Advancements in droplet assisted ionization for the characterization of organic aerosols
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Atmospheric aerosol, particulate matter suspended in the air, is ubiquitous in the world around us having a strong impact on human health and the environment. There are many ways particles can form by both natural and anthropogenic processes. A substantial portion of atmospheric aerosol is organic, and this organic matter is highly complex on a molecular scale, encompassing hundreds to thousands of unique molecular species that can either exist in the gas or particle phases. Insight into the sources and composition of atmospheric aerosol, and its evolution through the atmosphere can be gained through chemical composition measurements. Because of the complexity of atmospheric particulate matter, measuring the composition is a task that challenges most analytical techniques as the composition may change under a wide range of atmospheric or experimental conditions (e.g. relative humidity, temperature, pressure, etc.). However, mass spectrometry plays a crucial role owing to its combination of high sensitivity and molecular specificity. In this dissertation, I address understanding new ways to measure the molecular composition of aerosols in the laboratory through mass spectrometry, the significance of the composition in relation to real-world atmospheric aerosol, and how to expand on the analytical technologies available for understanding atmospheric aerosols. There is an ever-evolving need to improve existing analytical techniques for aerosol characterization and to develop new methods to further advance the knowledge of how to mitigate the unwanted health and environmental impacts of particulate matter in the atmosphere. ☐ Both offline (collection of aerosols onto a filter and extracted in solution) and online (direct measurement of aerosols from source to mass spectrometer) measurement techniques are discussed, and molecular measurements are performed in the context of identifying the molecular species detected and elucidating the underlying chemical processes of particles in the atmosphere. In this dissertation, droplet assisted ionization (DAI) is developed and utilized to measure the molecular composition of organic aerosols online. DAI is an inlet ionization technique that allows for the detection of intact molecular ions from preformed aerosol droplets. In our laboratory, DAI is performed by passing aerosol droplets through a temperature-controlled capillary tube that serves as the inlet to a Waters SYNAPT G2-S mass spectrometer. DAI is well suited for the analysis of aerosol particles in the Aitken mode size range (< 100 nm in diameter) where the mass concentration is low and the number concentration is high. Previous work with DAI has determined the optimum operating conditions for a variety of test compounds. Ion formation depends strongly on droplet solvent composition and capillary temperature. Aqueous droplets give orders of magnitude higher signal intensity than dry particles or droplets consisting of other solvent compositions. Droplet generation is achieved with the use of the condensation growth chamber (CGC). The CGC allows for sensitive detection of the aerosol droplets whether or not the original aerosol is aqueous. Experiments are performed to gain insight into the ion formation mechanisms of DAI when different temperatures applied to the capillary inlet. Size-selected particles show that aerodynamic droplet breakup leads to ion formation with a capillary temperature of 25°C, while thermal droplet breakup leads to ion formation at capillary temperatures greater than 250°C. Furthermore, different analyte properties (e.g. volatility, volume of water in the droplet, etc.) impact ion detection via DAI, as well as the addition of salt to the aerosol composition. DAI is also adapted to perform extractive electrospray ionization (E-ESI) to enhance existing analytical methodologies for the study of atmospheric aerosols online. ☐ With the optimization of DAI and understanding of ion formation mechanisms, DAI is used to investigate the molecular composition of α-pinene SOA, a proxy atmospheric organic aerosol. Because the sensitivity of DAI analysis increases as the original aerosol particle diameter decreases, this method is particularly well suited to the study of nanoparticle formation and growth. In a previous study by our group using offline analysis to characterize β-pinene SOA, we noted a particle size dependence of the molecular composition, specifically an increasing oligomeric content with increasing particle size. Presented here is an investigation of the size dependence on SOA composition using online analysis by DAI, as this method reduces the possible impact of artifacts associated with offline analysis. This work has implications for understanding the relative roles of gas vs. particle phase processes that cause particle growth in the atmosphere. Experiments are reported where the molecular composition of freshly nucleated SOA is studied by DAI under a variety of conditions. SOA having a median diameter of 20 nm produced signal with high ion intensity. Molecular characterization of the SOA was consistent with condensation of highly oxidized molecules (HOMs) that subsequently reacted within the particle phase. Further characterization of particle-phase chemistry is performed in regards to understanding the molecular species detected and impacts due to DAI ion formation mechanisms under various conditions. The results of these studies have implications for understanding growth of newly formed particles in the atmosphere. ☐ The development and insight into the ion formation mechanisms of DAI will serve future members of our research group and the scientific community to characterize atmospheric organic aerosol through online DAI mass spectrometry. Current and future studies using DAI as an online technique to characterize organic aerosol are discussed at the conclusion of this dissertation.
Description
Keywords
Atmospheric nanoparticles, Droplet assisted ionization, Droplet phase, Ionization mechanism, Mass spectrometry, Organic aerosol