Nitrous acid (HONO) is a household pollutant exhibiting adverse health effects and a major source of indoor OH radicals under a variety of lighting conditions. The present study focuses on gas-phase HONO and condensed-phase nitrite and nitrate formation on indoor surface thin films following heterogeneous hydrolysis of NO₂, in the presence and absence of light, and nitrate (NO₃^-) photochemistry. These thin films are composed of common building materials including zeolite, kaolinite, painted walls, and cement. Gas-phase HONO is measured using an incoherent broadband cavity-enhanced ultraviolet absorption spectrometer (IBBCEAS), whereby condensed-phase products, adsorbed nitrite and nitrate, are quantified using ion chromatography. All of the surface materials used in this study can store nitrogen oxides as nitrate, but only thin films of zeolite and cement can act as condensed-phase nitrite reservoirs. For both the photo-enhanced heterogeneous hydrolysis of NO₂ and nitrate photochemistry, the amount of HONO produced depends on the material surface. For zeolite and cement, little HONO is produced, whereas HONO is the major product from kaolinite and painted wall surfaces. An important result of this study is that surface interactions of adsorbed nitrite are key to HONO formation, and the stronger the interaction of nitrite with the surface, the less gas-phase HONO produced.

Wildfires are increasing in frequency, raising concerns that smoke can permeate indoor environments and expose people to chemical air contaminants. To study smoke transformations in indoor environments and evaluate mitigation strategies, we added smoke to a test house. Many volatile organic compounds (VOCs) persisted days following the smoke injection, providing a longer-term exposure pathway for humans. Two time scales control smoke VOC partitioning: a faster one (1.0 to 5.2 hours) that describes the time to reach equilibrium between adsorption and desorption processes and a slower one (4.8 to 21.2 hours) that describes the time for indoor ventilation to overtake adsorption-desorption equilibria in controlling the air concentration. These rates imply that vapor pressure controls partitioning behavior and that house ventilation plays a minor role in removing smoke VOCs. However, surface cleaning activities (vacuuming, mopping, and dusting) physically removed surface reservoirs and thus reduced indoor smoke VOC concentrations more effectively than portable air cleaners and more persistently than window opening.