Atmospheric aerosols impact climate by scattering or absorbing solar radiation and by participating in the formation of clouds and ice crystals. Additionally, the inhalation of aerosol particles contributes significantly to cardiovascular and cardiopulmonary diseases, and is a leading cause of mortality on a global scale. Despite the climate and health implications of aerosol particles, the magnitude of their impacts is still highly uncertain. The physical and chemical (physicochemical) properties of aerosol particles determine their impacts, though these properties are analytically challenging to measure due to their small size, chemical complexity, and continuous evolution in the atmosphere. Single-particle methods are necessary to build upon our understanding of the multiphase aerosol processes occurring in the atmosphere, and to elucidate the impact of these particles on climate and health. In this dissertation, single-particle microscopic and spectroscopic methods were applied to study both ambient and laboratory-generated particles to enable better predictions of the climate and health impacts of aerosol particles.
Secondary organic aerosol (SOA), formed from multiphase reactions between aerosol particles and atmospheric gases, was systematically studied in an indoor atmospheric chamber. Particles were collected at multiple points during the multiphase reaction for physicochemical analysis using electron microscopy, atomic force microscopy, and Raman microspectroscopy. Particles underwent physicochemical transformation after heterogeneous reactions, leading to the formation of isoprene-derived organosulfate compounds in the particle phase that increased particle viscosity and altered the internal structure of particles. This study highlighted the dynamic physicochemical properties of SOA, a major fraction of organic aerosol in the atmosphere.
Particles emitted from freshwater lakes, known as lake spray aerosol (LSA), were studied through laboratory and ambient observations to determine the impacts of this newly identified particle type on climate and health. An aircraft campaign used microscopy to show that cloudwater and ambient LSA over Lake Michigan have very similar physicochemical properties, suggesting the incorporation of LSA into clouds with likely contributions to lake-effect precipitation. In a separate study, freshwater was collected during a severe harmful algal bloom (HAB) and analyzed for algal toxins using mass spectrometry. Aerosol particles were generated in the laboratory from the freshwater samples, with an observed enrichment of hydrophobic toxins in the aerosol phase. Following this laboratory-based study, a field campaign was performed at a lake experiencing a severe HAB. Freshwater and aerosol samples were analyzed for the presence of algal toxins using mass spectrometry and infrared spectroscopy, and the amount of aerosolized toxins in ambient environments was determined. These projects highlight a new route of exposure to HAB toxins that has implications for people living near or downwind of HABs globally.
Lastly, a new analytical spectroscopy method was developed and applied to characterize vibrational modes present in submicron aerosol particles. This method combined optical photothermal infrared (O-PTIR) and Raman spectroscopy for simultaneous spectroscopic acquisition. This project identified the ideal substrate for analysis, characterized single- and multi- component standards, and showed the first classification of ambient aerosol particles with O-PTIR + Raman. The spectroscopic analysis of submicron particles enabled exploration of samples previously unstudied by vibrational spectroscopy, significantly advancing the fields of atmospheric and analytical chemistry.
The methods and results obtained in this dissertation have resulted in detailed measurements of particle physicochemical properties, providing new insights into the mechanisms of multiphase atmospheric processing and improving understanding of the impacts of aerosols on climate and human health.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/168057/1/niolson_1.pd
