Organic aerosol is ubiquitous in the atmosphere, and impacts climate through the scattering and absorption of light and through the formation of nuclei for cloud droplets. These aerosol particles, which are composed of organic compounds and salts, are of great recent interest due to the complex chemistry that occurs within the particles as well as at the air-aerosol interface. Historically, organic aerosol was thought to undergo two phase transitions as the relative humidity around the particles is varied: efflorescence (crystallization) and deliquescence (water uptake). Recently, however, it was proposed that organic aerosol can undergo a phase transition in which liquid-liquid phase separation results in the formation of a particle with two liquid phases. This phenomenon has been recognized in the biophysical chemistry community for over a century, but atmospheric systems differ in several key aspects. Over the past 15 years, characterisation of the systems that undergo phase separation, the mechanisms by which this phase transition occurs, and the resultant morphologies have been investigated, sometimes with lingering questions. In addition, theory has been developed to model liquid-liquid phase separation in bulk systems. This review will cover these studies, focusing on experimental results, as well as covering recent results on the inhibition of liquid-liquid phase separation in nanoscale particles and studies that address the implications of this phase transition on climate-related properties of aerosol particles.
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