Marilena Gidarakou

Secondary ice production (SIP) strongly influences ice crystal number concentrations (ICNCs) and cloud microphysical properties, with important implications for radiative forcing, thermodynamics, cloud evolution, and precipitation. Here, we investigate four SIP mechanisms—rime splintering, collisional breakup, rain droplet shattering during freezing, and sublimation breakup—in the Integrated Community Limited Area Modeling System, a specialized version of the Regional Atmospheric Modeling System (RAMS-ICLAMS), during winter storms observed in November–December 2024 as part of the Cleancloud Helmos OrograPhic sIte experimeNt (CHOPIN). Activating SIP increases ICNCs by two to three orders of magnitude and enhances ice water content by up to a factor of 2–3, substantially modifying cloud structure, including a 25–35 % reduction in liquid water path and a doubling of ice water path. These changes improve agreement with observed radar signatures and precipitation patterns and alter the amount of precipitation reaching the ground. Among the represented mechanisms, collisional breakup is dominant, followed by rime splintering. Sublimation breakup is locally intense but at least one order of magnitude weaker, while droplet shattering is the weakest and is mainly activated in regions rich in raindrops. The simulated systems encompass synoptic-scale, convective, and stratiform clouds, including snowfall-producing and seeder–feeder configurations. These results highlight the importance of representing SIP—particularly collisional breakup and rime splintering—for realistic simulations of diverse orographic clouds and precipitation in weather- and climate-scale models.

Secondary ice production (SIP) strongly influences ice crystal number concentrations (ICNCs) and cloud microphysical properties, with important implications for radiative forcing, thermodynamics, cloud evolution, and precipitation. Here, we investigate four SIP mechanisms—rime splintering, collisional breakup, rain droplet shattering during freezing, and sublimation breakup—in the Integrated Community Limited Area Modeling System, a specialized version of the Regional Atmospheric Modeling System (RAMS-ICLAMS), during winter storms observed in November–December 2024 as part of the Cleancloud Helmos OrograPhic sIte experimeNt (CHOPIN). Activating SIP increases ICNCs by two to three orders of magnitude and enhances ice water content by up to a factor of 2–3, substantially modifying cloud structure, including a 25–35 % reduction in liquid water path and a doubling of ice water path. These changes improve agreement with observed radar signatures and precipitation patterns and alter the amount of precipitation reaching the ground. Among the represented mechanisms, collisional breakup is dominant, followed by rime splintering. Sublimation breakup is locally intense but at least one order of magnitude weaker, while droplet shattering is the weakest and is mainly activated in regions rich in raindrops. The simulated systems encompass synoptic-scale, convective, and stratiform clouds, including snowfall-producing and seeder–feeder configurations. These results highlight the importance of representing SIP—particularly collisional breakup and rime splintering—for realistic simulations of diverse orographic clouds and precipitation in weather- and climate-scale models.

During summer 2023, Greece experienced one of its most severe wildfire seasons in recent decades, with widespread fires across Evros, Rodopi, Attica, the Peloponnese, and several islands. This study investigates the aerosol optical and microphysical properties, as well as the impact on ground-level ultraviolet-B (UVB) radiation over Athens, focusing on two major wildfire episodes (18–21 July and 22–25 August). A synergistic approach was deployed, combining satellite imagery (MODIS), FLEXPART simulations, ground-based remoter sensing, in situ aerosol and radiation measurements. Elevated aerosol optical depths (AOD) up to 1.2, high fine-mode fractions (FMF) (> 0.85), and Ångström exponents (AE) above 1.5 indicated a strong dominance of fine biomass burning aerosols. The Single scattering albedo (SSA) ranged from 0.85 to 0.98, showing enhanced absorption during biomass burning periods and weaker absorption when smoke was mixed with dust. At 320 nm, dust presence resulted in stronger absorption, with SSA below 0.8 for pure dust cases compared to smoke mixtures. Particle linear depolarization ratios (PLDR), varied between 0.03 and 0.20, with higher values (∼ 0.10–0.20) reflecting the presence of non-spherical dust particles, and lower values (∼ 0.03–0.08) indicating spherical smoke particles. Ground-level UVB irradiance decreased by up to 50 % during peak smoke episodes, highlighting strong aerosol radiative impacts. Concurrently, PM10 and PM2.5 concentrations increased to 94 and 49 µg m−3, respectively, while organic aerosols peaked at 22.77 µg m−3, consistent with intense fire activity. FLEXPART simulations confirmed long-range transport of smoke from active fire regions, with additional contributions from regional pollution and Saharan dust.

The Cleancloud Helmos OrograPhic site experimeNt (CHOPIN) campaign took place at mount Helmos, Greece (37.98°N, 22.2°E; 1700-2314 m a.s.l.) to study the aerosol-cloud interactions during two distinct periods: autumn/winter (October–November 2024) and spring (April–May 2025). In situ aerosol sampling at the Helmos Atmospheric Aerosol and Climate Change Station (HAC)2 was performed at 2314 m a.s.l. along with aerosol lidar vertical measurements. (HAC)2 is located on a strategic site at a crossroad of different air masses containing various aerosol types (wildfire smoke, mineral dust, continental pollution, marine aerosols, and biogenic particles). Two lidar systems were deployed: the AIAS depolarization lidar (532 nm parallel and cross, 1064 nm) and the ATLAS-NEF multi-wavelength elastic-Raman-LIF lidar (355, 387, 407 and 420-520 nm). The vertically resolved aerosol optical properties (extinction and backscatter coefficient, lidar ratio, Ångström exponent, particle depolarization) and water vapor mixing ratios, alongside with fluorescence backscatter profiles, were provided from near-ground up to 5-7 km a.s.l. Lidar-inversion algorithms were used to retrieve the aerosol microphysical properties (effective radius, single scattering albedo, and complex refractive index). The aerosol chemical composition was retrieved using the ISORROPIA thermodynamic model. The aerosol fluorescence measurements highlighted enhanced presence of bioaerosols in selected cases. Saharan dust particles exhibited high depolarization ratios (δ532 ~0.20–0.25) and lidar ratios (LR ~40–55 sr), while biomass burning plumes showed distinct microphysical and chemical signatures. Comparison of in situ and lidar-derived optical, microphysical and chemical properties at 2.314 m a.s.l. was found to be quite satisfactory, paving the way for a novel synergistic approach to further elucidate the aerosols’ role in cloud formation and radiative forcing. These lidar data are used to improve Machine Learning algorithms in the frame of the F-LIDAR-M project.Funding: The research project, entitled “Real-time detection/Speciation of bio-aerosols profiling using Fluorescence LiDAR techniques and Machine Learning (F-LIDAR-M)” is implemented in the framework of H.F.R.I call “3rd Call for H.F.R.I.’s Research Projects to Support Faculty Members & Researchers” (H.F.R.I. Project Number: 25096).

Aerosol hygroscopicity play a fundamental role on cloud activation, radiative transfer, and particle–light interactions, yet its impact on fluorescence properties remains poorly understood. During the CleanCloud Helmos OrograPhic site experimeNt (CHOPIN) campaign at Mount Helmos, Greece (38.0°N, 22.2°E; 1700–2314 m a.s.l.), aerosol hygroscopicity and fluorescence were investigated across two periods: autumn (Oct–Nov 2024) and spring (Apr–May 2025). The high altitude and strategic location of the site allow the observation of a wide variety of aerosol types, including Saharan dust, biomass burning smoke, urban pollution, and biogenic particles.A multi-wavelength elastic-Raman–fluorescence lidar (ATLAS-NEF) operating at 355, 387, 407 and 470 nm, provided vertically resolved aerosol optical properties (extinction, backscatter, lidar ratios) and water vapor mixing ratios, as well as fluorescence backscatter profiles.Hygroscopic growth factors were derived from Raman-based backscatter following Hänel-type parameterizations, supported by measurements (pressure, temperature, and relative humidity) from radiosondes, a tethered helikite, and Unmanned Aerial Vehicles (UAVs). Fluorescence quenching was quantified as a function of relative humidity and compared to the optical growth exponent γ, while the Wideband Integrated Bioaerosol Sensor (WIBS) and Multiparameter Bioaerosol Sensor (MBS) provided information on bioaerosol concentrations and types.Aerosol backscatter generally increased with relative humidity, while fluorescence decreased, indicating humidity-dependent quenching. Biogenic particles showed strong fluorescence but limited hygroscopic growth, whereas dust and urban aerosols were moderately hygroscopic with reduced fluorescence. Synergies with MBS and WIBS highlighted temporal variability in bioaerosol concentrations, linking lidar fluorescence changes to particle composition and aging. These results demonstrate that this synergistic approach provides a robust framework to assess humidity-driven changes in optical and microphysical properties, with implications for cloud formation and radiative forcing.

During summer 2023, Greece experienced one of its most severe wildfire seasons in recent decades, with widespread fires across Evros, Rodopi, Attica, the Peloponnese, and several islands. This study investigates the aerosol optical and microphysical properties, as well as the impact on ground-level ultraviolet-B (UVB) radiation over Athens, focusing on two major wildfire episodes (18–21 July and 22–25 August). A synergistic approach was deployed, combining satellite imagery (MODIS), FLEXPART simulations, ground-based remote sensing, and in situ aerosol and radiation measurements. Elevated aerosol optical depths (AOD) up to 1.2, high fine-mode fractions (> 0.85), and Ångström exponents above 1.5 indicated a strong dominance of fine biomass burning aerosols. The Single scattering albedo (SSA) ranged from 0.85 to 0.98, showing enhanced absorption during biomass burning periods and weaker absorption when smoke was mixed with dust. At 320 nm, dust presence resulted in stronger absorption, with SSA below 0.8 for pure dust cases compared to smoke mixtures. Particle linear depolarization ratios (PLDR), varied between 0.03 and 0.20, with higher values (~0.10–0.20) reflecting the presence of non-spherical dust particles, and lower values (~0.03–0.08) indicating spherical smoke particles. Ground-level UVB irradiance decreased by up to 50 % during peak smoke episodes, highlighting strong aerosol radiative impacts. Concurrently, PM₁₀ and PM_2.5 concentrations increased to 94 and 49 µg m^-3, respectively, while organic aerosols peaked at 22.77 µg m^-3, consistent with intense fire activity. FLEXPART simulations confirmed long-range transport of smoke from active fire regions, with additional contributions from regional pollution and Saharan dust.

During summer 2023, Greece experienced one of its most severe wildfire seasons in recent decades, with widespread fires across Evros, Rodopi, Attica, the Peloponnese, and several islands. This study investigates the aerosol optical and microphysical properties, as well as the impact on ground-level ultraviolet-B (UVB) radiation over Athens, focusing on two major wildfire episodes (18–21 July and 22–25 August). A synergistic approach was deployed, combining satellite imagery (MODIS), FLEXPART simulations, ground-based remote sensing, and in situ aerosol and radiation measurements. Elevated aerosol optical depths (AOD) up to 1.2, high fine-mode fractions (> 0.85), and Ångström exponents above 1.5 indicated a strong dominance of fine biomass burning aerosols. The Single scattering albedo (SSA) ranged from 0.85 to 0.98, showing enhanced absorption during biomass burning periods and weaker absorption when smoke was mixed with dust. At 320 nm, dust presence resulted in stronger absorption, with SSA below 0.8 for pure dust cases compared to smoke mixtures. Particle linear depolarization ratios (PLDR), varied between 0.03 and 0.20, with higher values (~0.10–0.20) reflecting the presence of non-spherical dust particles, and lower values (~0.03–0.08) indicating spherical smoke particles. Ground-level UVB irradiance decreased by up to 50 % during peak smoke episodes, highlighting strong aerosol radiative impacts. Concurrently, PM₁₀ and PM_2.5 concentrations increased to 94 and 49 µg m^-3, respectively, while organic aerosols peaked at 22.77 µg m^-3, consistent with intense fire activity. FLEXPART simulations confirmed long-range transport of smoke from active fire regions, with additional contributions from regional pollution and Saharan dust.

During summer 2023, Greece experienced one of its most severe wildfire seasons in recent decades, with widespread fires across Evros, Rodopi, Attica, the Peloponnese, and several islands. This study investigates the aerosol optical and microphysical properties, as well as the impact on ground-level ultraviolet-B (UVB) radiation over Athens, focusing on two major wildfire episodes (18–21 July and 22–25 August). A synergistic approach was deployed, combining satellite imagery (MODIS), FLEXPART simulations, ground-based remote sensing, and in situ aerosol and radiation measurements. Elevated aerosol optical depths (AOD) up to 1.2, high fine-mode fractions (> 0.85), and Ångström exponents above 1.5 indicated a strong dominance of fine biomass burning aerosols. The Single scattering albedo (SSA) ranged from 0.85 to 0.98, showing enhanced absorption during biomass burning periods and weaker absorption when smoke was mixed with dust. At 320 nm, dust presence resulted in stronger absorption, with SSA below 0.8 for pure dust cases compared to smoke mixtures. Particle linear depolarization ratios (PLDR), varied between 0.03 and 0.20, with higher values (~0.10–0.20) reflecting the presence of non-spherical dust particles, and lower values (~0.03–0.08) indicating spherical smoke particles. Ground-level UVB irradiance decreased by up to 50 % during peak smoke episodes, highlighting strong aerosol radiative impacts. Concurrently, PM₁₀ and PM_2.5 concentrations increased to 94 and 49 µg m^-3, respectively, while organic aerosols peaked at 22.77 µg m^-3, consistent with intense fire activity. FLEXPART simulations confirmed long-range transport of smoke from active fire regions, with additional contributions from regional pollution and Saharan dust.