Nature of Temperature-Induced Phase Transitions in Secondary Organic Aerosol Particles

This study investigates how changes in temperature affect the secondary organic aerosol (SOA) phase state. SOA was formed by α-pinene ozonolysis in an atmospheric simulation chamber at temperatures (<i>T</i>₀) in the range of 257-283 K at RH₀ < 20%. After more than 14 h of SOA aging, one or more heating and cooling ramps were performed. Upon heating, we observe that the onset of evaporative SOA shrinkage is delayed by up to ∼20 K relative to <i>T</i>₀. Our observations are supported by aerosol dynamics and kinetic multilayer model simulations, relating observed changes to an effectively reversible temperature- and SOA-composition-dependent phase transition from solid glassy to semisolid. We demonstrate that the SOA content of highly oxygenated organic molecules (HOMs) increases with <i>T</i>₀ and at lower α-pinene concentrations. Higher HOM SOA content results in more viscous SOA with a higher glass transition temperature (<i>T</i>_g). The model was used to quantify how <i>T</i>_g varies with <i>T</i>₀ and the amount of α-pinene being oxidized. Because the SOA phase state is influenced by the conditions under which it forms, and affects SOA lifetime, reactivity, water uptake, and potentially ice nucleating properties, the results presented herein may have wide implications for the design of future SOA experiments, air quality, and climate.