Surface-Mediated Chemical Transformations of Aerosol Particles and Droplets

Atmospheric aerosol particles and droplets play a crucial role in Earth’s climate system by absorbing or scattering incoming solar radiation and nucleating cloud formation. In the atmosphere, particles and droplets can undergo significant changes in chemical composition, volatility and hygroscopicity via photochemical reactions, heterogeneous oxidation, condensation of low-volatility organic species, and homogeneous chemistry. Understanding these aging mechanisms is vital to evaluate the broader impacts of aerosol in the atmosphere. Because aerosol particles have a larger fraction of molecules at the surface compared to bulk solution, interface-mediated chemical transformations are expected to play a larger role in particles and droplets, potentially influencing the atmospheric aging mechanisms. Both heterogeneous and homogeneous reactions in aerosol particles and droplets can be altered by changing the chemical nature of the interface. This thesis addresses the fundamental role of the interface in altering heterogeneous reactions in diffusion-limited particles and in altering homogeneous reaction rates in droplets./nIn the first part of this thesis, aerosol photoemission is used to probe the evolving chemical nature of the surface of diffusion-limited solid aerosol particles. X-ray absorption (XAS) and X-ray photoelectron (XPS) spectroscopy are used to study the surface of squalene particles as they heterogeneously react with ozone. This is the first demonstration of aerosol photoemission to study free nanoparticle reactivity, providing insights into the ozonolysis mechanism (e.g., measuring a 16% yield of secondary ozonides). To characterize the surface sensitivity of the photoemission probes, the low kinetic energy (<5 eV) electron attenuation lengths in organic aerosol are measured. Electrons with >2 eV kinetic energy (which are used in the XAS measurements) are found to have electron attenuation lengths of 3-4 nm. Finally, the photoemission is used to study surfaces of diffusion-limited particles as they are heterogeneously oxidized by the OH radical. Even though bulk reactivity is low, the surfaces of diffusion-limited particles are measured to oxidize much more rapidly than the surfaces of particles that mix more rapidly. The chemical gradients that form in the diffusion-limited particles from oxidation are much steeper than initially expected, resulting in particles with highly oxidized surfaces at relatively low oxidant exposures./nThe second part of this thesis introduces new methods to study how interfaces can alter homogeneous reaction rates in droplets. Reactions in droplets have typically been probed in electrospray ionization droplets using mass spectrometry, which convolutes many different factors that can influence reaction rates. A new method which separates droplet generation from ionization is presented to help isolate the role of the interface in altering homogeneous reaction rates. Furthermore, a branched quadrupole trap (BQT), which can contactlessly levitate and merge droplets with precisely known compositions is described. Preliminary results from the BQT suggest that the synthesis of a fluorescent isoindole product is 20% faster in a 60-μm droplet compared to bulk solution, suggesting adsorption to and reaction at the interface can alter reaction rates in droplets.

Advisor: Wilson, Kevin R
Saykally, Richard J