The impact of synoptic forcing and vehicle traffic on atmospheric transport processes and NO₂ concentrations in a highly polluted urban street canyon

Ongoing air quality issues persist in Munich, Germany’s third most populous city, where annual nitrogen dioxide (NO 2) concentrations have exceeded national standards at a key monitoring station prior to 2024 since measurements began. According to European legislation, air quality plans must be implemented to address such exceedance. Additionally, upcoming revisions to EU regulations aim to halve the current annual threshold of 40 μg m−3 making effective reduction strategies essential. This study investigates the role of atmospheric transport processes in contributing to NO 2 concentration exceedances within a densely built urban street canyon characterized by heavy traffic and poor ventilation. Using a two-year observational campaign (2021–2023) alongside high-resolution Large Eddy Simulations (LES) with the PALM-4U model, we analyze the effects of wind direction, speed, turbulent kinetic energy (TKE), and traffic dynamics on pollutant concentrations. Ultrasonic anemometers and simulation scenarios focus on the two most relevant local flow regimes: a common northerly flow (44) and a less frequent westerly flow in the urban canyon. Findings show that the dominant northerly flow is decoupled of large-scale synoptic conditions and leads to pollutant accumulation and daily averaged NO 2 concentration reaching up to 72 μg m−3. Vehicle-induced turbulence has a non-linear relationship with NO 2 concentrations, highlighting the strong influence of diurnal traffic patterns. These results underscore that both emission reduction and enhanced air mixing are vital to mitigating urban air pollution in this street canyon. Addressing this dual challenge is crucial to meeting future air quality targets and safeguarding public health in urban environments.