ALBERT

Description: North Africa is the world’s largest source of dust, a large part of which is transported across the Atlantic to the Caribbean and beyond where it can impact radiation and clouds. Many aspects of this transport and its climate effects remain speculative. The Saharan Aerosol Long-Range Transport and Aerosol–Cloud-Interaction Experiment (SALTRACE; www.pa.op.dlr.de/saltrace) linked ground-based and airborne measurements with remote sensing and modeling techniques to address these issues in a program that took place in 2013/14. Specific objectives were to 1) characterize the chemical, microphysical, and optical properties of dust in the Caribbean, 2) quantify the impact of physical and chemical changes (“aging”) on the radiation budget and cloud microphysical processes, 3) investigate the meteorological context of transatlantic dust transport, and 4) assess the roles of removal processes during transport. SALTRACE was a German-led initiative involving scientists from Europe, Cabo Verde, the Caribbean, and the United States. The Falcon research aircraft of the Deutsches Zentrum für Luft- und Raumfahrt (DLR), equipped with a comprehensive aerosol and wind lidar payload, played a central role. Several major dust outbreaks were studied with 86 h of flight time under different conditions, making it by far the most extensive investigation on long-range transported dust ever made. This article presents an overview of SALTRACE and highlights selected results including data from transatlantic flights in coherent air masses separated by more than 4,000-km distance that enabled measurements of transport effects on dust properties. SALTRACE will improve our knowledge on the role of mineral dust in the climate system and provide data for studies on dust interactions with clouds, radiation, and health.

Description: Lidar depolarization measurements distinguish between spherical and non-spherical aerosol particles based on the change of the polarization state between the emitted and received signal. The particle shape information in combination with other aerosol optical properties allow the characterization of different aerosol types and the retrieval of aerosol particle microphysical properties. Regarding the microphysical inversions, lidar depolarization technique is becoming a key method since particle shape information can be used by algorithms based on spheres and spheroids, optimizing the retrieval procedure. Thus, the identification of the depolarization error sources and the quantification of their effects are crucial. This work presents a new tool to assess the lidar polarizing sensitivity and to estimate the systematic error of the volume linear depolarization ratio (δ), combining the Stokes–Müller formalism and the Monte Carlo technique. This tool is applied to a synthetic lidar system and to several EARLINET lidars with depolarization capabilities at 355 or 532 nm. The results evidence that the lidar polarization sensitivity can lead to δ relative errors larger than 100 %, being more probable its overestimation. The lidar systems show δ relative errors larger than 100 % for δ values around the molecular one (~0.004), decreasing up to ~10 % for δ = 0.45. However, among them, POLIS system shows the best behaviour with δ relative errors of 25 % and 0.22 % for δ = 0.004 and δ = 0.45, respectively, evidencing how a proper characterization of the lidar polarizing sensitivity can drastically reduce the δ systematic errors. In this regard, we provide some indications to reduce the lidar polarizing sensitivity and to improve its characterization.

Description: Volcanic ash plumes, emitted by the Eyjafjallajökull volcano (Iceland) in spring 2010, were observed by the lidar systems MULIS and POLIS in Maisach (near Munich, Germany), and by a CIMEL Sun photometer and a JenOptik ceilometer in Munich. We retrieve mass concentrations of volcanic ash from the lidar measurements; spectral optical properties, i.e. extinction coefficients, backscatter coefficients, and linear depolarization ratios, are used as input for an inversion. The inversion algorithm searches for model aerosol ensembles with optical properties that agree with the measured values within their uncertainty ranges. The non-sphericity of ash particles is considered by assuming spheroids. Optical particle properties are calculated using the T-matrix method supplemented by the geometric optics approach. The lidar inversion is applied to observations of the pure volcanic ash plume in the morning of 17 April 2010. We find 1.45 g m−2 for the ratio between the mass concentration and the extinction coefficient at λ = 532 nm, assuming an ash density of 2.6 g cm−3. The uncertainty range for this ratio is from 0.87 g m−2 to 2.32 g m−2. At the peak of the ash concentration over Maisach the extinction coefficient at λ = 532 nm was 0.75 km−1 (1-h-average), which corresponds to a maximum mass concentration of 1.1 mg m−3 (0.65 to 1.8 mg m−3). Model calculations show that particle backscatter at our lidar wavelengths (λ ≤ 1064 nm), and thus the lidar retrieval, is hardly sensitive to large particles (r ≳ 3 μm); large particles, however, may contain significant amounts of mass. Therefore, as an independent cross check of the lidar retrieval and to investigate the presence of large particles in more detail, we model ratios of sky radiances in the aureole of the Sun and compare them to measurements of the CIMEL. These ratios are sensitive to particles up to r ≈ 10 μm. This approach confirms the mass concentrations from the lidar retrieval. We conclude that synergistic utilization of high quality lidar and Sun photometer data, in combination with realistic aerosol models, is recommended for improving ash mass concentration retrievals.

Description: In this paper we describe the EARLINET Single Calculus Chain (SCC) a tool for the automatic analysis of lidar measurements. The development of this tool started in the framework of EARLINET-ASOS (European Aerosol Research Lidar Network – Advanced Sustainable Observation System) project and it is still continuing within ACTRIS (Aerosol, Clouds and Trace gases Research InfraStructure Network) project. The main idea was to develop a chain which allows all EARLINET stations to retrieve in a full automatic way the aerosol backscatter and extinction profiles starting from the raw lidar data of the lidar systems they operate. The calculus subsystem of the SCC is composed by two modules: a pre-processor module that handles the raw lidar data and corrects them for instrumental effects and an optical processing module for the retrieval of aerosol optical products from the pre-processed data. All the input parameters needed to perform the lidar analysis are stored in a database to get them in an efficient way and also to keep track of all the changes that may occur on any EARLINET lidar system over the time. The two calculus modules and the data are coordinated and synchronized by a further module (deamon) which makes fully automatic the whole analysis process. The end-user can interact with the SCC using a user-friendly web interface. All the SCC modules are developed using open source and free available software packages. The final products retrieved by the SCC fulfill all constraints fixed in the framework of the EARLINET quality assurance programs on both instrumental and algorithm levels. Moreover the man power needed to provide aerosol optical products is greatly reduced improving the near-real time availability of lidar data. The high quality of the SCC products is demonstrated by the good agreement between the SCC analysis and the corresponding independent manual retrievals. Finally, a real example of the applicability of the SCC in providing high quality aerosol optical products in case of intense observation period is provided.

Description: Airborne lidar and in-situ measurements of aerosols and trace gases were performed in volcanic ash plumes over Europe between Southern Germany and Iceland with the Falcon aircraft during the eruption period of the Eyjafjalla volcano between 19 April and 18 May 2010. Flight planning and measurement analyses were supported by a refined Meteosat ash product and trajectory model analysis. The volcanic ash plume was observed with lidar directly over the volcano and up to a distance of 2700 km downwind, and up to 120 h plume ages. Aged ash layers were between a few 100 m to 3 km deep, occurred between 1 and 7 km altitude, and were typically 100 to 300 km wide. Particles collected by impactors had diameters up to 20 μm diameter, with size and age dependent composition. Ash mass concentrations were derived from optical particle spectrometers for a particle density of 2.6 g cm−3 and various values of the refractive index (RI, real part: 1.59; 3 values for the imaginary part: 0, 0.004 and 0.008). The mass concentrations, effective diameters and related optical properties were compared with ground-based lidar observations. Theoretical considerations of particle sedimentation constrain the particle diameters to those obtained for the lower RI values. The ash mass concentration results have an uncertainty of a factor of two. The maximum ash mass concentration encountered during the 17 flights with 34 ash plume penetrations was below 1 mg m−3. The Falcon flew in ash clouds up to about 0.8 mg m−3 for a few minutes and in an ash cloud with approximately 0.2 mg m−3 mean-concentration for about one hour without engine damage. The ash plumes were rather dry and correlated with considerable CO and SO2 increases and O3 decreases. To first order, ash concentration and SO2 mixing ratio in the plumes decreased by a factor of two within less than a day. In fresh plumes, the SO2 and CO concentration increases were correlated with the ash mass concentration. The ash plumes were often visible slantwise as faint dark layers, even for concentrations below 0.1 mg m−3. The large abundance of volatile Aitken mode particles suggests previous nucleation of sulfuric acid droplets. The effective diameters range between 0.2 and 3 μm with considerable surface and volume contributions from the Aitken and coarse mode aerosol, respectively. The distal ash mass flux on 2 May was of the order of 500 (240–1600) kg s−1. The volcano induced about 10 (2.5–50) Tg of distal ash mass and about 3 (0.6–23) Tg of SO2 during the whole eruption period. The results of the Falcon flights were used to support the responsible agencies in their decisions concerning air traffic in the presence of volcanic ash.

Description: The eruption of the Icelandic volcano Eyjafjallajökull in April–May 2010 represents a "natural experiment" to study the impact of volcanic emissions on a continental scale. For the first time, quantitative data about the presence, altitude, and layering of the volcanic cloud, in conjunction with optical information, are available for most parts of Europe derived from the observations by the European Aerosol Research Lidar NETwork (EARLINET). Based on multi-wavelength Raman lidar systems, EARLINET is the only instrument worldwide that is able to provide dense time series of high-quality optical data to be used for aerosol typing and for the retrieval of particle microphysical properties as a function of altitude. In this work we show the four-dimensional (4-D) distribution of the Eyjafjallajökull volcanic cloud in the troposphere over Europe as observed by EARLINET during the entire volcanic event (15 April–26 May 2010). All optical properties directly measured (backscatter, extinction, and particle linear depolarization ratio) are stored in the EARLINET database available at http://www.earlinet.org. A specific relational database providing the volcanic mask over Europe, realized ad hoc for this specific event, has been developed and is available on request at http://www.earlinet.org. During the first days after the eruption, volcanic particles were detected over Central Europe within a wide range of altitudes, from the upper troposphere down to the local planetary boundary layer (PBL). After 19 April 2010, volcanic particles were detected over southern and south-eastern Europe. During the first half of May (5–15 May), material emitted by the Eyjafjallajökull volcano was detected over Spain and Portugal and then over the Mediterranean and the Balkans. The last observations of the event were recorded until 25 May in Central Europe and in the Eastern Mediterranean area. The 4-D distribution of volcanic aerosol layering and optical properties on European scale reported here provides an unprecedented data set for evaluating satellite data and aerosol dispersion models for this kind of volcanic events.

Description: The spatial structure and the progression speed of the first ash layer from the Icelandic Eyjafjallajökull volcano which reached Germany on 16/17 April is investigated from remote sensing data and numerical simulations. The ceilometer network of the German Meteorological Service was able to follow the progression of the ash layer over the whole of Germany. This first ash layer turned out to be a rather shallow layer of only several hundreds of metres thickness which was oriented slantwise in the middle troposphere and which was brought downward by large-scale sinking motion over Southern Germany and the Alps. Special Raman lidar measurements, trajectory analyses and in-situ observations from mountain observatories helped to confirm the volcanic origin of the detected aerosol layer. Ultralight aircraft measurements permitted the detection of the arrival of a second major flush of volcanic material in Southern Germany. Numerical simulations with the Eulerian meso-scale model MCCM were able to reproduce the temporal and spatial structure of the ash layer. Comparisons of the model results with the ceilometer network data on 17 April and with the ultralight aircraft data on 19 April were satisfying. This is the first example of a model validation study from this ceilometer network data.

Description: Particle depolarization ratio retrieved from lidar measurements are commonly used for aerosol typing studies, microphysical inversion, or mass concentration retrievals. The particle depolarization ratio is one of the primary parameters that can differentiate several major aerosol components, but only if the measurements are accurate enough. The uncertainties related to the retrieval of particle depolarization ratios are the main factor in determining the accuracy of the derived parameters in such studies. This paper presents an extended analysis of different depolarization calibration procedures, in order to reduce the related uncertainties. The calibration procedures are specific to each lidar system of the European Aerosol Research Lidar Network – EARLINET with polarising capabilities. The results illustrate a significant improvement of the depolarization lidar products for all the selected lidar stations. The calibrated volume and particle depolarization profiles at 532 nm show values that agree with the theory for all selected atmospheric constituents (several aerosol species, ice particles and molecules in the aerosol free regions).