Wavelength-Resolved Photon Fluxes of Indoor Light Sources: Implications for HOx Production.

Photochemistry is a largely unconsidered potential source of reactive species such as hydroxyl and peroxy radicals (OH and HO2, "HOx") 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 HOx 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 HOx 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.