Atmosperic and interfacial reaction dynamics

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Surfaces exposed to the atmosphere control many key atmospheric processes, such as activation of halides at snow and sea surfaces, loss of nitrogen oxide compounds, inhibition of heterogeneous aqueous chemistry by organic coatings and photochemical recycling of nitrates deposited on urban surfaces. An overarching theme of our research program is to relate the chemical morphology of various atmospheric surfaces to  heterogeneous reactivity. This work will build on the advances we have made during the past and will introduce some new tools as well. We will concentrate on aqueous systems, both liquid and frozen. (1) We will explore whether differences in solution photochemistry observed for natural organic material in seawater manifest as different interfacial photochemistry at sea surfaces. Glancing-angle LIF methods will measure adsorption isotherms for the samples at the freshwater and seawater surfaces, in the absence and presence of an artificial surface microlayer, to establish how effectively they may act as photosensitizers at the air-water boundary. In collaboration with the Lyon group, we will use PTR-MS to measure gas phase organic compound emission from illuminated freshwater and seawater samples containing these compounds. We will infer the production of organic radicals adsorbed at the water surface via the reaction of NO with R-O2. These experiments will provide key data concerning differences in the formation and chemical reactions of atmospheric oxidants and radicals at seawater and freshwater surfaces. (2) We will use quantum chemical and molecular dynamics tools to understand the reason(s) for the differences between the exclusion behaviors of nitrate and sulfate anions during freezing of aqueous solutions. We will combine different Raman probes with gas phase nitrogen oxide detection to relate the solvation state and specific location of the nitrate in frozen solutions to its photoreactivity. New experiments will be initiated using an x-ray beamline of a synchrotron source (at the Canadian Light Source or at the Paul Sherer Institute in Switzerland) to determine whether nitrate is co-located with other ions differently at the air interface than within the ice matrix. (3) We will monitor heterogeneous atmospheric chemistry within droplets, in real time. We intend to: (a) fully explore the morphology changes associated with evaporating mixed organic-inorganic drops, to understand the processes responsible. (b) Further develop spectroscopic probes to measure heterogeneous kinetics within droplets. Specifically, we will use absorption and Raman probing within the droplets (to monitor pH and composition) and gas phase NOx measurements in the surrounding air to explore the reason(s) for the different responses of NO2 and CO2 to organic coatings during interfacial transport. (c) Use laser-based Raman and fluorescence probes to interrogate composition (and kinetics) differences in the bulk vs. near-surface regions of the droplet.