Wavelength-Resolved Photon Fluxes of Indoor Light Sources: Implications for HO_<i>x</i> Production

Abstract

Photochemistry is a largely unconsidered potential source of reactive species such as hydroxyl and peroxy radicals (OH and HO₂, "HO_x") indoors. We present measured wavelength-resolved photon fluxes and distance dependences of indoor light sources including halogen, incandescent, and compact fluorescent lights (CFL) commonly used in residential buildings; fluorescent tubes common in industrial and commercial settings; and sunlight entering buildings through windows. We use these measurements to predict indoor HO_x production rates from the photolysis of nitrous acid (HONO), hydrogen peroxide (H₂O₂), ozone (O₃), formaldehyde (HCHO), and acetaldehyde (CH₃CHO). Our results suggest that while most lamps can photolyze these molecules, only sunlight and fluorescent tubes will be important to room-averaged indoor HO_x levels due to the strong distance dependence of the fluxes from compact bulbs. Under ambient conditions, we predict that sunlight and fluorescent lights will photolyze HONO to form OH at rates of 10⁶-10⁷ molecules cm^-3 s^-1, and that fluorescent lights will photolyze HCHO to form HO₂ at rates of ∼10⁶ molecules cm^-3 s^-1; rates could be 2 orders of magnitude higher under high precursor concentrations. Ozone and H₂O₂ will not be important photochemical OH sources under most conditions, and CH₃CHO will generally increase HO₂ production rates only slightly. We also calculated photolysis rate constants for nitrogen dioxide (NO₂) and nitrate radicals (NO₃) in the presence of the different light sources. Photolysis is not likely an important fate for NO₃ indoors, but NO₂ photolysis could be an important source of indoor O₃.