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The mixing layer height (MLH) is a measure for the verticalturbulent exchange within the boundary layer, which is one of the controllingfactors for the dilution of pollutants emitted near the ground. Based oncontinuous MLH measurements with a Vaisala CL51 ceilometer and measurementsfrom an air quality network, the relationship between MLH and near-surfacepollutant concentrations has been investigated. In this context theuncertainty of the MLH retrievals and the representativeness of ground-basedin situ measurements are crucial. We have investigated this topic by usingdata from the BAERLIN2014 campaign in Berlin, Germany, conducted from June toAugust 2014. To derive the MLH, three versions of the proprietary softwareBL-VIEW and a novel approach COBOLT were compared. It was found that theoverall agreement is reasonable if mean diurnal cycles are considered. Themain advantage of COBOLT is the continuous detection of the MLH with atemporal resolution of 10 min and a lower number of cases when the residuallayer is misinterpreted as mixing layer. We have calculated correlationsbetween MLH as derived from the different retrievals and concentrations ofpollutants (PM10, O3 and NOx) for different locations in themetropolitan area of Berlin. It was found that the correlations withPM10 are quite different for different sites without showing a clearpattern, whereas the correlation with NOx seems to depend on the vicinityof emission sources in main roads. In the case of ozone as a secondarypollutant,a clear correlation was found. We conclude that the effects of theheterogeneity of the emission sources, chemical processing and mixing duringtransport exceed the differences due to different MLH retrievals. Moreover,it seems to be unrealistic to find correlations between MLH and near-surfacepollutant concentrations representative for a city like Berlin (flatterrain), in particular when traffic emissions are dominant. Nevertheless itis worthwhile to use advanced MLH retrievals for ceilometer data, for example asinput to dispersion models and for the validation of chemical transportmodels

Mixing layer height as an indicator for urban air quality?