Rapid Wintertime Formation of Phenolic Secondary Organic Aerosol from Brown Carbon Photochemistry.

Biomass burning (BB) is a major organic aerosol source during urban winter pollution events. BB particles are rich in brown carbon (BrC) and absorb light to produce photooxidants such as oxidizing triplet excited states of organic chromophores (3C*). In midlatitudes, semivolatile phenols (ArOH) from BB are oxidized to form phenolic secondary organic aerosol (SOA) under moderate temperatures and sunlight. Here, we explore whether phenolic SOA is also formed in a cold, relatively dark, high-latitude city during winter. We use measurements from the 2022 ALPACA field campaign in Fairbanks, Alaska, incorporated into the PACT-1D model with added emissions, partitioning, and chemistry for 34 semivolatile BB phenols. The model includes phenol oxidation in the gas phase and in two populations of particles─aqueous and organic. Despite short pollutant residence times within the polluted surface layer, simulated phenolic SOA formation is fast, generating up to 4.5 μg m-3, with 99% produced by 3C* in organic particles. The modeled phenolic SOA explains up to 34% of measured daytime oxidized organic aerosol (OOA), indicating it is a significant portion of the poorly understood OOA in Fairbanks. Our results reveal that BrC photochemistry can be an important source of atmospheric oxidizing capacity even during high-latitude winters.