Toward a comprehensive and efficient thermodynamic model of organic aerosols indoors

Bryan Edward Cummings

doi:10.17918/00000408

Within residential buildings, organic aerosols (OA) often constitute the majority of particulate matter (PM) pollution, which is known to cause adverse cardiovascular and respiratory conditions. OA is composed of thousands of unique organic compounds, many of which are susceptible to partitioning between the aerosol and the gas phase. Until relatively recently, indoor air pollution models have largely neglected OA thermodynamic considerations, although certain organic thermodynamics modeling tools have been used with narrow applications to indoor PM studies over the past decade. Most of these cases have investigated particular processes, such as secondary organic aerosol (SOA) formation indoors or the repartitioning of outdoor OA. The need for the development of a comprehensive indoor OA thermodynamic model motivated the work done for this dissertation. Organic aerosol thermodynamics was modeled by the Indoor Model of Aerosols, Gases, Emissions, and Surfaces (IMAGES) using the volatility basis set (VBS). Explicitly representing indoor OA volatility allowed for errors associated with baseline, traditional particle models to be quantified across various model types and domains. For instance, traditional estimates of indoor particle emission rates for activities such as cooking may yield erroneous concentration predictions when used in other models. In such cases, error is driven by differences between model and experimental building conditions. Such errors were found to reach up to ~80% for typical stir-fry activities, associated with a magnitude of ~15-20 [mu]g/m³ depending on the particular emission strength. Epidemiological models that seek to predict indoor exposure to ambient pollution also have traditionally neglected volatility considerations. Such models fail to account for repartitioning driven by temperature and mass-loading gradients between the indoors and outdoors, leading to errors up to ~60% for total ambient PM, or about 3 [mu]g/m³ in the urban U.S. simulation domain that was considered. The two-dimensional volatility basis set (2D-VBS) was also incorporated into the underlying IMAGES model framework, representing its first known application to indoor air studies. Using the 2D-VBS to account for oxidation state in addition to volatility allowed OA aging transformations and water uptake to be modeled in addition to gas-to-particle partitioning. Simulation results showed that aging reactions are not likely to affect indoor OA composition and character from a day-averaged perspective, but may enhance peak OA concentrations under certain SOA-forming conditions on the order of ~10 [mu]g/m³. Also predicting the indoor humidity and aerosol water content in typical U.S. residences demonstrated that OA likely exists in a semisolid phase state indoors. Slow molecular diffusion within such particles challenges the implicit assumption often held by tradition indoor OA studies: that equilibrium thermodynamics holds, and that particles are typically liquid and well-mixed. A kinetic partitioning model of indoor organics was developed to more accurately represent the partitioning of material into and out of semisolid or glassy aerosols. This model was applied to a simulation of ambient aerosols that are transported into buildings and experience a temperature gradient that affects its effective volatility. Simulation results suggested that low diffusion inhibits repartitioning to at least some extent in the majority of simulated cases, representing residences in each of the 16 U.S. climate zones. Condensation may occur at equilibrium mostly in the southeastern U.S. in the summertime, where a hot and humid climate leads to a high indoor RH and therefore an indoor OA population in a liquid phase state. More northern locations along the east coast are typically associated with a drier indoor environment as the outdoor climate cools. In these cases, evaporation is often partially prohibited or fully prohibited. Dry climate zones from Arizona to Montana are more likely to experience limited or prohibited partitioning on hot outdoor days. And west coast marine climate zones are more likely to experience partial or equilibrium partitioning even in cooler regions.

Toward a comprehensive and efficient thermodynamic model of organic aerosols indoors

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Toward a comprehensive and efficient thermodynamic model of organic aerosols indoors

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