Backscatter Coefficient Profiles Research Articles

Pure rotational Raman lidar for accurate profiling of atmospheric temperature and aerosol/cloud backscatter coefficients

We have created a pure rotational Raman (PRR) lidar for round-the-clock and large-height-range quantitatively-physical measurements of atmospheric temperature profiles. A seeded frequency-doubled Nd:YAG laser is utilized as the light source. It has a typical power of ∼18 W and a pulse repetition rate of 30 Hz. The lidar uses a 0.3-m-diameter receiver and a 1.0-m-diameter receiver together. Each receiver contains a state-of-the-art three-channel polychromator that extracts respectively two individual Stokes N2 PRR lines with rotational quantum numbers J = 4 and 14, as well as a Rayleigh/Mie backscatter signal. The temperature profiles are derived from the strict analytical expression of temperature in terms of the ratio of the two PRR line signals without calibration. Furthermore, a strict analytical expression has been established to derive the aerosol/cloud backscatter coefficient profiles without assumptions on the lidar ratio. The lidar provides a rigorous and effective methodology for accurate profile measurements of the atmospheric temperature and optical properties of aerosol/cloud. Observational examples and statistics obtained at a subtropical site have demonstrated the high measurement accuracies, large measurement altitude range, good range/time resolution, and unprecedented daytime measurement capability of this single-line-extracted PRR lidar.

A long-term Doppler wind LiDAR study of heavy pollution episodes in western Yangtze River Delta region, China

Previous studies are primarily based on surface air pollution for a single event, leaving research on the temporal characteristics of aerosols and wind fields in the atmospheric boundary layer (ABL) and their associations unclear due to a lack of long-term vertical profile data. This study was the first to conduct long-term Doppler wind LiDAR measurements from Sep. 2019 to Aug. 2022 in Hefei in western Yangtze River Delta (YRD) region, China. The spatiotemporal characteristics of retrieved aerosol backscatter coefficient (β) and wind profiles were analyzed from the perspective of long-term statistics and typical heavy pollution episodes (HPEs). The seasonal profile of β showed a peak in the near-surface layer meanwhile the overall β profile (<0.5 km) was the highest in winter but lowest in summer. Combined with ground-based meteorological and air quality observations, 12 HPEs were identified and classified into dust-related events and fog-haze episodes. The results showed a consistency between hourly variations in PM10 (particulate matter smaller than 10 μm) concentration and β retrieved at 300 m during a high PM10 concentration (>150 μg/m3) period. Different roles of horizontal wind at different altitudes and its associated time delay effect on surface PM10 pollution were evaluated based on correlation coefficients (Ф) and air pollution diffusion conditions. Significant positive Ф values were noticed below 0.5 km during the entire dust-related EP4 and EP6, indicating that the higher wind speeds could exacerbate PM10 pollution. In contrast, large negative Ф values meant the removal of PM10 pollutants by strong winds below 0.8 km during 24 h after peak in EP3. Combining with backward trajectory analysis and meteorological condition, notable transboundary pollution with positive contributions from upper winds (>1.5 km) was discovered in dust-related EP1, EP4, EP7, EP9, and EP10. Time delay effect (1-h/2-h lag) of upper winds (>0.8 km) on surface PM10 pollution was explored in EP5, EP6, EP7, EP8, and EP9. In spring dust-related HPEs, surface PM10 pollution was mostly contributed by long-range transport of aerosol particles at higher altitudes (>1.5 km) from the northwest direction, driven by the Mongolian cyclone and the cold front system. Transboundary aerosols originated from the northern part of Anhui province and the YRD region in the middle altitudes (∼0.5 km) was the main contributor to fog-haze HPEs. The findings demonstrated the ability of the real-time Doppler wind LiDAR system to monitor transboundary air pollution and provided a scientific reference for policy makers.

A near-global multiyear climate data record of the fine-mode and coarse-mode components of atmospheric pure dust

Abstract. A new four-dimensional, multiyear, and near-global climate data record of the fine-mode (submicrometer in terms of diameter) and coarse-mode (supermicrometer in terms of diameter) components of atmospheric pure dust is presented. The separation of the two modes of dust in detected atmospheric dust layers is based on a combination of (1) the total pure-dust product provided by the well-established European Space Agency (ESA) “LIdar climatology of Vertical Aerosol Structure” (LIVAS) database and (2) the coarse-mode component of pure dust provided by the first step of the two-step POlarization LIdar PHOtometer Networking (POLIPHON) technique, developed in the framework of the European Aerosol Research Lidar Network (EARLINET). Accordingly, the fine-mode component of pure dust is extracted as the residual between the LIVAS total pure dust and the coarse-mode component of pure dust. Intermediate steps involve the implementation of regionally dependent lidar-derived lidar ratio values and AErosol RObotic NETwork (AERONET)-based climatological extinction-to-volume conversion factors, facilitating conversion of dust backscatter into extinction and subsequently extinction into mass concentration. The decoupling scheme is applied to observations from the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) at 532 nm. The final products consist of the fine mode and coarse mode of atmospheric pure dust, quality-assured profiles of backscatter coefficient at 532 nm, extinction coefficient at 532 nm, and mass concentration for each of the two components. The datasets are established primarily with the original L2 horizontal (5 km) and vertical (60 m) resolution of the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) along the CALIPSO orbit path and secondly in averaged profiles of seasonal–temporal resolution, 1° × 1° spatial resolution, and the original vertical resolution of CALIPSO, focusing on the latitudinal band extending between 70° S and 70° N and covering more than 15 years of Earth observations (June 2006–December 2021). The quality of the CALIPSO-based fine-mode and coarse-mode dust products is assessed through the use of AERONET fine-mode and coarse-mode aerosol optical thickness (AOT) interpolated to 532 nm and the AERosol properties – Dust (AER-D) campaign airborne in situ particle size distributions (PSDs) as reference datasets during atmospheric conditions characterized by dust presence. The near-global fine-mode and coarse-mode pure-dust climate data record is considered unique with respect to a wide range of potential applications, including climatological, time series, and trend analysis over extensive geographical domains and temporal periods, validation of atmospheric dust models and reanalysis datasets, assimilation activities, and investigation of the role of airborne dust in radiation and air quality.

Tropospheric sulfate from Cumbre Vieja (La Palma) observed over Cabo Verde contrasted with background conditions: a lidar case study of aerosol extinction, backscatter, depolarization and lidar ratio profiles at 355, 532 and 1064 nm

Abstract. In September 2021, volcanic aerosol (mainly freshly formed sulfate plumes) originating from the eruption of Cumbre Vieja on La Palma, Canary Islands, Spain, crossed Cabo Verde at altitudes below 2 km. On 24 September 2021, an extraordinary large aerosol optical depth (AOD) close to 1 (daily mean at 500 nm) was observed at Mindelo, Cabo Verde. This event provided favorable conditions to obtain lidar-derived profiles of extinction and backscatter coefficients, lidar ratio, and depolarization ratio at 355, 532 and 1064 nm in the sulfate aerosol plume. A novel feature of the lidar system operated at Mindelo is the availability of extinction, lidar ratio and depolarization measurements at 1064 nm in addition to the standard wavelengths of 355 and 532 nm. Having measurements of these parameters at all three wavelengths is a major advantage for the aerosol characterization and in aerosol typing efforts as the lidar ratio and the particle linear depolarization ratio are key parameters for this purpose. In this article, we present the key results of the lidar observations obtained on one specific day, namely on 24 September 2021 at 04:38–05:57 UTC, including the first ever measurements of the particle extinction coefficient, the lidar ratio and the depolarization ratio at 1064 nm for volcanic sulfate, and discuss the findings in terms of aerosol optical properties and mass concentrations by comparison with a reference observation (16 September 2021) representing the typical background conditions before the start of the eruptions. We found an unusual high particle extinction coefficient of 721 ± 51, 549 ± 38 and 178 ± 13 Mm−1, as well as an enhanced lidar ratio of 66.9 ± 10.1, 60.2 ± 9.2 and 30.8 ± 8.7 sr at 355, 532 and 1064 nm, respectively, in the sulfate-dominated planetary boundary layer (PBL). The particle linear depolarization ratio was ≤ 0.9 % at all respective wavelengths. It is the first time that lidar-derived intensive aerosol optical properties could be derived for volcanic sulfate at all three wavelengths, and thus it is a highly valuable data set for global aerosol characterization. The lidar analysis also revealed a sulfate-related AOD of about 0.35 ± 0.03 at 532 nm of the total PBL-related AOD of 0.43. The rest of the AOD contribution was caused by a lofted Saharan dust layer extending from 1.4 to 5 km and leading to a total AOD of 0.79 at 532 nm. Volcanic ash contribution to the observed aerosol plumes could be mostly excluded based on trajectory analysis and the observed optical properties. Peak mass concentration was 178.5 ± 44.6 µg m−3 in the volcanic-influenced and sulfate-dominated polluted PBL, showing the hazardous potential of such sulfate plumes to significantly worsen local air quality even at remote locations.

Saharan Dust Loads over Sofia, Bulgaria: Lidar Observations on Winter and Spring Dust Events

Aerosols have significant impacts on the climate, ecosystems, and human health. Sahara Desert is the largest source of natural mineral aerosols (dust) on Earth. Enormous amounts of Saharan particulate matter are frequently transported to Europe through advection of warm air masses, being typical for spring and summer. Over the last years, winter Saharan dust load events have also been reported for various locations over Europe, including Sofia. In this work, we give a description of lidar observations of atmospheric aerosols present above Sofia, Bulgaria, during episodes of Saharan dust intrusions in periods of minimum (in winter) and maximum (in spring) activity of dust transport to the region. Measurements were conducted in February and April 2021, with the two aerosol channels (at 532 nm and 1064 nm) of the Nd:YAG-laser based lidar. Aerosol/dust layering and optical and microphysical properties are characterized. Colourmaps of the vertical aerosol distribution and its temporal dynamics were presented. Time-averaged vertical profiles of the aerosol backscatter coefficients and corresponding backscatter-related Ångström exponents were retrieved and analyzed. The Saharan origin of the aerosols detected was identified using online available air/dust tracking and mapping web resources. Results obtained show that the considered here dust load events have affected significantly both the structure and aerosol composition of the local troposphere.

Use of lidar aerosol extinction and backscatter coefficients to estimate cloud condensation nuclei (CCN) concentrations in the southeast Atlantic

Abstract. Accurately capturing cloud condensation nuclei (CCN) concentrations is key to understanding the aerosol–cloud interactions that continue to feature the highest uncertainty amongst numerous climate forcings. In situ CCN observations are sparse, and most non-polarimetric passive remote sensing techniques are limited to providing column-effective CCN proxies such as total aerosol optical depth (AOD). Lidar measurements, on the other hand, resolve profiles of aerosol extinction and/or backscatter coefficients that are better suited for constraining vertically resolved aerosol optical and microphysical properties. Here we present relationships between aerosol backscatter and extinction coefficients measured by the airborne High Spectral Resolution Lidar 2 (HSRL-2) and in situ measurements of CCN concentrations. The data were obtained during three deployments in the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) project, which took place over the southeast Atlantic (SEA) during September 2016, August 2017, and September–October 2018. Our analysis of spatiotemporally collocated in situ CCN concentrations and HSRL-2 measurements indicates strong linear relationships between both data sets. The correlation is strongest for supersaturations (S) greater than 0.25 % and dry ambient conditions above the stratocumulus deck, where relative humidity (RH) is less than 50 %. We find CCN–HSRL-2 Pearson correlation coefficients between 0.95–0.97 for different parts of the seasonal burning cycle that suggest fundamental similarities in biomass burning aerosol (BBA) microphysical properties. We find that ORACLES campaign-average values of in situ CCN and in situ extinction coefficients are qualitatively similar to those from other regions and aerosol types, demonstrating overall representativeness of our data set. We compute CCN–backscatter and CCN–extinction regressions that can be used to resolve vertical CCN concentrations across entire above-cloud lidar curtains. These lidar-derived CCN concentrations can be used to evaluate model performance, which we illustrate using an example CCN concentration curtain from the Weather Research and Forecasting Model coupled with physics packages from the Community Atmosphere Model version 5 (WRF-CAM5). These results demonstrate the utility of deriving vertically resolved CCN concentrations from lidar observations to expand the spatiotemporal coverage of limited or unavailable in situ observations.

Added Value of Aerosol Observations of a Future AOS High Spectral Resolution Lidar with Respect to Classic Backscatter Spaceborne Lidar Measurements

In the context of the Atmosphere Observing System (AOS) international program, a new-generation spaceborne lidar is expected to be in polar orbit for deriving new observations of aerosol and clouds. In this work, we analyze the added values of these new observations for characterizing aerosol vertical distribution. For this, synthetic observations are simulated using the BLISS lidar simulator in terms of the backscatter coefficient at 532 nm. We consider two types of lidar instruments, an elastic backscatter lidar instrument and a high spectral resolution lidar (HSRL). These simulations are performed with atmospheric profiles from a nature run (NR) modeled by the MOCAGE chemical transport model. In three case studies involving large events of different aerosol species, the added value of the HSRL channel (for measuring aerosol backscatter profiles with respect to simple backscatter measurements) is shown. Observations independent of an a priori lidar ratio assumption, as done typically for simple backscattering instruments, allow probing the vertical structures of aerosol layers without divergence, even in cases of intense episodes. A 5-day study in the case of desert dust completes the study of the added value of the HSRL channel with relative mean bias from the NR of the order of 1.5%. For low abundances, relative errors in the backscatter coefficient profiles may lay between +40% and −40%, with mean biases between +5% and −5%.

Analysis of the air quality in upper atmospheric boundary layer in a high-density city in Asia using 3-year vertical profiles measured by the 3-Dimensional Real-Time Atmospheric Monitoring System (3DREAMS).

Past studies focused on ground-level air quality, whereas air quality in upper atmospheric boundary layer (ABL) remains unclear due to lack of long-term and high time-resolution profile data. This study utilized the 3-Dimensional Real-Time Atmospheric Monitoring System (3DREAMS) to provide vertical profiles of aerosol backscatter coefficients and wind for three years (2019-2021), along with DustTrak to describe and analyze the characteristics of aerosols in the upper ABL in a high-density city in Asia (Hong Kong, China). It is the first study to assess the long-term record and spatial variations of upper-level aerosol in a high-density city using a LiDAR network. Results show an opposite diurnal profile of aerosol comparing between ground-level and upper ABL, which is different with the diurnal pattern observed in ground measurements (higher air pollutant concentration level in daytime and lower in nighttime). The co-location vertical wind measurements provided the explanation of the opposite diurnal patterns. The 3-year vertical profiles also show the significant spatial variation of vertical distribution of aerosol at different locations, whereas the temporal variations can be affected by various factors such as emissions and transboundary air pollution. Our episode analysis clearly demonstrated the capability of 3DREAMS to monitor transboundary air pollution with detailed information of horizontal transport and vertical convection. 3DREAMS is therefore proved as a suitable system for transboundary air pollution monitoring. Our findings provide a critical reference for atmospheric scientists and decisionmakers to understand transboundary air pollution.

Calibration method of Fernald inversion for aerosol backscattering coefficient profiles via multi-wavelength Raman–Mie lidar

Mie scattering lidar is widely used in the detection of atmospheric aerosol optical properties, however, the inversion algorithm is lack of accuracy, which is mainly caused by the inaccurate calibration of reference condition in Fernald inversion. Here, we propose a multi-wavelength aerosol backscattering ratio parameterized-calibration method (MABP-CM) based on parameterized equations for reference condition calibration of Fernald inversion. The aerosol backscatter ratios at 355 nm, 532 nm, and 1064 nm wavelengths are respectively calculated from the aerosol size distribution of aerial survey data. We find that there is a positive correlation between the backscatter ratios at different wavelengths, and the parameterized equations are established. The parameterized equations can be used as conversion factors of aerosol backscatter ratio at different wavelengths, which is beneficial to improving the calibration accuracy of multi-wavelength backscatter ratios at reference height in Fernald inversion. The Raman inversion is used to retrieve the 355 nm aerosol backscattering coefficient and aerosol backscatter ratio. Then, combined with MABP-CM, the 532 nm and 1064 nm aerosol backscatter ratios at reference height are respectively derived, to obtain the backscattering coefficients with Fernald backward integration equation. With the improved method, the reference height is allowed not to be at clear atmosphere, and the reference condition can still be accurately calibrated. Simulation verification of multi-wavelength backscattering coefficients is carried out with aerial survey data. Our simulation suggests that MABP-CM can reduce the error of 532 nm backscattering coefficient to ±20%, and reduce the error of 1064 nm backscattering coefficient to ±40%. Multi-wavelength aerosol backscattering coefficients can be simultaneously retrieved with high accuracy on cloudy and heavily haze days with MABP-CM. Finally, the multi-wavelength Raman–Mie lidar is used to observe the short-term and long-term changes of aerosol backscattering coefficient on clear and cloudy days.

Statistical validation of Aeolus L2A particle backscatter coefficient retrievals over ACTRIS/EARLINET stations on the Iberian Peninsula

Abstract. The Global Observing System (GOS) has encountered some limitations due to a lack of worldwide real-time wind measurements. In this context, the European Space Agency (ESA) has developed the Aeolus satellite mission, based on the ALADIN (Atmospheric Laser Doppler Instrument) Doppler wind lidar; this mission aims to obtain near-real-time wind retrievals at the global scale. As spin-off products, the instrument retrieves aerosol optical properties such as particle backscatter and extinction coefficients. In this work, a validation of Aeolus reprocessed (baseline 10) co-polar backscatter coefficients (βAeoluspart) is presented through an intercomparison with analogous ground-based measurements taken at the ACTRIS (Aerosols, Clouds and Trace gases Research InfraStructure Network)/EARLINET (European Aerosol Research Lidar Network) stations of Granada (Spain), Évora (Portugal) and Barcelona (Spain) over the period from July 2019 until October 2020. Case studies are first presented, followed by a statistical analysis. The stations are located in a hot spot between Africa and the rest of Europe, which guarantees a variety of aerosol types, from mineral dust layers to continental/anthropogenic aerosol, and allows us to test Aeolus performance under different scenarios. The so called Aeolus-like profiles (βAeoluslike,355part) are obtained from total particle backscatter coefficient and linear particle depolarization ratio (δlinearpart) profiles at 355 and 532 nm measured from the surface, through a thorough bibliographic review of dual-polarization measurements for relevant aerosol types. Finally, the study proposes a relation for the spectral conversion of δlinearpart, which is implemented in the Aeolus-like profile calculation. The statistical results show the ability of the satellite to detect and characterize significant aerosol layers under cloud-free conditions, along with the surface effect on the lowermost measurements, which causes the satellite to largely overestimate co-polar backscatter coefficients. Finally, the Aeolus standard correct algorithm middle bin (SCAmb) shows a better agreement with ground-based measurements than the standard correct algorithm (SCA), which tends to retrieve negative and meaningless coefficients in the clear troposphere. The implementation of Aeolus quality flags entails a vast reduction in the number of measurements available for comparison, which affects the statistical significance of the results.

Vertical Profiling of Fresh Biomass Burning Aerosol Optical Properties over the Greek Urban City of Ioannina, during the PANACEA Winter Campaign

Vertical profiling of aerosol particles was performed during the PANhellenic infrastructure for Atmospheric Composition and climatE chAnge (PANACEA) winter campaign (10 January 2020–7 February 2020) over the city of Ioannina, Greece (39.65° N, 20.85° E, 500 m a.s.l.). The middle-sized city of Ioannina suffers from wintertime air pollution episodes due to biomass burning (BB) domestic heating activities. The lidar technique was applied during the PANACEA winter campaign on Ioannina city, to fill the gap of knowledge of the spatio-temporal evolution of the vertical mixing of the particles occurring during these winter-time air pollution episodes. During this campaign the mobile single-wavelength (532 nm) depolarization Aerosol lIdAr System (AIAS) was used to measure the spatio-temporal evolution of the aerosols’ vertical profiles within the Planetary Boundary Layer (PBL) and the lower free troposphere (LFT; up to 4 km height a.s.l.). AIAS performed almost continuous lidar measurements from morning to late evening hours (typically from 07:00 to 19:00 UTC), under cloud-free conditions, to provide the vertical profiles of the aerosol backscatter coefficient (baer) and the particle linear depolarization ratio (PLDR), both at 532 nm. In this study we emphasized on the vertical profiling of very fresh (~hours) biomass burning (BB) particles originating from local domestic heating activities in the area. In total, 33 out of 34 aerosol layers in the lower free troposphere were characterized as fresh biomass burning ones of local origin, showing a mean particle linear depolarization value of 0.04 ± 0.02 with a range of 0.01 to 0.09 (532 nm) in a height region 1.21–2.23 km a.s.l. To corroborate our findings, we used in situ data, particulate matter (PM) concentrations (PM2.5) from a particulate sensor located close to our station, and the total black carbon (BC) concentrations along with the respective contribution of the fossil fuel (BCff) and biomass/wood burning (BCwb) from the Aethalometer. The PM2.5 mass concentrations ranged from 5.6 to 175.7 μg/m3, while the wood burning emissions from residential heating were increasing during the evening hours, with decreasing temperatures. The BCwb concentrations ranged from 0.5 to 17.5 μg/m3, with an extremely high mean contribution of BCwb equal to 85.4%, which in some cases during night-time reached up to 100% during the studied period.

Capabilities of an Automatic Lidar Ceilometer to Retrieve Aerosol Characteristics within the Planetary Boundary Layer

Continuous observation and quantitative retrieval of aerosol backscatter coefficients are important in the study of air quality and climate in metropolitan areas such as New York City. Ceilometers are ideal for this application, but aerosol backscatter coefficient retrievals from ceilometers are challenging and require proper calibration. In this study, we calibrate the ceilometer (Lufft CHM15k, 1064 nm) system constant with the molecular backscatter coefficient and evaluate the calibrated profiles with other independent methods, including the water-phase cloud method and comparison with the NASA Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) attenuated backscatter coefficient profile. Multiple-day calibration results show a stable system constant with a relative uncertainty of about 7%. We also evaluate the overlap correction for the CHM15k ceilometer (provided by Lufft) with a Vaisala CL-31 ceilometer, and the results show good consistency between two ceilometers’ range-corrected signal (RCS) profiles above 200 m. Next, we implement a forward iterative method to retrieve aerosol backscatter coefficients from continuous ceilometer measurements. In the retrieval, the lidar ratio is constrained by the co-located NASA AERONET radiometer aerosol optical depth (AOD) retrieval and agrees with the AERONET lidar-ratio products, derived from aerosol microphysical parameters. The aerosol backscatter coefficient retrievals are validated with co-located elastic-Raman lidar retrievals and indicate a good correlation (R2≥0.95) in the planetary boundary layer (PBL). Furthermore, a case study shows that the ceilometer retrieved aerosol extinction coefficient profiles can be used to estimate the AOD of the PBL and the aloft plumes. Finally, simulations of the uncertainty of aerosol backscatter coefficient retrieval show that a calibration error of 10% results in 10–20% of relative error in the aerosol backscatter coefficient retrievals, while relative error caused by a lidar-ratio error of 10% is less than 4% in the PBL.

Aerosol radiative impact during the summer 2019 heatwave produced partly by an inter-continental Saharan dust outbreak – Part 1: Short-wave dust direct radiative effect

Abstract. The short-wave (SW) direct radiative effect (DRE) during the summer 2019 heatwave produced partly by a moderate, long-lasting Saharan dust outbreak over Europe is analysed in this study. Two European sites (periods) are considered: Barcelona, Spain (23–30 June), and Leipzig, Germany (29 and 30 June), 1350 km apart from each other. Major data are obtained from AERONET and polarised Micro-Pulse Lidar (P-MPL) observations. Modelling is used to describe the different dust pathways, as observed at both sites. The coarse dust (Dc) and fine dust (Df) components (with total dust, DD = Dc + Df) are identified in the profiles of the total particle backscatter coefficient using the POLIPHON (POlarisation LIdar PHOtometer Networking) method in synergy with P-MPL measurements. This information is used to calculate the relative mass loading and the centre-of-mass height, as well as the contribution of each dust mode to the total dust DRE. Several aspects of the ageing of dust are put forward. The mean dust optical depth and its Df/DD ratios are, respectively, 0.153 and 24 % in Barcelona and 0.039 and 38 % in Leipzig; this Df increase in Leipzig is attributed to a longer dust transport path in comparison to Barcelona. The dust produced a cooling effect on the surface with a mean daily DRE of −9.1 and −2.5 W m−2, respectively, in Barcelona and Leipzig, but the Df/DD DRE ratio is larger for Leipzig (52 %) than for Barcelona (37 %). Cooling is also observed at the top of the atmosphere (TOA), although less intense than on the surface. However, the Df/DD DRE ratio at the TOA is even higher (45 % and 60 %, respectively, in Barcelona and Leipzig) than on the surface. Despite the predominance of Dc particles under dusty conditions, the SW radiative impact of Df particles can be comparable to, even higher than, that induced by the Dc ones. In particular, the Df/DD DRE ratio in Barcelona increases by +2.4 % d−1 (surface) and +2.9 % d−1 (TOA) during the dusty period. This study is completed by a second paper about the long-wave and net radiative effects. These results are especially relevant for the next ESA EarthCARE mission (planned in 2022) as it is devoted to aerosol–cloud–radiation interaction research.

Lidar detection and characterization of multi-type aerosol layers in the troposphere above Sofia, Bulgaria

Aerosols can directly and strongly affect the atmospheric energy budget, the climatic processes, the ecological state of the environment, and the biosphere and human health. This is why the detection and identification of aerosols in terms of type and origin and their optical and microphysical characterization are of great scientific and practical importance. Lidars are a widely recognized and proven tool for detecting and studying atmospheric aerosols. In this paper, we present results of lidar detection and characterization of aerosol layers present in the troposphere over the city of Sofia. The measurements were performed at the wavelengths of 1064 nm and 532 nm by using the Nd:YAG-laser based lidar of Sofia lidar station, part of the European Aerosol Research Lidar Network (EARLINET) and the Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS). Time-averaged vertical profiles of the aerosol backscatter coefficient and the backscatter-related Ångström exponent were retrieved and analyzed. Color-coded height-time diagrams of the vertical mass distribution in the observed layers and its temporal dynamics were also obtained. Based on information from forecast models, it is concluded that the aerosol layers registered contained marine aerosols, forest fire smoke, indirectly and directly transported dust from Sahara, as well as continental, regional and local aerosols from different sources.

Lidar observations of massive Saharan dust intrusion above Sofia, Bulgaria, in April 2019

A large-scale massive transport of Saharan dust over all of Europe was observed in the last third of April 2019. As part of the European Aerosol Research Lidar Network (EARLINET) and the Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS), Sofia lidar station conducted lidar observations of the atmospheric aerosol load in this period. Some of the results obtained are described in this work. Remote measurements were implemented using two elastic-scatter lidar channels (1064 nm/532 nm). The paper presents results of studying and analyzing the optical and microphysical properties of the aerosols detected, as well as the topological and dynamical features of the aerosol/dust layers observed. The height profiles of the atmospheric aerosol backscattering coefficient are retrieved and discussed. The microphysical properties of aerosol/dust particles are characterized qualitatively based on an analysis of the backscatter-related Ångström exponents profiles and distributions obtained for particular aerosol layers. Dust mixing with other aerosol types of local origin or such captured during the transport is also considered and discussed based on local meteorological radiosonde data and air-transport modeling and forecasting data. Conclusions are drawn concerning the experimentally ascertained and the possible impacts of the massive dust intrusion event on the local tropospheric aerosol structure and meteorological conditions.

The electrical activity of Saharan dust as perceived from surface electric field observations

Abstract. We report on the electric field variations during Saharan dust advection over two atmospheric remote stations in Greece, using synergistic observations of the vertical atmospheric electric field strength (Ez) at ground level and the lidar-derived particle backscatter coefficient profiles. Both parameters were monitored for the first time with the simultaneous deployment of a ground-based field mill electrometer and a multi-wavelength polarization lidar. The field mill time series are processed to extract the diurnal variations of the global electric circuit and remove fast field perturbations due to peak lightning activity. In order to identify the influence of the elevated dust layers on the ground Ez, we extract a localized reference electric field from the time series that reflects the local fair-weather activity. Then, we compare it with the reconstructed daily average behaviour of the electric field and the Saharan dust layers' evolution, as depicted by the lidar. The observed enhancement of the vertical electric field (up to ∼100 V m−1), for detached pure dust layers, suggests the presence of in-layer electric charges. Although higher dust loads are expected to result in such an electric field enhancement, episodic cases that reduce the electric field are also observed (up to ∼60 V m−1). To quantitatively approach our results, we examine the dependency of Ez against theoretical assumptions for the distribution of separated charges within the electrified dust layer. Electrically neutral dust is approximated by atmospheric conductivity reduction, while charge separation areas within electrically active dust layers are approximated as finite-extent cylinders. This physical approximation constitutes a more realistic description of the distribution of charges, as opposed to infinite-extent geometries, and allows for analytical solutions of the electric field strength so that observed variations during the monitored dust outbreaks can be explained.

Ground-based lidar processing and simulator framework for comparing models and observations (ALCF 1.0)

Abstract. Automatic lidars and ceilometers (ALCs) provide valuable information on cloud and aerosols but have not been systematically used in the evaluation of general circulation models (GCMs) and numerical weather prediction (NWP) models. Obstacles associated with the diversity of instruments, a lack of standardisation of data products and open processing tools mean that the value of large ALC networks worldwide is not being realised. We discuss a tool, called the Automatic Lidar and Ceilometer Framework (ALCF), that overcomes these problems and also includes a ground-based lidar simulator, which calculates the radiative transfer of laser radiation and allows one-to-one comparison with models. Our ground-based lidar simulator is based on the Cloud Feedback Model Intercomparison Project (CFMIP) Observation Simulator Package (COSP), which has been extensively used for spaceborne lidar intercomparisons. The ALCF implements all steps needed to transform and calibrate raw ALC data and create simulated attenuated volume backscattering coefficient profiles for one-to-one comparison and complete statistical analysis of clouds. The framework supports multiple common commercial ALCs (Vaisala CL31, CL51, Lufft CHM 15k and Droplet Measurement Technologies MiniMPL), reanalyses (JRA-55, ERA5 and MERRA-2) and models (the Unified Model and AMPS – the Antarctic Mesoscale Prediction System). To demonstrate its capabilities, we present case studies evaluating cloud in the supported reanalyses and models using CL31, CL51, CHM 15k and MiniMPL observations at three sites in New Zealand. We show that the reanalyses and models generally underestimate cloud fraction. If sufficiently high-temporal-resolution model output is available (better than 6-hourly), a direct comparison of individual clouds is also possible. We demonstrate that the ALCF can be used as a generic evaluation tool to examine cloud occurrence and cloud properties in reanalyses, NWP models, and GCMs, potentially utilising the large amounts of ALC data already available. This tool is likely to be particularly useful for the analysis and improvement of low-level cloud simulations which are not well monitored from space. This has previously been identified as a critical deficiency in contemporary models, limiting the accuracy of weather forecasts and future climate projections. While the current focus of the framework is on clouds, support for aerosol in the lidar simulator is planned in the future.

Tropospheric vertical profiling of the aerosol backscatter coefficient and the particle linear depolarization ratio for different aerosol mixtures during the PANACEA campaign in July 2019 at Volos, Greece

The PANhellenic infrastructure for Atmospheric Composition and climate change (PANACEA) campaign took place in July 2019, to assess the presence of aerosols both in situ and at different heights, in selected cities of Greece suffering from high air pollution levels. One of these sites was the city of Volos (39.21° N., 22.57° E). Vertical profiles of the aerosol backscatter coefficient (baer) and the particle linear depolarization ratio (δp) of maritime polluted, urban, smoke and dust mixtures were measured during using the mobile depolarization Aerosol lIdAr System (AIAS). 42 aerosol layers were characterized, out of which 40% were of urban origin, 26% of smoke origin, 21% maritime polluted aerosol layers and 12% dust mixtures. The maritime polluted and urban aerosols showed a mean δp value at 532 nm of 4 ± 1%, within a range of 2–6% and 2–5%, respectively. The smoke aerosols showed a mean δp value of 6 ± 1%, within a range of 3–9%, while the dust mixtures showed a mean δp of 20 ± 4%, ranging from 13 to 29%. Finally, the majority of these layers were observed above 250 up to 2000 m height asl., although the dust mixtures reached 3000 m height asl.

Monitoring atmospheric particulate matters using vertically resolved measurements of a polarization lidar, in-situ recordings and satellite data over Tehran, Iran

The highly polluted atmosphere above Tehran has been investigated by using a polarization lidar operating at 532 nm, in-situ particulate matter suites distributed over the city, and meteorological observations. The measurement campaign is conducted from Nov. 2014 to Jan. 2016. Three typical cases are studied in detail where, the atmosphere is polluted with urban pollution, mixture of urban pollution and dust particles from local sources, and long range transported dust from the Arabian Peninsula. For these cases, vertical profiles of the lidar backscatter coefficient, extinction coefficients, particle depolarization ratio (delta _{text {p}}) and mass concentrations of atmospheric aerosols (separated into dust and non-dust particles) are presented. Using the lidar recordings, variations of the planetary boundary layer height above the city are investigated along the year. During November to February, lidar profiles frequently show polluted boundary layers that are reaching up to 1 km above the ground level. The depolarization ratio (delta _{text {p}}) varies between 0.04 and 0.08 in the polluted boundary layer. During the campaign, for 103 days the urban pollution was dominant, 45 recorded dust events (0.15<delta _{text {p}}<0.20) were originated from the dry regions in the south of Tehran and 15 dust events (0.20<delta _{text {p}}<0.35) impacted the city that were originated from the Arabian Desert and Mesopotamia.

Biomass burning events measured by lidars in EARLINET – Part 1: Data analysis methodology

Abstract. The methodology of analysing the biomass burning events recorded in the database of the European Aerosol Research Lidar Network in the framework of the Aerosol, Clouds and Trace Gases Research Infrastructure is presented. The period of 2008–2017 was chosen to analyse all of the events stored in the database under the Forest Fire category for a total of 14 stations available. The data provided ranged from complete datasets (particle backscatter, extinction and linear depolarization ratio profiles) to single profiles (particle backscatter coefficient profile). Smoke layers geometry was evaluated and the mean optical properties within each layer were computed. The back-trajectory technique was used to double-check the source of all pollution layers. The biomass burning layers were identified by taking into account the presence of the fires along the back trajectory. The biomass burning events are analysed by the means of the intensive parameters. The analysis was structured in three directions: (I) common biomass burning source (fire) recorded by at least two stations, (II) long-range transport from North America, and (III) analysis over four geographical regions (south-eastern Europe, north-eastern Europe, central Europe, and south-western Europe). Based on back-trajectory calculations and fire locations, the lidar measurements can be labelled either as measurements of a “single fire” or “mixed fires” (case I), measurements of North American fires, or measurements of mixed North American and local fires (case II). The histogram of the fire locations reveals the smoke sources for each region. For each region, statistics on intensive parameters are performed. The source origin of the intensive parameters is categorized based on the continental origin of the air mass (European, African, Asian, North American, or a combination of them). The methodology presented here is meant to provide a perspective to explore a large number of lidar data and deliver novel approaches to analyse the intensive parameters based on the assigned biomass burning sources. A thorough consideration of all potential fire sources reveals that most of the time the lidar measurements characterize the smoke from a mixture of fires. A comprehensive discussion of all the results (based on the intensive parameters and the source locations) will be given in a companion paper submitted to the ACP EARLINET special issue.