Date of Award
Summer 9-13-2023
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Aerosols have a major influence on Earth’s climate and cause adverse health effects by degrading ambient air quality. Aerosols impact Earth’s climate directly, by scattering and absorbing incoming shortwave radiation, and indirectly by altering cloud optical properties, cloud formation, and precipitation. Uncertainties in our understanding of these processes limit our current knowledge about the effect of aerosols on Earth’s climate. Poor air quality is globally the largest environmental health risk. Epidemiological studies have uncovered clear relationships of gaseous pollutants and particulate matter (PM) with adverse health outcomes, including increased risk of cardiovascular and respiratory diseases. However, the effects of PM exposure on other health risks are an area of ongoing research. I address a few of these challenges through observationally constrained ecological regression and aerosol optical modeling in this dissertation, which is divided into two parts. The onset of a global pandemic in early 2020 led to the first branch of this dissertation. The unfolding of the COVID-19 pandemic presented a unique opportunity to investigate possible associations between long-term PM exposure in humans and the rapid spread of the disease among US population. This branch of research began with the development of a robust and state-of-the-art COVID-19 epidemic progression model. The epidemic model allowed for systematic evaluation of several disease containment strategies, and the development of methods to infer important epidemic parameters based on the number of confirmed cases in an area. The most important epidemic parameter which the model infers is the basic reproduction ratio (R0). The epidemic progression model was then used to show that long-term exposure to air pollution is a statistically significant predictor of R0 at the state level, and that exposure to secondary inorganic aerosols increases R0. The epidemic model framework was then extended to evaluate associations between R0 and long-term air pollution exposure at finer scales. This work showed that racial and ethnic minority groups in 12 metropolitan cities have been historically exposed to high population density and high concentrations of PM, which contributed to rapid COVID-19 spread in these communities. This work concludes by showing which racial and ethnic minority groups were most impacted in each of the 12 metropolitan cities investigated, in the hopes that this will provide guidance for targeted air pollution reduction policy. The second part of this dissertation addresses lingering uncertainties in the contribution of aerosols to shortwave radiative forcing. Aerosol contribution to global radiative forcing has long been one of the largest sources of uncertainty in climate models. Black carbon (BC) is assumed to be largely responsible for short-wave radiation absorption in polluted air, but its light absorption properties remain poorly represented in current climate models. Poor representation of BC light absorption in climate models stems from incorrect parametrization of BC optical properties, which are highly dependent on the complex morphology and composition of BC aerosols. Field observations have shown that BC aerosols exist largely as fractal aggregates and are often coated or internally mixed with organic material. The coating of BC with organic material has been shown to enhance its light absorption properties and alter its nascent morphology. The magnitude of enhancement in shortwave light absorption can range from a factor of 1.1 to 3 – but this phenomenon had not yet been systematically parameterized. This work establishes universal scaling laws for optical properties of BC aerosol as a function of BC morphology and amount of non-absorbing and weakly absorbing organic coating. The developed scaling laws were then combined with particle-scale measurements of a wildfire-driven pyrocumulonimbus (pyroCb) plume to evaluate shortwave light absorption enhancement by this sub-class of BC. We use the scaling laws developed in this work to show that pyroCb BC absorbs up to twice as much shortwave radiation as ambient BC, and establish the limits of shortwave absorption enhancement by atmospheric BC. This work concludes by developing a new tool which allows for detailed simulation of aerosolcloud interactions. During their atmospheric lifetime, the properties of aerosols can be altered by aqueous chemistry and wet removal within clouds (termed cloud processing). The ways in which a particle interacts with clouds is highly dependent on its size and composition. Given the difficulty of direct measurement, our knowledge of the effects of cloud processing on BC particles relies heavily on models. However, current models are not capable of representing the compositional diversity of BC particles. This work addresses this gap by developing of the first particle-resolved cloud parcel model. The model was validated using in-situ particle resolved measurements of particle size and composition. The developed model accurately replicates observed changes to aerosol composition and size due to cloud processing, and a vital tool for future investigations of the effects of cloud processing on BC properties.
Language
English (en)
Chair
Rajan Chakrabarty