ALBERT

Description: Atmospheric aerosols are the precondition for the formation of cloud droplets and have thus large influence on the microphysical and radiative properties of clouds. In this work four different methods to derive potential cloud condensation nuclei (CCN) number concentrations were analyzed and compared: A model parameterization based on simulated particle concentrations, the same parameterization based on gravimetrical particle measurements, direct CCN measurements with a CCN counter at a certain observation site and lidar derived CCN profiles. In order to allow for sensitivity studies of the anthropogenic impact, a scenario for the maximum CCN concentration under peak aerosol conditions (1985) was estimated as well. In general, the simulations are in good agreement with the observation. At ground level, an average value of around 1 × 109 CCN/m3 at a supersaturation of 0.2 % was found with all methods. The discrimination of the chemical species revealed an almost equal contribution of ammonium sulfate and ammonium nitrate to the total number of potential CCN. This was not the case for the peak aerosol scenario, where almost no nitrate particles were formed. The potential activation at five different supersaturation values has been compared to the measurements. The discrepancies were lowest for the lowest and highest supersaturations, since chemical composition and the size distribution of the particles are less important in this range. In the mid supersaturation regime, the model overestimated the potentially activated particle fraction by around 30 %. The analysis of the modern (2013) and the peak aerosol scenario (1985) resulted in a scaling factor, which was defined as the quotient of the average vertical profile of the peak aerosol and present day CCN concentration. This factor was found to be around 2 close to the ground, increasing to around 3.5 between 2 and 5 km and approaching 1 (i.e., no difference between present day and peak aerosol conditions) with increasing height. By comparing the simulation with observed profiles, the vertical distribution of the potential CCN was found to be reasonable.