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.

Model Framework for Predicting Semivolatile Organic Material Emissions Indoors from Organic Aerosol Measurements: Applications to HOMEChem Stir-Frying.

Cooking activities emit myriad low-volatility, semivolatile, and highly volatile organic compounds that together form particles that can accumulate to large indoor concentrations. Absorptive partitioning thermodynamics governs the particle-phase organic aerosol concentration mainly via temperature and sorbing mass impacts. Cooking activities can increase the organic sorbing mass by 1-2 orders of magnitude, increasing particle-phase concentrations and affecting emission rate calculations. Although recent studies have begun to probe the volatility characteristics of indoor cooking particles, parametrizations of cooking particle mass emissions have largely neglected these thermodynamic considerations. Here, we present an improved thermodynamics-based model framework for estimating condensable organic material emission rates from a time series of observed concentrations, given that adequate measurements or assumptions can be made about the volatility of the emitted species. We demonstrate the performance of this methodology by applying data from stir-frying experiments performed during the House Observations of Microbial and Environmental Chemistry (HOMEChem) campaign to a two-zone box model representing the UTest House. Preliminary estimates of organic mass emitted on a per-stir-fry basis for three types of organic aerosol factors are presented. Our analysis highlights that using traditional nonvolatile particle models and emission characterizations for some organic aerosol emitting activities can incorrectly attribute concentration changes to emissions rather than thermodynamic effects.

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.

Partitioning of reactive oxygen species from indoor surfaces to indoor aerosols.

Reactive oxygen species (ROS) are among the species thought to be responsible for the adverse health effects of particulate matter (PM) inhalation. Field studies suggest that indoor sources of ROS contribute to measured ROS on PM in indoor air. We hypothesize that ozone reacts on indoor surfaces to form semi-volatile ROS, in particular organic peroxides (OPX), which partition to airborne particles. To test this hypothesis, we modeled ozone-induced formation of OPX, its decay and its partitioning to PM in a residential building and compared the results to field measurements. Simulations indicate that, while ROS of outdoor origin is the primary contributor to indoor ROS (in PM), a substantial fraction of ROS present in indoor PM is from ozone-surface chemistry. At an air change rate equal to 1/h, and an outdoor ozone mixing ratio of 35 ppb, 25% of the ROS concentration in air is due to indoor formation and partitioning of OPX to PM. For the same conditions, but with a modest indoor source of PM (1.5 mg h-1), 44% of indoor ROS on PM is of indoor origin. An indoor source of ozone, such as an electrostatic air cleaner, also increases OPX present in indoor PM. The results of the simulations support the hypothesis that ozone-induced formation of OPX on indoor surfaces, and subsequent partitioning to aerosols, is sufficient to explain field observations. Therefore, indoor sourced ROS could contribute meaningfully to total inhaled PM-ROS.

Phase state of organic aerosols may limit temperature-driven thermodynamic repartitioning following outdoor-to-indoor transport.

Ambient aerosols often experience temperature and humidity gradients following outdoor-to-indoor transport, causing organic aerosols (OA) to either gain or lose mass via gas-particle repartitioning. Recent models have sought to quantify these effects using equilibrium partitioning thermodynamics. However, evidence suggests some indoor OA may possess glassy or semisolid phase states with higher viscosities than liquid OA. Characteristic partitioning timescales of higher-viscosity particles are significantly longer than for liquid particles, which may either fully or partially inhibit repartitioning. For outdoor OA experiencing a temperature change during transport indoors, the ultimate repartitioning state depends on the relationship between the gas-particle partitioning rate coefficient (kgp) of semivolatile organics and the indoor particle loss rate coefficient (lp). That is, thermodynamic equilibrium partitioning may occur when semivolatile kgp ≫ lp, no repartitioning when semivolatile kgp ≪ lp, and partial repartitioning when their magnitudes are similar. Longer indoor particle lifetimes, higher particle number, and larger particle sizes all raise kgp (driving repartitioning towards equilibrium). For simulated U.S. residences, equilibrium condensation was likely reached in humid climate zones during warm meteorological conditions. In colder regions, the degree of evaporative repartitioning depended on whether organics could repartition before the particle phase state adjusts to indoor conditions, which is uncertain. When an appreciable temperature gradient exists, this study not only confirmed that all outdoor-originating OA that is liquid indoors will reach thermodynamic equilibrium, but also concluded that a plurality (46% for this domain) of such OA that is semisolid may also achieve thermodynamic equilibrium during its indoor lifetime.

Simulating indoor inorganic aerosols of outdoor origin with the inorganic aerosol thermodynamic equilibrium model ISORROPIA.

Outdoor aerosols can transform and have their composition altered upon transport indoors. Herein, IMAGES, a platform that simulates indoor organic aerosol with the 2-dimensional volatility basis set (2D-VBS), was extended to incorporate the inorganic aerosol thermodynamic equilibrium model, ISORROPIA. The model performance was evaluated by comparing aerosol component predictions to indoor measurements from an aerosol mass spectrometer taken during the summer and winter seasons. Since ammonia was not measured in the validation dataset, outdoor ammonia was estimated from aerosol measurements using a novel pH-based algorithm, while nitric acid was held constant. Modeled indoor ammonia sources included temperature-based occupant and surface emissions. Sensitivity to the nitric acid indoor surface deposition rate ß g , HNO 3 , g was explored by varying it in model runs, which did not affect modeled sulfate due to its non-volatile nature, though the fitting of a filter efficiency was required for good correlations of modeled sulfate with measurements in both seasons. Modeled summertime nitrate well-matched measured observations when ß g , HNO 3 , g = 2.75 h - 1 , but wintertime comparisons were poor, possibly due to missing thermodynamic processes within the heating, ventilating, and air-conditioning (HVAC) system. Ammonium was consistently overpredicted, potentially due to neglecting thirdhand smoke impacts observed in the field campaign, as well as HVAC impacts.

Improving Predictions of Indoor Aerosol Concentrations of Outdoor Origin by Considering the Phase Change of Semivolatile Material Driven by Temperature and Mass-Loading Gradients.

Outdoor aerosols experience environmental changes as they are transported indoors, including outdoor-to-indoor temperature and mass-loading gradients, which can reduce or enhance their indoor concentrations due to repartitioning driven by changes in thermodynamic equilibrium states. However, the complexity required to model repartitioning typically hinders its inclusion in studies predicting indoor exposure to ambient aerosols. To facilitate exposure predictions, this work used an explicit thermodynamic indoor aerosol model to simulate outdoor-to-indoor aerosol repartitioning typical for residential and office buildings across the 16 U.S. climate zones over an annual time horizon. Results demonstrate that neglecting repartitioning when predicting indoor concentrations can produce errors of up to 80-100% for hydrocarbon-like organic aerosol, 40-60% for total organic aerosol, 400% for ammonium nitrate, and 60% (typically 3 µg/m3) for the total PM2.5 aerosol. Underpredictions were more likely for buildings in hotter than colder regions, and for residences than offices, since both cooler indoor air and more meaningful residential organic aerosol concentrations encourage condensation of semivolatile organics. Furthermore, a method for computing correction factors to more easily account for thermodynamic repartitioning is provided. Applying these correction factors to mechanical-only aerosol predictions significantly reduced errors to <0.5 µg/m3 for the total indoor PM2.5 while bypassing explicit thermodynamic simulations.

Indoor aerosol water content and phase state in U.S. residences: impacts of relative humidity, aerosol mass and composition, and mechanical system operation.

Hygroscopic particulate matter (PM) constituents promote uptake of aerosol water (AW), depending on relative humidity (RH), which can constrain qualities such as organic aerosol (OA) phase state and inorganic aerosol (IA) deliquescence and efflorescence. This work provides a first incorporation of AW predictions into residential indoor PM simulations. The indoor model, IMAGES, which simulates factored OA concentrations and thermodynamics using the 2D-volatility basis set, was expanded to predict speciated IA concentrations, AW with κ-Köhler theory of hygroscopic growth, and OA phase state with glass transition temperatures. Since RH is the largest driver of AW and varies with meteorology, simulations were conducted using a database of historical ambient weather and pollution records spanning the sixteen U.S. climate zones, facilitating assessment of seasonal and regional trends. Over this diverse simulation set, the residential indoor AW mass was ∼10 to 100 times smaller than dry PM mass. This relative AW amount indoors was about ∼10 times smaller than outdoors, since indoor-emitted aerosol is likely less hygroscopic. The indoor OA phase state was typically semisolid, suggesting kinetic limitations might inhibit thermodynamic OA partitioning equilibrium from being established indoors. Residences in hot and humid climates during the summertime may have liquid indoor OA, while amorphous solid indoor OA can exist in cold climates. Deliquescence and efflorescence of recirculated IA within HVAC systems during cooling or heating, respectively, was also modeled. Oftentimes, two IA populations with different histories existing as wet or dry aerosol were generated by HVAC operation depending on indoor and outdoor environmental conditions and the HVAC operating mode.

Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies.

Potted plants have demonstrated abilities to remove airborne volatile organic compounds (VOC) in small, sealed chambers over timescales of many hours or days. Claims have subsequently been made suggesting that potted plants may reduce indoor VOC concentrations. These potted plant chamber studies reported outcomes using various metrics, often not directly applicable to contextualizing plants' impacts on indoor VOC loads. To assess potential impacts, 12 published studies of chamber experiments were reviewed, and 196 experimental results were translated into clean air delivery rates (CADR, m3/h), which is an air cleaner metric that can be normalized by volume to parameterize first-order loss indoors. The distribution of single-plant CADR spanned orders of magnitude, with a median of 0.023 m3/h, necessitating the placement of 10-1000 plants/m2 of a building's floor space for the combined VOC-removing ability by potted plants to achieve the same removal rate that outdoor-to-indoor air exchange already provides in typical buildings (~1 h-1). Future experiments should shift the focus from potted plants' (in)abilities to passively clean indoor air, and instead investigate VOC uptake mechanisms, alternative biofiltration technologies, biophilic productivity and well-being benefits, or negative impacts of other plant-sourced emissions, which must be assessed by rigorous field work accounting for important indoor processes.

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.

Predicting the importance of oxidative aging on indoor organic aerosol concentrations using the two-dimensional volatility basis set (2D-VBS).

Organic aerosol (OA) is chemically dynamic, continuously evolving by oxidative chemistry, for instance, via hydroxyl radical (OH) reactions. Studies have explored this evolution (so-called OA aging) in the atmosphere, but none have investigated it indoors. Aging organic molecules in both particle and gas-phases undergo changes in oxygen content and volatility, which may ultimately either enhance or reduce the condensed-phase OA concentration (COA ). This work models OH-induced aging using the two-dimensional volatility basis set (2D-VBS) within an indoor model and explores its significance on COA relative to prior modeling methodologies which neglect aging transformations. Lagrangian, time-averaged, and transient indoor simulations were conducted. The time-averaged simulations included a Monte Carlo procedure and sensitivity analysis, using input distributions typical of U.S. residences. Results demonstrate that indoors, aging generally leads to COA augmentation. The extent to which this is significant is conditional upon several factors, most notably temperature, OH exposure, and OA mass loading. Time-averaged COA was affected minimally in typical residences (<5% increase). However, some plausible cases may cause stronger COA enhancements, such as in a sunlit room where photolysis facilitates significant OH production (~20% increase), or during a transient OH-producing cleaning event (~35% peak increase).