Surprisingly Robust Photochemistry in Subarctic Particles During Winter: Evidence from Photooxidants

Abstract. Subarctic cities notoriously experience severe winter pollution episodes with PM_2.5 concentrations above 35 µg m^-3, the US EPA’s 24-hour standard. While winter sources of primary particles in Fairbanks, Alaska have been studied, the chemistry driving secondary particle formation is elusive. Biomass burning is a major source of wintertime primary particles, making the PM_2.5 rich in light-absorbing brown carbon (BrC). When BrC absorbs sunlight, it produces photooxidants – reactive species potentially important for secondary sulfate and secondary organic aerosol formation – yet photooxidant measurements in high-latitude PM_2.5 remain scarce. During the winter 2022 ALPACA field campaign in Fairbanks, we collected PM filters, extracted the filters into water, and exposed the extracts to simulated sunlight to characterize the production of three photooxidants: oxidizing triplet excited states of BrC, singlet molecular oxygen, and hydroxyl radical. Next, we used our measurements to model photooxidant production in highly concentrated aerosol liquid water. While conventional wisdom indicates photochemistry is limited during high-latitude winters, we find that BrC photochemistry is significant: we predict high triplet and singlet oxygen daytime particle concentrations up to 2x10^-12 M and 3x10^-11 M, respectively, with moderate hydroxyl radical concentrations up to 5x10^-15 M. Although our modeling predicts triplets account for 0.4–10 % of daytime secondary sulfate formation, particle photochemistry cumulatively dominates, generating 76 % of daytime secondary sulfate formation largely due to in-particle hydrogen peroxide, which contributes 25–54 %. Finally, we estimate triplet production rates year-round revealing highest rates in late winter when Fairbanks experiences severe pollution and in summer when wildfires generate BrC.