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

Review of animal transmission experiments of respiratory viruses: Implications for transmission risk of SARS-COV-2 in humans via different routes

Exploring transmission risk of different routes has major implications for epidemic control. However, disciplinary boundaries have impeded the dissemination of epidemic information, have caused public panic about "air transmission," "air-conditioning transmission," and "environment-to-human transmission," and have triggered "hygiene theater." Animal experiments provide experimental evidence for virus transmission, but more attention is paid to whether transmission is driven by droplets or aerosols and using the dichotomy to describe most transmission events. Here, according to characteristics of experiment setups, combined with patterns of human social interactions, we reviewed and grouped animal transmission experiments into four categories-close contact, short-range, fomite, and aerosol exposure experiments-and provided enlightenment, with experimental evidence, on the transmission risk of severe acute respiratory syndrome coronavirus (SARS-COV-2) in humans via different routes. When referring to "air transmission," context should be showed in elaboration results, rather than whether close contact, short or long range is uniformly described as "air transmission." Close contact and short range are the major routes. When face-to-face, unprotected, horizontally directional airflow does promote transmission, due to virus decay and dilution in air, the probability of "air conditioning transmission" is low; the risk of "environment-to-human transmission" highly relies on surface contamination and human behavior based on indirect path of "fomite-hand-mucosa or conjunctiva" and virus decay on surfaces. Thus, when discussing the transmission risk of SARS-CoV-2, we should comprehensively consider the biological basis of virus transmission, environmental conditions, and virus decay. Otherwise, risk of certain transmission routes, such as long-range and fomite transmission, will be overrated, causing public excessive panic, triggering ineffective actions, and wasting epidemic prevention resources.

Modeling Indoor Inorganic Aerosol Concentrations During the ATHLETIC Campaign with IMAGES

In 2018, the ATHLETIC campaign was conducted at the University of Colorado Dal Ward Athletic Center and characterized dynamic indoor air composition in a gym environment. Among other parameters, inorganic particle and gas-phase species were alternatingly measured in the gym's supply duct and weight room. The Indoor Model of Aerosols, Gases, Emissions, and Surfaces (IMAGES) uses the inorganic aerosol thermodynamic equilibrium model, ISORROPIA, to estimate the partitioning of inorganic aerosols and corresponding gases. In this study herein, measurements from the ATHLETIC campaign were used to evaluate IMAGES' performance. Ammonia emission rates, nitric acid deposition, and particle deposition velocities were related to observed occupancy, which informed these rates in IMAGES runs. Initially, modeled indoor inorganic aerosol concentrations were not in good agreement with measurements. A parametric investigation revealed that lowering the temperature or raising the relative humidity used in the ISORROPIA model drove the semivolatile species more toward the particle phase, substantially improving modeled-measured agreement. One speculated reason for these solutions is that aerosol water was enhanced by increasing the RH or decreasing the temperature. Another is that thermodynamic equilibrium was not established in this indoor setting or that the thermodynamic parametrizations in ISORROPIA are less accurate for typical indoor settings. This result suggests that applying ISORROPIA indoors requires further careful experimental validation.

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

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

Indoor-Outdoor Oxidative Potential of PM2.5 in Wintertime Fairbanks, Alaska: Impact of Air Infiltration and Indoor Activities

The indoor air quality of a residential home during winter in Fairbanks, Alaska, was investigated and contrasted with outdoor levels. Twenty-four-hour average indoor and outdoor filter samples were collected from January 17 to February 25, 2022, in a residential area with high outdoor PM2.5 concentrations. The oxidative potential of PM2.5 was determined using the dithiothreitol-depletion assay (OPDTT). For the unoccupied house, the background indoor-to-outdoor (I/O) ratio of mass-normalized OP (OPmDTT), a measure of the intrinsic health-relevant properties of the aerosol, was less than 1 (0.53 ± 0.37), implying a loss of aerosol toxicity as air was transported indoors. This may result from transport and volatility losses driven by the large gradients in temperature (average outdoor temperature of -19°C/average indoor temperature of 21 °C) or relative humidity (average outdoor RH of 78%/average indoor RH of 11%), or both. Various indoor activities, including pellet stove use, simple cooking experiments, incense burning, and mixtures of these activities, were conducted. The experiments produced PM2.5 with a highly variable OPmDTT. PM2.5 from cooking emissions had the lowest OP values, while pellet stove PM2.5 had the highest. Correlations between volume-normalized OPDTT (OPvDTT), relevant to exposure, and indoor PM2.5 mass concentration during experiments were much lower compared to those in outdoor environments. This suggests that mass concentration alone can be a poor indicator of possible adverse effects of various indoor emissions. These findings highlight the importance of considering both the quantity of particles and sources (chemical composition), as health metrics for indoor air quality.

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.

Comparison of Phase States of PM2.5 over Megacities, Seoul and Beijing, and Their Implications on Particle Size Distribution

Although the particle phase state is an important property, there is scant information on it, especially, for real-world aerosols. To explore the phase state of fine mode aerosols (PM_2.5) in two megacities, Seoul and Beijing, we collected PM_2.5 filter samples daily from Dec 2020 to Jan 2021. Using optical microscopy combined with the poke-and-flow technique, the phase states of the bulk of PM_2.5 as a function of relative humidity (RH) were determined and compared to the ambient RH ranges in the two cities. PM_2.5 was found to be liquid to semisolid in Seoul but mostly semisolid to solid in Beijing. The liquid state was dominant on polluted days, while a semisolid state was dominant on clean days in Seoul. These findings can be explained by the aerosol liquid water content related to the chemical compositions of the aerosols at ambient RH; the water content of PM_2.5 was much higher in Seoul than in Beijing. Furthermore, the overall phase states of PM_2.5 observed in Seoul and Beijing were interrelated with the particle size distribution. The results of this study aid in a better understanding of the fundamental physical properties of aerosols and in examining how these are linked to PM_2.5 in polluted urban atmospheres.

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.

Exploring the Nanostructures Accessible to an Organic Surfactant Atmospheric Aerosol Proxy

The composition of atmospheric aerosols varies with time, season, location, and environment. This affects key aerosol properties such as hygroscopicity and reactivity, influencing the aerosol’s impact on the climate and air quality. The organic fraction of atmospheric aerosol emissions often contains surfactant material, such as fatty acids. These molecules are known to form three-dimensional nanostructures in contact with water. Different nanostructures have marked differences in viscosity and diffusivity that are properties whose understanding is essential when considering an aerosol’s atmospheric impact. We have explored a range of nanostructures accessible to the organic surfactant oleic acid (commonly found in cooking emissions), simulating variation that is likely to happen in the atmosphere. This was achieved by changing the amount of water, aqueous phase salinity and by addition of other commonly coemitted compounds: sugars and stearic acid (the saturated analogue of oleic acid). The nanostructure was observed by both synchrotron and laboratory small/wide angle X-ray scattering (SAXS/WAXS) and found to be sensitive to the proxy composition. Additionally, the spacing between repeat units in these nanostructures was water content dependent (i.e., an increase from 41 to 54 Å in inverse hexagonal phase d-spacing when increasing the water content from 30 to 50 wt %), suggesting incorporation of water within the nanostructure. A significant decrease in mixture viscosity was also observed with increasing water content from ∼10⁴ to ∼10² Pa s when increasing the water content from 30 to 60 wt %. Time-resolved SAXS experiments on levitated droplets of this proxy confirm the phase changes observed in bulk phase mixtures and demonstrate that coexistent nanostructures can form in droplets. Aerosol compositional and subsequent nanostructural changes could affect aerosol processes, leading to an impact on the climate and urban air pollution.

The dynamics of SARS-CoV-2 infectivity with changes in aerosol microenvironment

Understanding the factors that influence the airborne survival of viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in aerosols is important for identifying routes of transmission and the value of various mitigation strategies for preventing transmission. We present measurements of the stability of SARS-CoV-2 in aerosol droplets (∼5 to 10 µm equilibrated radius) over timescales spanning 5 s to 20 min using an instrument to probe survival in a small population of droplets (typically 5 to 10) containing ∼1 virus/droplet. Measurements of airborne infectivity change are coupled with a detailed physicochemical analysis of the airborne droplets containing the virus. A decrease in infectivity to ∼10% of the starting value was observable for SARS-CoV-2 over 20 min, with a large proportion of the loss occurring within the first 5 min after aerosolization. The initial rate of infectivity loss was found to correlate with physical transformation of the equilibrating droplet; salts within the droplets crystallize at relative humidities (RHs) below 50%, leading to a near-instant loss of infectivity in 50 to 60% of the virus. However, at 90% RH, the droplet remains homogenous and aqueous, and the viral stability is sustained for the first 2 min, beyond which it decays to only 10% remaining infectious after 10 min. The loss of infectivity at high RH is consistent with an elevation in the pH of the droplets, caused by volatilization of CO₂ from bicarbonate buffer within the droplet. Four different variants of SARS-CoV-2 were compared and found to have a similar degree of airborne stability at both high and low RH.

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.

Nanoscopic Study of Water Uptake on Glass Surfaces with Organic Thin Films and Particles from Exposure to Indoor Cooking Activities: Comparison to Model Systems

Water uptake by thin organic films and organic particles on glass substrates at 80% relative humidity was investigated using atomic force microscopy-infrared (AFM-IR) spectroscopy. Glass surfaces exposed to kitchen cooking activities show a wide variability of coverages from organic particles and organic thin films. Water uptake, as measured by changes in the volume of the films and particles, was also quite variable. A comparison of glass surfaces exposed to kitchen activities to model systems shows that they can be largely represented by oxidized oleic acid and carboxylate groups on long and medium hydrocarbon chains (i.e., fatty acids). Overall, we demonstrate that organic particles and thin films that cover glass surfaces can take up water under indoor-relevant conditions but that the water content is not uniform. The spatial heterogeneity of the changes in these aged glass surfaces under dry (5%) and wet (80%) conditions is quite marked, highlighting the need for studies at the nano- and microscale.

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.

On the Water-Soluble Organic Matter in Inhalable Air Particles: Why Should Outdoor Experience Motivate Indoor Studies?

The current understanding of water-soluble organic aerosol (OA) composition, sources, transformations, and effects is still limited to outdoor scenarios. However, the OA is also an important component of particulate matter indoors, whose complexity impairs a full structural and molecular identification. The current limited knowledge on indoor OA, and particularly on its water-soluble organic matter (WSOM) fraction is the basis of this feature paper. Inspired by studies on outdoor OA, this paper discusses and prioritizes issues related to indoor water-soluble OA and their effects on human health, providing a basis for future research in the field. The following three main topics are addressed: (1) what is known about the origin, mass contribution, and health effects of WSOM in outdoor air particles; (2) the current state-of-the-art on the WSOM in indoor air particles, the main challenges and opportunities for its chemical characterization and cytotoxicity evaluation; and (3) why the aerosol WSOM should be considered in future indoor air quality studies. While challenging, studies on the WSOM fraction in air particles are highly necessary to fully understand its origin, fate, toxicity, and long-term risks indoors.

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.