| Summary: | The acidity of sea ice and sea ice aerosols plays a key role in the reactivity of the cryosphere, leading to or mediating processes such as the oxidation of bromine, which results in tropospheric ozone depletion events. We performed laboratory experiments to assess the acidity and subsequently used an environmental scanning electron microscope to observe the particles that emanate from the sublimating sea ices. For the acidity assessment, we propose a spectrophotometric method based on sulfonephthalein indicators employed in the frozen state. The diffuse reflectance UV-Vis approach thus allows estimating the local acidity at the level of molecular interactions and at environmentally relevant temperatures. Our results show a strong freezing-induced acidity increase in sea water, especially as regards solutions of low salinity. Importantly, the microscopic observation of sea ice sublimation revealed a major dependence of the temperature and concentration on the emanating aerosol-sized salt particles. In this context, the sublimation temperature of the ice is a dominant physical factor to determine the size of the residua: Below −20 °C, micron-sized pieces of salt emerge, whereas above this temperature large chunks of salt are detected. Concentration also plays a role in particle size distribution: Micron-sized particles are observed exclusively at salinities below 3.5 psu, while below 0.085 psu particles with a median smaller than 6 μm arise from sea ices at any subzero temperature.Generally, we relate our findings to the production of the polar atmospheric sea salt aerosols and acid-catalyzed reactivity reactions (e.g., photochemical bromine recycling).
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