Nature of Temperature-InducedPhase Transitions inSecondary Organic Aerosol Particles

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This study investigates how changes in temperature affectthe secondaryorganic aerosol (SOA) phase state. SOA was formed by α-pineneozonolysis in an atmospheric simulation chamber at temperatures (T0) in the range of 257–283 K at RH0 < 20%. After more than 14 h of SOA aging, one or moreheating and cooling ramps were performed. Upon heating, we observethat the onset of evaporative SOA shrinkage is delayed by up to ∼20K relative to T0. Our observations aresupported 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 tosemisolid. We demonstrate that the SOA content of highly oxygenatedorganic molecules (HOMs) increases with T0 and at lower α-pinene concentrations. Higher HOM SOA contentresults in more viscous SOA with a higher glass transition temperature(Tg). The model was used to quantify how Tg varies with T0 and the amount of α-pinene being oxidized. Because the SOAphase state is influenced by the conditions under which it forms,and affects SOA lifetime, reactivity, water uptake, and potentiallyice nucleating properties, the results presented herein may have wideimplications for the design of future SOA experiments, air quality,and climate.