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This article introduces a MATLAB-based radiometric model as a crucial initial step in evaluating the feasibility Differential Absorption Lidar (DIAL) systems for remote sensing of minor gaseous atmospheric constituents such as methane, a potent greenhouse gas. This model offers flexibility in parameter customization, considering factors like target gas, wavelength range, and atmospheric conditions, enabling to optimize system parameters, assess performance, and gain insights into improving measurement accuracy. We validate the model and find it capable of retrieving lidar signals for methane sensing concentrations of $2.085$ ppm over 800-meter distances using an OPO lidar system at around $3 \ \mathrm{\mu m}$. We explore two DIAL approaches, range-resolved and IPDA, each suited for specific scenarios. Furthermore, we highlight that this simulation framework extends beyond methane, from $\mathrm{CO_2}$ to alcohol vapors. This article serves as a valuable resource for researchers and practitioners in atmospheric science and remote sensing, offering insights into DIAL system feasibility and optimization.

The versatile Generalized Retrieval of Aerosol and Surface Properties (GRASP) algorithm exploits the advantages of synergic ground-based aerosol observations such as radiometric (sensitive to columnar aerosol optical and microphysical properties) and lidar (sensitive to vertical distribution of the optical properties) observations. The synergy is possible when the complementary data is mutually constrained by GRASP parametrization that includes, for the first time ever, the degree of linear polarization (DoLP) parameter measured by a polarized sun-sky-lunar AERONET photometer (380, 440, 500, 675, 870, 1020, and 1640 nm) in synergy with the vertical profiles from an elastic lidar (355, 532, and 1064 nm). First, a series of numerical tests is performed using simulated data generated using a climatology of data and ground-based measurements. The inversions are performed with and without random noise for five different combinations of input data, starting from the AERONET-like dataset and increasing to the complex one by adding more information for three aerosol scenarios: I—high aerosol optical depth (AOD) with dominant coarse mode; II—low AOD with dominant coarse mode; III—high AOD with dominant fine mode. The inclusion of DoLP improves (i) the retrieval accuracy of the fine-mode properties when it is not dominant; (ii) the retrieval accuracy of the coarse-mode properties at longer wavelengths and that of the fine-mode properties at shorter wavelengths; (iii) the retrieval accuracy of the coarse-mode real part of the refractive index (up to 36% reduction), but has no effect on the retrieval of the imaginary part; (iv) reduces up to 83% the bias of the sphere fraction (SF) retrieval in coarse-mode dominated regimes; and (v) the root mean square error (RMSE) of the retrieval for most of the parameters in all scenarios. In addition, the addition of more photometer channels in synergy with a three-wavelength elastic lidar reduces the RMSE for the real part (67% in the coa, Funding: This research was funded by: the Spanish Ministry of Science and Innovation (grant no. PID2019-103886RB-I00); H2020 (grant nos. 654109, 778349, 871115 and 101008004); and Horizon Europe REALISTIC project (grant no. 101086690)., Peer Reviewed, Postprint (published version)

Three volcanic plumes were detected during the Tajogaite volcano eruptive activity (Canary Islands, Spain, September–December 2021) over the Iberian Peninsula. The spatiotemporal evolution of these events is characterised by combining passive satellite remote sensing and ground-based lidar and sun-photometer systems. The inversion algorithm GRASP is used with a suite of ground-based remote sensing instruments such as lidar/ceilometer and sun-photometer from eight sites at different locations throughout the Iberian Peninsula. Satellite observations showed that the volcanic ash plumes remained nearby the Canary Islands covering a mean area of 120 ± 202 km2 during the whole period of eruptive activity and that sulphur dioxide plumes reached the Iberian Peninsula. Remote sensing observations showed that the three events were mainly composed of sulphates, which were transported from the volcano into the free troposphere. The high backscatter-related Ångström exponents for wavelengths 532–1064 nm (1.17 ± 0.20 to 1.40 ± 0.24) and low particle depolarization ratios (0.08 ± 0.02 to 0.09 ± 0.02), measured by the multi-wavelength Raman lidar, hinted at the presence of spherical small particles. The layer aerosol optical depth at 532 nm (AODL532) obtained from lidar measurements contributed between 49% and 82% to the AERONET total column AOD at 532 nm in event II (11–13 October). According to the GRASP retrievals, the layer aerosol optical depth at 440 nm (AODL440) was higher in all sites during event II with values between 0.097 (Badajoz) and 0.233 (Guadiana-UGR) and lower in event III (19–21 October) varying between 0.003 (Granada) and 0.026 (Évora). Compared with the GRASP retrievals of total column AOD at 440 nm, the AODL440 had contributions between 21% and 52% during event II. In the event I (25–28 September), the mean volume concentrations (VC) varied between 5 ± 4 µm3cm-3 (El-Arenosillo/Huelva) and 17 ± 10 µm3cm-3 (Guadiana-UGR), while in event II this variation was fro, Évora team funded by national funds through FCT - Fundação para a Ciência e Tecnologia, I.P., in the framework of the ICT project with the references UIDB/04683/2020 and UIDP/04683/2020 and by TOMAQAPA (PTDC/CTAMET/29678/2017). The authors acknowledge the GRASP-ACE (Grant agreement ID: 778349), ACTRIS-IMP (Grant agreement ID:871115), ATMO-ACCESS (Grant Agreement ID: 101008004), PROBE (COST Action number: CA18235), HARMONIA (COST Action number: CA21119), EUMETNET through the E-PROFILE program and REALISTIC (Grant agreement ID:101086690) projects. The authors acknowledge the support through ACTRIS-2 under grant agreement no.654109. This work was partially supported by the Spanish national projects PID2019-103886RB-I00/AEI/10.13039/501100011033, INTEGRATYON3 (PID2020-117825GB-C21 and PID2020-117825GB-C22), ELPIS (PID2020-120015RB-I00), CLARIN (CGL2016-81092-R), ePOLAAR (RTI2018-097864-B-I00), CAMELIA (PID2019-104205GB-C21/AEI/10.13039/501100011033) and ACTRIS-España (CGL2017-90884REDT), by University of Granada Plan Propio through Singular Laboratory (LS2022-1) program, by the Andalusia Autonomous Government projects AEROPRE (P18-RT-3820) and ADAPNE (P20_00136), by the UGR-FEDER projects DEM3TRIOS (A-RNM-430-UGR20) and MOGATRACO (A-RNM-524-UGR20) and partially by the Scientific Units of Excellence Program (grant no. UCE-PP2017-02), and by the R+D+i grant RTI 2018-097332-B-C22 funded by MCIN/AEI/ 10.13039/501100011033/ and “ERDF A Way of Doing Europe”. M.-Á. López-Cayuela and C.V. Carvajal-Pérez are supported by the INTA predoctoral contract program., Peer Reviewed, Postprint (published version)

An intense and long-lasting Saharan dust outbreak crossed the Iberian Peninsula (IP) from the southwest (SW) to the northeast (NE) from 25 March until 7 April 2021. This work aims to assess the optical and mass contribution of both fine and coarse dust particles along their transport. Five Iberian lidar stations were monitoring the transport and evolution of the Saharan dust particles, i.e. El Arenosillo/Huelva, Granada, Torrejón/Madrid and Barcelona in Spain, and Évora in Portugal. The particular meteorological conditions determined the aerosol scenario along the overall dust event, differing in the first part of the event (25–31 March), in which the strongest dust incidence occurred on 29–31 March at the south and central stations and 1 April at Barcelona, from the second one (1–7 April). The use of the two-step POLIPHON algorithm showed the relevance of using polarized lidar measurements for separating the aerosol properties of dust fine and coarse particles as an added value. Both the fine dust (Df) and coarse dust (Dc) components of the total particle backscatter coefficient (total dust, DD¿=¿Dc¿+¿Df) were separately derived. The dust plume was well-mixed with height and no significant differences were found in the vertical structure of both the Dc and Df particle backscatter coefficients. From the beginning of the dust outbreak until 1 April, the vertical Df¿¿DD mass ratio was nearly constant in time at each station and also in altitude with values of ~¿10¿%. Moreover, the mean dust optical depth at 532¿nm was decreasing along that dust pathway, reporting values from SW to NE stations of 0.34 at El Arenosillo/Huelva, 0.28 at Granada, 0.20 at Évora, 0.28 at Torrejón/Madrid, and 0.14 at Barcelona, although its Df¿¿DD ratio remained almost constant (28¿%–30¿%). A similar pattern was found for the total dust mass loading and its Df¿¿DD ratio, i.e. mostly decreasing mean mass values were reported, being constant in its Df¿¿DD ratio (~¿10¿%) along the SW–NE dust pat, This research has been supported by the Spanish Ministry of Science and Innovation (grant no. PID2019-104205GB-C21/AEI/10.13039/501100011033)., Peer Reviewed, Postprint (published version)

Aerosols are significant atmospheric constituents that modulate radiation and cloud processes. We evaluated 17-year aerosol profile trends in Barcelona, Spain, from lidar measurements. In summer aerosol reaches 5 km, while in the other seasons it exhibits clear exponential decay. Sahara dust transport affects all seasons, with winter layers above and others penetrating the boundary layer. This study informs the formation of haze and urban preservation strategies in the Mediterranean. The analysis puts in evidence that the averaged net radiative effect is of cooling at both surface level and top of the atmosphere., This work has been made possible through the efforts of many people, whom it would be too long to mention, and the funding of many grants through the years, in particular European projects of different framework programmes (FP5 EARLINET project (ID EVR1-CT-1999-40003), FP6 EARLINET-ASOS (ID: 25991), FP7 ACTRIS (ID: 262254), H2020 ACTRIS-2 (ID: 654109), ACTRIS-PPP (ID: 739530), ACTRIS IMP (ID: 871115) and ATMO-ACCESS (ID: 101008004)), projects of the Spanish National Research Programmes (refs. TIC 431/93, AMB96-1144-C02-01, REN2000-1907-CE, REN2000-1754-C02-02 / CLI, REN2003-09753-C02-C02 / CLI, REN2003-09753-C02-C CGL2008- 01330-E/CLI 02 / CLI, REN2002-12784-E, CGL2005-5131-E, CGL2006-27108-E/CLI, CGL2006-26149-E/CLI, CGL2007-28871-/CLI, CTM2006-27154-E/TECNO, TEC2006-07850/TCM, TEC2009-09106, TEC2012-34575, TEC2015-63832-P and PID2019-103886RB-I00), the project of the Catalan Regional Government IMMPACTE, and the ESA project No. nº 21487/08/NL/HE. The support of the European Union through NextGenerationEU funds is also gratefully acknowledged., Peer Reviewed, Postprint (author's final draft)

Aerosols are one of the most important pollutants in the atmosphere and have been monitored for the past few decades by remote sensing and in situ observation platforms to assess the effectiveness of government-managed reduction emission policies and assess their impact on the radiative budget of the Earth's atmosphere. In fact, aerosols can directly modulate incoming short-wave solar radiation and outgoing long-wave radiation and indirectly influence cloud formation, lifetime, and precipitation. In this study, we quantitatively evaluated long-term temporal trends and seasonal variability from a climatological point of view of the optical and microphysical properties of atmospheric particulate matter at the Universitat Politècnica de Catalunya (UPC), Barcelona, Spain, over the past 17 years, through a synergy of lidar, sun photometer, and in situ concentration measurements. Interannual temporal changes in aerosol optical and microphysical properties are evaluated through the seasonal Mann–Kendall test. Long-term trends in the optical depth of the recovered aerosol; the Ångström exponent (AE); and the concentrations of PM10, PM2.5, and PM1 reveal that emission reduction policies implemented in the past decades were effective in improving air quality, with consistent drops in PM concentrations and optical depth of aerosols. The seasonal analysis of the 17-year average vertically resolved aerosol profiles obtained from lidar observations shows that during summer the aerosol layer can be found up to an altitude of 5 km, after a sharp decay in the first kilometer. In contrast, during the other seasons, the backscatter profiles fit a pronounced exponential decay well with a well-defined scale height. Long-range transport, especially dust outbreaks from the Sahara, is likely to occur throughout the year. During winter, the dust aerosol layers are floating above the boundary layer, while during the other seasons they can penetrate the layer. The analysis also revealed that, This research has been supported by the European Union through NextgenerationEU funds and by the following projects along the years: FP5 EARLINET project (grant no. ID EVR1-CT-1999-40003), FP6 EARLINET-ASOS (ID: 25991), FP7 ACTRIS (ID: 262254), H2020 ACTRIS-2 (ID: 654109), ACTRIS-PPP (ID: 739530), ACTRIS IMP (ID: 871115) and ATMO-ACCESS (ID: 101008004), projects of the Spanish National Research programs (grant nos. TIC 431/93, AMB96-1144-C02-01, REN2000-1907-CE, REN2000-1754- C02-02/CLI, REN2003-09753-C02-C02/CLI, REN2003-09753- C02-C CGL2008-01330-E/CLI 02/CLI, REN2002-12784-E, CGL2005-5131-E, CGL2006-27108-E/CLI, CGL2006-26149- E/CLI, CGL2007-28871-/CLI, CTM2006-27154-E/TECNO, TEC2006-07850/TCM, TEC2009-09106, TEC2012-34575, TEC2015-63832-P and PID2019-103886RB-I00), the project of the Catalan Regional Government IMMPACTE, and the ESA project (grant no. 21487/08/NL/HE), Peer Reviewed, Postprint (published version)

Aerosols are one of the most important pollutants in the atmosphere and have been monitored for the past few decades by remote sensing and in situ observation platforms to assess the effectiveness of government-managed reduction emission policies and assess their impact on the radiative budget of the Earth's atmosphere. In fact, aerosols can directly modulate incoming short-wave solar radiation and outgoing long-wave radiation and indirectly influence cloud formation, lifetime, and precipitation. In this study, we quantitatively evaluated long-term temporal trends and seasonal variability from a climatological point of view of the optical and microphysical properties of atmospheric particulate matter at the Universitat Politècnica de Catalunya (UPC), Barcelona, Spain, over the past 17 years, through a synergy of lidar, sun photometer, and in situ concentration measurements. Interannual temporal changes in aerosol optical and microphysical properties are evaluated through the seasonal Mann-Kendall test. Long-term trends in the optical depth of the recovered aerosol; the Ångström exponent (AE); and the concentrations of PM10, PM2.5, and PM1 reveal that emission reduction policies implemented in the past decades were effective in improving air quality, with consistent drops in PM concentrations and optical depth of aerosols. The seasonal analysis of the 17-year average vertically resolved aerosol profiles obtained from lidar observations shows that during summer the aerosol layer can be found up to an altitude of 5 km, after a sharp decay in the first kilometer. In contrast, during the other seasons, the backscatter profiles fit a pronounced exponential decay well with a well-defined scale height. Long-range transport, especially dust outbreaks from the Sahara, is likely to occur throughout the year. During winter, the dust aerosol layers are floating above the boundary layer, while during the other seasons they can penetrate the layer. The analysis also revealed that

We derive an explicit (i.e., non-iterative) formula for the retrieval of the overlap function in an aerosol lidar with both elastic and Raman N2 and/or O2 channels used for independent measurements of aerosol backscatter and extinction coefficients. The formula requires only the measured, range-corrected elastic and the corresponding Raman signals, plus an assumed lidar ratio. We assess the influence of the lidar ratio error in the overlap function retrieval and present retrieval examples., This research has been supported by the Agencia Estatal de Investigación (grant no. PID2019-103886RB-I00) as well as the H2020 Environment (grant nos. 871115 and 101008004) and the H2020 Excellent Science (grant no. 778349) programs., Peer Reviewed, Postprint (published version)

Context. The Carina Nebula is one of the major massive star-forming regions in the Galaxy. Its relatively nearby distance (2.35 kpc) makes it an ideal laboratory for the study of massive star formation, structure, and evolution, both for individual stars and stellar systems. Thanks to the high-quality spectra provided by the Gaia-ESO survey and the LiLiMaRlin library, as well as Gaia EDR3 astrometry, a detailed and homogeneous spectroscopic characterization of its massive stellar content can be carried out. Aims. Our main objective is to spectroscopically characterize all massive members of the Carina Nebula in the Gaia-ESO survey footprint to provide an updated census of massive stars in the region and an updated estimate of the binary fraction of O stars. Methods. We performed accurate spectral classification using an interactive code that compares spectra with spectral libraries of OB standard stars, as well as line-based classic methods. We calculated membership using our own algorithm based on Gaia EDR3 astrometry. To check the correlation between the spectroscopic n-qualifier and the rotational velocity, we used a semi-automated tool for the line-broadening characterization of OB stars based on a combined Fourier transform and goodness-of-fit methodology. Results. The Gaia-ESO survey sample of massive OB stars in the Carina Nebula consists of 234 stars. The addition of brighter sources from the Galactic O-Star Spectroscopic Survey and additional sources from the literature allows us to create the most complete census of massive OB stars so far in the region. It contains a total of 316 stars, with 18 of them in the background and 4 in the foreground. Of the 294 stellar systems in Car OB1, 74 are of O type, 214 are of nonsupergiant B type, and 6 are of WR or nonO supergiant (II to Ia) spectral class. We identify 20 spectroscopic binary systems with an O-star primary, of which 6 are reported for the first time, and another 18 with a B-star primary, of which 13 ar

For the first time in fifty years, the Cumbre Vieja volcanic area (La Palma, Canary Islands, Spain) erupted on September 19, 2021, giving birth to a new volcano. Fresh volcanic aerosols were continuously injected into the troposphere at different height levels, decreasing with time until the end of December 2021 (15 weeks duration). A wide set of different instrumentation was deployed all over the Island in order to evaluate the effects of the volcanic plumes on the atmosphere and the air quality. For the first time, a long-term study of the relative mass contribution and vertical impact of the volcanic components, ash and non-ash particles separately, during the eruptive activity was carried out in this work. In particular, a polarized Micro-Pulse Lidar (P-MPL) was deployed at Tazacorte (at around 8 km west from the volcano) in 24/7 operation from October 17, 2021 until the end of the volcano activity (11 weeks) for vertical monitoring of the volcanic particles. First, a statistical study of the mass conversion factors for mass concentration estimation of the volcanic (ash and non-ash) particles was performed by using the AERONET sun/sky-photometer dataset at Fuencaliente (at around 18 km south from the volcano). A representative mass conversion factor was obtained for ash and non-ash particles: 1.89 ± 0.53 and 0.31 ± 0.06 g m-2, respectively, with no dependence on time and optical depth. Second, these factors were used to calculate the ash and non-ash mass concentrations from P-MPL observations. Ash particles dominated 11% of the time and mostly until week 3 (i.e. week 7 from the volcanic eruption). Their mass concentration decreased by one order of magnitude: the relative ash mass contribution was 73 ± 18% with a total mass loading of 566 ± 281 mg m-2 at week 1, reducing gradually down to 38 ± 32% and 120 ± 49 mg m-2, respectively, at week 11. Layer-to-layer, it decreased with increasing layer-height; no ash was detected above 4 km at the end of the volcanic peri, This research was funded by the Spanish Ministry of Science and Innovation (PID2020- 118793GA-I00, PID2019-104205GB-C21/AEI/10.13039/501100011 033, PID2019-103886RB-I00, and EQC2018-004686-P) and the Unit of Excellence “María de Maeztu” (MDM-2017-0737) and ACTRIS-Spain (CGL2017-90884-REDT) by the Spanish State Research Agency (AEI). The authors wish to thank the support from the H2020 program of the European Union and ACTRIS (GA n. 19ENV04, 654109, 778349, 871115, 101008004, and 101086690), Peer Reviewed, Postprint (published version)

An intense and long-lasting Saharan dust outbreak crossed the Iberian Peninsula (IP) from the southwest (SW) to the northeast (NE) from 25 March until 7 April 2021. This work aims to assess the optical and mass contribution of both fine and coarse dust particles along their transport. Five Iberian lidar stations were monitoring the transport and evolution of the Saharan dust particles, i.e. El Arenosillo/Huelva, Granada, Torrejón/Madrid and Barcelona in Spain, and Évora in Portugal. The particular meteorological conditions determined the aerosol scenario along the overall dust event, differing in the first part of the event (25–31 March), in which the strongest dust incidence occurred on 29–31 March at the south and central stations and 1 April at Barcelona, from the second one (1–7 April). The use of the two-step POLIPHON algorithm showed the relevance of using polarized lidar measurements for separating the aerosol properties of dust fine and coarse particles as an added value. Both the fine dust (Df) and coarse dust (Dc) components of the total particle backscatter coefficient (total dust, DD = Dc + Df) were separately derived. The dust plume was well-mixed with height and no significant differences were found in the vertical structure of both the Dc and Df particle backscatter coefficients. From the beginning of the dust outbreak until 1 April, the vertical Df /DD mass ratio was nearly constant in time at each station and also in altitude with values of ∼ 10 %. Moreover, the mean dust optical depth at 532 nm was decreasing along that dust pathway, reporting values from SW to NE stations of 0.34 at El Arenosillo/Huelva, 0.28 at Granada, 0.20 at Évora, 0.28 at Torrejón/Madrid, and 0.14 at Barcelona, although its Df / DD ratio remained almost constant (28 %–30 %). A similar pattern was found for the total dust mass loading and its Df/ DD ratio, i.e. mostly decreasing mean mass values were reported, being constant in its Df /DD ratio (∼ 10 %) along the SW–NE dust pa

For the first time in fifty years, the Cumbre Vieja volcanic area (La Palma, Canary Islands, Spain) erupted on September 19, 2021, giving birth to a new volcano. Fresh volcanic aerosols were continuously injected into the troposphere at different height levels, decreasing with time until the end of December 2021 (15 weeks duration). A wide set of different instrumentation was deployed all over the Island in order to evaluate the effects of the volcanic plumes on the atmosphere and the air quality. For the first time, a long-term study of the relative mass contribution and vertical impact of the volcanic components, ash and non-ash particles separately, during the eruptive activity was carried out in this work. In particular, a polarized Micro-Pulse Lidar (P-MPL) was deployed at Tazacorte (at around 8 km west from the volcano) in 24/7 operation from October 17, 2021 until the end of the volcano activity (11 weeks) for vertical monitoring of the volcanic particles. First, a statistical study of the mass conversion factors for mass concentration estimation of the volcanic (ash and non-ash) particles was performed by using the AERONET sun/sky-photometer dataset at Fuencaliente (at around 18 km south from the volcano). A representative mass conversion factor was obtained for ash and non-ash particles: 1.89 ± 0.53 and 0.31 ± 0.06 g m−2, respectively, with no dependence on time and optical depth. Second, these factors were used to calculate the ash and non-ash mass concentrations from P-MPL observations. Ash particles dominated 11% of the time and mostly until week 3 (i.e. week 7 from the volcanic eruption). Their mass concentration decreased by one order of magnitude: the relative ash mass contribution was 73 ± 18% with a total mass loading of 566 ± 281 mg m−2 at week 1, reducing gradually down to 38 ± 32% and 120 ± 49 mg m−2, respectively, at week 11. Layer-to-layer, it decreased with increasing layer-height; no ash was detected above 4 km at the end of the volcanic peri

Context. The Carina Nebula is one of the major massive star-forming regions in the Galaxy. Its relatively nearby distance (2.35 kpc) makes it an ideal laboratory for the study of massive star formation, structure, and evolution, both for individual stars and stellar systems. Thanks to the high-quality spectra provided by the Gaia-ESO survey and the LiLiMaRlin library, as well as Gaia EDR3 astrometry, a detailed and homogeneous spectroscopic characterization of its massive stellar content can be carried out. Aims. Our main objective is to spectroscopically characterize all massive members of the Carina Nebula in the Gaia-ESO survey footprint to provide an updated census of massive stars in the region and an updated estimate of the binary fraction of O stars. Methods. We performed accurate spectral classification using an interactive code that compares spectra with spectral libraries of OB standard stars, as well as line-based classic methods. We calculated membership using our own algorithm based on Gaia EDR3 astrometry. To check the correlation between the spectroscopic n-qualifier and the rotational velocity, we used a semi-automated tool for the line-broadening characterization of OB stars based on a combined Fourier transform and goodness-of-fit methodology. Results. The Gaia-ESO survey sample of massive OB stars in the Carina Nebula consists of 234 stars. The addition of brighter sources from the Galactic O-Star Spectroscopic Survey and additional sources from the literature allows us to create the most complete census of massive OB stars so far in the region. It contains a total of 316 stars, with 18 of them in the background and 4 in the foreground. Of the 294 stellar systems in Car OB1, 74 are of O type, 214 are of nonsupergiant B type, and 6 are of WR or nonO supergiant (II to Ia) spectral class. We identify 20 spectroscopic binary systems with an O-star primary, of which 6 are reported for the first time, and another 18 with a B-star primary, of which 13 ar

The European Aerosol Research Lidar Network (EARLINET), part of the Aerosols, Clouds and Trace gases Research Infrastructure (ACTRIS), organized an intensive observational campaign in May 2020, with the objective of monitoring the atmospheric state over Europe during the COVID-19 lockdown and relaxation period. Besides the standard operational processing of the lidar data in EARLINET, for seven EARLINET sites having collocated sun-photometric observations in the Aerosol Robotic Network (AERONET), a network exercise was held in order to derive profiles of the concentration and effective column size distributions of the aerosols in the atmosphere, by applying the GRASP/GARRLiC (from Generalized Aerosol Retrieval from Radiometer and Lidar Combined data – GARRLiC – part of the Generalized Retrieval of Atmosphere and Surface Properties – GRASP) inversion algorithm. The objective of this network exercise was to explore the possibility of identifying the anthropogenic component and of monitoring its spatial and temporal characteristics in the COVID-19 lockdown and relaxation period. While the number of cases is far from being statistically significant so as to provide a conclusive description of the atmospheric aerosols over Europe during this period, this network exercise was fundamental to deriving a common methodology for applying GRASP/GARRLiC to a network of instruments with different characteristics. The limits of the approach are discussed, in particular the missing information close to the ground in the lidar measurements due to the instrument geometry and the sensitivity of the GRASP/GARRLiC retrieval to the settings used, especially for cases with low aerosol optical depth (AOD) like the ones we show here. We found that this sensitivity is well-characterized in the GRASP/GARRLiC products, since it is included in their retrieval uncertainties., This research was financially supported by D- TECT (grant agreement no. 725698) funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme and by PANGEA4CalVal (grant agreement no. 101079201) funded by the European Union . The Barcelona site has been funded by the RE- ALISTIC project (grant agreement no. 101086690) under the Eu- ropean Union’s Horizon Widera 2022 Talents programme. The Warsaw site has been supported by the European Commission H2020 (ACTRIS IMP (grant no. 871115)). The Belsk site has been supported by the National Science Centre, Poland (grant no. 2021/41/B/ST10/03660)., Peer Reviewed, Postprint (published version)