Liquid-Liquid Phase Separation in Secondary Organic Aerosols from Phenolic Compounds and Terpenes: Contrasting Daytime and Nighttime Generation

SEPEHR NIKKHO, Melika Javanmardi, Julia Zaks, Allan K. Bertram, University of British Columbia

     Abstract Number: 142
     Working Group: Aerosol Physics

Abstract
Secondary organic aerosol (SOA) forms when volatile organic compounds (VOCs) are oxidized in the atmosphere by hydroxyl radicals (OH), ozone (O₃), or nitrate radicals (NO₃). OH dominates as an oxidant during the day, while NO₃ is more important at night. Key VOC sources include vegetation (e.g., α-pinene) and biomass burning (e.g., guaiacol and syringol). While many studies have investigated the phase behavior of SOA formed from OH and O₃ oxidation, there is limited research on the phase behavior of SOA produced by NO₃ oxidation.

We investigated liquid–liquid phase separation (LLPS) of secondary organic aerosol (SOA) using an optical microscope coupled to a flow cell with controlled relative humidity (RH). SOA was generated using NO₃ and OH oxidation of guaiacol, syringol, and α-pinene. For all VOCs oxidized by NO₃, the resulting SOA underwent LLPS at high RH, forming distinct organic-rich and water-rich phases. Similarly, SOA formed from α-pinene and OH also exhibited LLPS at high RH. In contrast, SOA generated from guaiacol and syringol with OH remained as a single phase across the entire RH range tested. These results show that nighttime chemistry (NO₃ oxidation) can produce SOA with two phases at high RH and highlight substantial differences in SOA phase behavior between day and night, with important implications for their cloud-nucleating potential.