Headline: Mixing layer heights derived from ceilometers: Useful for air quality studies?

Ceilometers have been used for several meteorological applications, often in the framework of air qualitystudies. Whereas the particle backscatter coefficient can be retrieved in a quantitative way only recently (withthe improvements of the hardware, Wiegner et al., 2014), mixing layer heights (MLH) have been derived formore than two decades. Several approaches are documented in the literature, however, automated proceduresare still prone to errors because of difficulties in the recognition and attribution of discontinuities (“steps”) inthe backscatter profiles. For example, it is often not clear whether a detected change in the vertical distributionof aerosol backscatter indicates the residual layer, the stable boundary layer, or an elevated layer. Thus, anassessment of the reliability and accuracy of MLH-retrievals is relevant for air quality studies: on the one handMLHs are often inversely correlated with ground-based in-situ measurements of particulate matter and gaseouspollutant concentrations, and on the other hand, MLH-retrievals can be used to validate chemistry transport models.To understand the benefit of MLH in the context of air quality, we have compared several retrievals of theMLH from ceilometer measurements during a field-campaign in summer 2014 (Bonn et al., 2016) for backgroundand polluted sites in Berlin. Correlations between the concentrations of several pollutants and MLH are analyzed,and how they are influenced by the uncertainty of the derived MLH.