Multiphase sulfur chemistry facilitates particle growth in a cold and dark urban environment.

Sulfate comprises an average of 20% of the ambient PM_2.5 mass during the winter months in Fairbanks, based on 24-hour filter measurements. During the ALPACA 2022 field campaign (Jan 15th-Feb 28th of 2022), we deployed two aerosol mass spectrometers (AMS) and one aerosol chemical speciation monitor (ACSM) at three urban sites, combined with Scanning Mobility Particle Sizers (SMPS), to examine the evolution of aerosol composition and size distribution at a sub-hourly time scale. During an intense pollution episode with ambient temperature between -25 and -35 °C, all three instruments (two AMS and one ACSM) recorded a sharp increase in sulfate mass, ranging from 5 to 40 μg m^-3 within a few hours. This increase contributed up to half of the observed rise in ambient PM_2.5 mass concentration and coincided with a substantial shift in the number distribution from particle sizes less than 100 nm diameter (D_p < 100 nm) to larger particles (D_p > 100 nm) with little increase in number concentration. The corresponding increase in the volume concentration and distribution shift to larger particle size suggests the secondary formation of sulfate and organic aerosol onto pre-existing aerosols. Comparing AMS-sulfate (all sulfur species) to inorganic sulfate measured by online particle-into-liquid sampler-ion chromatography (PILS-IC), we find roughly 80% of sulfate increase was due to organic sulfur, consistent with the observation of mass spectral signatures in the AMS of organosulfur compounds. The rapid formation of sulfate appears to coincide with spikes in ambient aldehyde concentrations (formaldehyde and acetaldehyde) and an increase in S(IV) in ambient PM_2.5. This likely results from multiphase chemistry, where hydroxymethanesulfonate (HMS) and other aldehyde-S(IV) adducts are formed through reactions between aldehydes and SO₂ in deliquesced aerosols. We estimate that all S(IV) species, including HMS, contribute an average of 30% to aerosol sulfur, with a dominant fraction occurring during rapid sulfate increase events. Our work highlights the crucial role of controlling aldehydes to mitigate severe air pollution events in Fairbanks and may apply to other urban areas. It also emphasizes the significance of multiphase chemistry in driving particle growth from Aitken mode to accumulation mode, a key step for aerosol-cloud interactions.