Unexpectedly High Indoor HONO Concentrations Associated with Photochemical NO₂ Transformation on Glass Windows

Nitrous acid (HONO) is an important gaseous pollutant contributing to indoor air pollution because it causes adverse health effects and is the main source of hydroxyl radicals (OH). Here, we present direct measurements of HONO produced through light-induced heterogeneous reactions of NO₂ with grime adsorbed on glass window. The uptake coefficients of NO₂ [γ(NO₂)] on the glass plates from the kitchen increased markedly from (2.3 ± 0.1) × 10^-6 at 0% RH to (4.1 ± 0.5) × 10^-6 at 90% RH. We report a significant quantity of daytime HONO produced in the kitchen, compared to the living room and bedroom. Kinetic modeling suggests that phase state and bulk diffusivity play important roles in the NO₂ uptake; the best fit to the measured uptake coefficients is obtained with fixed diffusion coefficients. Photon scattering may be occurring at the surface of the films, leading to enhanced photon-excitation rates of polycyclic aromatic hydrocarbons. By taking these effects into account, the results from this study indicate that the HONO yields obtained in this study can explain the missing HONO in the photochemical models describing the indoor air chemistry.