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.

Design and quantitative evaluation of 'Aerosol Bio-Containment Device (ABCD)' for reducing aerosol exposure during infectious aerosol-generating events

The Coronavirus Disease 2019 (COVID-19) pandemic renewed interest in infectious aerosols and reducing risk of airborne respiratory pathogen transmission, prompting development of devices to protect healthcare workers during airway procedures. However, there are no standard methods for assessing the efficacy of particle containment with these protective devices. We designed and built an aerosol bio-containment device (ABCD) to contain and remove aerosol via an external suction system and tested the aerosol containment of the device in an environmental chamber using a novel, quantitative assessment method. The ABCD exhibited a strong ability to control aerosol exposure in experimental and computational fluid dynamic (CFD) simulated scenarios with appropriate suction use and maintenance of device seals. Using a log-risk-reduction framework, we assessed device containment efficacy and showed that, when combined with other protective equipment, the ABCD can significantly reduce airborne clinical exposure. We propose this type of quantitative analysis serves as a basis for rating efficacy of aerosol protective enclosures.

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 (k_gp) of semivolatile organics and the indoor particle loss rate coefficient (l_p). That is, thermodynamic equilibrium partitioning may occur when semivolatile k_gp ≫ l_p, no repartitioning when semivolatile k_gp ≪ l_p, and partial repartitioning when their magnitudes are similar. Longer indoor particle lifetimes, higher particle number, and larger particle sizes all raise k_gp (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 β 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 β 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 PM_2.5 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 PM_2.5 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.

Review of indoor aerosol generation, transport, and control in the context of COVID-19

The coronavirus disease-2019 (COVID-19) pandemic has heightened the awareness of aerosol generation by human expiratory events and their potential role in viral respiratory disease transmission. Concerns over high severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) viral burden of mucosal surfaces has raised questions about the aerosol-generating potential and dangers of many otorhinolaryngologic procedures. However, the risks of aerosol generation and associated viral transmission by droplet or airborne routes for many otorhinolaryngology procedures are largely unknown. Indoor aerosol and droplet viral respiratory transmission risk is influenced by 4 factors: (1) aerosol or droplet properties; (2) indoor airflow; (3) virus-specific factors; and (4) host-specific factors. Herein we elaborate on known aerosol vs droplet properties, indoor airflow, and aerosol-generating events to provide context for risks of aerosol infectious transmission. We also provide simple but typically effective measures for mitigating the spread and inhalation of viral aerosols in indoor settings. Understanding principles of infectious transmission, aerosol and droplet generation, as well as concepts of indoor airflow, will assist in the integration of new data on SARS-CoV-2 transmission and activities that can generate aerosol to best inform on the need for escalation or de-escalation from current societal and institutional guidelines for protection during aerosol-generating procedures.

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.

Chemical and Physical Characterization of 3D Printer Aerosol Emissions with and without a Filter Attachment

Fused filament fabrication three-dimensional (3D) printers have been shown to emit ultrafine particles (UFPs) and volatile organic compounds (VOCs). Previous studies have quantified bulk 3D printer particle and VOC emission rates, as well as described particle chemical composition via ex situ analysis. Here, we present size-resolved aerosol composition measurements from in situ aerosol mass spectrometry and ex situ transmission electron microscopy (TEM). Particles were sampled for in situ analysis during acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) 3D printing activities and ex situ analysis during ABS printing. We examined the effect of a high-efficiency particulate air filter attachment on ABS emissions and particle chemical composition and demonstrate that filtration was effective in preventing UFP emissions and that particles sampled during filtered prints did not have a high contribution (∼4% vs ∼10%) from aromatic ions in the mass spectrum. Ex situ analysis of particles collected during ABS printing was performed via TEM and electron energy loss spectroscopy, which indicated a high level of sp² bonding type consistent with polymeric styrene. One 3D print with PLA resulted in an aerosol mass size distribution with a peak at ∼300 nm. Unfiltered ABS prints resulted in particle mass size distributions with peak diameters of ∼100 nm.

Correction: Seasonal variation in aerosol composition and concentration upon transport from the outdoor to indoor environment

Correction for 'Seasonal variation in aerosol composition and concentration upon transport from the outdoor to indoor environment' by Anita M. Avery et al., Environ. Sci.: Processes Impacts, 2019, 21, 528-547.

Human occupant contribution to secondary aerosol mass in the indoor environment

Humans impact indoor air quality directly via emissions from skin, breath, or personal care products, and indirectly via reactions of oxidants with skin constituents, or with skin that has been shed. However, separating the influence of the many emissions and their oxidation products from the influence of outdoor-originated aerosols has been a challenge. Indoor and outdoor aerosols were alternatively sampled at 4 minute time resolution with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) in a classroom with student occupants at regular intervals per university class schedule. Mass spectral analysis showed aerosol enhancements of oxidized and unoxidized hydrocarbon ion families during occupied periods, especially at ion fragments larger than m/z 100 and double bond equivalents consistent with squalene (C30H50) and its oxidized products from reaction with ozone, indicative of the secondary nature of the aerosol mass. Individual hydrocarbon mass fragments consistent with squalene fragmentation, including C5H9+, and C6H9+ were especially enhanced with room occupancy. Emissions of individual organic fragment ions were estimated using a model accounting for outdoor aerosols and air exchange. This showed occupancy related emissions at smaller fragments (C3H5+, C4H9+) that despite reflecting mostly outdoor-originated aerosols transported indoors, also show enhancements from occupant emissions indoors. Total emission of all fragments was 17.6 μg β-1 h-1 above unoccupied levels, translating to approximately 25% increase in organic aerosol mass concentration in the classroom during an occupied hour with a median occupied ozone loss (β). Human occupants, therefore, represent an additional mass burden of organic aerosol, especially in poorly ventilated or highly occupied indoor spaces.

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.

Observations and Contributions of Real-Time Indoor Ammonia Concentrations during HOMEChem

Although ammonia (NH₃) is usually found at outdoor concentrations of 1-5 ppb, indoor ammonia concentrations can be much higher. Indoor ammonia is strongly emitted from cleaning products, tobacco smoke, building materials, and humans. Because of ammonia's high reactivity, solubility in water, and tendency to sorb to a variety of surfaces, it is difficult to measure, and thus a comprehensive evaluation of indoor ammonia concentrations remains an understudied topic. During HOMEChem, which was a comprehensive indoor chemistry study occurring in a test house during June 2018, the real-time concentration of ammonia indoors was measured using cavity ring-down spectroscopy. A mean unoccupied background concentration of 32 ppb was observed, with further enhancements of ammonia occurring during cooking, cleaning, and occupancy activities, reaching maximum concentrations during these activities of 130, 1592, and 99 ppb, respectively. Furthermore, ammonia concentrations were strongly influenced by indoor temperatures and heating, ventilation, and air conditioning (HVAC) operation. In the absence of activity-based sources, the HVAC operation was the main modulator of ammonia concentration indoors.

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 (C_OA ). This work models OH-induced aging using the two-dimensional volatility basis set (2D-VBS) within an indoor model and explores its significance on C_OA 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 C_OA augmentation. The extent to which this is significant is conditional upon several factors, most notably temperature, OH exposure, and OA mass loading. Time-averaged C_OA was affected minimally in typical residences (<5% increase). However, some plausible cases may cause stronger C_OA 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).

Seasonal variation in aerosol composition and concentration upon transport from the outdoor to indoor environment

Outdoor-originated aerosols are an important component impacting indoor air quality. Since outdoor aerosols vary over short (diurnal) and long (seasonal) timescales, we examined how the variation in outdoor aerosol concentration and composition impact indoor aerosol. Measurements of both indoor and outdoor aerosol composition in real time in an urban classroom in winter and summer seasons were performed using an aerosol mass spectrometer (AMS), aethalometer, and a suite of gas phase instruments. Factor analysis of the organic aerosol components identified three factors in common between seasons, including hydrocarbon-like, cooking, and oxidized organic aerosol (HOA, COA, and OOA). Since sulfate is non-volatile, we report a sulfate-normalized indoor-outdoor ratio (I/O)i/SO4 for measured aerosol i components, allowing us to estimate aerosol component-based effects of seasonal and other variations in ventilation and HVAC operation, indoor emission sources, and chemically-based loss processes between outdoor and indoor environments. These chemical loss processes are interpreted in terms of changes in temperature and relative humidity (RH) between environments, which fluctuate on a daily and seasonal basis. The degree to which any effect is observed depends on the particular outdoor aerosol population and the magnitude of temperature or RH change. In wintertime, when aerosols were warmed upon transport indoors and loss of volatile components is favored, median (I/O)i/SO4 values for nitrate, total organics, HOA, and BC were smaller (0.35, 1.00, 1.24, and 1.18, respectively) than summertime values (0.75, 1.17, 1.96, and 1.80). For COA and OOA, however, (I/O)i/SO4 values were higher in the winter than in summer. Calculated aerosol liquid water (ALW), which is a function of temperature and RH and the relative contribution of hygroscopic components, varied significantly by season. Summertime ALW indoors provides a medium for aqueous processing, which is necessary for some hydrophilic gas phase reaction products that are important to indoor air quality and occupant exposure. This work describes the linkages between seasonal variability in aerosol composition outdoors and the subsequent chemically-specific variation observed when that aerosol is brought indoors.

Outcome-based ventilation: A framework for assessing performance, health, and energy impacts to inform office building ventilation decisions

This article presents an outcome-based ventilation (OBV) framework, which combines competing ventilation impacts into a monetized loss function ($/occ/h) used to inform ventilation rate decisions. The OBV framework, developed for U.S. offices, considers six outcomes of increasing ventilation: profitable outcomes realized from improvements in occupant work performance and sick leave absenteeism; health outcomes from occupant exposure to outdoor fine particles and ozone; and energy outcomes from electricity and natural gas usage. We used the literature to set low, medium, and high reference values for OBV loss function parameters, and evaluated the framework and outcome-based ventilation rates using a simulated U.S. office stock dataset and a case study in New York City. With parameters for all outcomes set at medium values derived from literature-based central estimates, higher ventilation rates' profitable benefits dominated negative health and energy impacts, and the OBV framework suggested ventilation should be ≥45 L/s/occ, much higher than the baseline ~8.5 L/s/occ rate prescribed by ASHRAE 62.1. Only when combining very low parameter estimates for profitable impacts with very high ones for health and energy impacts were all outcomes on the same order. Even then, however, outcome-based ventilation rates were often twice the baseline rate or more.

Thirdhand smoke uptake to aerosol particles in the indoor environment

Aerosol composition measurements made in an indoor classroom indicate the uptake of thirdhand smoke (THS) species to indoor particles, a novel exposure route for THS to humans indoors. Chemical speciation of the organic aerosol fraction using mass spectrometric data and factor analysis identified a reduced nitrogen component, predominantly found in the indoor environment, contributing 29% of the indoor submicron aerosol mass. We identify this factor as THS compounds partitioning from interior surfaces to gas phase and then aerosol phase. Partitioning of THS vapors to aerosols requires an aqueous phase for reactive uptake of the reduced nitrogen species (RdNS), leading to seasonal differences in THS concentration indoors. RdNS protonate under the acidic conditions expected for indoor aerosols of outdoor origin. Controlled laboratory measurements performed using cigarette smoke deposited into a Pyrex vessel showed a similar partitioning behavior to aerosol of outdoor origin and mass spectral features comparable to the measured indoor THS factor after 1 week of residence time in the closed vessel. This study reports a new, potentially large THS exposure route from partitioning of surface volatile organic compounds into the aerosol phase and subsequent dispersion in a mechanically ventilated building.

Reactive indoor air chemistry and health-A workshop summary

The chemical composition of indoor air changes due to the reactive nature of the indoor environment. Historically, only the stable parent compounds were investigated due to their ease of measurement by conventional methods. Today, however, scientists can better characterize oxidation products (gas and particulate-phase) formed by indoor chemistry. An understanding of occupant exposure can be developed through the investigation of indoor oxidants, the use of derivatization techniques, atmospheric pressure detection, the development of real-time technologies, and improved complex modeling techniques. Moreover, the connection between exposure and health effects is now receiving more attention from the research community. Nevertheless, a need still exists for improved understanding of the possible link between indoor air chemistry and observed acute or chronic health effects and long-term effects such as work-related asthma.

Airborne particles in indoor environment of homes, schools, offices and aged care facilities: The main routes of exposure

It has been shown that the exposure to airborne particulate matter is one of the most significant environmental risks people face. Since indoor environment is where people spend the majority of time, in order to protect against this risk, the origin of the particles needs to be understood: do they come from indoor, outdoor sources or both? Further, this question needs to be answered separately for each of the PM mass/number size fractions, as they originate from different sources. Numerous studies have been conducted for specific indoor environments or under specific setting. Here our aim was to go beyond the specifics of individual studies, and to explore, based on pooled data from the literature, whether there are generalizable trends in routes of exposure at homes, schools and day cares, offices and aged care facilities. To do this, we quantified the overall 24h and occupancy weighted means of PM₁₀, PM_2.5 and PN - particle number concentration. Based on this, we developed a summary of the indoor versus outdoor origin of indoor particles and compared the means to the WHO guidelines (for PM₁₀ and PM_2.5) and to the typical levels reported for urban environments (PN). We showed that the main origins of particle metrics differ from one type of indoor environment to another. For homes, outdoor air is the main origin of PM₁₀ and PM_2.5 but PN originate from indoor sources; for schools and day cares, outdoor air is the source of PN while PM₁₀ and PM_2.5 have indoor sources; and for offices, outdoor air is the source of all three particle size fractions. While each individual building is different, leading to differences in exposure and ideally necessitating its own assessment (which is very rarely done), our findings point to the existence of generalizable trends for the main types of indoor environments where people spend time, and therefore to the type of prevention measures which need to be considered in general for these environments.

Real-time transformation of outdoor aerosol components upon transport indoors measured with aerosol mass spectrometry

Outdoor aerosols are transported indoors, where their component concentrations depend on aerosol size, physiochemical properties, indoor sources and losses, and cross-environment gradients of temperature and relative humidity. We explored these dependencies by measuring real-time outdoor and indoor non-refractory, submicron (PM₁ ) aerosol component mass concentrations in a mixed-use laboratory space with an Aerodyne mini-aerosol mass spectrometer (AMS) and black carbon (BC) with an aethalometer. The median indoor/outdoor (I/O) ratios were 0.60 for sulfate, 0.25 for nitrate, 0.52 for ammonium, 0.73 for organics, and 0.61 for BC. Positive matrix factorization (PMF) on organic aerosol data identified hydrocarbon-like (HOA), cooking (COA), and oxygenated (OOA) factors. By assuming sulfate was nonvolatile, lost only by mechanical processes, and without indoor sources, the transformations of other components i due to partitioning changes or indoor sources were parameterized by normalizing their I/O ratios by sulfate's I/O ratio, that is, (I/O)_i/SO4 . Component-specific behavior was quantified by regressions of (I/O)_i/SO4 to outdoor-to-indoor temperature differences. Nitrate and HOA strongly and OOA weakly showed losses with increasing temperatures indoors vs. outdoors, and HOA likely had an indoor source. To our knowledge, this is the first reported deployment of an AMS to analyze real-time indoor aerosol composition and outdoor-to-indoor transformation.

Secondary organic aerosol formation initiated by α-terpineol ozonolysis in indoor air

Secondary organic aerosol (SOA) owing to reactive organic gas (ROG) ozonolysis can be an important indoor particle source. However, SOA formation owing to ozonolysis of α-terpineol, which is emitted by consumer product usage and reacts strongly with ozone, has not been systematically quantified. Therefore, we conducted 21 experiments to investigate the SOA formation initiated by α-terpineol ozonolysis for high (0.84 h^-1 ), moderate (0.61 h^-1 ), and low (0.36 h^-1 ) air exchange rates (AER), which is the frequency with which indoor is replaced by outdoor air. α-Terpineol concentrations of 6.39 to 226 ppb were combined with high ozone (~25 ppm) to ensure rapid and complete ozonolysis. No reactants were replenished, so SOA peaked quickly and then decreased due to AER and surface losses, and peak SOA ranged from 2.03 to 281 μg/m³ at unit density. SOA mass formation was parameterized with the aerosol mass fraction (AMF), a.k.a. the SOA yield, and AMFs ranged from 0.056 to 0.24. The AMFs strongly and positively correlated with reacted α-terpineol, whereas they weakly and negatively correlated with higher AERs. One-product, two-product, and volatility basis set (VBS) models were fit to the AMF data. Predictive modeling demonstrated that α-terpineol ozonolysis could meaningfully form SOA in indoor air.

Do time-averaged, whole-building, effective volatile organic compound (VOC) emissions depend on the air exchange rate? A statistical analysis of trends for 46 VOCs in U.S. offices

We used existing data to develop distributions of time-averaged air exchange rates (AER), whole-building 'effective' emission rates of volatile organic compounds (VOC), and other variables for use in Monte Carlo analyses of U.S. offices. With these, we explored whether long-term VOC emission rates were related to the AER over the sector, as has been observed in the short term for some VOCs in single buildings. We fit and compared two statistical models to the data. In the independent emissions model (IEM), emissions were unaffected by other variables, while in the dependent emissions model (DEM), emissions responded to the AER via coupling through a conceptual boundary layer between the air and a lumped emission source. For 20 of 46 VOCs, the DEM was preferable to the IEM and emission rates, though variable, were higher in buildings with higher AERs. Most oxygenated VOCs and some alkanes were well fit by the DEM, while nearly all aromatics and halocarbons were independent. Trends by vapor pressure suggested multiple mechanisms could be involved. The factors of temperature, relative humidity, and building age were almost never associated with effective emission rates. Our findings suggest that effective emissions in real commercial buildings will be difficult to predict from deterministic experiments or models.

Perceptions in the U.S. building industry of the benefits and costs of improving indoor air quality

How building stakeholders (e.g. owners, tenants, operators, and designers) understand impacts of Indoor Air Quality (IAQ) and associated energy costs is unknown. We surveyed 112 stakeholders across the United States to ascertain their perceptions of their current IAQ and estimates of benefits and costs of, as well as willingness to pay for, IAQ improvements. Respondents' perceived IAQ scores correlated with the use of high-efficiency filters but not with any other IAQ-improving technologies. We elicited their estimates of the impacts of a ventilation-filtration upgrade (VFU), that is, doubling the ventilation rate from 20 to 40 cfm/person (9.5 to 19 l/s/person) and upgrading from a minimum efficiency reporting value 6 to 11 filter, and compared responses to estimates derived from IAQ literature and energy modeling. Minorities of respondents thought the VFU would positively impact productivity (45%), absenteeism (23%), or health (39%). Respondents' annual VFU cost estimates (mean = $257, s.d. = $496, median = $75 per person) were much higher than ours (always <$32 per person), and the only yearly cost a plurality of respondents said they would pay for the VFU was $15 per person. Respondents holding green building credentials were not more likely to affirm the IAQ benefits of the VFU and were less likely to be willing to pay for it.

Volatile organic compound conversion by ozone, hydroxyl radicals, and nitrate radicals in residential indoor air: Magnitudes and impacts of oxidant sources

Indoor chemistry may be initiated by reactions of ozone (O₃), the hydroxyl radical (OH), or the nitrate radical (NO₃) with volatile organic compounds (VOC). The principal indoor source of O₃ is air exchange, while OH and NO₃ formation are considered as primarily from O₃ reactions with alkenes and nitrogen dioxide (NO₂), respectively. Herein, we used time-averaged models for residences to predict O₃, OH, and NO₃ concentrations and their impacts on conversion of typical residential VOC profiles, within a Monte Carlo framework that varied inputs probabilistically. We accounted for established oxidant sources, as well as explored the importance of two newly realized indoor sources: (i) the photolysis of nitrous acid (HONO) indoors to generate OH and (ii) the reaction of stabilized Criegee intermediates (SCI) with NO₂ to generate NO₃. We found total VOC conversion to be dominated by reactions both with O₃, which almost solely reacted with d-limonene, and also with OH, which reacted with d-limonene, other terpenes, alcohols, aldehydes, and aromatics. VOC oxidation rates increased with air exchange, outdoor O₃, NO₂ and d-limonene sources, and indoor photolysis rates; and they decreased with O₃ deposition and nitric oxide (NO) sources. Photolysis was a strong OH formation mechanism for high NO, NO₂, and HONO settings, but SCI/NO₂ reactions weakly generated NO₃ except for only a few cases.

Secondary organic aerosol in residences: predicting its fraction of fine particle mass and determinants of formation strength

Indoor secondary organic aerosol (SOA) formation may contribute to particle concentrations within residences, but little systematic work has investigated its magnitude or the determinants of its formation. This work uses a time-averaged modeling approach to predict the indoor SOA mass formed in residences due to the oxidation of 66 reactive organic compounds by ozone or the hydroxyl radical, parameterizing SOA formation with the aerosol mass fraction. Other organic and inorganic aerosols owing to outdoor and indoor sources were also predicted. Model inputs were represented as distributions within a Monte Carlo analysis, so that result distributions and sensitivity of results to inputs could be quantified, using a dataset developed from the study of Relationships between Indoor, Outdoor and Personal Air and other sources. SOA comprised a large amount of indoor organic and total fine particles for a subset of the results (e.g., >47% of indoor organic and >30% of fine aerosol for 10% of the modeled cases), but was often a small fraction. The sensitivity analysis revealed that SOA formation is driven by high terpene emission rates (particularly by d-limonene) and outdoor ozone, along with low air exchange and ozone and particle deposition rates.

PRACTICAL IMPLICATIONS

This study predicts that indoor SOA formation can be a substantial fraction of indoor aerosols in residences, for certain combinations of building and reactant parameters. The model herein can predict SOA for risk analyses or be used to design experiments to study indoor SOA formation. The terpene, d-limonene, contributes by far the most to formation, and eliminating this particular compound indoors would be impactful on indoor aerosol concentrations.

Transient secondary organic aerosol formation from limonene ozonolysis in indoor environments: impacts of air exchange rates and initial concentration ratios

Secondary organic aerosol (SOA) results from the oxidation of reactive organic gases (ROGs) and is an indoor particle source. The aerosol mass fraction (AMF), a.k.a. SOA yield, quantifies the SOA forming potential of ROGs and is the ratio of generated SOA to oxidized ROG. The AMF depends on the organic aerosol concentration, as well as the prevalence of later generation reactions. AMFs have been measured in unventilated chambers or steady-state flow through chambers. However, indoor settings have outdoor air exchange, and indoor SOA formation often occurs when ROGs are transiently emitted, for instance from emissions of cleaning products. Herein, we quantify "transient AMFs" from ozonolysis of pulse-emitted limonene in a ventilated chamber, for 18 experiments at low (0.28 h(-1)), moderate (0.53 h(-1)), and high (0.96 h(-1)) air exchange rates (AER) with varying initial ozone-limonene ratios. Transient AMFs increased with the amount of ROG reacted; AMFs also increased with decreasing AERs and increasing initial ozone-limonene ratios, which together likely promoted more ozone reactions with the remaining exocyclic bond of oxidized limonene products in the SOA phase. Knowing the AER and initial ozone-limonene ratio is crucial to predict indoor transient SOA behavior accurately.

Indoor secondary organic aerosol formation initiated from reactions between ozone and surface-sorbed D-limonene

Reactions between ozone and terpenoids produce numerous products, some of which may form secondary organic aerosol (SOA). This work investigated the contribution to gas-phase SOA formation of ozone reactions with surface-sorbed D-limonene, which is common indoors. A model framework was developed to predict SOA mass formation because of ozone/terpenoid surface reactions, and it was used with steady state experiments in a 283 L chamber to determine the aerosol mass fraction of SOA resulting from surface reactions, ξs (the ratio of mass of SOA formed and mass of ozone consumed by ozone/terpenoid surface reactions), for ozone/D-limonene reactions on stainless steel. The ξs = 0.70-0.91, with lower relative humidity leading to both higher mass and number formation. Also, surface reactions promoted nucleation more than gas-phase reactions, and number formation due to surface reactions and gas-phase reactions were 126-339 and 51.1-60.2 no./cm(3) per μg/m(3) of formed SOA, respectively. We also used the model framework to predict that indoor spaces in which ozone/D-limonene surface reactions would likely lead to meaningful gas-phase SOA formation are those with surfaces that have low original reactivity with ozone, such as glass, sealed materials, or smooth metals.

Indoor secondary organic aerosol formation initiated from reactions between ozone and surface-sorbed D-limonene

Reactions between ozone and terpenoids produce numerous products, some of which may form secondary organic aerosol (SOA). This work investigated the contribution to gas-phase SOA formation of ozone reactions with surface-sorbed D-limonene, which is common indoors. A model framework was developed to predict SOA mass formation because of ozone/terpenoid surface reactions, and it was used with steady state experiments in a 283 L chamber to determine the aerosol mass fraction of SOA resulting from surface reactions, ξs (the ratio of mass of SOA formed and mass of ozone consumed by ozone/terpenoid surface reactions), for ozone/D-limonene reactions on stainless steel. The ξs = 0.70-0.91, with lower relative humidity leading to both higher mass and number formation. Also, surface reactions promoted nucleation more than gas-phase reactions, and number formation due to surface reactions and gas-phase reactions were 126-339 and 51.1-60.2 no./cm(3) per μg/m(3) of formed SOA, respectively. We also used the model framework to predict that indoor spaces in which ozone/D-limonene surface reactions would likely lead to meaningful gas-phase SOA formation are those with surfaces that have low original reactivity with ozone, such as glass, sealed materials, or smooth metals.

Predicting secondary organic aerosol formation from terpenoid ozonolysis with varying yields in indoor environments

The ozonolysis of terpenoids generates secondary organic aerosol (SOA) indoors. Models of varying complexity have been used to predict indoor SOA formation, and many models use the SOA yield, which is the ratio of the mass of produced SOA and the mass of consumed reactive organic gas. For indoor simulations, the SOA yield has been assumed as a constant, even though it depends on the concentration of organic particles in the air, including any formed SOA. We developed two indoor SOA formation models for single terpenoid ozonolysis, with yields that vary with the organic particle concentration. The models have their own strengths and were in agreement with published experiments for d-limonene ozonolysis. Monte Carlo analyses were performed, which simulated different residential and office environments to estimate ranges of SOA concentrations and yields for d-limonene and α-pinene ozonolysis occurring indoors. Results indicate that yields are highly variable indoors and are most influenced by background organic particles for steady-state formation and indoor ozone concentration for transient peak formation. Additionally, a review of ozonolysis yields for indoor-relevant terpenoids in the literature revealed much uncertainty in their values at low concentrations typical of indoors.

PRACTICAL IMPLICATIONS

The results in this study suggest important factors that govern indoor secondary organic aerosol (SOA) formation and yields, in typical residential and office spaces. This knowledge informs the development and comparison of control strategies to reduce indoor-generated SOA. The ranges of SOA concentrations predicted indoors allow the quantification of the effects of sorptive interactions of semi-volatile organic compounds or reactive oxygen species with SOA, filter loading owing to SOA formation, and impacts of SOA on health, if links are established.

The effect of an ion generator on indoor air quality in a residential room

Ion generators charge particles with a corona prior to their removal on collector plates or indoor surfaces and also emit ozone, which can react with terpenes to yield secondary organic aerosol, carbonyls, carboxylic acids, and free radicals. This study characterized the indoor air quality implications of operating an ion generator in a 27 m(3) residential room, with four different test room configurations. Two room configurations had carpet overlaying the original flooring of stained/sealed concrete, and for one configuration with and without carpet, a plug-in air freshener was used as a terpene source. Measurements included airborne sampling of particulate matter (0.015-20 μm), terpenes and C(1) -C(4) and C(6) -C(10) aldehydes, ozone concentrations, and air exchange rates. When the heating, ventilating, and air-conditioning system was not operating (room air exchange rate = ∼0.5/h), the use of the ion generator in the presence of the air freshener led to a net increase in ultrafine particles (<0.1 μm). Also, increased concentrations of ozone were observed regardless of air freshener presence, as well as increases in formaldehyde and nonanal, albeit within measurement uncertainty in some cases. Thus, it may be prudent to limit ion generator use indoors until evidence of safety can be ascertained.

PRACTICAL IMPLICATIONS

Portable ion generators are intended to clean the air of particles, but they may emit ozone as a byproduct of their operation, which has the potential to degrade indoor air quality. This study showed that under certain conditions in a residential room, the use of a portable ion generator can increase concentrations of ozone and, to a lesser degree, potentially aldehydes. Also, if operated in the presence of a plug-in air freshener that emits terpenes, its use can increase concentrations of secondary organic aerosol in the ultrafine size range.

Particle loading rates for HVAC filters, heat exchangers, and ducts

The rate at which airborne particulate matter deposits onto heating, ventilation, and air-conditioning (HVAC) components is important from both indoor air quality (IAQ) and energy perspectives. This modeling study predicts size-resolved particle mass loading rates for residential and commercial filters, heat exchangers (i.e. coils), and supply and return ducts. A parametric analysis evaluated the impact of different outdoor particle distributions, indoor emission sources, HVAC airflows, filtration efficiencies, coils, and duct system complexities. The median predicted residential and commercial loading rates were 2.97 and 130 g/m(2) month for the filter loading rates, 0.756 and 4.35 g/m(2) month for the coil loading rates, 0.0051 and 1.00 g/month for the supply duct loading rates, and 0.262 g/month for the commercial return duct loading rates. Loading rates are more dependent on outdoor particle distributions, indoor sources, HVAC operation strategy, and filtration than other considered parameters. The results presented herein, once validated, can be used to estimate filter changing and coil cleaning schedules, energy implications of filter and coil loading, and IAQ impacts associated with deposited particles.

PRACTICAL IMPLICATIONS

The results in this paper suggest important factors that lead to particle deposition on HVAC components in residential and commercial buildings. This knowledge informs the development and comparison of control strategies to limit particle deposition. The predicted mass loading rates allow for the assessment of pressure drop and indoor air quality consequences that result from particle mass loading onto HVAC system components.

An evaluation of the indoor air quality in bars before and after a smoking ban in Austin, Texas

This study assessed differences in the indoor air quality and occupancy levels in seventeen bars due to a city-wide smoking ban that took effect on September 1, 2005 in Austin, Texas, USA. We measured the following in each venue before and after the smoking ban: mean number of occupants, mean number of lit cigarettes, temperature, relative humidity, room volume, and PM(2.5), CO, and CO(2) concentrations. Additionally, VOC measurements were conducted at three of the venues. There was not a statistically significant change in occupancy, but the best estimate PM(2.5) concentrations in the venues decreased 71-99%, a significant reduction in all venues, relative to the pre-ban levels; CO concentrations decreased significantly in all but one venue; and concentrations of VOCs known to be emitted from cigarettes decreased to below the detection limit for all but two common compounds. These results suggest that the smoking ban has effectively improved indoor air quality in Austin bars without an associated decrease in occupancy.