Atmospheric Mixing Controls Air Pollution Chemistry and Mitigation Efficacy in High-Latitude Cities

Urban air pollution in cold environments poses a significant public health risk. However, the physicochemical processes determining the concentrations of primary and secondary pollutants remain poorly understood. This is due to fundamentally different conditions compared to warmer environments, such as extremely shallow polluted surface layers (PSLs) and low ultraviolet radiation. We apply an observation-driven chemical transport model to a multiday persistent PSL event during the 2022 Alaskan Layered Pollution and Chemical Analysis (ALPACA) experiment in Fairbanks, AK, USA. The simulations account for pollutant emissions, multiphase chemical kinetics, and turbulent and advective exchange of PSL air with the clean background atmosphere. This exchange is continuous, occurs on a time scale of 30 min to 3.5 h, and is essential for an accurate representation of the PSL composition. We find that measured diurnal cycles of particulate nitrate reflect the interplay between photochemical nitric acid formation and the loss of nitrate aerosol from of the PSL through mixing with clean background air. The continuous removal and replenishment of PSL aerosol counteracts self-acidification and prevents coagulation with acidic primary sulfate aerosol, thus sustaining sulfate and hydromethanesulfonate (HMS) formation throughout the pollution event. Sensitivity calculations show that within the coupled chemistry and transport system of a shallow PSL, reductions of nitrogen oxide emissions can change the multiphase oxidation regime and thereby even increase the levels of secondary nitrate and sulfate in the PSL.