Laboratory investigations of Aitken mode particle growth by α-pinene ozonolysis
Date
2019
Authors
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Publisher
University of Delaware
Abstract
Atmospheric aerosols represent the greatest remaining uncertainty in the overall understanding of climate change. Currently climate models do not accurately predict cloud condensation nuclei (CCN) concentrations leading to high uncertainty in the overall cooling effect on global climate by aerosols. Small nanoparticles (10-100 nm in diameter) have also been shown to have negative effects on human health. The growth of particles in this size range is complex and many processes are poorly understood leading to over simplifications in climate models. Growth in this size range is dominated by low volatility organics that are present in the gas phase of the ambient atmosphere. These gas phase molecules are oxidation products of organic precursors that can be emitted from various natural and anthropogenic sources. These precursors are oxidized by species such as ozone, OH and NO3 to produce a large number of oxidation products with a wide range of chemical properties. The wide range of chemical properties cause particles to grow by several processes including condensation, equilibrium partitioning and particle phase reaction, further increasing complexity. Finally, the simultaneous presence of many precursors and oxidants in the atmosphere even further increases the number of potential oxidation products that can be created. ☐ The work described in this dissertation aims to improve the understanding of particle growth in this size range by simulating different atmospheric conditions in a flow tube reactor and measuring the resulting particle diameter and composition changes. The flow tube reactor used in this work was custom built from a quartz tube with stainless steel funnels on either end. The reactor was characterized both experimentally and using computational fluid dynamic (CFD) simulations to ensure a known and reproducible residence time of particles, which was approximately four minutes under the conditions used in these experiments. Knowing the reaction time along with the concentrations of reactants allowed for kinetic modeling of oxidation products formed in the flow tube. Growth of size-selected ammonium sulfate seed particles by α-pinene ozonolysis was studied under dry conditions (RH 10%) at room temperature to determine the amount of condensable material needed to explain the measured growth. Based on the results, a molar yield of highly oxidized molecules (HOMs) of 13±1% is reported for α-pinene ozonolysis. ☐ The experimental setup was then modified to allow experiments to be done under high relative humidity and with seed particles containing aerosol liquid water (ALW). The results showed that growth of dry seed particles was independent of relative humidity (RH), providing evidence that formation of condensable molecules in the gas phase is independent of water vapor. However, when aerosol liquid water was present in the particles, a significant growth enhancement was observed, especially at smaller seed particle diameters where seeds were more likely to be homogeneous. The work described here provides valuable insight into the mechanisms by which aerosol particles grow when exposed to α-pinene oxidation products and will help to increase the understanding of particle growth into CCN size ranges in order to improve predictions made by climate models.