​​​​​​​Vertically resolved information on aerosol particles represents a key aspect in many atmospheric studies, including aerosol–climate interactions and aerosol impacts on air quality and human health. This information is primarily derived by lidar active remote sensing, in particular with extensive networks currently in operation worldwide. In Italy, the Institute of Atmospheric Sciences and Climate (ISAC) of the National Research Council (CNR) established the ALICENET network of automated lidar ceilometers (ALCs) in 2015. Since then, ALICENET has grown as a cooperative effort of Italian institutions dealing with atmospheric science and monitoring, and it currently includes instruments run by regional environmental protection agencies, universities, research centres, and private companies. In the current configuration, the network makes use of both single-channel ALCs and dual-channel, polarisation-sensitive-system ALCs (referred to as PLCs). The systems operate in very different environments (urban, coastal, mountainous, and volcanic areas) from northern to southern Italy, thus allowing the continuous monitoring of the aerosol vertical distribution across the country. ALICENET also contributes to the EUMETNET programme E-PROFILE, filling an Italian observational gap compared to other EU member states, which generally run extended ALC networks through national meteorological services. In this work, we present the ALICENET infrastructure and the specifically developed data processing centralised at CNR-ISAC, converting raw instrumental data into quantitative, quality-controlled information on aerosol properties ranging from attenuated backscatter to aerosol mass and vertical stratifications. This setup allows us to get insights into the 4D aerosol field over Italy with applications from near-real-time monitoring to long-term analyses, examples of which are reported in this work. Specific comparisons of the ALICENET products to independent measurements obtained with different techniques, such as particulate matter (PM) concentrations from in situ samplers and aerosol optical depth (AOD) from sun photometers, are also included here, revealing the good performances of the ALICENET algorithms. Overall, ALICENET represents a valuable resource to extend the current aerosol observational capabilities in Italy and in the Mediterranean area, and it contributes to bridging the gap between atmospheric science and its application to specific sectors, among which are air quality, solar energy, and aviation safety.

Aerosol vertical stratification significantly influences the Earth’s radiative balance and particulate-matter-related air quality. Continuous vertically resolved observations remain scarce compared to surface-level and column-integrated measurements. This work presents and makes available a novel, long-term (2016–2022) aerosol dataset derived from continuous (24/7) vertical profile observations from three selected stations (Aosta, Rome, Messina) of the Italian Automated Lidar-Ceilometer (ALC) Network (ALICENET). Using original retrieval methodologies, we derive over 600,000 quality-assured profiles of aerosol properties at the 15 min temporal and 15 metre vertical resolutions. These properties include the particulate matter mass concentration (PM), aerosol extinction and optical depth (AOD), i.e., air quality legislated quantities or essential climate variables. Through original ALICENET algorithms, we also derive long-term aerosol vertical layering data, including the mixed aerosol layer (MAL) and elevated aerosol layers (EALs) heights. Based on this new dataset, we obtain an unprecedented, fine spatiotemporal characterisation of the aerosol vertical distributions in Italy across different geographical settings (Alpine, urban, and coastal) and temporal scales (from sub-hourly to seasonal). Our analysis reveals distinct aerosol daily and annual cycles within the mixed layer and above, reflecting the interplay between site-specific environmental conditions and atmospheric circulations in the Mediterranean region. In the lower troposphere, mixing processes efficiently dilute particles in the major urban area of Rome, while mesoscale circulations act either as removal mechanisms (reducing the PM by up to 35% in Rome) or transport pathways (increasing the loads by up to 50% in Aosta). The MAL exhibits pronounced diurnal variability, reaching maximum (summer) heights of >2 km in Rome, while remaining below 1.4 km and 1 km in the Alpine and coastal sites, respectively. The vertical build-up of the AOD shows marked latitudinal and seasonal variability, with 80% (30%) of the total AOD residing in the first 500 m in Aosta-winter (Messina-summer). The seasonal frequency of the EALs reached 40% of the time (Messina-summer), mainly in the 1.5–4.0 km altitude range. An average (wet) PM > 40 μg m⁻³ is associated with the EALs over Rome and Messina. Notably, 10–40% of the EAL-affected days were also associated with increased PM within the MAL, suggesting the entrainment of the EALs in the mixing layer and thus their impact on the surface air quality. We also integrated ALC observations with relevant, state-of-the-art model reanalysis datasets (ERA5 and CAMS) to support our understanding of the aerosol patterns, related sources, and transport dynamics. This further allowed measurement vs. model intercomparisons and relevant examination of discrepancies. A good agreement (within 10–35%) was found between the ALICENET MAL and the ERA5 boundary layer height. The CAMS PM₁₀ values at the surface level well matched relevant in situ observations, while a statistically significant negative bias of 5–15 μg m⁻³ in the first 2–3 km altitude was found with respect to the ALC PM profiles across all the sites and seasons.

Aerosols play a key role in air quality, weather, and climate. Ground-based active remote sensing can contribute to the continuous monitoring of aerosol vertical profiles, especially when operating within regional, national and international networks. In fact, networked Automated-Lidar-Ceilometers (ALC) are now widely used to this purpose, monitoring the low and middle troposphere. However, conversion of their raw data into quantitative geophysical information is not straightforward.In this work, we present a model-supported approach to retrieve vertically-resolved aerosol optical and physical properties (aerosol backscatter and extinction coefficients, surface area, volume and mass concentrations) from elastic lidar systems. It extends previous results and processing capabilities of lidar and/or ALC data developed and employed within the Italian ALC network ALICENET (Dionisi et al., 2018; Bellini et al., 2024). In particular, we present here an upgraded version of the model, which relies on a Monte Carlo framework generating a large ensemble of light-scattering computations at multiple, lidar-relevant wavelengths (355, 532, 910, and 1064 nm) and targeted to reproduce a continental aerosol type mixed to low-to-moderate contributions of desert dust. With respect to previous model configurations (e.g., Dionisi et al., 2018), the new version simulate the coarse, dust particles as spheroids, taking advantage of the open-access spheroid package GRASP (Dubovik et al., 2006). This also allows computation of the aerosol depolarization ratio in addition to the other aerosol optical and physical properties. The model simulations are then used to derive mean functional relationships linking aerosol backscatter and particle depolarization ratio to the other aerosol properties. This upgraded version of the model was indeed developed within ALICENET to assist inversion of new commercially available ALC systems with polarization capability (PLC, as the Vaisala CL61). In this work, we will present: a) the numerical model simulations results, b) their evaluation through independent aerosol data from AERONET sun-photometers and 3) their practical use within the operative ALICENET inversion of PLC data to derive aerosol optical and physical properties. In fact, application of the new functional relationships shows improved agreement of PLC-retrievals with columnar aerosol optical depth and in situ mass measured at ground level in dust-loaded conditions. These results suggest that the proposed methodology could be applied to operational ALC/PLC networks operating in low-to-moderate dust-affected conditions, thus supporting radiative transfer, atmospheric chemistry, and air quality studies.References:Dionisi, et al., A multiwavelength numerical model in support of quantitative retrievals of aerosol properties from automated lidar ceilometers and test applications for AOT and PM10 estimation, Atmos. Meas. Tech., 11, 6013–6042, https://doi.org/10.5194/amt-11-6013-2018, 2018.Bellini, et al., ALICENET– an Italian network of automated lidar ceilometers for four-dimensional aerosol monitoring: infrastructure, data processing, and applications, Atmos. Meas. Tech., 17, 6119–6144, https://doi.org/10.5194/amt 17-6119-2024, 2024.Dubovik et al., Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust, J. Geophys. Res., 111, D11208, https://doi.org/10.1029/2005JD006619, 2006.

The vertically-resolved information on aerosol particles represents a key aspect in many atmospheric studies, ranging from aerosol-climate interactions to those investigating aerosol impacts on air quality and human health. This kind of information can be primarily derived by lidar active remote sensing and extended networks of these systems are currently run at the global scale. A network of Automated Lidar-Ceilometers (ALCs), Alicenet, was set up in Italy in 2015 by the Institute of Atmospheric Sciences and Climate (ISAC) of the National Research Council (CNR). Alicenet grew up in these years as a cooperative effort of Italian institutions dealing with atmospheric science and monitoring, and includes regional Environmental Protection Agencies, Universities and Research Centres. In the current configuration, the network runs both single-channel ALCs and dual channel, polarisation-sensitive systems (PLCs) operating in very different environments (urban, coastal, mountainous and volcanic areas) from Northern to Southern Italy, thus allowing the continuous monitoring of the aerosol vertical distribution across the country. Alicenet also contributes to the EUMETSAT program E-PROFILE, filling an Italian observational gap compared to other EU Member States. In this work, we present the Alicenet infrastructure and a detailed description of the specifically-developed data processing chain converting raw instrumental (Level 0) data into quantitative information on aerosol properties, with output products ranging from attenuated backscatter to aerosol mass and vertical stratification from the surface up to the upper troposphere (output data Levels 1–3). Overall, this setup provides from near real-time to long-term overviews of the 4D aerosol field over Italy. Examples of both are reported in this work. Specific comparisons of the Alicenet products to relevant independent measurements of different parameters (e.g. surface PM10, sunphotometer AOD) are also included, revealing the good performances of the Alicenet algorithms. Overall, Alicenet represents a valuable resource to extend the current aerosol observational capabilities in Italy and in the Central European Mediterranean area, and contributes to bridge a gap between atmospheric science and its application to specific sectors, among which: a) air quality, b) solar energy, c) aviation safety. More in general, the maturity of the ALC/PLC instrumentation and of the data processing tools available within the wider international scientific/technical atmospheric community suggest lidar-ceilometer networks could usefully integrate current EU Atmospheric Research Infrastructures for aerosol studies.