Argon reacts with water under ambient conditions

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 timescales, 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 cation ([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. Given the ubiquitous presence of water microdroplets in nature, this unexpected reaction redefines the landscape of noble gas chemistry on Earth and may serve as a potential marker of exotic aqueous processes in the cosmos.