Unveiling Argon's Hidden Reactivity: Discovery of Argon Hydroxide Cations at the Air-Water Interface of Microdroplets

Argon is the third most abundant gas in the atmosphere and is regarded as chemically inert. Unlike many other atmospheric gases that have undergone fixation over geological time scales, argon has not been known to form stable compounds. Contrary to this century-old belief, we report that argon can react at the surface of tiny water droplets suspended in air to form argon hydroxide cations ([ArOH]⁺) at room temperature. Remarkably, this species can be generated and maintained in equilibrium within an aerosol vial for extended periods, following real-time monitoring by mass spectrometry. Mechanistic analysis suggests that the water microdroplet surface carries a net charge and, during its evolution in air into smaller droplets, generates corona discharge, producing [ArOH]⁺ at the air-water interface. Given the ubiquitous presence of water microdroplets in nature, this reaction of argon with water redefines the landscape of noble gas chemistry on Earth and beyond.