENERATION OF NITRATE RADICALS, DINITROGEN PENTAOXIDE, AND NITRIC-ACID INDOORS

Outdoors, in the absence of direct sunlight, significant concentrations of the nitrate radical can result from the reaction between ozone and nitrogen dioxide. The nitrate radical will rapidly equilibrate with dinitrogen pentaoxide, and both of these species can further react to produce nitric acid. We suggest that similar chemistry can occur indoors. A recent study supports this suggestion. Measurements of acidic species in the Boston area indicate significant indoor sources of nitric acid during the summer, but not during the winter. A simple mass balance model (incorporating an O3/NO2 reaction sequence) yields indoor HNO3 values in reasonable agreement with measured values. Whereas outdoors, at night, the major route to gas-phase nitric acid appears to be the homogeneous hydrolysis of N2O5, indoors the dominant path may often be NO3 abstraction of H atoms from volatile organic compounds (VOCs). Such chemistry may also be an important source of free radicals indoors-not just the nitrate radical, but also peroxy and hydroxyl radicals derived from reactions among the nitrate radical and VOCs.