Volatile organic compounds and their role in secondary aerosol chemistry in a cold and dark urban environment

Abstract. Wintertime PM_2.5 pollution is a longstanding issue in the urban subarctic environment such as Fairbanks, Alaska. While previous studies suggest that aldehydes may serve as precursors of S(IV) species in aerosols, the role of volatile organic compound (VOC) emissions in secondary aerosol chemistry remains poorly understood. Here, we use measurements from an online proton transfer reaction time of flight mass spectrometer (PTR-ToF-MS), combined with complementary gas and aerosol measurements from the the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in 2022, to examine VOC sources and their roles in aerosol chemistry in downtown Fairbanks. We find that alcohols, aromatics and carbonyls together account for ~70% of measured VOCs, with methanol, ethanol, formaldehyde, benzene and toluene as dominant species. Positive matrix factorization (PMF) analysis indicate that approximately 56% of VOCs are associated with vehicle emissions, while wood heating and heating oil together contribute about 14%. Formaldehyde is primarily linked to diesel emissions, as well as primary and secondary sources associated with aged air masses. By comparing PMF factors with measured PM_2.5 S(IV) species, we find that vehicle-related emissions of ammonia and formaldehyde likely play a key role in the formation of hydroxymethanesulfonate (HMS) in Fairbanks.