Aerosol composition, air quality, and boundary layer dynamics in the urban background of Stuttgart in winter

Abstract. Aerosol distribution is of great relevance for air quality especially for the cities like Stuttgart which is located in a basin. To understand the impact of boundary layer mixing processes on local air quality and to validate the large eddy simulation (LES) model PAML-4U, we collected a comprehensive set of data from remote sensing, in-situ methods including radiosondes for the urban background of downtown Stuttgart. Stagnant meteorological conditions caused accumulation of aerosols during winter. Case studies show that clouds during previous nights can weaken temperature inversion and accelerate boundary layer mixing after sunrise. This is important for ground-level aerosol dilution during morning rush hours. Furthermore, our observations validate results of the LES model PALM-4U in terms of boundary layer heights and aerosol mixing for 48 hours. The simulated aerosol concentrations follow the trend of our observations but are still underestimated by a factor of 4.5 ± 2.1. This underestimation is mainly due to uncertainties of emissions and boundary conditions of the model. This paper firstly evaluates the PALM-4U model performance in simulating aerosol temporal-spatial distribution, which can help to improve the LES model and to better understand sources and sinks for air pollution as well as the role of horizontal and vertical transport.