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Abstract. Fairbanks, Alaska, is a sub-Arctic city that frequently suffers from the non-attainment of national air quality standards in the wintertime due to the coincidence of weak atmospheric dispersion and increased local emissions. As part of the Alaskan Layered Pollution and Chemical Analysis (ALPACA) campaign, we deployed a Chemical Analysis of Aerosol Online (CHARON) inlet coupled with a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF MS) and an Aerodyne high-resolution aerosol mass spectrometer (AMS) to measure organic aerosol (OA) and non-refractory submicron particulate matter (NR-PM1), respectively. We deployed a positive matrix factorization (PMF) analysis for the source identification of NR-PM1. The AMS analysis identified three primary factors: biomass burning, hydrocarbon-like, and cooking factors, which together accounted for 28 %, 38 %, and 11 % of the total OA, respectively. Additionally, a combined organic and inorganic PMF analysis revealed two further factors: one enriched in nitrates and another rich in sulfates of organic and inorganic origin. The PTRCHARON factorization could identify four primary sources from residential heating: one from oil combustion and three from wood combustion, categorized as low temperature, softwood, and hardwood. Collectively, all residential heating factors accounted for 79 % of the total OA. Cooking and road transport were also recognized as primary contributors to the overall emission profile provided by PTRCHARON. All PMF analyses could apportion a single oxygenated secondary organic factor. These results demonstrate the complementarity of the two instruments and their ability to describe the complex chemical composition of PM1 and related sources. This work further demonstrates the capability of PTRCHARON to provide both qualitative and quantitative information, offering a comprehensive understanding of the OA sources. Such insights into the sources of submicron aerosols can ultimately assist environmental regulators and citizens in improving the air quality in Fairbanks and in rapidly urbanizing regional sub-Arctic areas. 

The prevailing view for aqueous secondary aerosol formation is that it occurs in clouds and fogs, owing to the large liquid water content compared to minute levels in fine particles. Our research indicates that this view may need reevaluation due to enhancements in aqueous reactions in highly concentrated small particles. Here, we show that low temperature can play a role through a unique effect on particle pH that can substantially modulate secondary aerosol formation. Marked increases in hydroxymethanesulfonate observed under extreme cold in Fairbanks, Alaska, demonstrate the effect. These findings provide insight on aqueous chemistry in fine particles under cold conditions expanding possible regions of secondary aerosol formation that are pH dependent beyond conditions of high liquid water. 

Fairbanks-North Star Borough, Alaska (FNSB) regularly experiences some of the worst wintertime air quality in the United States.