Surface Emissions Modulate Indoor SVOC Concentrations through Volatility-Dependent Partitioning.

Measurements by semivolatile thermal desorption aerosol gas chromatography (SV-TAG) were used to investigate how semivolatile organic compounds (SVOCs) partition among indoor reservoirs in (1) a manufactured test house under controlled conditions (HOMEChem campaign) and (2) a single-family residence when vacant (H2 campaign). Data for phthalate diesters and siloxanes suggest that volatility-dependent partitioning processes modulate airborne SVOC concentrations through interactions with surface-laden condensed-phase reservoirs. Airborne concentrations of SVOCs with vapor pressures in the range of C13 to C23 alkanes were observed to be correlated with indoor air temperature. Observed temperature dependencies were quantitatively similar to theoretical predictions that assumed a surface-air boundary layer with equilibrium partitioning maintained at the air-surface interface. Airborne concentrations of SVOCs with vapor pressures corresponding to C25 to C31 alkanes correlated with airborne particle mass concentration. For SVOCs with higher vapor pressures, which are expected to be predominantly gaseous, correlations with particle mass concentration were weak or nonexistent. During primary particle emission events, enhanced gas-phase emissions from condensed-phase reservoirs partitioned to airborne particles, contributing substantially to organic particulate matter. An emission event related to oven-usage was inferred to deposit siloxanes in condensed-phase reservoirs throughout the house, leading to the possibility of reemission during subsequent periods with high particle loading

Characterizing sources and emissions of volatile organic compounds in a northern California residence using space‐ and time‐resolved measurements

We investigate source characteristics and emission dynamics of volatile organic compounds (VOCs) in a single‐family house in California utilizing time‐ and space‐resolved measurements. About 200 VOC signals, corresponding to more than 200 species, were measured during 8 weeks in summer and five in winter. Spatially resolved measurements, along with tracer data, reveal that VOCs in the living space were mainly emitted directly into that space, with minor contributions from the crawlspace, attic, or outdoors. Time‐resolved measurements in the living space exhibited baseline levels far above outdoor levels for most VOCs; many compounds also displayed patterns of intermittent short‐term enhancements (spikes) well above the indoor baseline. Compounds were categorized as “high‐baseline” or “spike‐dominated” based on indoor‐to‐outdoor concentration ratio and indoor mean‐to‐median ratio. Short‐term spikes were associated with occupants and their activities, especially cooking. High‐baseline compounds indicate continuous indoor emissions from building materials and furnishings. Indoor emission rates for high‐baseline species, quantified with 2‐hour resolution, exhibited strong temperature dependence and were affected by air‐change rates. Decomposition of wooden building materials is suggested as a major source for acetic acid, formic acid, and methanol, which together accounted for ~75% of the total continuous indoor emissions of high‐baseline species

Aerosol single scattering albedo dependence on biomass combustion efficiency: Laboratory and field studies

Single scattering albedo (ω) of fresh biomass burning (BB) aerosols produced from 92 controlled laboratory combustion experiments of 20 different woods and grasses was analyzed to determine the factors that control the variability in ω. Results show that ω varies strongly with fire-integrated modified combustion efficiency (MCEFI)—higher MCEFI results in lower ω values and greater spectral dependence of ω. A parameterization of ω as a function of MCEFI for fresh BB aerosols is derived from the laboratory data and is evaluated by field observations from two wildfires. The parameterization suggests that MCEFI explains 60% of the variability in ω, while the 40% unexplained variability could be accounted for by other parameters such as fuel type. Our parameterization provides a promising framework that requires further validation and is amenable for refinements to predict ω with greater confidence, which is critical for estimating the radiative forcing of BB aerosols

Surface reservoirs dominate dynamic gas-surface partitioning of many indoor air constituents

Human health is affected by indoor air quality. One distinctive aspect of the indoor environment is its very large surface area that acts as a poorly characterized sink and source of gas-phase chemicals. In this work, air-surface interactions of 19 common indoor air contaminants with diverse properties and sources were monitored in a house using fast-response, on-line mass spectrometric and spectroscopic methods. Enhanced-ventilation experiments demonstrate that most of the contaminants reside in the surface reservoirs and not, as expected, in the gas phase. They participate in rapid air-surface partitioning that is much faster than air exchange. Phase distribution calculations are consistent with the observations when assuming simultaneous equilibria between air and large weakly polar and polar absorptive surface reservoirs, with acid-base dissociation in the polar reservoir. Chemical exposure assessments must account for the finding that contaminants that are fully volatile under outdoor air conditions instead behave as semivolatile compounds indoors

Chemical Evolution of Atmospheric Organic Carbon over Multiple Generations of Oxidation

The evolution of atmospheric organic carbon (OC) as it undergoes oxidation has a controlling influence on concentrations of key atmospheric species, including particulate matter, ozone, and oxidants. However, the full characterization of OC over hours to days of atmospheric processing has been stymied by its extreme chemical complexity. Here we study the multigenerational oxidation of -pinene in the laboratory, characterizing products with several state-of-the-art analytical techniques. While quantification of some early-generation products remains elusive, full carbon closure is achieved (within uncertainty) by the end of the experiments. This enables new insights into the effects of oxidation on OC properties (volatility, oxidation state, and reactivity), and the atmospheric lifecycle of OC. Following an initial period characterized by functionalization reactions and particle growth, fragmentation reactions dominate, forming smaller species. After approximately one day of atmospheric aging, most carbon is sequestered in two long-lived reservoirs, volatile oxidized gases and low-volatility particulate matter

Oxidation and Emission of Volatile Organic Compounds Indoors

The vast majority of earth’s atmosphere is outside, yet humans spend ~90% of their time indoors so the air they breathe is dominantly indoor air. Indoor air differs substantially from outdoor air in terms of its organic chemical composition and transformation processes, driven by key features including lower oxidant levels, less intense sunlight, higher surface area to volume environment, and more direct human influence While this dissertation looks at indoor air at a variety of scales, a consistent focus is that the human occupant is a defining feature of indoor air. This dissertation investigates how human activity affects indoor oxidant levels, how the human body directly influences the composition of indoor volatile organic compounds (VOCs), and how human activities control the emission of VOCs into the indoor environment. Chapter 1 introduces the motivation for this work, provides a brief overview of key issues in atmospheric chemistry associated with the study of indoor air, reviews key prior knowledge of indoor air VOCs and oxidants that set the groundwork for the following chapters. A brief description of the instrumentation used to measure indoor VOCs is discussed, and a roadmap to this dissertation is provided. In Chapter 2 we present direct indoor measurements of the nitrate radical (NO3¬) and dinitrogen pentoxide (N2O5) produced from combustion cooking emissions in a residential kitchen. The presence and importance of NO3 indoors had been hypothesized as early as 1986, however this chapter presents the first ever direct measurement made indoors. When indoor ozone (O3) concentration was low (~4 ppbv), nitric oxide (NO) emitted from gas-stove combustion suppressed NO3 formation. However, at moderate O3 levels (~40 ppbv), measured NO3 concentrations reached 3 to 4 pptv, and the indoor NO3 reactivity loss rate coefficient reached 0.8 s-1. A box model of known chemistry agrees with the reactivity estimate and shows that moderate O3 levels led to a nitrate radical production rate of 7 ppbv h-1¬. These indoor NO3 production rates and reactivities are much higher than is typical outdoors. At low O3 levels indoor combustion suppresses nitrate radical chemistry, but when sufficient O3 enters residences from outdoors or is emitted directly from indoor sources, gas stove combustion emissions promote indoor NO3 chemistry. Therefore in polluted regions with high levels of ozone, indoor NO3 chemistry will take on a greater importance.Chapter 3 presents a laboratory study of the ozonolysis of squalene, a major component of human skin oil. Rather than directly study skin oil, oxidation experiments are carried out on pure squalene particles passing through a flow tube reactor, allowing for the quantification of gas phase products from a single starting material with varying conditions of water vapor and O3. Previous work examining the condensed-phase products of pure squalene particle ozonolysis in a flow tube reactor found that an increase in water vapor concentration led to lower concentrations of secondary ozonides, increased concentrations of carbonyls, and smaller particle diameter, suggesting that water changes the fate of the Criegee intermediate. To determine if this loss of volume corresponds to an increase in gas-phase products, we measured gas-phase VOC concentrations via proton transfer reaction time of flight mass spectrometry (PTR-TOF-MS). Studies were conducted at atmospherically relevant O3 exposure levels (5-30 ppb h).An increase in water vapor concentration led to strong enhancement of gas-phase oxidation products at all tested O3 exposures. An increase in water vapor from ~0% to 70% relative humidity (RH) at high O3 exposure increased the total mass concentration of gas-phase VOCs by a factor of three. The observed fraction of carbon in the gas phase correlated well with the fraction of particle volume lost. Experiments involving O3 oxidation of shirts soiled with skin oil confirms that the RH dependence of gas-phase reaction product generation occurs similarly on surfaces containing skin oil under realistic indoor conditions. Relative humidity changes the fate of the Criegee intermediates and the volatility of the oxidation products, resulting in a RH dependence of the product distribution and amount of gas-phase VOCs emitted from ozonolysis of unsaturated carbon bonds in skin oil. Similar behavior is expected for O3 reactions with surface bound organics containing unsaturated carbon bonds.Chapter 4 presents VOC emission profiles from scripted cooking, cleaning and human occupancy experiments performed during the HOMEChem study at the University of Texas, Austin test house. Quantifying indoor VOC speciation and emissions is critical to understanding and modeling the processes controlling indoor concentration dynamics, human exposure, and the chemistry of indoor air. Much of previous research on indoor VOCs has utilized broad surveys of different structures with low time resolution measurements of concentrations for specific target compounds. Such studies necessarily average over the dynamics driven by events and processes controlling variability in concentrations, and focus on the resulting concentrations rather than the actual emission rates. This study was designed to quantify VOC emission profiles from the building and its contents, and from the scripted experiments with multiple replicates. Measurements of VOCs were performed with a PTR-TOF-MS which continuously measured the time-resolved mass spectrum of indoor and outdoor air. Continuous tracer releases enabled determination of air change rates (ACR) and thus calculation of speciated, time resolved net VOC emissions. The building and its contents were the dominant emission source into the house, with large emissions of acetic acid, methanol, and formic acid. Cooking emissions are greater than cleaning emissions, and comprised mainly of ethanol. Bleach cleaning leads to high emissions of reactive chlorinated compounds, while cleaning with a natural product emits predominantly monoterpenes and terpenoids. Emissions from occupancy experiments show large enhancements of siloxanes from personal care products in the morning which are nearly depleted by the afternoon when the products had already mostly evaporated. These results are used to construct VOC emissions for a hypothetical 24 hours, and show emissions from the house and its contents make up nearly half of the indoor VOC emissions, while the rest come from occupants and their typical activities.In Chapter 5 we provide preliminary evidence that indoor emissions escaping to outdoors are an increasingly important fraction of the fuel for outdoor air pollution in urban areas of developed countries, and suggest this as a promising new research direction for the field of atmospheric chemistry and air pollution control

Oxidation and Emission of Volatile Organic Compounds Indoors

The vast majority of earth’s atmosphere is outside, yet humans spend ~90% of their time indoors so the air they breathe is dominantly indoor air. Indoor air differs substantially from outdoor air in terms of its organic chemical composition and transformation processes, driven by key features including lower oxidant levels, less intense sunlight, higher surface area to volume environment, and more direct human influence While this dissertation looks at indoor air at a variety of scales, a consistent focus is that the human occupant is a defining feature of indoor air. This dissertation investigates how human activity affects indoor oxidant levels, how the human body directly influences the composition of indoor volatile organic compounds (VOCs), and how human activities control the emission of VOCs into the indoor environment. Chapter 1 introduces the motivation for this work, provides a brief overview of key issues in atmospheric chemistry associated with the study of indoor air, reviews key prior knowledge of indoor air VOCs and oxidants that set the groundwork for the following chapters. A brief description of the instrumentation used to measure indoor VOCs is discussed, and a roadmap to this dissertation is provided. In Chapter 2 we present direct indoor measurements of the nitrate radical (NO3¬) and dinitrogen pentoxide (N2O5) produced from combustion cooking emissions in a residential kitchen. The presence and importance of NO3 indoors had been hypothesized as early as 1986, however this chapter presents the first ever direct measurement made indoors. When indoor ozone (O3) concentration was low (~4 ppbv), nitric oxide (NO) emitted from gas-stove combustion suppressed NO3 formation. However, at moderate O3 levels (~40 ppbv), measured NO3 concentrations reached 3 to 4 pptv, and the indoor NO3 reactivity loss rate coefficient reached 0.8 s-1. A box model of known chemistry agrees with the reactivity estimate and shows that moderate O3 levels led to a nitrate radical production rate of 7 ppbv h-1¬. These indoor NO3 production rates and reactivities are much higher than is typical outdoors. At low O3 levels indoor combustion suppresses nitrate radical chemistry, but when sufficient O3 enters residences from outdoors or is emitted directly from indoor sources, gas stove combustion emissions promote indoor NO3 chemistry. Therefore in polluted regions with high levels of ozone, indoor NO3 chemistry will take on a greater importance.Chapter 3 presents a laboratory study of the ozonolysis of squalene, a major component of human skin oil. Rather than directly study skin oil, oxidation experiments are carried out on pure squalene particles passing through a flow tube reactor, allowing for the quantification of gas phase products from a single starting material with varying conditions of water vapor and O3. Previous work examining the condensed-phase products of pure squalene particle ozonolysis in a flow tube reactor found that an increase in water vapor concentration led to lower concentrations of secondary ozonides, increased concentrations of carbonyls, and smaller particle diameter, suggesting that water changes the fate of the Criegee intermediate. To determine if this loss of volume corresponds to an increase in gas-phase products, we measured gas-phase VOC concentrations via proton transfer reaction time of flight mass spectrometry (PTR-TOF-MS). Studies were conducted at atmospherically relevant O3 exposure levels (5-30 ppb h).An increase in water vapor concentration led to strong enhancement of gas-phase oxidation products at all tested O3 exposures. An increase in water vapor from ~0% to 70% relative humidity (RH) at high O3 exposure increased the total mass concentration of gas-phase VOCs by a factor of three. The observed fraction of carbon in the gas phase correlated well with the fraction of particle volume lost. Experiments involving O3 oxidation of shirts soiled with skin oil confirms that the RH dependence of gas-phase reaction product generation occurs similarly on surfaces containing skin oil under realistic indoor conditions. Relative humidity changes the fate of the Criegee intermediates and the volatility of the oxidation products, resulting in a RH dependence of the product distribution and amount of gas-phase VOCs emitted from ozonolysis of unsaturated carbon bonds in skin oil. Similar behavior is expected for O3 reactions with surface bound organics containing unsaturated carbon bonds.Chapter 4 presents VOC emission profiles from scripted cooking, cleaning and human occupancy experiments performed during the HOMEChem study at the University of Texas, Austin test house. Quantifying indoor VOC speciation and emissions is critical to understanding and modeling the processes controlling indoor concentration dynamics, human exposure, and the chemistry of indoor air. Much of previous research on indoor VOCs has utilized broad surveys of different structures with low time resolution measurements of concentrations for specific target compounds. Such studies necessarily average over the dynamics driven by events and processes controlling variability in concentrations, and focus on the resulting concentrations rather than the actual emission rates. This study was designed to quantify VOC emission profiles from the building and its contents, and from the scripted experiments with multiple replicates. Measurements of VOCs were performed with a PTR-TOF-MS which continuously measured the time-resolved mass spectrum of indoor and outdoor air. Continuous tracer releases enabled determination of air change rates (ACR) and thus calculation of speciated, time resolved net VOC emissions. The building and its contents were the dominant emission source into the house, with large emissions of acetic acid, methanol, and formic acid. Cooking emissions are greater than cleaning emissions, and comprised mainly of ethanol. Bleach cleaning leads to high emissions of reactive chlorinated compounds, while cleaning with a natural product emits predominantly monoterpenes and terpenoids. Emissions from occupancy experiments show large enhancements of siloxanes from personal care products in the morning which are nearly depleted by the afternoon when the products had already mostly evaporated. These results are used to construct VOC emissions for a hypothetical 24 hours, and show emissions from the house and its contents make up nearly half of the indoor VOC emissions, while the rest come from occupants and their typical activities.In Chapter 5 we provide preliminary evidence that indoor emissions escaping to outdoors are an increasingly important fraction of the fuel for outdoor air pollution in urban areas of developed countries, and suggest this as a promising new research direction for the field of atmospheric chemistry and air pollution control

Multiphase Mechanism for the Production of Sulfuric Acid from SO2 by Criegee Intermediates Formed During the Heterogeneous Reaction of Ozone with Squalene.

Here we report a new multiphase reaction mechanism by which Criegee intermediates (CIs), formed by ozone reactions at an alkene surface, convert SO2 to SO3 to produce sulfuric acid, a precursor for new particle formation (NPF). During the heterogeneous ozone reaction, in the presence of 220 ppb SO2, an unsaturated aerosol (squalene) undergoes rapid chemical erosion, which is accompanied by NPF. A kinetic model predicts that the mechanism for chemical erosion and NPF originate from a common elementary step (CI + SO2) that produces both gas phase SO3 and small ketones. At low relative humidity (RH = 5%), 20% of the aerosol mass is lost, with 17% of the ozone-surface reactions producing SO3. At RH = 60%, the aerosol shrinks by 30%, and the yield of SO3 is <5%. This multiphase formation mechanism of H2SO4 by CIs is discussed in the context of indoor air quality and atmospheric chemistry