Factors affecting wavelength‐resolved ultraviolet irradiance indoors and their impacts on indoor photochemistry

Abstract

Solar photons in the high-energy ultraviolet (UV) region are responsible for the photolysis of gas molecules leading to the production of highly reactive atoms and radicals, which are the main drivers of oxidation chemistry in the atmosphere. Hydroxyl radicals (OH), which react rapidly and indiscriminately with most atmospheric trace compounds and determine the lifetimes of many atmospheric constituents, are generated in the troposphere primarily via ozone photolysis in a narrow wavelength band between 290 and 320 nm. 1,2 Nitrous acid (HONO) photolysis is an important source of OH in the early morning and late evening, when photons at wavelengths shorter than ~320 nm are attenuated by the atmosphere. 1 Photolysis reactions of reactive chlorine gas species such as hypochlorous acid (HOCl), nitryl chloride (ClNO 2 ), and molecular chlorine (Cl 2 ) can be important sources of chlorine atoms (Cl), which rapidly oxidize volatile organic compounds. 3 Considerable effort has been expended to study photochemistry in the ambient atmosphere. Conversely, the role of photochemistry in indoor air has received little attention, 4 despite the fact that people spend more than 85% of their time

Spatiotemporal characterization of irradiance and photolysis rate constants of indoor gas‐phase species in the UTest house during HOMEChem

“…A reduction of 38% in UV transmittance was observed as the solar incident angle increased from 27° (6:35 pm) to 59° (11:15 am; Figure 1B). The angular dependence of UV transmittance observed in the UTest house is consistent with the decreasing trends previously predicted and measured in the laboratory, 26,27 though with greater observed attenuation.…”

“…Measurement details of gas species can be found in several papers 12,23,38 . Since calibrated HCHO measurements were not available, we used HCHO data obtained from the most recent real‐time measurements in another single‐family house in the US 26 . To validate our calculations, we used Equation 4 to predict OH ss under the conditions (gas‐phase concentrations and photon fluxes) reported by Gomez Alvarez et al in a classroom under sunlit conditions; 1 the OH ss we predict agrees with their measured OH concentration within 30% (details in Text S3).…”

“…On average, J HONO during the directly irradiated hours (ie, morning by the westward window and afternoon by the eastward window) was ~4 times that when photolysis was driven by diffuse light (i.e., after 12 p.m. by the eastward window and earlier than 5 p.m. by the westward window), in agreement with previous studies. 26,27 The difference in J values of other molecules under direct and diffuse irradia-…”

“…Simulations show that indoor HO x radical concentrations under a typical range of indoor lighting conditions are confined within the volume of HONO‐photolyzing light 7 . Zhou et al also reported spatial heterogeneity of UV photon fluxes from indoor sunlight and fluorescent tubes and demonstrated that indoor photon fluxes and predicted OH concentrations under the indoor sunlight conditions depend on the time of day 26 . Most studies, however, use photon fluxes measured at a single location and time to calculate photolysis rate constants ( J ).…”

“…3 A few other recent studies measured solar UV photon fluxes in various indoor environments and provided detailed characterizations of the dependence of indoor UV photon flux on window transmittance, distance from window, solar incident angle, direct vs. diffuse sunlight, and surface reflectance. [26][27][28][29] Quantifying the effect of photochemistry on indoor oxidative capacity and air composition is further complicated by the fact that indoor photon fluxes are highly spatially and temporally variable.…”