Chemical Composition, Spatial Homogeneity, and Growth of Indoor Surface Films

Ozone Chemistry on Greasy Glass Surfaces Affects the Levels of Volatile Organic Compounds in Indoor Environments

The chemistry of ozone (O3) on indoor surfaces leads to secondary pollution, aggravating the air quality in indoor environments. Here, we assess the heterogeneous chemistry of gaseous O3 with glass plates after being 1 month in two different kitchens where Chinese and Western styles of cooking were applied, respectively. The uptake coefficients of O3 on the authentic glass plates were measured in the dark and under UV light irradiation typical for indoor environments (320 nm < λ < 400 nm) at different relative humidities. The gas-phase product compounds formed upon reactions of O3 with the glass plates were evaluated in real time by a proton-transfer-reaction quadrupole-interface time-of-flight mass spectrometer. We observed typical aldehydes formed by the O3 reactions with the unsaturated fatty acid constituents of cooking oils. The formation of decanal, 6-methyl-5-hepten-2-one (6-MHO), and 4-oxopentanal (4-OPA) was also observed. The employed dynamic mass balance model shows that the estimated mixing ratios of hexanal, octanal, nonanal, decanal, undecanal, 6-MHO, and 4-OPA due to O3 chemistry with authentic grime-coated kitchen glass surfaces are higher in the kitchen where Chinese food was cooked compared to that where Western food was cooked. These results show that O3 chemistry on greasy glass surfaces leads to enhanced VOC levels in indoor environments.

Composition of indoor organic surface films in residences: simulating the influence of sources, partitioning, particle deposition, and air exchange

Indoor surfaces are coated with organic films that modulate thermodynamic interactions between the surfaces and room air. Recently published models can simulate film formation and growth via gas-surface partitioning, but none have statistically investigated film composition. The Indoor Model of Aerosols, Gases, Emissions, and Surfaces (IMAGES) was used here to simulate ten years of nonreactive film growth upon impervious indoor surfaces within a Monte Carlo procedure representing a sub-set of North American residential buildings. Film composition was resolved into categories reflecting indoor aerosol (gas + particle phases) factors from three sources: outdoor-originating, indoor-emitted, and indoor-generated secondary organic material. In addition to gas-to-film partitioning, particle deposition was modeled as a vector for organics to enter films, and it was responsible for a majority of the film mass after ∼1000 days of growth for the median simulation and is likely the main source of LVOCs within films. Therefore, the organic aerosol factor possessing the most SVOCs contributes most strongly to the composition of early films, but as the film ages, films become more dominated by the factor with the highest particle concentration. Indoor-emitted organics (e.g. from cooking) often constituted at least a plurality of the simulated mass in developed films, but indoor environments are diverse enough that any major organic material source could be the majority contributor to film mass, depending on building characteristics and indoor activities. A sensitivity analysis suggests that rapid film growth is most likely in both newer, more air-tight homes and older homes near primary pollution sources.

Chemical Fate of Oils on Indoor Surfaces: Ozonolysis and Peroxidation

Unsaturated triglycerides found in food and skin oils are reactive in ambient air. However, the chemical fate of such compounds has not been well characterized in genuine indoor environments. Here, we monitored the aging of oil coatings on glass surfaces over a range of environmental conditions, using mass spectrometry, nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) techniques. Upon room air exposure (up to 17 ppb ozone), the characteristic ozonolysis products, secondary ozonides, were observed on surfaces near the cooking area of a commercial kitchen, along with condensed-phase aldehydes. In an office setting, ozonolysis is also the dominant degradation pathway for oil films exposed to air. However, for indoor enclosed spaces such as drawers, the depleted air flow makes lipid autoxidation more favorable after an induction period of a few days. Forming hydroperoxides as the major primary products, this radical-mediated peroxidation behavior is accelerated by indoor direct sunlight, but the initiation step in dark settings is still unclear. These results are in accord with radical measurements, indicating that indoor photooxidation facilitates radical formation on surfaces. Overall, many intermediate and end products observed are reactive oxygen species (ROS) that may induce oxidative stress in human bodies. Given that these species can be widely found on both food and household surfaces, their toxicological properties are worth further attention.

The persistence of smoke VOCs indoors: Partitioning, surface cleaning, and air cleaning in a smoke-contaminated house

Wildfires are increasing in frequency, raising concerns that smoke can permeate indoor environments and expose people to chemical air contaminants. To study smoke transformations in indoor environments and evaluate mitigation strategies, we added smoke to a test house. Many volatile organic compounds (VOCs) persisted days following the smoke injection, providing a longer-term exposure pathway for humans. Two time scales control smoke VOC partitioning: a faster one (1.0 to 5.2 hours) that describes the time to reach equilibrium between adsorption and desorption processes and a slower one (4.8 to 21.2 hours) that describes the time for indoor ventilation to overtake adsorption-desorption equilibria in controlling the air concentration. These rates imply that vapor pressure controls partitioning behavior and that house ventilation plays a minor role in removing smoke VOCs. However, surface cleaning activities (vacuuming, mopping, and dusting) physically removed surface reservoirs and thus reduced indoor smoke VOC concentrations more effectively than portable air cleaners and more persistently than window opening.

Effective mass accommodation for partitioning of organic compounds into surface films with different viscosities

Indoor surfaces can act as reservoirs and reaction media influencing the concentrations and type of species that people are exposed to indoors. Mass accommodation and partitioning are impacted by the phase state and viscosity of indoor surface films. We developed the kinetic multi-layer model KM-FILM to simulate organic film formation and growth, but it is computationally expensive to couple such comprehensive models with indoor air box models. Recently, a novel effective mass accommodation coefficient (αeff) was introduced for efficient and effective treatments of gas-particle partitioning. In this study, we extended this approach to a film geometry with αeff as a function of penetration depth into the film, partitioning coefficient, bulk diffusivity, and condensed-phase reaction rate constant. Comparisons between KM-FILM and the αeff method show excellent agreement under most conditions, but with deviations before the establishment of quasi-equilibrium within the penetration depth. We found that the deposition velocity of species and overall film growth are impacted by bulk diffusivity in highly viscous films (Db ∼<10-15 cm2 s-1). Reactions that lead to non-volatile products can increase film thicknesses significantly, with the extent of film growth being dependent on the gas-phase concentration, rate coefficient, partitioning coefficient and diffusivity. Amorphous semisolid films with Db > ∼10-17-10-19 cm2 s-1 can be efficient SVOC reservoirs for compounds with higher partitioning coefficients as they can be released back to the gas phase over extended periods of time, while glassy solid films would not be able to act as reservoirs as gas-film partitioning is impeded.

Ozone Loss: A Surrogate for the Indoor Concentration of Ozone-Derived Products

Ozone concentrations tend to be substantially lower indoors than outdoors, largely because of ozone reactions with indoor surfaces. When there are no indoor sources of ozone, a common condition, the net concentration of gaseous products derived from indoor ozone chemistry scales linearly with the difference between outdoor and indoor ozone concentrations, termed "ozone loss." As such, ozone loss is a metric that might be used by epidemiologists to disentangle the adverse health effects of ozone's oxidation products from those of exposure to ozone itself. The present paper examines the characteristics, potential utility, and limitations of the ozone loss concept. We show that for commonly occurring indoor conditions, the ozone loss concentration is directly proportional to the total rate constant for ozone removal on surfaces (ksum) and inversely proportional to the net removal of ozone by air exchange (λ) plus surface reactions (ksum). It follows that the ratio of indoor ozone to ozone loss is equal to the ratio of λ to ksum. Ozone loss is a promising metric for probing potential adverse health effects resulting from exposures to products of indoor ozone chemistry. Notwithstanding its virtues, practitioners using it should be mindful of the limitations discussed in this paper.

Distribution of polycyclic aromatic hydrocarbons in indoor/outdoor window films and the indoor film/air partition of northeastern Chinese college dormitories

Indoor window films can represent short-term air pollution conditions of indoor environment through rapidly capturing organic contaminants as effective passive air samplers. To investigate the temporal variation, influence factors of polycyclic aromatic hydrocarbons (PAHs) in indoor window films, and the exchange behavior with gas phase in college dormitories, 42 pairs window films of interior and exterior window surfaces and corresponding indoor gas phase and dust samples were collected monthly in six selected dormitories, Harbin, China, from August to December 2019 and September 2020. The average concentration of ∑₁₆PAHs in indoor window films (398 ng/m²) was significantly (p < 0.01) lower than that in outdoors (652 ng/m²). In addition, the median indoor/outdoor ∑₁₆PAHs concentration ratio was close to 0.5, showing that outdoor air acted as a major PAH source to indoor environment. The 5-ring PAHs were mostly dominant in window films whereas the 3-ring PAHs contributed mostly in gas phase. 3-ring PAHs and 4-ring PAHs were both important contributors for dormitory dust. Window films showed stable temporal variation, i.e. PAH concentrations in heating months were higher than those in non-heating months. The atmospheric O₃ concentration was the main influence factor of PAHs in indoor window films. PAHs with low molecular weight in indoor window films rapidly reached film/air equilibrium phase within in dozens of hours. The large deviation in the slope of the log K_F-A versus log K_OA regression line from that in reported equilibrium formula might be the difference between the window film composition and octanol.

Indoor partitioning and potential thirdhand exposure to carbonyl flavoring agents added in e-cigarettes and hookah tobacco

Flavoring agents added to the e-cigarettes and hookah tobacco have increased the attractiveness of novel nicotine products. Many widely used flavorings are carbonyls, which are toxic to humans. In an indoor environment, residents can be exposed to such harmful flavorings previously emitted to the surrounding environment, through a process termed thirdhand exposure. The recent discovery of a large volume of indoor reservoirs emphasizes the importance of indoor partitioning, which is responsible for thirdhand exposure. Indoor partitioning can be expressed with partitioning coefficients, such as Henry's law solubility constant (H). However, reliable H values for many key flavorings are currently lacking. To better understand their environmental behavior, this study experimentally determined the effective Henry's law constant (Hcps,eff) using the inert gas stripping (IGS) method. Further, the influence of the hydration process for target flavorings was quantified using proton nuclear magnetic resonance (¹H NMR) spectroscopy. We found that hydration of α-dicarbonyls (diacetyl and 2,3-pentanedione) enhanced their Hcps,eff from their intrinsic Henry's law constant (Hcps) by a factor of 3.52 and 2.88, respectively. The two-dimensional partitioning plots were employed to simulate the indoor phase distribution and evaluate the pathways of human exposure. Our findings show that the indoor partitioning of many harmful flavorings is highly sensitive to temperature and the size of indoor reservoirs, indicating that residents are likely to experience third-hand exposure.

Temperature and indoor environments

From the thermodynamic perspective, the term temperature is clearly defined for ideal physical systems: A unique temperature can be assigned to each black body via its radiation spectrum, and the temperature of an ideal gas is given by the velocity distribution of the molecules. While the indoor environment is not an ideal system, fundamental physical and chemical processes, such as diffusion, partitioning equilibria, and chemical reactions, are predictably temperature-dependent. For example, the logarithm of reaction rate and equilibria constants are proportional to the reciprocal of the absolute temperature. It is therefore possible to have non-linear, very steep changes in chemical phenomena over a relatively small temperature range. On the contrary, transport processes are more influenced by spatial temperature, momentum, and pressure gradients as well as by the density, porosity, and composition of indoor materials. Consequently, emergent phenomena, such as emission rates or dynamic air concentrations, can be the result of complex temperature-dependent relationships that require a more empirical approach. Indoor environmental conditions are further influenced by the thermal comfort needs of occupants. Not only do occupants have to create thermal conditions that serve to maintain their core body temperature, which is usually accomplished by wearing appropriate clothing, but also the surroundings must be adapted so that they feel comfortable. This includes the interaction of the living space with the ambient environment, which can vary greatly by region and season. Design of houses, apartments, commercial buildings, and schools is generally utility and comfort driven, requiring an appropriate energy balance, sometimes considering ventilation but rarely including the impact of temperature on indoor contaminant levels. In our article, we start with a review of fundamental thermodynamic variables and discuss their influence on typical indoor processes. Then, we describe the heat balance of people in their thermal environment. An extensive literature study is devoted to the thermal conditions in buildings, the temperature-dependent release of indoor pollutants from materials and their distribution in the various interior compartments as well as aspects of indoor chemistry. Finally, we assess the need to consider temperature holistically with regard to the changes to be expected as a result of global emergencies such as climate change.

Partitioning of airborne PAEs on indoor impermeable surfaces: A microscopic view of the sorption process

Organic films were widely found on indoor impermeable surfaces exposed to gaseous organic compounds, but few studies have addressed the film growth details on different indoor substrates. In this study, we observed the topography evolution of phthalic acid ester (PAE) organic films on three impermeable substrates: polished glass (G-P), mirror-polished stainless steel (SS-M) and drawn stainless steel (SS-D). PAE organic films were preferentially formed upon the flat surface with sparse inherent nano-peaks of substrate G-P and in valleys of substrate SS-M and SS-D. Surface uniformity of substrates and viscosity of PAE molecules were inferred as critical parameters determining the surface average adhesion forces. We obtained the partition coefficients of DEP, DnBP, BBP and DEHP on substrate G-P, SS-M and SS-D by fitting the initial monolayer adsorption process. Organic films continuously grew instead of reaching adsorption equilibrium after long-term PAE exposure, indicating that multilayer adsorption may occur. The organic film growth rates in saturated gas-phase PAE concentrations were quantified as about one-tenth of the results in previous studies where substrates were simultaneously exposed to multiple pollutants. To sum up, the results outline PAE adsorption details on impermeable materials and provide a reference for better estimation on PAE exposure assessment.

Indoor ozone: Concentrations and influencing factors

Because people spend most of their time indoors, much of their exposure to ozone occurs in buildings, which are partially protective against outdoor ozone. Measurements in approximately 2000 indoor environments (residences, schools, and offices) show a central tendency for average indoor ozone concentration of 4-6 ppb and an indoor to outdoor concentration ratio of about 25%. Considerable variability in this ratio exists among buildings, as influenced by seven building-associated factors: ozone removal in mechanical ventilation systems, ozone penetration through the building envelope, air-change rates, ozone loss rate on fixed indoor surfaces, ozone loss rate on human occupants, ozone loss by homogeneous reaction with nitrogen oxides, and ozone loss by reaction with gas-phase organics. Among these, the most important are air-change rates, ozone loss rate on fixed indoor surfaces, and, in densely occupied spaces, ozone loss rate on human occupants. Although most indoor ozone originates outdoors and enters with ventilation air, indoor emission sources can materially increase indoor ozone concentrations. Mitigation technologies to reduce indoor ozone concentrations are available or are being investigated. The most mature of these technologies, activated carbon filtration of mechanical ventilation supply air, shows a high modeled health-benefit to cost ratio when applied in densely occupied spaces.

Influence of Mechanical Ventilation Systems and Human Occupancy on Time-Resolved Source Rates of Volatile Skin Oil Ozonolysis Products in a LEED-Certified Office Building

Building mechanical ventilation systems are a major driver of indoor air chemistry as their design and operation influences indoor ozone (O₃) concentrations, the dilution and transport of indoor-generated volatile organic compounds (VOCs), and indoor environmental conditions. Real-time VOC and O₃ measurements were integrated with a building sensing platform to evaluate the influence of mechanical ventilation modes and human occupancy on the dynamics of skin oil ozonolysis products (SOOPs) in an office in a LEED-certified building during the winter. The ventilation system operated under variable recirculation ratios (RRs) from RR = 0 (100% outdoor air) to RR = 1 (100% recirculation air). Time-resolved source rates for 6-methyl-5-hepten-2-one (6-MHO), 4-oxopentanal (4-OPA), and decanal were highly dynamic and changed throughout the day with RR and occupancy. Total SOOP source rates during high-occupancy periods (10:00-18:00) varied from 2500-3000 μg h^-1 when RR = 0.1 to 6300-6700 μg h^-1 when RR = 1. Source rates for gas-phase reactions, outdoor air, and occupant-associated emissions generally decreased with increasing RR. The recirculation air source rate increased with RR and typically became the dominant source for RR > 0.5. SOOP emissions from surface reservoirs were also a prominent source, contributing 10-50% to total source rates. Elevated per person SOOP emission factors were observed, potentially due to multiple layers of soiled clothing worn during winter.

Kinetic multi-layer model of film formation, growth, and chemistry (KM-FILM): Boundary layer processes, multi-layer adsorption, bulk diffusion, and heterogeneous reactions

Large surface area-to-volume ratios indoors cause heterogeneous interactions to be especially important. Semi-volatile organic compounds can deposit on impermeable indoor surfaces forming thin organic films. We developed a new model to simulate the initial film formation by treating gas-phase diffusion and turbulence through a surface boundary layer and multi-layer reversible adsorption on rough surfaces, as well as subsequent film growth by resolving bulk diffusion and chemical reactions in a film. The model was applied with consistent parameters to reproduce twenty-one sets of film formation measurements due to multi-layer adsorption of multiple phthalates onto different indoor-relevant surfaces, showing that the films should initially be patchy with the formation of pyramid-like structures on the surface. Sensitivity tests showed that highly turbulent conditions can lead to the film growing by more than a factor of two compared to low turbulence conditions. If surface films adopt an ultra-viscous state with bulk diffusion coefficients of less than 10^-18  cm² s^-1 , a significant decrease in film growth is expected. The presence of chemical reactions in the film has the potential to increase the rate of film growth by nearly a factor of two.

Observing ozone chemistry in an occupied residence

Outdoor ozone transported indoors initiates oxidative chemistry, forming volatile organic products. The influence of ozone chemistry on indoor air composition has not been directly quantified in normally occupied residences. Here, we explore indoor ozone chemistry in a house in California with two adult inhabitants. We utilize space- and time-resolved measurements of ozone and volatile organic compounds (VOCs) acquired over an 8-wk summer campaign. Despite overall low indoor ozone concentrations (mean value of 4.3 ppb) and a relatively low indoor ozone decay constant (1.3 h^-1), we identified multiple VOCs exhibiting clear contributions from ozone-initiated chemistry indoors. These chemicals include 6-methyl-5-hepten-2-one (6-MHO), 4-oxopentanal (4-OPA), nonenal, and C8-C12 saturated aldehydes, which are among the commonly reported products from laboratory studies of ozone interactions with indoor surfaces and with human skin lipids. These VOCs together accounted for ≥12% molecular yield with respect to house-wide consumed ozone, with the highest net product yield for nonanal (≥3.5%), followed by 6-MHO (2.7%) and 4-OPA (2.6%). Although 6-MHO and 4-OPA are prominent ozonolysis products of skin lipids (specifically squalene), ozone reaction with the body envelopes of the two occupants in this house are insufficient to explain the observed yields. Relatedly, we observed that ozone-driven chemistry continued to produce 6-MHO and 4-OPA even after the occupants had been away from the house for 5 d. These observations provide evidence that skin lipids transferred to indoor surfaces made substantial contributions to ozone reactivity in the studied house.