Mechanisms and kinetics of enhanced reactivity in droplets relative to bulk solution
Date
2022
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Publisher
University of Delaware
Abstract
Atmospheric aerosol, particulate matter in solid and/or liquid phase suspended in air, has important impacts on climate and human health. Atmospheric aerosol can be biogenic or anthropogenic in origin and is ubiquitous in the environment. A great portion of atmospheric aerosol is organic and can arise from either primary or secondary sources. Primary organic aerosol (POA) refers to these particles that are directly emitted into the atmosphere such as vehicle exhaust, and during cooking and biomass burning. Secondary organic aerosol (SOA) is produced by oxidation of volatile organic compounds (VOCs) in air. The chemical composition of SOA is highly complex, because multiple reaction pathways exist, encompassing several hundred distinct molecular formulas. Droplet chemistry may play a significant role in the composition and growth of atmospheric particulate matter, which represents one of the uncertainties that need to be reduced for model development to accurately estimate the mass concentration and rate of organic aerosol formation. ☐ It is known that various chemical reactions can be accelerated in droplets relative to bulk solution. Reaction acceleration in droplets has been studied using a variety of mass spectrometry ionization methods, for example, electrospray ionization, desorption electrospray ionization, and microdroplet fusion mass spectrometry. Most of these ionization methods require applying a high voltage for droplet generation as a part of the ionization process, making it difficult to differentiate the conditions that are unique to droplet environment from ionization. In this dissertation, the mechanism of reaction acceleration in droplets was investigated using droplet assisted ionization (DAI), where droplet generation was completely separated from ionization, and no high voltage was required. Product formation was studied as a function of various parameters (droplet residence time, temperature, relative humidity, and droplet size) that could be controlled independently of the conditions needed for efficient ionization by DAI. ☐ The reaction chosen for the study was Girard’s T (GT) reaction. GT reagent could selectively derivatize and thereby identify carbonyls that are abundant in the SOA generated by -pinene ozonolysis. GT-carbonyl products formed from this reaction were readily observed when droplets containing -pinene SOA and GT were generated, allowed to react for a short period of time (seconds), and then analyzed with DAI. Varying the residence time between droplet formation and analysis by mass spectrometry allowed the reaction rate constant to be determined, which was found to be 4 orders of magnitude faster than what would be expected from bulk solution kinetics. Decreasing the water content of the droplets, either by heating the capillary inlet to the mass spectrometer or by decreasing the relative humidity of the air surrounding the droplets, enhanced product formation. The results suggest that reaction enhancement probably occurs at the droplet surface. ☐ Reaction on the droplet surface was further validated from size dependence experiments where product formation was studied for both heterogeneous and homogeneous systems. For reaction of gas phase pinonaldehyde (a major product of -pinene ozonolysis) with both dry and wet (aqueous) nanosized particles containing GT reagent, the signal intensity ratio of product to GT reagent was shown to be inversely proportional to the particle size (i.e. linear with 1/R where R is the particle radius). The apparent equilibrium constant for wet particles was found to be enhanced by at least 4 to 5 orders of magnitude relative to bulk aqueous solution. Product formation in nanosized wet particles containing SOA and GT reagent also increased as surface-to-volume ratio, but a plot of signal intensity ratio vs. 1/R was not linear. This nonlinearity was attributed to the multiphase character (organic and aqueous phases) of these particles. The size dependence experiments highlight the impact of a high surface-to-volume ratio for enhancing product formation in droplets. ☐ Lastly, a new ionization source, extractive droplet assisted ionization (E-DAI), was designed, and constructed for the purpose of studying the effect of charge on the formation of products in droplets and online (derivatization) analysis of organic aerosol. In this ionization method, aerosol particles containing analytes collide with electrospray droplets, where analyte components are extracted into charged electrospray droplets before entering capillary inlet where ionization occurs. Results of preliminary experiments showed that formation of products was not observed at room temperature when aerosol particles of cortisone (a ketosteroid) were mixed with electrospray droplets containing GT reagent. Therefore, heating was then applied on the capillary inlet to assist the reaction, yet it did not turn out to be an efficient method either. Neither the signal intensity of reactants nor the signal intensity ratio (Products/GT) was efficiently enhanced as the temperature increased. Future work would involve optimization of this ionization method as well as ionization mechanism study of E-DAI.
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Keywords
Atmospheric aerosol