Two different methods for the determination of the mixing layer height (MLH) are investigated with a one-year data set from central Europe: (i) based on a significant gradient of backscatter and (ii) on the vertical velocity. The aerosol-based method shows significant over-estimation in the morning hours when the ML grows into the residual layer and late afternoon hours when turbulent mixing decays. This results in systematic over-estimation of average characteristcs as e.g. maximum MLH.