Environmental Interfaces, such as air-water interfaces and air-ice interfaces, are ubiquitous in the environment. For large systems, such as the sea surface, the surface to volume ratio is small, which makes the interface negligible. However, for small systems, such as aerosols and small domains within the ice and snow structures, the surface to volume ratio is large, thus the significance of interfaces can no longer be overlooked. Chemical and physical properties at interfaces could be thoroughly different from bulk phase properties. Such unique properties enable environmental interfaces to affect all kinds of environmental processes, such as ice and cloud nucleation, the formation of secondary aerosols as well as the reactivity of chemical processes. Chemical and physical properties at interfaces are critical to environmental processes yet understudied due to limited characterization approaches. In our study, we attempted to characterize: 1) the chemical moiety distribution using Raman spectroscopy, 2) pH distribution by applying pH nanoprobe enabled surface-enhanced Raman spectroscopy (SERS), at different types of interfaces. The interfaces include the air-water interface of the aerosol droplets as well as the air-ice interface of frozen aerosol droplets, frozen bulk solutions and snow. The droplets and solutions consisted of environmentally abundant inorganic electrolytes such as ammonium salts and sodium chloride.<br/>For micron-size aerosolized droplets, a 2D Raman map with a spatial resolution of 1 µm<sup>2</sup> was generated for single droplets. The distribution of each chemical moiety within the droplet can be represented by the contour map generated according to the corresponding peak. The pH inside droplets was measured by putting a SERS-active pH nanoprobe into the droplet and generating a 2D pH map. Droplets were frozen by a temperature-control stage to observe the distribution change during the phase transition.  Frozen bulk solutions and snow field sample was studied by the combined system of the temperature-control stage and the Raman microspectroscopy. The characterization method for chemical moiety and pH distribution was the same for droplets. The distributions within small domains of ice crystals and snow were observed. A localized distribution of chemical moieties and pH values indicated the interfacial effect was observed in this study. Such observation can provide molecular-level implications for the interfacial effects on environmental processes.

Characterize the chemical moiety and pH distribution at dynamic environmental interfaces with Raman spectroscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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