The potential for atmospheric aerosol particles to exist as metastable aqueous droplets instead of in their thermodynamically favoured crystalline solid phase has many implications for atmospheric physics and chemistry. Aqueous droplets are larger than their dry counterparts1 and generally scatter more light2,3. Also, the existence of an aqueous phase influences the oxidation, equilibrium vapour pressure and mass-transfer kinetics of trace gases4-9. Here we report observations at three urban and rural sites, showing that metastable droplets existed more than 50% of the time when the ambient relative humidity was between ∼45 and 75%. At one site, more than 10% of the aerosol's light-scattering coefficient was attributed to metastable liquid H2O when the relative humidity was greater than 55%. These results indicate that metastable droplets are ubiquitous in nature, although the full ramifications of these observations are still unclear.
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