Nitrous acid, nitrogen dioxide, and ozone concentrations in residential environments.

Nitrous acid (HONO) may be generated by heterogeneous reactions of nitrogen dioxide and direct emission from combustion sources. Interactions among nitrogen oxides and ozone are important for outdoor photochemical reactions. However, little is known of indoor HONO levels or the relationship between residential HONO, NO(2), and O(3) concentrations in occupied houses. Six-day integrated indoor and outdoor concentrations of the three pollutants were simultaneously measured in two communities in Southern California using passive samplers. The average indoor HONO concentration was 4.6 ppb, compared to 0.9 ppb for outdoor HONO. Average indoor and outdoor NO(2)concentrations were 28 and 20.1 ppb, respectively. Indoor O(3) concentrations were low (average 14.9 ppb) in comparison to the outdoor levels (average 56.5 ppb). Housing characteristics, including community and presence of a gas range, were significantly associated with indoor NO(2) and HONO concentrations. Indoor HONO levels were closely correlated with indoor NO(2) levels and were about 17% of indoor NO(2) concentrations. Indoor HONO levels were inversely correlated with indoor O(3) levels. The measurements demonstrated the occurrence of substantial residential indoor HONO concentrations and associations among the three indoor air pollutants.