Mixing layer height as an indicator for urban air quality?

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 (PM 10 , O 3 and NO x ) for different locations in the metropolitan area of Berlin. It was found that the correlations with PM 10 are quite different for different sites without showing a clear pattern, whereas the correlation with NO x 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.

Quantifying the relationship between PM<sub>2.5</sub> concentration, visibility and planetary boundary layer height for long-lasting haze and fog–haze mixed events in Beijing

“…The PBL heights retrieved by measuring the attenuated backscatter profile of MPL and CL31 still exist some uncertainties (Tang et al, 2016;Geiß et al, 2017). Tang et al (2016) found that PBL height cannot be correctly obtained through sudden changes in the attenuated backscatter profiles.…”

BAERLIN2014 – stationary measurements and source apportionment at an urban background station in Berlin, Germany

“…All MLH data presented follow this method (for more detail see Schäfer et al, 2015) unless otherwise noted. The influence of different options of the proprietary software and an comparison with the more sophisticated approach COBOLT (COntinuous BOundary Layer Tracing) on the retrieved MLH is discussed in detail by Geiß et al (2017). It was found that the proprietary software slightly tends to overestimate the MLH compared to COBOLT.…”

“…Complementing the BLUME measurements (see Stülpnagel et al, 2015, or Geiß et al, 2017, for details) were additional PM 10 filter samples collected for elemental carbon (EC) and organic carbon (OC), ions, and organic tracer analysis; intermittent canister and cartridge samples for the quantification of NMVOCs from an inlet next to the PM 10 inlet on the roof of the measurement station; a quadrupole proton transfer reaction mass spectrometer (high-sensitivity PTR-MS; Ionicon) up in the van for the measurement of NMVOCs; a set of particle instruments to measure number concentration, size distribution, and surface area also located in the van (Sect. 2.2.4); and a ceilometer CL51 (Vaisala GmbH, Hamburg) situated on the roof of the kindergarten.…”

“…In addition to this investigation for the reference site, a more detailed correlation analysis of the MLH with PM 10 , O 3 , and NO x taking into account all 16 BLUME stations in Berlin was carried out using the MATLAB approach outlined here as well as an alternative approach, COBOLT (Geiß et al, 2017). In this context it was assumed that the MLH derived for the reference site in Neukölln is representative for the entire metropolitan area of Berlin.…”

“…There are numerous approaches to estimate the mixing layer height (MLH) from the measured profile; the underlying assumption is that at the top of the mixing layer aerosol concentration drastically drops resulting in a pronounced decrease of backscattered signal intensity. Measurements in the framework of BAERLIN2014 were performed with a Vaisala ceilometer CL51 (Münkel, 2007;Geiß et al, 2017). This instrument is eye-safe (class 1M) with fully automated and unattended operation.…”