Formation and Transport of Secondary Contaminants Associated with Germicidal Ultraviolet Light Systems in an Occupied Classroom.

Germicidal ultraviolet light (GUV) systems are designed to control airborne pathogen transmission in buildings. However, it is important to acknowledge that certain conditions and system configurations may lead GUV systems to produce air contaminants including oxidants and secondary organic aerosols (SOA). In this study, we modeled the formation and dispersion of oxidants and secondary contaminants generated by the operation of GUV systems employing ultraviolet C 254 and 222 nm. Using a three-dimensional computational fluid dynamics model, we examined the breathing zone concentrations of chemical species in an occupied classroom. Our findings indicate that operating GUV 222 leads to an approximate increase of 10 ppb in O3 concentration and 5.2 µg·m-3 in SOA concentration compared to a condition without GUV operation, while GUV 254 increases the SOA concentration by about 1.2 µg·m-3, with a minimal impact on the O3 concentration. Furthermore, increasing the UV fluence rate of GUV 222 from 1 to 5 µW·cm-2 results in up to 80% increase in the oxidants and SOA concentrations. For GUV 254, elevating the UV fluence rate from 30 to 50 µW·cm-2 or doubling the radiating volume results in up to 50% increase in the SOA concentration. Note that indoor airflow patterns, particularly buoyancy-driven airflow (or displacement ventilation), lead to 15-45% lower SOA concentrations in the breathing zone compared to well-mixed airflow. The results also reveal that when the ventilation rate is below 2 h-1, operating GUV 254 has a smaller impact on human exposure to secondary contaminants than GUV 222. However, GUV 254 may generate more contaminants than GUV 222 when operating at high indoor O3 levels (>15 ppb). These results suggest that the design of GUV systems should consider indoor O3 levels and room ventilation conditions.

The human oxidation field.

Hydroxyl (OH) radicals are highly reactive species that can oxidize most pollutant gases. In this study, high concentrations of OH radicals were found when people were exposed to ozone in a climate-controlled chamber. OH concentrations calculated by two methods using measurements of total OH reactivity, speciated alkenes, and oxidation products were consistent with those obtained from a chemically explicit model. Key to establishing this human-induced oxidation field is 6-methyl-5-hepten-2-one (6-MHO), which forms when ozone reacts with the skin-oil squalene and subsequently generates OH efficiently through gas-phase reaction with ozone. A dynamic model was used to show the spatial extent of the human-generated OH oxidation field and its dependency on ozone influx through ventilation. This finding has implications for the oxidation, lifetime, and perception of chemicals indoors and, ultimately, human health.

Spatial and temporal scales of variability for indoor air constituents.

Historically air constituents have been assumed to be well mixed in indoor environments, with single point measurements and box modeling representing a room or a house. Here we demonstrate that this fundamental assumption needs to be revisited through advanced model simulations and extensive measurements of bleach cleaning. We show that inorganic chlorinated products, such as hypochlorous acid and chloramines generated via multiphase reactions, exhibit spatial and vertical concentration gradients in a room, with short-lived ⋅OH radicals confined to sunlit zones, close to windows. Spatial and temporal scales of indoor constituents are modulated by rates of chemical reactions, surface interactions and building ventilation, providing critical insights for better assessments of human exposure to hazardous pollutants, as well as the transport of indoor chemicals outdoors.

Spatial distributions of ozonolysis products from human surfaces in ventilated rooms.

Ozone has adverse effects on human health. Skin oil on the human surface acts as an ozone sink indoors, producing oxidation products that can cause skin and respiratory irritations. Concentrations of ozone and oxidation products near human surfaces, including the breathing zone, can be modulated by indoor ventilation modes and human surface conditions. The objective of this study is to examine concentrations and spatial heterogeneity of ozone and ozonolysis products under representative ranges of indoor ventilation, clothing, and breathing conditions. Using computational fluid dynamics (CFD) simulation in conjunction with a chemical kinetic model, details of ozone reactions with the human surface and subsequent chemical reactions are examined. The results show that primary ozonolysis products are concentrated near the soiled clothing, while the secondary products are relatively well distributed throughout the room. Increasing indoor air mixing enhances the ozone deposition to the human surface, thereby resulting in higher emission rates of oxidation products in the room. Soiled clothing consumes more ozone than clean clothing and accordingly produces ~ 65% more primary products and ~15% more secondary products. The results also reveal that unsaturated hydrocarbons from the human breath, such as isoprene, contribute to only ~0.5% of ozone removal compared to ozone deposition to the human surface.

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

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.

Modelling consortium for chemistry of indoor environments (MOCCIE): integrating chemical processes from molecular to room scales.

We report on the development of a modelling consortium for chemistry in indoor environments that connects models over a range of spatial and temporal scales, from molecular to room scales and from sub-nanosecond to days, respectively. Our modeling approaches include molecular dynamics (MD) simulations, kinetic process modeling, gas-phase chemistry modeling, organic aerosol modeling, and computational fluid dynamics (CFD) simulations. These models are applied to investigate ozone reactions with skin and clothing, oxidation of volatile organic compounds and formation of secondary organic aerosols, and mass transport and partitioning of indoor species to surfaces. MD simulations provide molecular pictures of limonene adsorption on SiO2 and ozone interactions with the skin lipid squalene, providing kinetic parameters such as surface accommodation coefficient, desorption lifetime, and bulk diffusivity. These parameters then constrain kinetic process models, which resolve mass transport and chemical reactions in gas and condensed phases for analysis of experimental data. A detailed indoor chemical box model is applied to simulate α-pinene ozonolysis with improved representation of gas-particle partitioning. Application of 2D-volatility basis set reveals that OH-induced aging sometimes drives increases in indoor organic aerosol concentrations, due to organic mass functionalization and enhanced partitioning. CFD simulations show that concentrations of ozone and primary product change near the human surface rapidly, indicating non-uniform spatial distributions from the occupant surface to ambient air, while secondary ozone product is relatively well-mixed throughout the room. This development establishes a framework to integrate different modeling tools and experimental measurements, opening up an avenue for development of comprehensive and integrated models with representations of various chemistry in indoor environments.