The strong Antarctic vortex plays a crucial role in forming an expansive region with significant stratospheric ozone depletion during austral spring, commonly referred to as the Antarctic “ozone hole”. This study examines daily ozone column behavior during this phenomenon using ERA5 reanalysis data and ground-based observations from 10 Antarctic stations collected between September and December from 2008 to 2022. A preliminary analysis of these datasets revealed smoothly varying patterns with quasi-uniform gradients in the ozone distribution within the ozone hole. This observation led to the hypothesis that average ozone columns over zones, defined as concentric areas around the South Pole, can be estimated using mean values of the measurements derived from station observations. This study aims to evaluate the validity of this hypothesis. The results indicate that the mean ozone levels calculated from daily measurements at two stations—Belgrano and Dome Concordia, or Belgrano and Arrival Heights—provide a reliable approximation of the average ozone levels over the zone spanning 70°S to 90°S. Including additional stations extended the zone of reliable approximation northward to 58°S. The approximation error was estimated to range from 5% to 7% at 1σ and from 6% to 8% at the 10th–90th percentile levels. Furthermore, the geographical distribution of the stations enabled a schematic reconstruction of the ozone hole’s position and shape. On the other hand, the high frequency of ground-based measurements contributed to studying the ozone hole variability in both the inner area and edges on an hourly time scale. These findings have practical implications for the near-real-time monitoring of ozone hole development, along with satellite observations, considering ground-based measurements as a source of information about ozone layer in the South Pole region. The results also suggest the possible role of observations from the ground in the analyses of pre-satellite-era hole behavior. Additionally, this study found a high degree of consistency between ground-based measurements and corresponding ERA5 reanalysis data, further supporting the reliability of the observations.

Clouds modulate the net radiative flux that interacts with both shortwave (SW) and longwave (LW) radiation, but the uncertainties regarding their effect in polar regions are especially high because ground observations are lacking and evaluation through satellites is made difficult by high surface reflectance. In this work, sky conditions for six different polar stations, two in the Arctic (Ny-Ålesund and Utqiagvik [formerly Barrow]) and four in Antarctica (Neumayer, Syowa, South Pole, and Dome C) will be presented, considering the decade between 2010 and 2020. Measurements of broadband SW and LW radiation components (both downwelling and upwelling) are collected within the frame of the Baseline Surface Radiation Network (BSRN). Sky conditions—categorized as clear sky, cloudy, or overcast—were determined using cloud fraction estimates obtained through the RADFLUX method, which integrates shortwave (SW) and longwave (LW) radiative fluxes. RADFLUX was applied with daily fitting for all BSRN stations, producing two cloud fraction values: one derived from shortwave downward (SWD) measurements and the other from longwave downward (LWD) measurements. The variation in cloud fraction used to classify conditions from clear sky to overcast appeared consistent and reasonable when compared to seasonal changes in shortwave downward (SWD) and diffuse radiation (DIF), as well as longwave downward (LWD) and longwave upward (LWU) fluxes. These classifications served as labels for a machine learning-based classification task. Three algorithms were evaluated: Random Forest, K-Nearest Neighbors (KNN), and XGBoost. Input features include downward LW radiation, solar zenith angle, surface air temperature ($T_a$), relative humidity, and the ratio of water vapor pressure to $T_a$. Among these models, XGBoost achieved the highest balanced accuracy, with the best scores of 0.78 at Ny-Ålesund (Arctic) and 0.78 at Syowa (Antarctica). The evaluation employed a leave-one-year-out approach to ensure robust temporal validation. Finally, the results from cross-station models highlighted the need for deeper investigation, particularly through clustering stations with similar environmental and climatic characteristics to improve generalization and transferability across locations. Additionally, the use of feature normalization strategies proved effective in reducing inter-station variability and promoting more stable model performance across diverse settings.

A multi-year analysis of aerosol optical depth (AOD, τ) and Ångström exponent (α) was conducted using ground-based photometer data from 15 Arctic and 11 Antarctic sites. Extending the dataset of Tomasi et al. 2015 through December 2024, the study incorporates stellar and lunar photometric observations to fill data gaps during the polar night. Daily mean values of τ at 0.500 μm and α (0.440–0.870 μm) were used to derive monthly means and seasonal histograms. In the Arctic, persistent haze events in winter and early spring lead to peak τ values. A decreasing trend in Arctic τ suggests the impact of European emission regulations, while biomass-burning aerosols are becoming more significant. In Antarctica, τ increases from the plateau to the coast. Fine-mode aerosols dominate in summer-autumn, while coarse-mode particles are more prevalent in winter-spring. Shipborne photometer data align well with ground-based measurements, confirming the reliability of mobile observations. Trend analyses using the Mann-Kendall test and Theil-Sen regression indicate a significant negative trend in τ at Andenes (-2.43 % per year), likely driven by reduced anthropogenic emissions. Antarctic stations such as Syowa and South Pole show positive trends (+3.84 % and +3.54 % per year), though these are subject to uncertainties from data limitations and instrument changes. This work contributes to the Polar-AOD network (https://polaraod.net/; last access 15/05/2025), enhancing the understanding of aerosol variability and long-term trends in polar regions while promoting open data access for the scientific community.

In May-June 2024, an intercomparison campaign is scheduled to take place in Valladolid, Spain, comparing different photometer models. Specifically, the campaign will involve the PREDE POM-02L from the Institute of Polar Sciences (CNR-ISP), the CIMEL CE318-T from the Atmospheric Optics Group (GOA) at Valladolid University, and the PFR from the Physikalisch-Meteorologisches Observatorium Davos (PMOD). The primary objective of this campaign is to assess the effectiveness of a personalized algorithm for determining Aerosol Optical Depth (AOD) and Angstrom Exponent (α), while also evaluating the quality of these measurements by comparing them with data from the AERONET station operated by the GOA. AOD measurements will be conducted at various wavelengths: 440, 500, 675, and 870 nm for CIMEL; 400, 500, 675, and 870 nm for PREDE; and 440, 500, and 870 nm for PFR; α will be evaluated between 500 and 870 nm for all the photometers. For the multi-instrument intercomparison, the recommendations defined in 2005 by the WMO during the event ’WMO/GAW expert workshop on a global surface network for long-term observations of columnar aerosol optical properties' will be followed. These consist of: (i) acquiring more than 1000 points over at least 5 days of measurements, with AOD500 between 0.04 and 0.20; (ii) ensuring traceability with 95% uncertainty within 0.005 + 0.01/m; (iii) using the 500±3 nm and 865±5 nm channels, with a bandwidth of 15 nm or less, since these channels are mostly free from absorption effects. Valladolid, a medium-size city in the northern Castilian plateau, typically experiences continental aerosols as the predominant aerosol type. AOD values tend to be lowest during winter, gradually increasing to peak levels in late spring and summer. Maximum AOD values in Valladolid are often associated with the presence of dust particles from Saharan dust outbreaks, which are characterized by low α values. This intercomparison campaign, spanning late spring and early summer, provides an opportunity to observe the first significant desert dust transport events at the site. Additionally, biomass-burning events, associated with high AOD and high α values, may also be observed during this period. Detecting and studying these events will be another key focus of the campaign. In conclusion, this campaign offers us a chance to assess how well different sun-photometers work. Additionally, we’ll be able to study how desert dust and biomass burning events affect the site during this period.