Understanding the Spatial Heterogeneity of Indoor OH and HO₂ due to Photolysis of HONO Using Computational Fluid Dynamics Simulation

Abstract

Indoor photolysis of nitrous acid (HONO) generates hydroxyl radicals (OH), and since OH is fast reacting, it may be confined within the HONO-photolyzing indoor volume of light. This study investigated the HONO-photolysis-induced formation of indoor OH, the transformation of OH to hydroperoxy radicals (HO2), and resulting spatial distributions of those radicals and their oxidation products. To do so, a computational fluid dynamics (CFD) model framework was established to simulate HONO photolysis in a room and subsequent reactions associated with OH and HO2 under a typical range of indoor lighting and ventilation conditions. The results showed that OH and HO2 were essentially confined in the volume of HONO-photolyzing light, but oxidation products were relatively well distributed throughout the room. As the light volume increased, more total in-room OH was produced, thereby increasing oxidation product concentrations. Spatial distributions of OH and HO2 varied by the type of artificial light (e.g., fluorescent versus incandescent), due to differences in photon flux as a function of light source and the distance from the source. The HO2 generation rate and air change rate made notable impacts on product concentrations.

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

“…In this work, we made spatially resolved measure- to 5.7 × 10 6 molec cm −3 under direct illumination from sunlight, and an order of magnitude higher than OH ss predicted in the dark (when OH is generated primarily from ozonolysis of alkenes). [10][11][12]16,20 Higher SZA near sunrise and/or sunset (80˚), or diffuse light at the same time on a sunny day led to a large reduction of OH ss of ~85%, although predicted OH ss remained on the same order of magnitude (10 6 molec cm −3 ) as levels commonly reported outdoors. 53 8 is also presented in Table 3.…”

“…Artificial lights including fluorescent, incandescent, and halogen lamps are additional light sources in indoor environments that emit wavelengths short enough to induce photolysis of several chemicals. These lights have been predicted to photolyze HONO, although radical levels were generally lower than those predicted for indoor sunlight 8,16,17 . Photolysis of Cl 2 and HOCl by artificial lights were calculated to be important photochemical sources of OH and Cl radicals, particularly near the light sources 18 .…”

Indoor Surface Chemistry: Developing a Molecular Picture of Reactions on Indoor Interfaces

“…Additionally, OH production reactions can be spatially inhomogeneous because of the wide range of photon fluxes found indoors. 62 Therefore, the formation of HONO must be understood in indoor environments. Furthermore, the formation of HONO and its dissociation is a topic of great interest, since much less is understood about HONO compared with NO and NO 2 .…”

Combining homogeneous and heterogeneous chemistry to model inorganic compound concentrations in indoor environments: the H<sup>2</sup>I model (v1.0)

“…Here, two different parts are considered: one irradiated by direct light and another illuminated indirectly. This is one of the first models to consider the evolution of light intensity in each part through the day, as well as the volumes that they occupy, without making use of computational fluid dynamics (CFD) simulations (Won et al, 2019).…”