ALBERT

Description: In recent years attention was increasingly paid to backscatter profiles of ceilometers as a new source of aerosol information. Several case studies have shown that – although originally intended for cloud detection only – ceilometers can provide the planetary boundary layer height and even quantitative information such as the aerosol backscatter coefficient βp, provided that the signals have been calibrated. It is expected that the retrieval of aerosol parameters will become widespread as the number of ceilometers is steadily increasing, and continuous and unattended operation is provided. In this context however one should be aware of the fact that the majority of ceilometers provides signals that are influenced by atmospheric water vapor. As a consequence, profiles of aerosol parameters can only be retrieved if water vapor absorption is taken into account. In this paper we describe the influence of water vapor absorption on ceilometer signals at wavelengths around λ = 910 nm. Spectrally high-resolved absorption coefficients are calculated from HITRAN on the basis of realistic emission spectra of ceilometers. These results are used as a reference to develop a methodology ("WAPL") for routine and near-real time corrections of the water vapor influence. Comparison of WAPL with the reference demonstrates its very high accuracy. Extensive studies with simulations based on measurements reveal that the error when water vapor absorption is ignored in the βp-retrieval can be in the order of 20 % for mid-latitudes and more than 50 % for the tropics. It is concluded that the emission spectrum of the laser source should be provided by the manufacturer to increase the accuracy of WAPL, and that 910 nm is better suited than 905 nm. With WAPL systematic errors can be avoided, that would exceed the inherent errors of the Klett solutions by far.

Description: In recent years attention was increasingly paid to backscatter profiles of ceilometers as a new source of aerosol information. Several case studies have shown that – although originally intended for cloud detection only – ceilometers can provide the planetary boundary layer height and even quantitative information such as the aerosol backscatter coefficient βp, provided that the signals have been calibrated. It is expected that the retrieval of aerosol parameters will become widespread as the number of ceilometers is steadily increasing, and continuous and unattended operation is provided. In this context however one should be aware of the fact that the majority of ceilometers emit wavelengths that are influenced by atmospheric water vapor. As a consequence, profiles of aerosol parameters can only be retrieved if water vapor absorption is taken into account. In this paper we describe the influence of water vapor absorption on ceilometer signals at wavelengths in the range around λ = 910 nm. Spectrally high resolved absorption coefficients are calculated from HITRAN on the basis of realistic emission spectra of ceilometers. These results are used as reference to develop a methodology ("WAPL") for routine and near real time corrections of the water vapor influence. Comparison of WAPL with the reference demonstrates its very high accuracy. Extensive studies with simulations based on measurements reveal that the error when water vapor absorption is ignored in the βp retrieval can be in the order of 20 % for mid-latitudes and more than 50 % for the tropics. It is concluded that the emission spectrum of the laser source should be provided by the manufacturer to increase the accuracy of WAPL, and that 910 nm is better suited than 905 nm. With WAPL systematic errors can be avoided, that would exceed the inherent random errors of the Klett solutions by far.

Description: The "Berlin Air quality and Ecosystem Research: Local and long-range Impact of anthropogenic and Natural hydrocarbons" (BAERLIN2014) campaign was conducted during the 3 summer months (June–August) of 2014. During this measurement campaign, both stationary and mobile measurements were undertaken to address complementary aims. This paper provides an overview of the stationary measurements and results that were focused on characterization of gaseous and particulate pollution, including source attribution, in the Berlin–Potsdam area, and quantification of the role of natural sources in determining levels of ozone and related gaseous pollutants. Results show that biogenic contributions to ozone and particulate matter are substantial. One indicator for ozone formation, the OH reactivity, showed a 31 % (0.82 ± 0.44 s−1) and 75 % (3.7 ± 0.90 s−1) contribution from biogenic non-methane volatile organic compounds (NMVOCs) for urban background (2.6 ± 0.68 s−1) and urban park (4.9 ± 1.0 s−1) location, respectively, emphasizing the importance of such locations as sources of biogenic NMVOCs in urban areas. A comparison to NMVOC measurements made in Berlin approximately 20 years earlier generally show lower levels today for anthropogenic NMVOCs. A substantial contribution of secondary organic and inorganic aerosol to PM10 concentrations was quantified. In addition to secondary aerosols, source apportionment analysis of the organic carbon fraction identified the contribution of biogenic (plant-based) particulate matter, as well as primary contributions from vehicles, with a larger contribution from diesel compared to gasoline vehicles, as well as a relatively small contribution from wood burning, linked to measured levoglucosan.

Description: 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.

Description: 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.