Sources, Physicochemical Transformations, and Inhalation Exposures of Indoor Organic Chemicals
Modern human populations spend ~90% of their time indoors. Moreover, concentrations of well-studied organic chemicals in indoor air are often orders of magnitude higher than equivalent outdoor concentrations. Considering that airborne organic pollutants can impact human and environmental health, it is critical to understand the sources, physicochemical behavior and modes of exposure of airborne organic chemicals in the indoor environment. This dissertation improves scientific understanding of these topics via analysis of time-resolved measurements of organic pollutants acquired in three normally-occupied residences and test houses over several months.
Until recently, researchers have largely used offline measurement approaches to study indoor air quality. Time-averaged samples are collected over a period spanning hours to weeks and then returned to the laboratory for subsequent analysis. While this approach is well-suited for survey-based analyses where pollutant concentrations are measured at many different sampling sites, the limited time-resolution inhibits study of dynamic processes that influence indoor air quality. Recent advances in instrumentation permit continuous monitoring of airborne organic chemicals with high chemical specificity on minute-by-minute and hourly time-scales over extended observation periods. This dissertation reports key findings from several months of time-resolved measurements of volatile organic compounds (VOCs) as measured by proton-transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) and semivolatile organic compounds (SVOCs) as measured by semivolatile thermal desorption aerosol gas chromatography in two normally-occupied residences (H1, H2) and a test house with scripted experiments (HOMEChem).
In Chapter 2, time-varying concentrations and gas-particle phase partitioning of phthalate diesters, a class of compounds of public-health interest, are reported at the H2 site. The dynamic behavior of four reported phthalates is observed to be related to their vapor pressure and physicochemical parameters such as temperature and particle mass concentration. These findings are generalized in Chapter 3 where it is suggested that volatility-dependent partitioning processes are the principle drivers of SVOC dynamic behavior. As part of this analysis, observations are connected to a theoretical model that assumes kinetic equilibrium with organic surface films commonly found indoors. Furthermore, it is observed that certain SVOCs can be deposited on surfaces during large emission events and then re-emitted into bulk air during future particle-emission events. In these scenarios, particles were inferred to enhance mass transport from condensed-phase reservoirs to bulk air. A high-resolution exposure assessment is conducted for VOCs in Chapters 4 and 5. Source apportionment analysis, a risk-based prioritization analyses and experimental estimates of intake fractions are reported for >200 VOCs. In contrast to expectations, a key finding suggests that for most VOCs, time-averaged exposure are attributable to the building and its static contents rather than episodic emission-events like cooking related to occupants or outdoor-to-indoor transport.