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

Aerosol distributions are of great relevance for air quality, especially for cities like Stuttgart, which
has limited air exchange due to its location in a basin. We collected a comprehensive set of data from remote
sensing and in situ methods including radiosondes for the urban background of downtown Stuttgart to determine
the impact of boundary layer mixing processes on local air quality and to evaluate the simulation results of the
high-resolution large eddy simulation (LES) model PALM-4U at 10 m grid spacing. Stagnant meteorological
conditions caused accumulation of aerosols, and chemical composition analysis shows that ammonium nitrate
(37 ± 9 %) and organic aerosol (OA; 34 ± 9 %) dominated during this winter study. Case studies show that clouds
during previous nights can weaken temperature inversion and accelerate boundary layer mixing after sunrise by
up to 3 h. This is important for ground-level aerosol dilution during the morning rush hour. Furthermore, our
observations validate results of the LES model PALM-4U in terms of boundary layer heights and aerosol mixing
for 48 h. The simulated aerosol concentrations follow the trend of our observations but are still underestimated
... mehr
by a factor of 4.5 ± 2.1 due to missing secondary aerosol formation processes and uncertainties of emissions and
boundary conditions in the model. This paper firstly evaluates the PALM-4U model performance in simulating
aerosol spatio-temporal distributions, 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