Understanding secondary atmospheric chemistry in extremely cold and dark environments through hydroxymethanesulfonate (HMS) measurements in wintertime Fairbanks, Alaska

Abstract

Fairbanks, Alaska is a subarctic city that regularly violates PM2.5 air quality standards in the winter, exacerbated by strong wintertime inversion layers and wood stove heating. While primary sources of PM2.5 in Fairbanks have been studied extensively, secondary chemistry remains not well understood. Here, we focus on the aqueous secondary product hydroxymethanesulfonate (HMS), a major sulfur(IV) particulate species, to help understand what drives secondary pollution in Fairbanks. In 2020 and 2021, we used a particle-into-liquid sampler coupled with ion chromatography (PILS-IC) to measure HMS and sulfate concentrations. We report that HMS concentrations are high compared to other highly polluted regions like Beijing, and HMS makes up a significant portion of PM2.5 mass. In 2022, we repeated these measurements as part of the Alaska Layered Pollution And Chemical Analysis (ALPACA) campaign, and examined the processes that allow accelerated formation of HMS in Fairbanks. We find that HMS formation is likely driven by the extremely cold conditions of Fairbanks. HMS formation occurs in aqueous droplets and is highly pH dependent, forming much faster at higher pH. The extreme cold of Fairbanks lowers the volatility of gas-phase ammonia, which leads to increased solubility of aqueous-phase ammonia and increased partitioning to aqueous-phase ammonium, raising particle pH and causing rapid production of PM2.5 HMS. We show how this mechanism applies to other regions globally, and how it can affect pH-dependent sulfate production. To better understand the HMS precursors formaldehyde and SO2 gas, as well as gas-phase pollutants in general, we made volatile organic compound (VOC) measurements in 2022. Many VOC concentrations are much higher in wintertime Fairbanks compared to wintertime measurements in other US cites. We find that diesel vehicles and air mass transport (possibly from nearby areas with more wood heating) are large sources of formaldehyde. Aromatic pollutants like benzene, toluene, and C8 aromatics largely come from gasoline vehicles and heating oil. Methanol, the single largest VOC, was related to windshield wiper fluid. These measurements and calculations lead to a better understanding of secondary pollutant products and VOC concentrations in cold and dark environments like wintertime Fairbanks, AK, and can hopefully contribute to pollution mitigation strategies in the future.