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From 19 September to 13 December 2021, volcanic eruptions took place at the Cumbre Vieja ridge, Las Palmas, Canary Islands. Thereby, fine ash and volatiles, like sulfur dioxide (SO2), were emitted and transported over hundreds to thousands of kilometers away from the island. Continuous lidar observations with the multiwavelength-Raman-polarization lidar PollyXT were performed at the Ocean Science Center at Mindelo, Cabo Verde, in the framework of the Joint Aeolus-Tropical Atlantic Campaign (JATAC) 2021/2022 enabling the characterization of the atmospheric state above Mindelo during the eruption period. A special feature of the system operated at Mindelo is, that measurements of the particle extinction coefficient, the particle extinction-to-backscatter ratio (lidar ratio) and the particle linear depolarization ratio are available at all three wavelengths (355, 532 and 1064 nm). The typical aerosol conditions over Mindelo are a clean marine planetary boundary layer (PBL) up to approx. 1 km and above a Saharan dust layer (SAL, up to 6 km) during northern hemispheric summer and fall. A particle extinction coefficient smaller than 200 Mm−1, a lidar ratio smaller than 30 sr and a particle linear depolarization ratio close to 0 % have been typically observed within the planetary boundary layer, while a lidar ratio between 40 and 60 sr and a linear depolarization ratio between 20 and 30 % are characteristic for the SAL above. In contrast, during the time of the volcanic eruptions, a strongly polluted PBL was observed on specific days beginning on the 23 September 2021, whereby the particle extinction coefficient and the lidar ratio increased up to 800 Mm−1 and 80 sr (at 355 nm), respectively. On 24 September, the aerosol optical depth, determined by an AERONET (Aerosol Robotic Network) sun photometer, was as high as 0.9 and 1.1 (daily averages at 500 and 340 nm). HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) trajectories indicate air mass transport from Canary Islands to Mindelo at heights below 2 km. The observed pollution in the PBL over Mindelo is attributed to sulfate aerosol from the volcanic eruption at Las Palmas as the particle linear depolarization ratio was low (≤ 3 %) and, thus, does not indicate non-spherical particles, such as Saharan dust or volcanic ash. We thus conclude that sulfate aerosol formed from gaseous precursors during the transport (2–3 days for a distance of 1500 km) from Las Palmas towards Cabo Verde. No indications of volcanic ash over Mindelo were found in the SAL. This finding is supported by the HYSPLIT trajectories, which show that air masses in higher altitudes originate from the African continent and not from the Canary Islands. [ABSTRACT FROM AUTHOR]

Six months of stratospheric aerosol observations with the European Aerosol Research Lidar Network (EARLINET) from August 2017 to January 2018 are presented. The decay phase of an unprecedented, record-breaking stratospheric perturbation caused by wild fire smoke is reported and discussed in terms of geometrical, optical, and microphysical aerosol properties. Enormous amounts of smoke (mainly soot particles) were injected into the upper troposphere and lower stratosphere over fire areas in western Canada on 12 August 2017 during strong thunderstorm-pyrocumulonimbus activity. The stratospheric smoke plumes spread over the entire northern hemisphere in the following weeks and months. Twenty-eight European lidar stations from northern Norway to southern Portugal and the Eastern Mediterranean monitored the strong stratospheric perturbation on a continental scale. The main smoke layer (over central, western, southern, and eastern Europe) was found between 15 and 20 km height since September 2017 (about two weeks after entering the stratosphere). Thin layers of smoke were detected to ascent to 22-24 km height. The stratospheric aerosol optical thickness at 532 nm decreased from values > 0.25 on 21-23 August 2017 to 0.005-0.03 until 5-10 September, and was mainly 0.003-0.004 from October to December 2017, and thus still significantly above the stratospheric background (0.001-0.002). Stratospheric particle extinction coefficients (532 nm) were as high as 50-200 Mm-1 until the beginning of September and of the order of 1 Mm-1 (0.5-5 Mm-1) from October 2017 until the end of January 2018. The corresponding layer mean particle mass concentration was of the order of 0.05-0.5 μg cm-3 over the months. Soot is an efficient ice-nucleating particle (INP) at upper tropospheric (cirrus) temperatures and available to influence cirrus formation when entering the tropopause from above. We estimated INP concentrations of 50-500 L-1 until the first days in September and afterwards 5-50 L-1 until the end of the year 2018 in the lower stratosphere for typical cirrus formation temperatures of -55 °C and ice supersaturation values of 1.15. The measured profiles of the particle linear depolarization rato indicated the predominance of non-spherical soot particles. The 532 nm depolarization ratio decreased with time in the main smoke layer from values of 0.15-0.25 (August-September) to values of 0.05-0.10 (October-November) and <0.05 (December-January). The decrease of the depolarization ratio is consistent with the steady removal of the larger smoke particles by gravitational settling and changes in the particle shape with time towards a spherical form. An ascending layer with a vertical depth of 500-1000 m was detected (over the Eastern Mediterranean at 32-35° N) that ascended from about 18-19 km to 22-23 km height from the beginning of October to the beginning of December 2017 (about 2 km per month) and may be related to the increasing build up of the winter-hemispheric Brewer-Dobson circulation system. [ABSTRACT FROM AUTHOR]

A unique 4-week ship cruise from Guadeloupe to Cabo Verde in April-May 2013 (see part 1, Rittmeister et al., 2017) is used for an in-depth comparison of dust profiles observed with a polarization/Raman lidar aboard the German research vessel Meteor over the remote tropical Atlantic and respective dust forecasts of a regional (SKIRON) and two global atmospheric (dust) transport models (NMMB/BSC-Dust, MACC/CAMS). New options of model-observation comparisons are presented. We analyze how well the modeled fine dust (submicrometer particles) and coarse dust contributions to light extinction and mass concentration match respective lidar observations, and to what extent models, adjusted to aerosol optical thickness observations, are able to reproduce the observed layering and mixing of dust and non-dust (mostly marine) aerosol components over the remote tropical Atlantic. Based on the coherent set of dust profiles at well defined distances from Africa (without any disturbance by anthropogenic aerosol sources over the ocean) we investigate how accurately the models handle dust removal at distances of 1500 km to more than 5000 km west of the Saharan dust source regions. It was found that (a) dust predictions are of acceptable quality for the first several days after dust emission up to 2000 km west of the African continent, (b) the removal of dust from the atmosphere is too strong for large transport paths in the global models, and (c) the simulated fine-to-coarse dust ratio (in terms of mass concentration and light extinction) is too high in the models compared to the observations. This deviation is already given close to the dust sources and then increases with distance from Africa. [ABSTRACT FROM AUTHOR]

Continuous vertically resolved monitoring of marine aerosol, Saharan dust, and marine/dust aerosol mixtures was performed with multiwavelength polarization/Raman lidar aboard the German research vessel R/V Meteor during a one-month transatlantic cruise from Guadeloupe to Cabo Verde over 4500 km (from 61.5° W to 2&deg W, mostly along 14.5° N) in April-May 2013, as part of SALTRACE (Saharan Aerosol Long-range Transport and Aerosol-Cloud Interaction Experiment). An overview of measured aerosol optical properties over the tropical Atlantic is given in terms of spectrally resolved particle backscatter and extinction coefficients, lidar ratio, and linear depolarization ratio. Height profiles from the marine boundary layer (MBL) up to the top of the Saharan Air Layer (SAL) are presented. MBL and SAL mean lidar ratios were around 20 and 40 sr. These values indicate clean marine conditions in the MBL and entrainment of marine particles into the lower part of the SAL. In the central and upper parts of the SAL, the lidar ratios were most frequently 50-60 sr and thus typical for Saharan dust. The MBL and SAL mean depolarization ratios were close to 0.05 and between 0.2-0.3, respectively, which reflects almost dust-free conditions in the MBL and the occurrence of a mixture of marine and dust particles in the SAL. The conceptual model, describing the long-range transport and removal processes of Saharan dust over the North Atlantic, is discussed and confronted with the lidar observations along the west-to-east track of the slowly moving research vessel. The role of turbulent downward mixing as an efficient dust removal process is illuminated. In a follow-up article (Rittmeister et al., 2017), the lidar observations of dust extinction coefficient and derived mass concentration profiles are compared with respective dust profiles simulated with three well-established European atmospheric aerosol and dust prediction models (MACC, NMMB/BSC-Dust, SKIRON). [ABSTRACT FROM AUTHOR]