Laboratory investigations of atmospheric nanoparticle formation and growth
Date
2019
Authors
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Publisher
University of Delaware
Abstract
Atmospheric nanoparticles are suspended particulate matter with diameters of 100 nm and smaller, and present a deleterious impact on human health and global climate. Particles within this size range can effectively alter the Earth’s radiative energy balance either directly by scattering or absorbing incident sunlight, or indirectly by modifying the formation and properties of cloud droplets. Nanoparticles represent the greatest fraction of ambient particulate matter by number, and the majority of these particles are formed directly within the atmosphere through a process by which gas molecules partition together to form molecular clusters on the order of 1 nm, which subsequently grow to larger sizes. The rapid growth of these particles up to a diameter of approximately 50 nm is crucial to their atmospheric survival and their climatic relevance, however the factors affecting this growth process remain poorly understood, leading to one of the greatest uncertainties in current climate models. Ultimately, a better understanding of the chemical processes underlying nanoparticle formation and growth is needed in order to refine future predictions in climate change due to atmospheric particles. ☐ This thesis aims to evaluate the potential for atmospheric nanoparticle formation and growth to occur by secondary chemical mechanisms that are currently under-represented or completely unaccounted for in today’s atmospheric climate models. First, the formation of particulate organic nitrogen-containing compounds occurring in aqueous nanodroplets was investigated using a combination of particle size characterization and mass spectrometry techniques. It is shown that aqueous reactions involving ammonium sulfate and atmospherically relevant dicarbonyls occur in nano-scale droplets, and can be a significant source of atmospheric organic particulate matter. Combined molecular and elemental composition measurements using mass spectrometry were utilized to determine the organic nitrogen-to-carbon ratio (N/C) of the droplets, and revealed a dependence on relative humidity, as well as organic precursor identity. Furthermore, this chemistry can help explain high amounts of particulate nitrogen measured in ambient nanoparticles during summertime new particle formation events that occurred in Lewes, Delaware, a location where gasphase concentrations of water-soluble organics are significant. ☐ Next, the implementation of a custom-built aerosol flow tube reactor is described in order to investigate the effect of the ubiquitous anthropogenic pollutant sulfur dioxide on nanoparticle formation and growth during the ozonolysis of various biogenic alkenes. Experiments were conducted in the presence of dry, monodisperse ammonium sulfate seed particles and an OH scavenger at a low relative humidity of 15%. Without sulfur dioxide, new particle formation was not observed, and seed particle growth was consistent with condensation of low-volatility oxidation products produced from each organic precursor. With sulfur dioxide, new particle formation was observed from every precursor studied, consistent with sulfuric acid formation by reaction of sulfur dioxide with carbonyl oxides (i.e. Criegee Intermediates) produced during alkene ozonolysis. The presence of SO2 was also found to alter the chemical composition of the particulate organic material via the formation of organosulfate compounds in the condensed phase. ☐ This investigation was then further expanded upon by conducting the same experiment for two of the organic precursors (α-pinene and β-pinene) under more atmospherically relevant conditions of higher relative humidity and aerosol liquid water content. In the absence of SO2, it was found that higher relative humidity led to a minor increase in seed particle growth from both precursors, whereas the effect of higher aerosol liquid water content on particle growth was precursor-dependent. In the presence of SO2, higher relative humidity led to a dramatic increase in the number of nucleated particles from both precursors when dry seed particles were used, however a decrease in nucleation was observed in the presence of deliquesced seeds. Additionally, organosulfates were detected in deliquesced particles from both α-pinene and β-pinene ozonolysis when SO2 was present, although their formation does not appear to depend on aerosol liquid water, and growth rates remained comparable to trials using effloresced seed particles. Finally, under all conditions studied, addition of SO2 was found to have no significant effect on particle growth by ozonolysis of α- pinene, but was found to dramatically increase particle growth by β-pinene. These results suggest that a previously unidentified pathway to particle growth activated by SO2 may alter aerosol climate effects in regions with significant anthropogenic-biogenic interactions.