Measuring and modeling the composition of secondary organic aerosol
Date
2020
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Publisher
University of Delaware
Abstract
Atmospheric aerosols are most broadly defined as any solid or liquid-like substance suspended in the air. Understanding the atmospheric aerosols is important because they affect both our global climate and human health. However, gaps in knowledge still exist for these subjects because of an overall lack in understanding of atmospheric nanoparticles. Atmospheric nanoparticles are the greatest of all atmospheric particulate matter in number and originate in the atmosphere through gas-to-particle conversion processes. Condensation of gas-phase molecular species is crucial in growing these nanoparticles to diameters between 50 and 100 nm at which point the nanoparticle becomes a seed for cloud formation in the atmosphere. Key contributors to the nanoparticle growth process include sulfuric acid, ammonia, and organic species. Of the three, the contribution of organic material is least understood. ☐ The contribution of organic material to the nanoparticle formation and growth process is poorly constrained due to the complexity of the oxidation of volatile organic compounds, which can produce multiple generations of products. These products enter the particle phase largely though processes of partitioning between gas and particle phases. However, certain chemical species can react within the particle phase to form new products making nanoparticle growth and composition both dynamic and complex. ☐ The scope of this dissertation is to create a link between molecular composition and nanoparticle growth to better understand the contribution of nanoparticles in the atmosphere. This is done two phases. The first phase is development of a fundamental modeling approach that examines how molecular composition changes nanoparticle growth rates. The modeling approach is then applied to laboratory experiments to explain mechanisms of nanoparticle growth using a home-built flow tube reactor. In one such experiment, the growth of different diameter nanoparticles is measured in the presence of α-pinene ozonolysis products. For each different nanoparticle size, a condensation growth mechanism of non-volatile organic compounds adequately explains the growth. ☐ In a second experiment, nanoparticle growth is measured in the presence of both β-pinene ozonolysis products and sulfur dioxide. Inside the flow tube reactor, sulfur dioxide is oxidized to sulfuric acid, which condenses onto preexisting particles and also nucleates new particles as well. Condensation of sulfuric acid does not account for the measured nanoparticle growth. Reactive uptake of sulfur dioxide onto nanoparticles is the only the mechanism that is able to explain the measured growth. In this mechanism, sulfur dioxide is oxidized to sulfuric acid by particle phase organic compounds containing functionalities such as peroxides. After reacting, most of the organic fraction evaporates back to the gas phase leaving only inorganic fraction in the particle phase. ☐ In the second phase of this dissertation, methods for measuring the composition of secondary organic aerosol are discussed. The method developed and discussed in this dissertation is a mass spectrometry method referred to as droplet assisted ionization (DAI). Although this method is developed with the intention of measuring organic aerosol molecular composition, work is first done to optimize and characterize the properties of how ions are produced using DAI. The ion yield, defined as ion counts per molecule entering the mass spectrometer, is generally highest when water is used as the solvent and the mass spectrometer inlet is heated above 500°C. The relationships of ion signal with mass flow into the mass spectrometer and nanoparticle diameter are also examined. ☐ Analysis by DAI is also used for investigations into the mechanism by which ions are ejected from droplets and into the gas phase for detection. Gas-phase ion formation is fundamentally achieved in two steps where the first is breakup of uncharged droplets to progeny droplets carrying a net charge and is followed by the second step where ions are ejected into the gas phase. This is done by computational fluid dynamics modeling to approximate changes in air pressure and temperature across the inlet of the mass spectrometer and is then combined with a numerical droplet evaporation model to examine the effect of inlet temperature on ion signal. Modeling studies indicate that the temperature dependence of droplet breakup does not explain the temperature dependence of the measured ion signal. The final interpretation is that the measured ion signal is representative of the temperature dependence of step two where ions are ejected into the gas phase. ☐ From the temperature dependence of the ion signal, the activation energy of ion formation is also determined. For nonionic test analytes, such as cortisone, the activation energy of ion formation is similar to the enthalpy of vaporization of water, suggesting that ion formation is determined by the evaporation of solvent. When nonionic analytes are mixed in solution with salts, the activation energy is lower than that of nonionic analytes in solution alone. This suggests changes in the ion formation mechanism in the presence of salts. The mechanism of ion formation for DAI is expected to be quite similar to the mechanism of ion formation by electrospray ionization. These results provide new insight into the ion formation mechanisms of not only DAI but also other methods such as electrospray ionization. ☐ The modeling and measurement approaches discussed in this dissertation are tools that future members of our research group will be able to utilize for studies of measuring and modeling the composition of secondary organic aerosol. Both current and future projects using measurement and modeling approaches are discussed at the conclusion of this dissertation.
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Keywords
Secondary organic aerosol