Snow may hold impurity deposits, such as sea salt aerosol, which show vivid chemistry. Snow's porosity guarantees the efficient gas exchange of reaction products with the overlaying air. That snow thus acts as a chemical reactor on Earth with impacts on air quality, climate, and geochemical cycles has long been recognized. X-ray excited electron spectroscopy at near ambient pressure offers the possibility to investigate the underlying processes at surfaces of atmospheric relevance with high sensitivity and chemical specificity. This approach thus fills a unique gap in atmospheric science.Here, I will present the results of our work at the Swiss Light Source tackling 2 seemingly simple questions: 1) At which temperature do aqueous solutions freeze, and 2) How do acidic trace gases dissociate at ice surfaces?
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