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Novel observations of aerosol and clouds by means of ground-based remote sensing have been performed in Antarctica over the Ekstrom Ice Shelf on the coast of Dronning Maud Land at Neumayer Station III (70.67[degrees]S, 8.27[degrees]W) from January to December 2023. The deployment of the OCEANET-Atmosphere remote sensing observatory in the framework of the Continuous Observations of Aerosol-Cloud Interaction (COALA) campaign has brought Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) aerosol and cloud profiling capabilities next to meteorological and air chemistry in situ observations at the Antarctic station. We present an overview of the site, the instrumental setup, and data analysis strategy and introduce 3 scientific highlights from austral fall and winter, namely, 1) observations of a persistent mixed-phase cloud embedded in a plume of marine aerosol. Remote sensing-based retrievals of cloud-relevant aerosol properties and cloud microphysical parameters confirm that the free-tropospheric mixed-phase cloud layer formed in an aerosol-limited environment. 2) Two extraordinary warm air intrusions: one with intense snowfall produced the equivalent of 10% of the yearly snow accumulation and a second one with record-breaking maximum temperatures and heavy icing due to supercooled drizzle. 3) Omnipresent aerosol layers in the stratosphere. Our profiling capabilities could show that 50% of the 500-nm aerosol optical depth of 0.06 was caused by stratospheric aerosol, while the troposphere was usually pristine. As demonstrated by these highlights, the 1-yr COALA observations will serve as a reference dataset for the vertical structure of aerosol and clouds above the region, enabling future observational and modeling studies to advance understanding of atmospheric processes in Antarctica. KEYWORDS: Antarctica; Ice shelves; Aerosols; Clouds; Remote sensing; Aerosol-cloud interaction, 1. Introduction The Antarctic continent and its surrounding Southern Ocean are key components of the global climate system. Having been considered rather stable over the last century, the climate of [...]

Between 14 March and 21 April 2022, an extensive investigation of an extraordinary Saharan dust intrusion over Europe was performed based on lidar measurements obtained by the European Aerosol Research Lidar Network (EARLINET). The dust episode was divided into two distinct periods, one in March and one in April, characterized by different dust transport paths. The dust aerosol layers were studied over 18 EARLINET stations, examining aerosol characteristics during March and April in four different regions (M-I, M-II, M-III, and M-IV and A-I, A-II, A-III, and A-IV, respectively), focusing on parameters such as aerosol layer thickness, center of mass (CoM), lidar ratio (LR), particle linear depolarization ratio (PLDR), and Ångström exponents (ÅE). In March, regions exhibited varying dust geometrical and optical properties, with mean CoM values ranging from approximately 3.5 to 4.8 km, and mean LR values typically between 36 and 54 sr. PLDR values indicated the presence of both pure and mixed dust aerosols, with values ranging from 0.20 to 0.32 at 355 nm and 0.24 to 0.31 at 532 nm. ÅE values suggested a range of particle sizes, with some regions showing a predominance of coarse particles. Aerosol Optical Depth (AOD) simulations from the NAAPS model indicated significant dust activity across Europe, with AOD values reaching up to 1.60. In April, dust aerosol layers were observed between 3.2 to 5.2 km. Mean LR values typically ranged from 35 to 51 sr at both 355 nm and 532 nm, while PLDR values confirmed the presence of dust aerosols, with mean values between 0.22 and 0.31 at 355 nm and 0.25 to 0.31 at 532 nm. The ÅE values suggested a mixture of particle sizes. The AOD values in April were generally lower, not exceeding 0.8, indicating a less intense dust presence compared to March. The findings highlight spatial and temporal variations in aerosol characteristics across the regions, during the distinctive periods. From 15 to 16 March 2022, Saharan dust significantly reduced UV-B radiation by approximately 14% over the ATZ station (Athens, GR). Backward air mass trajectories showed that the dust originated from the Western and Central Sahara when, during this specific case, the air mass trajectories passed over GRA (Granada, ES) and PAY (Payerne, CH) before reaching ATZ, maintaining high relative humidity and almost stable aerosol properties throughout its transport. Lidar data revealed elevated aerosol backscatter (baer) and PLDR values, combined with low LR and ÅE values, indicative of pure dust aerosols. [ABSTRACT FROM AUTHOR]

Marine aerosol affects the global energy budget and regional weather. The production of marine aerosol is primarily driven by wind at the sea–air interface. Previous studies have explored the effects of wind on marine aerosol, mostly by examining the relationships between aerosol optical depth (AOD) and surface wind speed. In this paper, utilizing the synergy of aerosol and wind observations from Aeolus, the relationships between the marine aerosol optical properties at 355 nm and the instantaneous co-located wind speeds of remote oceans are investigated at two vertical layers (within and above the marine atmospheric boundary layer (MABL)). The results show that the enhancements of the extinction and backscatter coefficients caused by wind are larger within the MABL than above it. The correlation models between extinction and backscatter with wind speed were established using power-law functions. The slope variation points occur during extinction and backscatter coefficients increasing with wind speed, indicating that the wind-driven enhancement of marine aerosol involves two phases: a rapid-growth phase with high wind dependence, followed by a slower-growth phase after the slope variation points. We also compared the AOD–wind relationship acquired from Aeolus with CALIPSO-derived results from previous research. The variation in the lidar ratio with wind speed is examined, suggesting a possible "increasing–decreasing–increasing" trend of marine aerosol particle size as wind speed increases. This study enhances the comprehension of the correlation between marine aerosol optical properties and wind speed by providing vertical information and demonstrating that their relationships are more complex than a linear or exponential relation. [ABSTRACT FROM AUTHOR]

In this study, we present a new approach for the determination of polarization parameters of the Nicosia Cimel CE376 lidar system, using the Polly XT in Limassol as a reference instrument. The method is applied retrospectively to the measurements obtained during the 2021 Cyprus Fall Campaign. Lidar depolarization measurements represent valuable information for aerosol typing and for the quantification of some specific aerosol types such as dust and volcanic ash. An accurate characterization is required for quality measurements and to remove instrumental artifacts. In this article, we use the Polly XT , a widely used depolarization lidar, as our reference to evaluate the CE376 system's gain ratio and channel cross-talk. We use observations of transported dust from desert regions for this approach, with layers in the free troposphere. Above the boundary layer and the highest terrain elevation of the region, we can expect that, for long-range transport of aerosols, local effects should not affect the aerosol mixture enough for us to expect similar depolarization properties at the two stations (separated by ∼ 60 km). Algebraic equations are used to derive polarization parameters from the comparison of the volume depolarization ratio measured by the two systems. The applied methodology offers a promising opportunity to evaluate the polarization parameters of a lidar system, in cases where a priori knowledge of the cross-talk parameters is not available, or to transfer the polarization parameters from one system to the other. [ABSTRACT FROM AUTHOR]

This study introduces a novel methodology for the characterization of atmospheric aerosol based on lidar-derived intensive optical properties. The proposed aerosol-typing scheme is based on the optimal estimation method (OEM) and allows the identification of up to four different aerosol components of an aerosol mixture, as well as the quantification of their contribution to the aerosol mixture in terms of relative volume. The four aerosol components considered in this typing scheme are associated with the most commonly observed aerosol particles in nature and are assumed to be physically separated from each other and, therefore, can create external mixtures. Two components represent absorbing and less-absorbing fine-mode particles, and the other two components represent spherical and non-spherical coarse-mode particles. These components reflect adequately the most frequently observed aerosol types in the atmosphere: combustion- and pollution-related aerosol, sea salt, and desert dust, respectively. In addition, to consolidate the calibration and validation efforts for the upcoming EarthCARE mission, the typing scheme proposed here is in accordance with the Hybrid End-To-End Aerosol Classification (HETEAC) model of EarthCARE. The lidar-derived optical parameters used in this typing scheme are the lidar ratio and the particle linear depolarization ratio at two distinct wavelengths (355 and 532 nm), the backscatter-related color ratio for the wavelength pair of 532/1064 nm and the extinction-related Ångström exponent for the wavelength pair of 355/532 nm. These intensive optical properties can be combined in different ways, making the methodology flexible, thus allowing its application to lidar systems with different configurations (e.g., single wavelength or multiwavelength, Raman, high spectral resolution). The typing scheme was therefore named HETEAC-Flex due to its compatibility with EarthCARE's HETEAC and its methodological flexibility. The functionality of the typing scheme is demonstrated by its application to three case studies based on layer-averaged optical properties. [ABSTRACT FROM AUTHOR]

The number of intense wildfires may increase further in upcoming years as a consequence of climate change. It is therefore necessary to improve our knowledge about the role of smoke in the climate system, with emphasis on the impact of smoke particles on the evolution of clouds, precipitation, and cloud radiative properties. Presently, one key aspect of research is whether or not wildfire smoke particles can initiate cirrus formation. In this study, we present lidar observations over Limassol, Cyprus, from 27 October to 3 November 2020, when extended wildfire smoke fields crossed the Mediterranean Basin from Portugal to Cyprus. We found strong evidence that aged smoke (organic aerosol particles) originating from wildfires in North America triggered significant ice nucleation at temperatures from -47 to -53 ∘ C and caused the formation of extended cirrus layers. The observations suggest that the ice crystals were nucleated just below the tropopause in the presence of smoke particles serving as ice-nucleating particles (INPs). The main part of the 2–3 km thick smoke layer was, however, in the lower stratosphere just above the tropopause. With actual radiosonde observations of temperature and relative humidity and lidar-derived smoke particle surface area concentrations used as starting values, gravity wave simulations show that the lofting of air by 100–200 m is sufficient to initiate significant ice nucleation on the smoke particles, leading to ice crystal number concentrations of 1–100 L -1. [ABSTRACT FROM AUTHOR]

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]

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]

This study introduces a novel methodology for the characterization of atmospheric aerosol based on lidar-derived intensive optical properties. The proposed aerosol-typing scheme is based on the optimal estimation method (OEM) and allows the identification of up to four different aerosol components of an aerosol mixture as well as the quantification of their contribution to the aerosol mixture in terms of relative volume. The four aerosol components considered in this typing scheme are associated with the most commonly observed aerosol particles in nature and are assumed to be physically separated from each other and, therefore, can create external mixtures. Two components represent absorbing and less absorbing fine-mode particles and the other two spherical and non-spherical coarse-mode particles. These components reflect adequately the most frequently observed aerosol types in the atmosphere: combustion- and pollution-related aerosol, sea salt and desert dust, respectively. In addition, to consolidate the calibration and validation efforts for the upcoming EarthCARE mission, the typing scheme proposed here is in accordance with the Hybrid End-To-End Aerosol Classification (HETEAC) model of EarthCARE. The lidar-derived optical parameters used in this typing scheme are the lidar ratio and the particle linear depolarization ratio at two distinct wavelengths (355 and 532 nm), the backscatter-related color ratio for the wavelength pair of 532/1064 nm and the extinction-related Ångström exponent for the wavelength pair of 355/532 nm. These intensive optical properties can be combined in different ways making the methodology flexible, allowing thus its application to lidar systems with different configurations (e.g., single wavelength or multiwavelength, Raman, high-spectral-resolution). The typing scheme was therefore named HETEAC-Flex, due to its compatibility with EarthCARE's HETEAC and methodological flexibility. The functionality of the typing scheme is demonstrated by its application to three case studies. [ABSTRACT FROM AUTHOR]

On 22nd August 2018, the European Space Agency (ESA) launched the first direct detection Doppler wind lidar into space. Operating at 355 nm and acquiring signals with a dual channel receiver, it allows wind observations in clear air and particle-laden regions of the atmosphere. Furthermore, particle optical properties can be obtained using the High Spectral Resolution Technique Lidar (HSRL) technique. Measuring with 87 km horizontal and 0.25-2 km vertical resolution between ground and up to 30 km in the stratosphere, the global coverage of Aeolus observations shall fill gaps in the global observing system and thus help improving numerical weather prediction. Within this contribution, first results from the German initiative for experimental Aeolus validation are presented and discussed. Ground-based wind and aerosol measurements from tropospheric radar wind profilers, Doppler wind lidars, radiosondes, aerosol lidars and cloud radars are utilized for that purpose.

We present smoke lidar measurements from the Canadian fires of 2017. The advected smoke layers over Europe are detected at both tropospheric and stratospheric heights, with the latter presenting non-typical values of the Particle Linear Depolarization Ratio (PLDR) with strong wavelength dependence from the UV to the Near-IR. Specifically, the PLDR values are of the order of 22, 18 and 4% at 355, 532 and 1064 nm respectively. In an attempt to interpret these results, we apply the hypothesis that smoke particles have near-spherical shapes. Scattering calculations with the T-matrix code support other findings in the literature ([1]- [2]), showing that the near-spherical shape (or closely similar shapes as in [2]), is the only shape that has been shown to reproduce the observed PLDR and Lidar Ratio (LR) values of the stratospheric smoke particles at the three measurement wavelengths.

Vertically resolved multiwavelength aerosol Raman lidar observations were conducted in the pristine environment of the Southern-hemisphere midlatitudes at Punta Arenas, Chile (53.1346°S, 70.8834°W). In contrast to the usually prevailing clean and pristine conditions at this site, two pronounced lofted aerosol layers were observed up to 4.2 and 4.4 km height on 4 and 5 February 2019, respectively. The layers mainly consisted of biomass burning aerosols originating from the region of Central Chile, where wildfires were also observed. Based on spectrally resolved backscatter and extinction coefficients, lidar ratios and depolarization ratio a detailed characterization of the aerosol optical properties is presented.

The biomass burning events are analyzed using the EARLINET-ACTRIS atmospheric profiling of aerosols using lidars. The period of 2008-2017 was chosen to analyze all the events assigned in the EARLINET database under Forest Fire category. A number of fourteen stations were considered. The data provided, ranged from complete data sets (backscatter, extinction and particle linear depolarization ratio) to single profiles (backscatter coefficient). A thorough quality control was performed. Smoke layers geometry was evaluated and the mean properties within each layer were computed. The Hysplit backward-trajectory technique and the FIRMS fire database were used to double check the source of each layer. Discussions were made under the following scenarios: fire events seen by two stations, long range transport from North America, and geographical clusters.

A new version of automatic lidar calibration and processing program was developed to process the data from multiwavelength Raman polarization lidar. The absolute lidar calibration and water vapor calibration algorithms were applied. The program can provide plentiful products, like aerosol backscatter and extinction coefficients, lidar ratio, Ångström exponent, volume and particle depolarization ratios, water vapor mixing ratio and aerosol target classification. Good agreement was found in the comparison with manual quality-assured profiles or radiosonde measurement. Lidar calibration based on the aerosol optical properties retrieved with Raman method, Klett method and AOD-Constrained method were implemented. Good consistency was found.