Simulation of aromatics in Fairbanks, Alaska during the wintertime ALPACA-2022 campaign

Benzene, toluene, ethylbenzene, and xylene (BTEX) are hazardous air pollutants with high toxicity and a strong potential for secondary pollutant formation. However, their occurrence and behavior in the Arctic remain poorly understood. During the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in Fairbanks, Alaska in January-February 2022. Surface observations in downtown Fairbanks revealed two major pollution periods, with extremely cold (down to -35°C) and warmer temperatures (around 0°C), respectively. BTEX concentrations reached 4–12 times higher than those reported in the US and European countries under dark, cold Arctic winter conditions at breathing level, posing a significant health risk. We simulated BTEX atmospheric distributions in the Fairbanks region using the FLEXible PARTicle-Weather Research and Forecasting (FLEXPART-WRF) Lagrangian particle dispersion model and anthropogenic emissions at the surface and aloft. Due to limited photochemical loss in to the dark polar winter conditions, we treat BTEX as an unreactive tracer in the model. The control run with the emission inventory developed by Alaska Department of Environmental Conservation (ADEC) substantially underestimates BTEX concentrations compared to observations during both polluted periods, indicating deficiencies in winter emissions and near-surface mixing. Enhancing cold-start gasoline vehicle emissions by a factor of 2 during very low-temperatures substantially improved model results during the cold polluted period, while introducing a relative humidity dependence for mobile emissions improved simulated BTEX during the warm, humid pollution period. Addition of emissions of residential heating oil aromatics, not taken into account in the ADEC inventory, also reduced normalized mean biases by 5-10%.The improved model simulation was used to investigate contributing source sectors. While mobile traffic emissions were identified as the dominant source of BTEX across the Fairbanks North Star Borough, residential heating and non-point sources contributed substantially in downtown Fairbanks. Replacing residential wood burning in the inventory with oil heating during severe pollution periods, in line with air quality control guidelines, was found to effectively reduce BTEX concentrations, particularly benzene by up to 30%. While persistent surface-based temperature inversions largely confined BTEX below ~20 m, upward transport, induced by wind shear, during severe episodes, sometimes lofted near-surface pollutants to higher altitudes, potentially contributing to regional pollution and background Arctic haze.The findings of this study emphasise the need to accurately account for temperature and humidity dependent vehicle emissions, residential oil heating emissions, and winter boundary-layer dynamics for improved simulations of air quality in cold wintertime environments, not only in the Arctic but also in mid-latitudes.