Measuring the Building Envelope Penetration Factor for Ambient Nitrogen Oxides

Per-Person and Whole-Building VOC Emission Factors in an Occupied School with Gas-Phase Air Cleaning

Using real-time measurements of CO₂ and volatile organic compounds (VOCs) in the air handler of an occupied middle school, we quantified source strengths for 249 VOCs and apportioned the source to the building, occupants and their activities, outdoor air, or recirculation air. For VOCs quantified in this study, there is a source to the outdoors of 8.6 ± 1.8 g/h in building exhaust air, of which 5.9 ± 1.7 g/h can be attributed to indoor sources (the building and occupants and their activities). The corresponding whole-building area emission factor from indoor sources is 1020 ± 300 μg/(m² h), including reactive VOCs like isoprene and monoterpenes (33 ± 5.1 and 29 ± 5.7 μg/(m² h), respectively). Per-person emission factors are calculated for compounds associated with occupants and their activities, e.g., monoterpenes are emitted at a rate of 280 ± 80 μg/(person h). The air handler included carbon scrubbing, reducing supply air concentrations of 125 compounds by 38 ± 19% (mean ± std. dev.) with a net removal of 2.4 ± 0.4 g/h of organic compounds from the building. This carbon scrubber reduces steady-state indoor concentrations of organics by 65 μg/m³ and the contribution of indoor sources of VOCs to the outdoor environment by ∼40%. These data inform the design and operation of buildings to reduce human exposure to VOCs inside buildings. These data indicate the potential for gas-phase air cleaning to improve both indoor air quality and reduce VOC emissions from buildings to the outdoor environment.

Determination of the optimal penetration factor for evaluating the invasion process of aerosols from a confined source space to an uncontaminated area

Due to the outbreak and spread of COVID-19, SARS-CoV-2 has been proven to survive in aerosols for hours. Virus-containing aerosols may intrude into an uncontaminated area from a confined source space under certain ventilated conditions. The penetration factor, which is the most direct parameter for evaluating the invasion process, can effectively reflect the penetration fraction of aerosols and the shielding efficiency of buildings. Based on the observed concentrations of aerosols combined with a widely used concentration model, four numerical calculations of the penetration factor are proposed in this study. A theoretical time-correction P_est was applied to a size-dependent P_avg by proposing a correction coefficient r, and the error analysis of the real-time P(t) and the derived P_d were also performed. The results indicated that P_avg supplied the most stable values for laboratory penetration simulations. However, the time-correction is of little significance under current experimental conditions. P(t) and P_d are suitable for rough evaluation under certain conditions due to the inevitability of particles detaching and re-entering after capture. The proposed optimal penetration factor and the error analysis of each method in this study can provide insight into the penetration mechanism, and also provide a rapid and accurate assessment method for preventing and controlling the spread of the epidemic.

High-Efficiency Air Cleaning Reduces Indoor Traffic-Related Air Pollution and Alters Indoor Air Chemistry in a Near-Roadway School

Schools in proximity to roadways expose students to traffic-related air pollution (TRAP). We investigate impacts of air-cleaning on indoor TRAP levels and indoor chemistry in a renovated school adjacent an interstate highway. We monitor air pollutants pre- and post-renovation and quantify efficiency of particle (MERV8 and 16 filters) and gas (functionalized activated carbon) air-cleaning. Time-resolved measurements show air-cleaning systems are effective, with in situ particle removal efficiency >94% across 10 nm to 10 μm. Activated carbon removed BTEX and NO₂ with variability in removal efficiency. Over eight months of monitoring, NO₂ removal efficiency was 96% initially and decreased to 61%; and BTEX removal efficiency was >80% or increased to >80%. Air-cleaning reduced indoor TRAP to below or near urban background. Air-cleaning systems suppressed indoor chemistry by reducing indoor levels of oxidants (NO₂, O₃) and reactive organics of indoor origin. When the air cleaning system was inactive, our data show that indoor SOA formation within the school was elevated. Loss rates of NO₂ and O₃ through the air-cleaning system were ∼1.5-2.4 h^-1 and ∼2.3 h^-1, respectively. Air-cleaning was 83% and 69% efficient, respectively, in removing monoterpenes and isoprene. By suppressing precursors, scaling calculations show air-cleaning prevented ∼3.4 mg/h of indoor SOA formation due to indoor ozone-monoterpene chemistry. For comparison, we estimate that filtration removed ∼130 mg/h of PM_0.01-0.3.

Indoor acids and bases

Numerous acids and bases influence indoor air quality. The most abundant of these species are CO₂ (acidic) and NH₃ (basic), both emitted by building occupants. Other prominent inorganic acids are HNO₃ , HONO, SO₂ , H₂ SO₄ , HCl, and HOCl. Prominent organic acids include formic, acetic, and lactic; nicotine is a noteworthy organic base. Sources of N-, S-, and Cl-containing acids can include ventilation from outdoors, indoor combustion, consumer product use, and chemical reactions. Organic acids are commonly more abundant indoors than outdoors, with indoor sources including occupants, wood, and cooking. Beyond NH₃ and nicotine, other noteworthy bases include inorganic and organic amines. Acids and bases partition indoors among the gas-phase, airborne particles, bulk water, and surfaces; relevant thermodynamic parameters governing the partitioning are the acid-dissociation constant (K_a ), Henry's law constant (K_H ), and the octanol-air partition coefficient (K_oa ). Condensed-phase water strongly influences the fate of indoor acids and bases and is also a medium for chemical interactions. Indoor surfaces can be large reservoirs of acids and bases. This extensive review of the state of knowledge establishes a foundation for future inquiry to better understand how acids and bases influence the suitability of indoor environments for occupants, cultural artifacts, and sensitive equipment.

The atmospheric chemistry of indoor environments

Through air inhalation, dust ingestion and dermal exposure, the indoor environment plays an important role in controlling human chemical exposure. Indoor emissions and chemistry can also have direct impacts on the quality of outdoor air. And so, it is important to have a strong fundamental knowledge of the chemical processes that occur in indoor environments. This review article summarizes our understanding of the indoor chemistry field. Using a molecular perspective, it addresses primarily the new advances that have occurred in the past decade or so and upon developments in our understanding of multiphase partitioning and reactions. A primary goal of the article is to contrast indoor chemistry to that which occurs outdoors, which we know to be a strongly gas-phase, oxidant-driven system in which substantial oxidative aging of gases and aerosol particles occurs. By contrast, indoor environments are dark, gas-phase oxidant concentrations are relatively low, and due to air exchange, only short times are available for reactive processing of gaseous and particle constituents. However, important gas-surface partitioning and reactive multiphase chemistry occur in the large surface reservoirs that prevail in all indoor environments. These interactions not only play a crucial role in controlling the composition of indoor surfaces but also the surrounding gases and aerosol particles, thus affecting human chemical exposure. There are rich research opportunities available if the advanced measurement and modeling tools of the outdoor atmospheric chemistry community continue to be brought indoors.