Time-Resolved Measurements of Nitric Oxide, Nitrogen Dioxide, and Nitrous Acid in an Occupied New York Home

Indoor oxidizing capacity in occupied residences is poorly understood. We made simultaneous continuous time-resolved measurements of ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2), and nitrous acid (HONO) for two months in an occupied detached home with gas appliances in Syracuse, NY. Indoor NO and HONO mixing ratios were higher than those outdoors, whereas O3 was much lower (sub-ppbv) indoors. Cooking led to peak NO, NO2, and HONO levels 20–100 times greater than background levels; HONO mixing ratios of up to 50 ppbv were measured. Our results suggest that many reported NO2 levels may have a large positive bias due to HONO interference. Nitrous acid, NO2, and NO were removed from indoor air more rapidly than CO2, indicative of reactive removal processes or surface uptake. We measured spectral irradiance from sunlight entering the residence through glass doors; hydroxyl radical (OH) production rates of (0.8–10) × 107 molecules cm–3 s–1 were calculated in sunlit areas due to HONO photolysis, in some cases exceeding rates expected from ozone–alkene reactions. Steady-state nitrate radical (NO3) mixing ratios indoors were predicted to be lower than 1.65 × 104 molecules cm–3. This work will help constrain the temporal nature of oxidant concentrations in occupied residences and will improve indoor chemistry models.