Sulfur and Oxygen Isotopes Show That Primary Sulfate is the Dominant Source of Particulate Sulfate in Fairbanks, AK

Abstract

Within and surrounding high-latitude cities, poor air quality disturbs Arctic ecosystems, influences climate, and harms human health. The Fairbanks North Star Borough has wintertime particulate matter (PM) concentrations that exceed the Environmental Protection Agency’s (EPA) threshold for public health. Particulate sulfate (SO42-) is the most abundant inorganic species and contributes approximately 20% of the total PM mass in Fairbanks, but air quality models underestimate observed sulfate concentrations. Here we quantify sulfate sources using size-resolved δ34S(SO42-), δ18O(SO42-), and Δ17O(SO42-) of particulate sulfate in Fairbanks from January 18th to February 25th, 2022 using a Bayesian isotope mixing model. Primary sulfate contributes 62 (±12%) of total sulfate mass on average. Most primary sulfate is found in the size bin with particle diameter < 0.7μm, which contains 90 (±5%) of total sulfate mass and poses the greatest risk to human health. Oxidation by all secondary formation pathways combined contributes 38 (±12%) of total sulfate mass on average, indicating that secondary sulfate formation is inefficient in this cold, dark environment. On average, the dominant secondary sulfate formation pathways are oxidation by H2O2 (12 ± 6%), O3 (8 ± 4%), NO2 (8 ± 3%), OH (8 ± 3%),, and TMI-O2 (2 ± 1%),. These findings will inform mitigation strategies to improve air quality and public health in Fairbanks and possibly other high-latitude urban areas during winter.