At the end of May 2025, extremely strong wildfires in Canada produced several pyrocumulonimbus clouds which lifted wildfire smoke particles up to the lower stratosphere (> 10 km height). A dense stratospheric smoke plume developed which reached stratospheric aerosol optical depths up to 3.2 which is comparable with a moderate volcanic eruption. EarthCARE’s lidar ATLID captured this event and enabled us to study stratospheric smoke shortly after emission and to track a single smoke plume on its transport way towards Europe.The spaceborne lidar allowed to precisely study the maximum plume height and revealed a lofting of the smoke plume top height from 13.6 km above Canada to 17.4 km above Europe and a further slight ascent during the transport towards Asia. The self-lofting of dense smoke plumes can be explained by the absorption of solar radiation which heats the ambient air and creates buoyancy. The self-lofting is strongest for optically thick smoke plumes close to the source region and gets weaker when the plume is horizontally more spread and thus optically thinner.ATLID detected an enhanced depolarization ratio of 0.26±0.02 which indicates non-spherical smoke particles in the stratosphere. This finding is in line with previous observations of stratospheric smoke layers, but clearly demonstrates a difference to tropospheric observations of Canadian smoke in Europe, which are characterized by a low depolarization ratio and hence a spherical shape (Haarig et al., 2018).The novel high-spectral-resolution lidar (HSRL) capability of ATLID allowed us for the first time to study the evolution of the lidar ratio of a stratospheric smoke layer during long-range transport. Higher values around 70 sr were observed shortly after emission, which decreased during the first days of transport to values of 49±7 sr.As another highlight, EarthCARE observed a significant downmixing of stratospheric smoke at a strong tropopause fold over the Mediterranean and North Africa (Haarig et al., 2025). These observations directly show a pathway of removal of the stratospheric smoke and closes the life cycle from injection to removal. Additionally, the synergistic EarthCARE observations will be used to estimate the radiative impact of this strong stratospheric smoke event.ReferencesHaarig, M., et al. (2018), Depolarization and lidar ratios at 355, 532, and 1064 nm and microphysical properties of aged tropospheric and stratospheric Canadian wildfire smoke. Atmospheric Chemistry and Physics, 18 (16), 11847–11861.Haarig, M. et al. The life cycle of a stratospheric smoke plume as seen from EarthCARE - tracking a plume from Canada to Europe. ESS Open Archive. October 22, 2025.

Abstract From January to June 2025, a stratospheric sulfate layer originating from the eruptions of the Ruang volcano in April 2024 could be observed with a multi‐wavelength Raman and fluorescence lidar over Leipzig, Germany. The aerosol‐optical depth of the stratospheric volcanic sulfate was <0.02, decreasing with time. The volcanic sulfate aerosol showed very low laser‐induced aerosol fluorescence, enabling an unambiguous distinction from wildfire smoke. By combining the fluorescence with the dual‐wavelength Raman lidar observations, pure volcanic sulfate layers could be identified and, thus, pure sulfate lidar ratios could be determined. The lidar ratios of the sulfate ranged between 29–32 sr at 355 nm and 58–63 sr at 532 nm.

In the framework of the Joint Aeolus Tropical Atlantic Campaign (JATAC), a scanning HALO photonics Doppler lidar has been deployed at Mindelo, Cabo Verde, since 2021 to study atmospheric dynamics and to validate the wind products of the Aeolus mission. The ground-based Doppler lidar was usually operating in stare mode (vertical profiling), but each 15 minutes a plan position indicator (PPI) scan was performed to derive vertical profiles of the horizontal wind speed and direction. Due to the vicinity of the measurement site in the middle of the Atlantic Ocean west of arid Africa, aerosol from the ground up to 4-7 km can be observed year-round, allowing to retrieve wind profiles up to this altitude. We used these profiles to validate the Aeolus L2B wind products from the direct overpasses at the site each Friday from June 2021 to April 2023 (end of Aeolus operations). The evaluation of the operational Aeolus wind products (Baseline 12 to 16) in the time period from July 2021 to April 2023 revealed a systematic error of 0.09 (0.88) m/s and a random error of 7 (3.5) m/s for the Rayleigh (Mie) products.

The extension of the Hybrid End-To-End Aerosol Classification (HETEAC) model with respect to stratospheric targets is presented. The methodology follows the HETEAC approach, i.e., a suitable set of basic feature types is defined to allow for scattering calculations. The most appropriate microphysical representation is selected based on the agreement between optical properties derived from scattering calculations and ground-based lidar observations. The extended HETEAC model (2.0), considers stratospheric features such as sulfate, volcanic ash, and smoke particles and also polar stratospheric clouds (PSCs). HETEAC 2.0 ensures consistency with the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) approach, which is of great importance for the harmonization of long-term satellite records.

The stationary lidar system MARTHA at TROPOS has undergone a major upgrade. With a new diode-pumped laser, the average laser power has more than doubled. In combination with the large far-range telescope, this will allow us to profile the atmosphere from the ground up to the stratosphere. A new spectrometer with an extra telescope enables spectrally resolved measurements of laser-induced aerosol fluorescence, which shall allow unambiguous aerosol typing and the characterization of biogenic aerosol particles. The upgrade and the features of the new system are described and range-corrected signals from a first test measurement are presented.

Updated POLIPHON (Polarization Lidar Photometer Networking) conversion factors for the laser wavelengths of 355, 532, 911, and 1064 nm are presented. The conversion factors allow us to transfer profiles of aerosol-type-dependent optical properties measured with lidars and ceilometers into profiles of microphysical particle properties, and to estimate cloud condensation nucleus (CCN) and ice-nucleating particle (INP) concentrations at observed cloud levels. These updates were necessary to permit a coherent and harmonized data analysis of different long-term spaceborne lidar observations at 355 and 532 nm and to support ground-based lidar and ceilometer network measurements and research on aerosol-cloud interaction. The POLIPHON conversion factors are obtained by analyzing long-term sun photometer observations conducted at 62 AERONET (Aerosol Robotic Network) stations. Conversion factors are now available for mineral dust, marine aerosol, urban and rural anthropogenic particles, tropospheric and stratospheric wildfire smoke and volcanic sulfate aerosol.

Abstract. During the wildfire season on the West Coast of the US in September 2020, biomass burning smoke aerosol was continually generated, and several transatlantic smoke transports occurred. The ALADIN lidar onboard Aeolus satellite measured global wind and aerosol profiles from August 2018 to April 2023. As a high spectral resolution lidar (HSRL), ALADIN could directly retrieve the extinction coefficient. Targeting a large-scale tropospheric smoke transport event originating from the western US on 14 September and arriving over Europe on 21 September, a method for constructing an Aeolus smoke dataset was developed based on ALADIN observations, in synergy with multi-platform data. The backscatter coefficient in the smoke dataset was corrected using a linear depolarization ratio of 0.15, since ALADIN detected only co-polarized backscattered signals and the averaged smoke depolarization ratio remained relatively stable throughout the transport. Utilizing the selected cross-sections from the Aeolus smoke dataset, we acquired the vertical structure of the smoke layers and characterized their features. These layers show good agreement with the HYSPLIT simulations and are considered representative of different transport phases. Statistical analyses of the complete Aeolus smoke dataset from 14 to 21 September reveal the evolution of the smoke plume throughout its transatlantic transport. We illustrated the variations in aerosol optical depth, layer altitude and lidar ratio throughout the transport process. To our knowledge, this paper represents the first use of a spaceborne HSRL for observations and analyses of a large-scale tropospheric smoke transport. The comprehensive characterization presented offers valuable information for advancing global aerosol research.

Ground-based lidar stations play a vital role in the validation of spaceborne lidar products. While ground-based measurements have a high temporal resolution, they have limited spatial coverage, which potentially imposes implications for the Calibration and Validation (Cal/Val) of the satellite products. Therefore, in this study, we assess the representativeness of a remote ground-based ACTRIS (Aerosol, Clouds, and Trace Gases Research Infrastructure) station, in Mindelo, Cabo Verde by utilizing the continuous observations of a ground-based PollyNET multiwavelength polarization Raman lidar. This station was selected since Cabo Verde has been a key location for the validation of two recent Earth Explorer missions of the European Space Agency (ESA), namely Aeolus and the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE). The islands are located in the Atlantic Ocean, in the outflow region of the African continent with frequent dust outbreaks, but also smoke advection and, thus, along with the local (marine) boundary layer provide an excellent atmospheric laboratory. Continuous, vertically-resolved aerosol measurements are being conducted with the state-of-the-art multiwavelength polarization Raman lidar PollyXT at Mindelo since June 2021. Based on these observations and in combination with the LIdar climatology of Vertical Aerosol Structure for space-based lidar simulation studie (LIVAS) products available at different radii around Mindelo, a statistical analysis of the optical properties was performed to evaluate the representativeness of the station in the context of aerosol profiling Cal/Val activities. Additionally, three case studies, focusing on different distances from the ground-based station, have been closely examined for a more complete and detailed comparison. Our study results indicate that overall the ground-based station in Mindelo can be considered conditionally representative. According to the monthly analysis, at altitudes where the lofted aerosol (dust) layers occur, lidar observations were very representative for radii up to 300 km around the island, while the boundary-layer characteristics varied. Case studies confirmed the long-term results and revealed that lidar observations of lofted aerosol layers can be representative for radii up to 100 km around Mindelo and at the same time highlighted the importance of spatiotemporal homogeneity of the target. From our findings and especially for the Cabo Verde region, we conclude that it is better to use monthly averaged aerosol profiles for the validation of spaceborne profiles over long times rather than using single overpasses, as representativeness cannot be guaranteed for the latter without additional measures. Thus, using fixed radii around a certain ground site (as e.g., the frequently used 100 km) for validation activities seems to be inappropriate for profile-to-profile comparison without any further considerations. However, we show in our case studies that if representativeness can be guaranteed, also single-profile validation is possible and has its own valuable potential. Additionally, the proposed study can serve as a calibration/validation tool for the remote sensing facilities of the European Aerosol Research Lidar Network (EARLINET).

Abstract. Dushanbe, located in Central Asia, resides in the global dust belt, fundamental for the emission and transport of dust. The Central Asian Dust EXperiment (CADEX, 2015 to 2016) provided foundational insights into the optical properties of atmospheric dust in Central Asia by means of lidar observations. A follow-up dataset of high-quality multi-year (2019 to 2025) lidar observations from a newly established Aerosol Clouds and Trace Gases Research Infrastructure (ACTRIS) site in Dushanbe, Tajikistan have been analyzed with the goal to deepen the knowledge of the extent, duration and interannual characteristics of the annual dust cycle in the Central Asian region. The dataset consists of automatically processed lidar data, including the application of the POlarization Lidar and PHOtometer Network (POLIPHON) method to separate the measured total particle backscatter coefficient into a dust and non-dust fraction. The study found a pronounced cycle of the dust backscatter coefficient over the course of several years with low values in winter, also in the planetary boundary layer, and a slow onset of the dust season in spring with an increased occurrence of lofted dust layers. The main dust season spans from July to September with a sharp end in September/October. During summer, non-dust particles, likely local pollution, additionally get lifted above the PBL. The PBL is normally well-mixed and shows rather constant contribution of local pollution. These results show the importance of long-term lidar observations in the Central Asian region and the value of high-quality automated processing of the retrieved data.

We are setting up a new scattering laboratory, the Optical Lab for Lidar Applications (OLALA) to measure the particle depolarization ratio of irregularly shaped mineral dust particles in the exact backscatter direction. For that, we will use size-segregated natural dust samples which will be observed at three wavelengths (355, 532 and 1064 nm) in order to assess the particle's size effect on the spectral slope of the depolarization ratio. The gained knowledge will be applied to a better understanding of particle's size in lidar field observations of mineral dust and to improve optical scattering models dealing with non-spherical particles at a scattering angle of 180°.

In this paper we discuss about the design, construction and working of two different experimental setups to measure the depolarization ratio of natural mineral dust samples in the laboratory in the exact back-scattering direction. The first setup involves a 50:50 beam splitter and the second setup involves a Faraday rotator to achieve the 180˚ backscattering angle. Both the experimental setups will be first tested at 532 nm wavelength. Later, the setup with better results will be extended to cover the full triple-wavelength (355, 532 and 1064 nm) capabilities for linear polarization and circular polarization, enabling ellipsometry measurements of mineral dust particles.

Aerosol-cloud interactions (ACI) are a major source of uncertainty in climate science, critically affecting our ability to project near-term climate evolution and assess societal risks. These interactions influence effective radiative forcing, cloud dynamics, and precipitation patterns, yet remain insufficiently constrained due to limitations in observations, modeling, and process understanding. This uncertainty hampers robust policy advice across multiple domains-from estimating remaining carbon budgets and climate sensitivity, to anticipating regional extreme events and evaluating climate interventions such as solar radiation modification. In many cases, the influence of ACI is either underappreciated or excluded from decision-making frameworks due to its complexity and lack of quantification. This perspective outlines a path forward to overcome these barriers by leveraging emerging opportunities in satellite remote sensing, ground-based and airborne observations, high-resolution climate modeling, and machine learning. We identify key areas where rapid progress is feasible, including improved retrievals of cloud microphysical properties, better representation of natural aerosols in a warming world, and enhanced integration of observational and modeling communities. Even as anthropogenic aerosol and its impacts on clouds is reducing owing to emissions controls, addressing ACI uncertainties remains essential for refining climate projections, supporting effective mitigation and adaptation strategies, and delivering actionable science to policymakers in a rapidly changing climate system.

Abstract. This technical note presents a comprehensive comparison of manually and automatically analyzed lidar profiles of aerosol optical properties retrieved from an 18-month measurement campaign with a continuously measuring, automated PollyXT multiwavelength polarization Raman lidar in Dushanbe, Tajikistan. The manual analysis was performed using a custom software (known as Verlauf) in multiple analyses steps based on visual inspection of the lidar signals. Its results serve as the reference dataset. The automatic analysis was performed using the PollyNET Processing Chain (PPC) (version 4.0). Retrieval parameters, derived layer-mean values, and seasonal mean and median profiles of aerosol optical properties (backscatter and extinction coefficients, particle depolarization and lidar ratios, and backscatter- and extinction-related Angstrom exponents) of the two datasets were compared. The absolute values of the percentage differences of the seasonal mean and median backscatter coefficient profiles at 1.5 km height are largely below 10 %, except at 532 nm in winter and at 1064 nm in spring and winter. The absolute values of the percentage differences of the seasonal mean and median extinction coefficient profiles at 1.5 km height are largely less than 5 %. The absolute values of the percentage differences of the seasonal mean and median particle depolarization ratios at 1.5 km are largely below 20 % at 355 and below 10 % at 532 nm wavelength. For the most part, these discrepancies are within the measurement uncertainties of the considered quantities despite challenges in the automatic retrieval such as reference height detection, depolarization calibration, and cloud screening. This supports the conclusion that the automatically analyzed profiles of aerosol optical properties are utilizable for common applications, such as extinction statistics, aerosol typing, and retrieval of microphysical and cloud-relevant aerosol properties. However, caution should be exercised when using the particle depolarization ratio at 355 nm and backscatter-related Angstrom exponents, which showed enhanced discrepancies. The results will assist further improvements of the PPC and the implementation of additional features (e.g., optimal estimation methods, retrievals of microphysics). Together with the growing network of PollyXT lidars, the automatic processing chain will remain under constant development with the goal of dissemination of near real-time data on product level with high quality to users and scientific databases.

At the end of May 2025, extremely strong wildfires in Canada produced several pyrocumulonimbus clouds lifting the smoke particles up to the lower stratosphere. Stratospheric aerosol optical depths of more than 2.5 were observed by the ATmospheric LiDAR (ATLID) onboard of the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) satellite. ATLID observations showed that the smoke layer top ascended from 13.6 km over Canada to 17.4 km over Europe. The enhanced depolarization ratio of 0.24 0.02 stayed constant during transport and indicated non-spherical smoke particles. ATLID is the first space lidar that is able to measure the extinction-to-backscatter ratio (lidar ratio) at 355 nm. A decrease of the lidar ratio with increasing transport time was observed from values of 68 5 to 49 5 sr. EarthCARE observed the substantial downmixing of the stratospheric smoke into the troposphere at tropopause folds over the Mediterranean revealing an important removal process of stratospheric smoke.

The potential impact of wildfire smoke on Arctic cirrus formation is discussed based on lidar and radar observations during the winter half year of the 1-year MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition. Aerosol and ice cloud observations were performed aboard the icebreaker Polarstern at latitudes > 85° N. Aged Siberian wildfire smoke polluted the tropopause region over the central Arctic during the entire winter half year of 2019–2020. The smoke particle surface area concentration at the tropopause was of the order of 5–15 µm2 cm−3 and indicated considerably enhanced levels of aerosol pollution for more than 6 months. Numerous cirrus systems with cloud-top temperatures between −60 and −75 °C developed in the polluted upper troposphere. We analyzed all MOSAiC winter cirrus layers with respect to their geometrical and optical properties and a subgroup of 20 cirrus events with respect to their ice water content (IWC) and ice crystal number concentration (ICNC). In individual ice fallstreaks that are connected to individual ice nucleation events, ICNCs typically ranged from 1 to 10 crystals L−1 but were frequently also as high as 20–50 L−1; however, observations > 100 L−1 were rare. Three observational facts corroborate our hypothesis that smoke significantly influenced Arctic cirrus formation: (1) the occurrence of a long-lasting, persistent smoke pollution layer in the upper troposphere so that favorable conditions for heterogeneous ice nucleation on smoke particles were always given and, at the same time, homogeneous freezing of background aerosol was probably widely suppressed; (2) the high smoke particle surface area concentrations, which were high enough to significantly trigger ice nucleation on smoke particles (as shown in Part 2, the companion paper to this article; Ansmann et al., 2025); and (3) the frequently found maximum cirrus ice saturation ratios of 1.3–1.5, which point to the dominance of heterogeneous ice nucleation processes, initiated by inefficient ice-nucleating particles (INPs), as expected when aged smoke particles (i.e., organic aerosol particles) serve as INPs. The studies are continued in the simulation portion of this work (Part 2; Ansmann et al., 2025).

One of the most powerful instruments for studying aerosol particles and their interactions with the environment is atmospheric lidar. In recent years, fluorescence lidar has emerged as a useful tool for identifying aerosol particles due to its link with biological content. Since 2022, this technique has been implemented in Leipzig, Germany. This paper describes the experimental setup and data analysis, with a special emphasis on the characterization of the new fluorescence channel centered at 466 nm. The new capabilities of the fluorescence lidar are examined and corroborated through several case studies. Most of the measurement cases considered are from the spring and summer of 2023, when large amounts of biomass-burning aerosol from huge forest fires in Canada were transported to Europe. The fluorescence of the observed aerosol layers is characterized. For wildfire smoke, the fluorescence capacity was typically in the range of 2×10-4–7 × 10−4, which aligns well with the values reported in the literature. The key aspects of this study are the capabilities of the fluorescence lidar technique, which can potentially improve not only the typing but even the detection of aerosol particles. In several measurement cases with an apparently low aerosol load, the fluorescence channel clearly revealed the presence of aerosol layers that were not detectable with the traditional elastic-backscatter channels. This capability is discussed in detail and linked to the fact that fluorescence backscattering is related to aerosol particles only. A second area of potential of the fluorescence technique is the distinction between non-activated aerosol particles and hydrometeors, given water's inability to exhibit fluorescence. A smoke–cirrus case study suggests an influence of the aerosol layer on cloud formation, as it seems to affect the elastic-backscatter coefficient within the cloud passing time. These aforementioned applications promise huge advancements towards a more detailed view of the aerosol–cloud interaction problem.

Novel ground-based remote sensing observations of aerosols and clouds have been carried out in Antarctica at the German Neumayer Station III (70.67°S, 8.27°W) for a whole year. The deployment of the mobile exploratory platform OCEANET-Atmosphere brought full ACTRIS aerosol and cloud profiling capabilities next to meteorological, radiation, and air chemistry in-situ observations at the Antarctic station. Neumayer III is currently the only station on a floating ice shelf that is manned throughout the year, providing excellent conditions for studying atmospheric effects on the Antarctic ice shelf. For that deployment the standard instrumentation of OCEANET-Atmosphere (PollyXT Raman polarization Lidar, a HATPRO microwave Radiometer, a Cimel sun and lunar photometer, and Radiation sensors) was extended by a Mira-35 cloud radar, a scanning LITRA-S Doppler lidar and a Parsivel² optical disdrometer. Together, these instruments brought the full ACTRIS aerosol and cloud profiling capabilities to a region where sophisticated ground-based observations were not available. The synergy of the different instruments allows for detailed retrievals of aerosol and cloud properties, such as cloud-relevant aerosol properties, liquid droplet properties and ice crystal concentrations. While data analysis is ongoing, three scientific highlights have already been identified during austral fall and winter, namely: Observations of a persistent shallow mixed-phase cloud embedded in a plume of advected marine aerosol. State of the art microphysical retrievals are used to obtain aerosol and cloud microphysical properties. Closure between cloud-relevant aerosol particles and precipitating ice crystals was achieved, demonstrating that the cloud formed in an aerosol-limited environment. Two extraordinary warm air intrusions: One with intense snowfall produced the equivalent of 10% of the yearly snow accumulation, a second one with record high temperatures and heavy icing due to supercooled drizzle. Omnipresent aerosol layers in the stratosphere, contributing almost 50% to the aerosol optical depth of around 0.06 at 500nm. Lidar-derived optical signatures revealed sulphate aerosol in the stratosphere - most likely linked to the Hunga Tonga eruption in 2022. We will present an overview of the campaign, the three highlights and provide an outlook on potential future usage of the dataset.

Vertically resolved observations of the temporal evolution of mixed-phase clouds (MPCs) were performed over the central Arctic during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition, which lasted from October 2019 to September 2020. The research icebreaker Polarstern, drifting with the pack ice for more than 7 months, mostly at latitudes > 85 degrees N, served as a platform for state-of-the-art remote sensing of aerosols and clouds. The use of the recently introduced dual field-of-view (FOV) polarization lidar technique in combination with the well-established lidar-radar retrieval technique provided, for the first time, a robust instrumental basis to monitor the evolution of the liquid and the ice phase of MPCs and the interplay between the two phases. Two long-lasting Arctic MPC events observed close to the North Pole in mid-winter (December 2019) and late summer (September 2020) are discussed to provide new insight into Arctic MPC evolution processes. In the second part of the article, cloud statistics, covering all seasons of a year, are presented. The focus is on the optical and microphysical properties of the liquid phase. These results are solely derived from the dual-FOV lidar observations. The key findings of the study can be summarized as follows: persistent activation of aerosol particles to form water droplets is of great importance for the longevity of MPCs. The observations confirm that ice formation occurs predominantly via immersion freezing. The field studies suggest that the free tropospheric reservoirs of cloud condensation nuclei (CCN) and of ice-nucleating particles (INPs) were always well filled, i.e., the clouds did not exhaust their supply of activatable and activated particles. The observation of long-lasting MPC events, low ice production rates, and a sufficiently large INP reservoir leads to the recommendation to use a time-dependent immersion freezing parameterization in MPC modeling efforts.

This study investigates the relationship between lidar-measured intensive optical properties of Saharan dust and simulated hematite content, using data collected during the Joint Aeolus Tropical Atlantic Campaign (JATAC) in 2021 and 2022. Measurements were taken in Mindelo, S & atilde;o Vicente, Cabo Verde. The study aims to determine how changes in hematite content influence the intensive optical properties of dust particles, particularly in the ultraviolet-visible (UV-Vis) spectrum. Given the well-documented impact of hematite on the absorption properties of dust, especially in the UV-Vis range, our hypothesis is that these effects will be detectable in lidar measurements. Specifically, this study focuses on the lidar ratio, particle depolarization ratio, and backscatter- and extinction-related & Aring;ngstr & ouml;m exponents at 355 and 532 nm wavelengths. By analyzing dust plume cases separately regarding their size differences, the strongest positive correlation was identified between the backscatter-related & Aring;ngstr & ouml;m exponent and hematite fraction (R2=0.63). These findings contribute to improving the representation of dust in atmospheric models, which often overlook the variability in mineralogical composition in their dust descriptions, and refining calculations of its direct radiative effect.