Abstract
Abstract. The mixing layer height (MLH) is a measure for the vertical turbulent exchange within the boundary layer, which is one of the controlling factors for the dilution of pollutants emitted near the ground. Based on continuous MLH measurements with a Vaisala CL51 ceilometer and measurements from an air quality network, the relationship between MLH and near-surface pollutant concentrations has been investigated. In this context the uncertainty of the MLH retrievals and the representativeness of ground-based in situ measurements are crucial. We have investigated this topic by using data from the BAERLIN2014 campaign in Berlin, Germany, conducted from June to August 2014. To derive the MLH, three versions of the proprietary software BL-VIEW and a novel approach COBOLT were compared. It was found that the overall agreement is reasonable if mean diurnal cycles are considered. The main advantage of COBOLT is the continuous detection of the MLH with a temporal resolution of 10 min and a lower number of cases when the residual layer is misinterpreted as mixing layer. We have calculated correlations between MLH as derived from the different retrievals and concentrations of pollutants (PM10, O3 and NOx) for different locations in the metropolitan area of Berlin. It was found that the correlations with PM10 are quite different for different sites without showing a clear pattern, whereas the correlation with NOx seems to depend on the vicinity of emission sources in main roads. In the case of ozone as a secondary pollutant, a clear correlation was found. We conclude that the effects of the heterogeneity of the emission sources, chemical processing and mixing during transport exceed the differences due to different MLH retrievals. Moreover, it seems to be unrealistic to find correlations between MLH and near-surface pollutant concentrations representative for a city like Berlin (flat terrain), in particular when traffic emissions are dominant. Nevertheless it is worthwhile to use advanced MLH retrievals for ceilometer data, for example as input to dispersion models and for the validation of chemical transport models.
Highlights
Air pollution is one of the major environmental issues in metropolitan areas because of its adverse effects on human health (e.g., Chen and Kann, 2008; Rückerl et al, 2011; Lelieveld et al, 2015)
These correlations are based on ceilometer measurements at one site and it was impossible in the framework of BAERLIN2014 to verify that the diurnal cycle of the mixing layer height (MLH) within the 20 km range of the air quality stations is identical
The MLH is expected to have an influence on air quality at the surface
Summary
Air pollution is one of the major environmental issues in metropolitan areas because of its adverse effects on human health (e.g., Chen and Kann, 2008; Rückerl et al, 2011; Lelieveld et al, 2015). Geiß et al.: Mixing layer height and air quality rective 2008/50/EC) may be exceeded For this reason several trace gases and particle mass concentrations (diameter below 10 μm, PM10) are continuously monitored by air pollution monitoring networks near the surface implemented by federal or state administrations. Since ceilometers are commercially available, including software providing “atmospheric products” (e.g., the MLH), we feel that it is necessary to scrutinize the application of such products This is the main motivation and objective of our paper: to investigate the potential of proprietary software to derive the MLH and the usefulness of correlations between such derived MLHs and surface concentrations of pollutants in an urban environment. The retrieval of MLH is an issue even though state-of-the-art ceilometers provide a clear identification of aerosol layers; often several atmospheric layers are detected but it remains ambiguous which one is the mixing layer.
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