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The number of intense wildfires may increase in the upcoming years as a consequence of climate change. Changing aerosol conditions may lead to changes in regional and global cloud and precipitation pattern. One key aspect of research is presently whether or not wildfire smoke particles can initiate ice nucleation. We found strong evidence that aged smoke particles (dominated by 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. Our study is based on 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. 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 as starting values, gravity wave simulations show that lofting by 90-180 m is sufficient to initiate significant ice nucleation on the smoke particles, expressed in ice crystal number concentrations of 1-100 L−1 The authors acknowledge the ‘EXCELSIOR’: ERATOSTHENES: EXcellence reseacrh Centre for Earth Surveillance and Space-Based Monitoring of the Environment H2020 Widespread Teaming project (www.excelsior2020.eu). The ‘EXCELSIOR’ project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 857510, from the Government of the Republic of Cyprus through the Directorate General for the European Programmes, Coordination and Development and the Cyprus University of Technology. The authors acknowledge support through the European Research Infrastructure for the observation of Aerosol, Clouds and Trace Gases ACTRIS under grant agreement no. 654109 and 739530 from the European Union’s Horizon 2020 research and innovation programme. The PollyXT-CYP was funded by the German Federal Ministry of Education and Research (BMBF) via the PoLiCyTa project (grant no. 1LK1603A).The study is supported by “ACCEPT” project (Prot. No: LOCALDEV-0008) co-financed by the Financial Mechanism of Norway (85%) and the Republic of Cyprus (15%) in the framework of the programming period 2014 - 2021. The lidar analysis on smoke-cirrus interaction was further supported by BMBF funding of the SCiAMO project (MOSAIC-FKZ 03F0915A). DAK acknowledges support by U.S. Department of Energy’s (DOE) Atmospheric System Research (ASR) program, Office of Biological and Environmental Research (OBER) (grant no. DE-SC0021034).

Fires are a widespread ecological factor since ancient times. It has a negative impact not only on the environment but on the economy, society and people. A forest fire can lead to a change in land surface, the destruction of large areas of vegetation and soil erosion. As a result, the economy is negatively affected, the balance of ecosystems is disturbed, and the flora, fauna and natural beauty are destructed. Also, biomass burning smoke affects air quality due to the large quantities of trace gases and aerosol particles that are emitted, leading to global climate change and playing a significant role in troposphere chemistry. A fundamental tool for forest fire management is the science of remote sensing. Remote sensing is commonly used for mapping burnt areas as well as for studying the effects of fire incidents and this statement is very well supported by the literature at local, regional and global levels. This study is mainly focused on burned area mapping and damage assessment on land surface and atmosphere for the case of the Arakapas fire in Cyprus. For the purposes of this study, the satellite images acquired from Sentinel-2 were used for the burnt area mapping and the fire severity estimation based on the dNBR (difference Normalized Burn Ratio) spectral index, and the Corine land cover was used for the assessment of the vegetation type that was disturbed. This event considered one of the largest in recent years is explored using data from Sentinel-5P, where carbon monoxide product is studied in the region affected by the fires. Furthermore, on the morning of the 5th of July, due to the change of wind direction, the smoke travelled from the centre of the island to the southwest, and it was detected by the multiwavelength Raman lidar, installed in Limassol. Thus, the optical properties of the smoke plume retrieved from the lidar are presented. The PollyXT-CYP lidar system of the ECoE, observed multiple layers between 500m and 2.5km with depolarization ratio of 5-8% and lidar ratio of 75sr for the upper layers.For the purposes of this study, the image processing was performed using custom scripts in the GEE (Google Earth Engine) platform with the JavaScript programming interface. The area affected by the fire was calculated to be ~40Km2. The spatial distribution map of the dNBR was classified according to the USGS fire severity levels, where high dNBR values indicate a more severe fire and values near zero and negative values indicate unburned and/or decreased vegetation after the fire. AcknowledgementsThe authors acknowledge the 'EXCELSIOR': ERATOSTHENES: Excellence Research Centre for Earth Surveillance and Space-Based Monitoring of the Environment H2020 Widespread Teaming project (www.excelsior2020.eu). The 'EXCELSIOR' project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant Agreement No 857510, from the Government of the Republic of Cyprus through the Directorate General for the European Programmes, Coordination and Development and the Cyprus University of Technology.

The presence of aerosol and clouds constitutes one of the highest uncertainties regarding the energy budget of the Earth. Therefore, their continuous observation can help reduce these uncertainties, by providing more information about aerosol-cloud interactions and how these atmospheric components contribute to climate change.To study the properties of aerosols and clouds, new infrastructure will soon be set up in Cyprus by the Eratosthenes Centre of Excellence, which was recently established through the ‘EXCELSIOR’ H2020 Widespread Teaming Project. Eratosthenes Centre of Excellence is a digital innovation hub for Earth Observation, Space Technology and Geospatial Information, aiming to become the reference centre in the East Mediterranean, north Africa and the Middle East (EMMENA region). The infrastructure will be installed in Limassol, on the south coast of the island. Apart from the fact that the site is less than 2 km from the island’s coastline, this location is extremely important in terms of the regional atmospheric composition, as the air masses affecting the site originate from the surrounding areas of EMMENA, as well as from south-eastern Europe.Infrastructure to sample aerosol is already represented by a state-of-the-art PollyXT lidar, with measurements being registered continuously since October 2020. By the end of 2023, a similar LACROS multi-instrument platform will be available for the continuous monitoring of clouds and aerosols. This new ground-based remote sensing platform consists of a 35GHz cloud radar, a ceilometer, a microwave radiometer, a Doppler lidar, and a disdrometer. The infrastructure will be integrated into the Cyprus Atmospheric Remote Sensing Observatory (CARO).An example of the value of the observations about cloud formation and the role of aerosol in the process of cloud formation that will be provided from the site in Limassol, can be reflected in the data collected during the CyCARE campaign, executed between October 2016 and March 2018, during which similar ground-based infrastructure was deployed. The authors acknowledge the ‘EXCELSIOR’: ERATOSTHENES: EΧcellence Research Centre for Earth Surveillance and Space-Based Monitoring of the Environment H2020 Widespread Teaming project (www.excelsior2020.eu). The ‘EXCELSIOR’ project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 857510, from the Government of the Republic of Cyprus through the Directorate General for the European Programmes, Coordination and Development and the Cyprus University of Technology.

In the summer of 2021, several wildfires were reported in the south of Turkey, fires that are considered one of the worst in the history of Turkey. Due to atmospheric conditions, the smoke plume travelled south between 27 July to 5 August 2021, and smoke layers arrived above Cyprus. In this work, the capabilities of the newly established ERATOSTHENES Centre of Excellence (CoE), to study large-scale atmospheric events is presented. The study is based on the synergistic use of different datasets of remote sensing techniques both from ground and space. The EARLINET multiwavelength-Raman-polarization lidar PollyXT-CYP hosted by the ERATOSTHENES CoE is continuously running since October 2020 in Limassol, and during summer 2021, the lidar observed smoke plumes from these extreme wildfires on the south coast of Turkey. The PollyXT-CYP is a key research infrastructure of the Cyprus Atmospheric Remote Sensing Observatory (CARO) of the ERATOSTHENES CoE established through the EXCELSIOR H2020 EU Teaming project coordinated by the Cyprus University of Technology. CARO will consist of two high-tech containers housing the PollyXT-CYP lidar and state-of-the art doppler lidar, cloud radar and radiometric equipment which will be used to measure the air quality, the dust transport, and the cloud properties over Cyprus. The CARO is a planning National Facility of the Republic of Cyprus for Aerosol and Cloud Remote Sensing Observations. Land cover information which shows the type of burned vegetation is used together with satellite products to capture additionally the burned area and to investigate the carbon monoxide of the smoke plume. The study is focusing on the optical characteristics of the plume, as it was detected by the PollyXT-CYP lidar at Limassol. An intense fresh smoke layer was detected on 28-29 July 2021, at an altitude between 2.5 to 4.0 km, having a volume depolarization ratio of ~15% at 355n and ~20% at 532nm, and lidar ratio of 75-80sr at 355nm and 65-70sr at 532nm. Acknowledgements The authors acknowledge the ‘EXCELSIOR’: ERATOSTHENES: EΧcellence Research Centre for Earth Surveillance and Space-Based Monitoring of the Environment H2020 Widespread Teaming project (www.excelsior2020.eu). The ‘EXCELSIOR’ project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 857510, from the Government of the Republic of Cyprus through the Directorate General for the European Programmes, Coordination and Development and the Cyprus University of Technology. The PollyXT-CYP was funded by the German Federal Ministry of Education and Research (BMBF) via the PoLiCyTa project (grant no. 01LK1603A). The study is supported by “ACCEPT” project (Prot. No: LOCALDEV-0008) co-financed by the Financial Mechanism of Norway (85%) and the Republic of Cyprus (15%) in the framework of the programming period 2014 - 2021.

This paper presents a collection of lidar-derived aerosol intensive optical properties for several aerosol types, namely the particle linear depolarization ratio, the extinction-to-backscatter ratio (lidar ratio) and the Ångström exponent. The data collection, named DeLiAn, is based on globally distributed, long-term, ground-based, multiwavelength, Raman and polarization lidar measurements, conducted mainly with lidars that have been developed at the Leibniz Institute for Tropospheric Research. The intensive optical properties are presented at two wavelengths, 355 and 532 nm, for 13 aerosol categories. The categories cover the basic aerosol types (i.e., marine, pollution, continental European background, volcanic ash, smoke, mineral dust), as well as the most frequently observed mixtures they form. This extensive collection also incorporates more peculiar aerosol categories, including dried marine aerosol that, compared to marine aerosol, exhibits a significantly enhanced depolarization ratio (up to 15 %). Besides Saharan dust, additional mineral dust types related to their source region were identified due to their lower lidar ratios (Central Asian and Middle Eastern dust). In addition, extreme wildfire events (such as in north America and Australia) emitted smoke into the stratosphere showing significantly different optical properties, i.e., high depolarization values (up to 25 %), compared to tropospheric smoke. The data collection reflects and underlines the variety of aerosol mixtures in the atmosphere and can be used for the development of aerosol-typing schemes. The paper contains the most up-to-date and comprehensive overview of optical properties from aerosol lidar measurements and, therefore, provides a solid basis for future aerosol retrievals in the frame of both spaceborne and ground-based lidars. Furthermore, DeLiAn can assist the efforts for the harmonization of satellite records of aerosol properties performed at different wavelengths.

Recent studies have shown that the Eastern Mediterranean and the Middle East are regions strongly affected by climate change, with a rate of warming two times faster than the global average. While climate change comes with major implications, including the fact that severe storms will become more virulent and more often, studies showed that the annual number of cyclones in the Mediterranean region tends to remain constant in the near future, with no visible trend. In this paper, we are studying the trends for convective indices in the region of Cyprus, with modelled, and also observational data. For the modelled data, we are using the CORDEX dataset for convective potential energy and convective inhibition with 3h temporal resolution, for the historical period (1971 - 2000), and for the near future (2021 - 2050), from which we extracted the daily maximum and mean values. Observational data is used from the ESWD database, reports on tornadoes, hail and heavy rain, together with radiosonde data from Athalassa station (radiosonde available in high-resolution mode from 2016, with two launches per day, at 05:00 and 11:00 UTC). Using the Python packages SHARPpy and MetPy we computed the convective indices from radiosounding data. It is estimated that two tornadoes and three waterspouts occur each year in Cyprus, and in a study from 2006 describing two tornadoes outbreaks from January 2003 and January 2004, these events had CAPE index 411 J/kg and 210 J/kg, and respectively lifted index of -2 and 0. The authors acknowledge the ‘EXCELSIOR’: ERATOSTHENES: EΧcellence Research Centre for Earth Surveillance and Space-Based Monitoring of the Environment H2020 Widespread Teaming project (www.excelsior2020.eu). The ‘EXCELSIOR’ project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 857510, from the Government of the Republic of Cyprus through the Directorate General for the European Programmes, Coordination and Development and the Cyprus University of Technology.

The purpose of this study is to investigate the ability of the Sentinel-5P TROPOspheric Monitoring Instrument (TROPOMI) to derive accurate geometrical features of lofted aerosol layers, selecting the Mediterranean Basin as the study area. Comparisons with ground-based correlative measurements constitute a key component in the validation of passive and active satellite aerosol products. For this purpose, we use ground-based observations from quality-controlled lidar stations reporting to the European Aerosol Research Lidar Network (EARLINET). An optimal methodology for validation purposes has been developed and applied using the EARLINET optical profiles and TROPOMI aerosol products, aiming at the in-depth evaluation of the TROPOMI aerosol layer height (ALH) product for the period 2018 to 2022 over the Mediterranean Basin. Seven EARLINET stations were chosen, taking into consideration their proximity to the sea, which provided 63 coincident aerosol cases for the satellite retrievals. In the following, we present the first validation results for the TROPOMI/S5P ALH using the optimized EARLINET lidar products employing the automated validation chain designed for this purpose. The quantitative validation at pixels over the selected EARLINET stations illustrates that the TROPOMI ALH product is consistent with the EARLINET lidar products, with a high correlation coefficient R = 0.82 (R = 0.51) and a mean bias of -0.51 +/- 0.77 km and -2.27 +/- 1.17 km over ocean and land, respectively. Overall, it appears that aerosol layer altitudes retrieved from TROPOMI are systematically lower than altitudes from the lidar retrievals. High-albedo scenes, as well as low-aerosol-load scenes, are the most challenging for the TROPOMI retrieval algorithm, and these results testify to the need to further investigate the underlying cause. This work provides a clear indication that the TROPOMI ALH product can under certain conditions achieve the required threshold accuracy and precision requirements of 1 km, especially when only ocean pixels are included in the comparison analysis. Furthermore, we describe and analyse three case studies in detail, one dust and two smoke episodes, in order to illustrate the strengths and limitations of the TROPOMI ALH product and demonstrate the presented validation methodology. The present analysis provides important additions to the existing validation studies that have been performed so far for the TROPOMI S5P ALH product, which were based only on satellite-to-satellite comparisons., "Panhellenic Infrastructure for Atmospheric Composition and Climate Change" project MIS 5021516, European Commission European Space Agency European Commission, SIROCCO project EXCELLENCE/1216/0217, AQ-SERVE project - Republic of Cyprus INTEGRATED/0916/0016, European Commission, Federal Ministry of Education & Research (BMBF), EU H2020 EXCELSIOR project, European Research Council (ERC) under Community's Horizon 2020 research and innovation framework programme (ERC) 725698

Due to its geographical location, Cyprus is often affected by dust storms arriving from the largest deserts of the planet, the Sahara, the Arabian Peninsula and the Syrian. In order to characterize dust properties, the Cyprus Atmospheric Observatory (CAO) and the Unmanned Research Laboratory (USRL) of the Cyprus Institute (CYI), in collaboration with the Cyprus Atmospheric Remote sensing Observatory (CARO) of the ERATOSTHENES Centre of Excellence (ECoE) of the Cyprus University of Technology (CUT), performed a research campaign in Fall 2021. Measurements were performed with ground-based aerosol remote sensing systems (lidars, ceilometers and sunphotometers), and UAV based in-situ instruments (OPCs, backscatter sondes, and impactors able to collect dust samples). As part of the remote sensing observations, two depolarized lidars performed measurements from different locations, one from CYI premises in Nicosia and the second one from ECoE-CUT premises in Limassol. The lidar signals provide information about the vertical aerosol profile at the two locations, which can be used to derive the optical properties of dust at different altitudes. Here, we will present first results on the synergy between the continuous vertically extended measurements of lidars and the in-situ measurements from UAV instrumentation during the periods of dust outbreaks. Two dust events occurred from 25 October to 1 November and from 13 to 18 November 2021. During these dust events, the lidars observed depolarized aerosol layers from ground up to 5 km above sea level. The lidar measurements provided the temporal and spatial development of these dust layers, and were also used in real-time for planning the UAV flight schedule. According to backward trajectory analyses, the two dust events had different origins with the first arriving from the Sahara and the second one from the Middle East.