Particle Linear Depolarization Ratio Research Articles

Enhancing mobile aerosol monitoring with CE376 dual-wavelength depolarization lidar

Abstract. We present the capabilities of a compact dual-wavelength depolarization lidar to assess the spatiotemporal variations in aerosol properties aboard moving vectors. Our approach involves coupling the lightweight Cimel CE376 lidar, which provides measurements at 532 and 808 nm and depolarization at 532 nm, with a photometer to monitor aerosol properties. The assessments, both algorithmic and instrumental, were conducted at ATOLL (ATmospheric Observatory of LiLle) platform operated by the Laboratoire d'Optique Atmosphérique (LOA), in Lille, France. An early version of the CE376 lidar co-located with the CE318-T photometer and with a multi-wavelength Raman lidar were considered for comparisons and validation. We developed a modified Klett inversion method for simultaneous two-wavelength elastic lidar and photometer measurements. Using this setup, we characterized aerosols during two distinct events of Saharan dust and dust smoke aerosols transported over Lille in spring 2021 and summer 2022. For validation purposes, comparisons against the Raman lidar were performed, demonstrating good agreement in aerosol properties with relative differences of up to 12 % in the depolarization measurements. Moreover, a first dataset of CE376 lidar and photometer performing on-road measurements was obtained during the FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality) field campaign deployed in summer 2019 over the northwestern USA. By lidar and photometer mapping in 3D, we investigated the transport of released smoke from active fire spots at William Flats (northeast WA, USA). Despite extreme environmental conditions, our study enabled the investigation of aerosol optical properties near the fire source, distinguishing the influence of diffuse, convective, and residual smoke. Backscatter, extinction profiles, and column-integrated lidar ratios at 532 and 808 nm were derived for a quality-assured dataset. Additionally, the extinction Ångström exponent (EAE), color ratio (CR), attenuated color ratio (ACR), and particle linear depolarization ratio (PLDR) were derived. In this study, we discuss the capabilities (and limitations) of the CE376 lidar in bridging observational gaps in aerosol monitoring, providing valuable insights for future research in this field.

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.

Optical and Microphysical Properties of the Aerosols during a Rare Event of Biomass-Burning Mixed with Polluted Dust

A rare event of mixed biomass-burning and polluted dust aerosols was observed over Athens, Greece (37.9° N, 23.6° E), during 21–26 May 2014. This event was studied using a synergy of a 6-wavelength elastic-Raman-depolarization lidar measurements, a CIMEL sun photometer, and in situ instrumentation. The FLEXPART dispersion model was used to identify the aerosol sources and quantify the contribution of dust and black carbon particles to the mass concentration. The identified air masses were found to originate from Kazakhstan and Saharan deserts, under a rare atmospheric pressure system. The lidar ratio (LR) values retrieved from the Raman lidar ranged within 25–89 sr (355 nm) and 35–70 sr (532 nm). The particle linear depolarization ratio (δaer) ranged from 7 to 28% (532 nm), indicating mixing of dust with biomass-burning particles. The aerosol optical depth (AOD) values derived from the lidar ranged from 0.09–0.43 (355 nm) to 0.07–0.25 (532 nm). An inversion algorithm was used to derive the mean aerosol microphysical properties (mean effective radius (reff), single scattering albedo (SSA), and mean complex refractive index (m)) inside selected atmospheric layers. We found that reff was 0.12–0.51 (±0.04) µm, SSA was 0.94–0.98 (±0.19) (at 532 nm), while m ranged between 1.39 (±0.05) + 0.002 (±0.001)i and 1.63 (±0.05) + 0.008 (±0.004)i. The polarization lidar photometer networking (POLIPHON) algorithm was used to estimate the vertical profile of the mass concentration for the dust and non-dust components. A mean mass concentration of 15 ± 5 μg m−3 and 80 ± 29 μg m−3 for smoke and dust was estimated for selected days, respectively. Finally, the retrieved aerosol microphysical properties were compared with column-integrated sun photometer CIMEL data with good agreement.

HETEAC-Flex: an optimal estimation method for aerosol typing based on lidar-derived intensive optical properties

Abstract. This study introduces a novel methodology for the characterization of atmospheric aerosol based on lidar-derived intensive optical properties. The proposed aerosol-typing scheme is based on the optimal estimation method (OEM) and allows the identification of up to four different aerosol components of an aerosol mixture, as well as the quantification of their contribution to the aerosol mixture in terms of relative volume. The four aerosol components considered in this typing scheme are associated with the most commonly observed aerosol particles in nature and are assumed to be physically separated from each other and, therefore, can create external mixtures. Two components represent absorbing and less-absorbing fine-mode particles, and the other two components represent spherical and non-spherical coarse-mode particles. These components reflect adequately the most frequently observed aerosol types in the atmosphere: combustion- and pollution-related aerosol, sea salt, and desert dust, respectively. In addition, to consolidate the calibration and validation efforts for the upcoming EarthCARE mission, the typing scheme proposed here is in accordance with the Hybrid End-To-End Aerosol Classification (HETEAC) model of EarthCARE. The lidar-derived optical parameters used in this typing scheme are the lidar ratio and the particle linear depolarization ratio at two distinct wavelengths (355 and 532 nm), the backscatter-related color ratio for the wavelength pair of 532/1064 nm and the extinction-related Ångström exponent for the wavelength pair of 355/532 nm. These intensive optical properties can be combined in different ways, making the methodology flexible, thus allowing its application to lidar systems with different configurations (e.g., single wavelength or multiwavelength, Raman, high spectral resolution). The typing scheme was therefore named HETEAC-Flex due to its compatibility with EarthCARE's HETEAC and its methodological flexibility. The functionality of the typing scheme is demonstrated by its application to three case studies based on layer-averaged optical properties.

Classification of aerosols using particle linear depolarization ratio (PLDR) over seven urban locations of Asia

Active lidar remote sensing has been used to obtain detailed and quantitative information about the properties of aerosols. We have analyzed the spatio-temporal classification of aerosols using the parameters of particle linear depolarization ratio and single scattering albedo from Aerosol Robotic Network (AERONET) over seven megacities of Asia namely; Lahore, Karachi, Kanpur, Pune, Beijing, Osaka, and Bandung. We find that pollution aerosols dominate during the winter season in all the megacities. The concentrations, however, vary concerning the locations, i.e., 70–80% pollution aerosols are present over Lahore, 40–50% over Karachi, 90–95% over Kanpur and Pune, 60–70% and over Beijing and Osaka. Pure Dust (PD), Pollution Dominated Mixture (PDM), and Dust Dominated Mixture (DDM) are found to be dominant during spring and summer seasons.This proposes that dust over Asia normally exists as a mixture with pollution aerosols instead of pure form. We also find that black carbon (BC) dominated pollution aerosols.

Investigating a Persistent Stratospheric Aerosol Layer Observed over Southern Europe during 2019

A persistent stratospheric aerosol layer first appeared during July 2019 above Thessaloniki, Greece (40.5°N, 22.9°E). It was initially at 12 km and, during August 2019, was even up to 20 km, with increased thickness and reduced attenuated backscatter levels till the end of the year. In this study, we analyze the geometrical and optical properties of this stratospheric layer, using ground-based Lidar measurements, CALIOP/CALIPSO & OMPS-LP space-borne observations, as well as CAMS/ECMWF assimilation experiments. The main aim of the paper is to present an overview of this atmospheric feature and to identify any temporal changes in the aerosol properties that would signify substantial changes in the composition of this long-lasting stratospheric plume over Thessaloniki. This aim is further enhanced by emphasizing the importance of the combined information based on active ground- and space-borne lidars, passive remote sensing, and models during the complex stratospheric aerosol conditions as those encountered during 2019. The layer’s origin is linked to the Raikoke volcanic eruption in the Kuril Islands in June 2019, yielding a particle linear depolarization ratio less than 0.05, while some indications exist that the intense forest fires at mid and high northern latitudes throughout the summer of 2019 also contributed to the persistent layer. We report that in July, mainly volcanic sulphate aerosol layers with a 1–3 km vertical extent were identified in the stratosphere at ~15 km over Thessaloniki, while after August and until the end of 2019, the plume heights showed a significant month-to-month variability and a broadening (with thickness greater than 3 km) towards lower altitudes. The aerosol optical thickness was found to be in the range between 0.004 and 0.125 (visible) and 0.001 and 0.095 (infrared) and the particle depolarization of the detected stratospheric plume was found to be 0.03 ± 0.04, indicative of spherical particles, such as sulphate aerosols.

Spectral dependence of birch and pine pollen optical properties using a synergy of lidar instruments

Abstract. Active remote sensors equipped with the capability to detect polarization, a shape-relevant parameter, are essential to aerosol particle identification in the vertical domain. Most commonly, the linear particle depolarization ratio has been available at the shorter wavelengths of 355 and/or 532 nm. Recently, linear particle depolarization ratios at longer wavelengths (910, 1064, and 1565 nm) have emerged in lidar aerosol research. In this study, a synergy of three lidars, namely a PollyXT lidar, a Vaisala CL61 ceilometer, and a HALO Photonics StreamLine Pro Doppler lidar, as well as in situ aerosol and pollen observations have been utilized to investigate the spectral dependence of birch and pine pollen particles. We found that, regardless of the pollen type, the linear particle depolarization ratio was subject to the amount of pollen and its relative contribution to the aerosol mixture in the air. More specifically, during birch pollination, characteristic linear particle depolarization ratios of 5 ± 2 % (355 nm), 28 ± 6 % (532 nm), 23 ± 6 % (910 nm), and 33 ± 4 % (1565 nm) were retrieved at the pollen layer. Regarding the pine-dominant period, characteristic linear particle depolarization ratios of 6 ± 2 %, 43 ± 11 %, 22 ± 6 %, and 26 ± 3 % were determined at wavelengths of 355, 532, 910, and 1565 nm, respectively. For birch, the linear particle depolarization ratio at 1565 nm was the highest, followed by the 532 and 910 nm wavelengths, respectively. A sharp decrease at 355 nm was evident for birch pollen. For pine pollen, a maximum at the 532 nm wavelength was observed. There was no significant change in the linear particle depolarization ratio at 910 nm for the pollen types considered in this study. Given the low concentration of pollen in the air, the inclusion of the longer wavelengths (910 and 1565 nm) for the detection of birch and pine can be beneficial due to their sensitivity to trace large aerosol particles.

Investigating an indirect aviation effect on mid-latitude cirrus clouds – linking lidar-derived optical properties to in situ measurements

Abstract. Aviation has a large impact on the Earth's atmosphere and climate by various processes. Line-shaped contrails and contrail cirrus clouds lead to changes in the natural cirrus cloud cover and have a major contribution to the effective radiative forcing from aviation. In addition, aviation-induced aerosols might also change the microphysical properties and optical properties of naturally formed cirrus clouds. Latter aerosol–cloud interactions show large differences in the resulting effective radiative forcing, and our understanding on how aviation-induced aerosols affect cirrus cloud properties is still poor. Up to now, observations of this aviation-induced aerosol effect have been rare. In this study, we use combined airborne lidar and in situ ice cloud measurements to investigate differences in the microphysical and optical properties of naturally formed cirrus clouds, which formed in regions that are highly affected by aviation-induced aerosol emissions and, of those, which formed in regions rather unaffected by aviation. Urbanek et al. (2018) showed that those cirrus clouds, which are more affected by aviation-induced soot emission, are characterized by larger values of the particle linear depolarization ratio (PLDR). In this follow-on study we relate collocated lidar measurements performed aboard HALO during the ML-CIRRUS mission of the particle linear depolarization ratio with in situ cloud probe measurements of the number concentration and effective diameter of the ice particles. In situ measurements for both cloud types (high-PLDR-mode – aviation-affected – and low-PLDR-mode – pristine – cirrus) can be reliably compared in a temperature range between 210 and 215 K. Within this temperature range we find that high-PLDR-mode cirrus clouds tend to show larger effective ice particle diameters with a median value of 61.4 compared to 50.7 µm for low-PLDR-mode pristine cirrus clouds. Larger effective ice particles in aviation-influenced (high-PLDR-mode) cirrus are connected to lower ice particle number concentration with a median value of 0.05 compared to 0.11 cm−3 (low-PLDR-mode), which evolved in more pristine regions with only little impact from aviation. We suspect that a suppression of homogeneous ice formation by the heterogeneously freezing soot aerosol particles included in the areas affected by air traffic is the cause of the reduced ice crystal concentrations.

Long-term analysis of AOD separated by aerosol type in East Asia

This study aimed to identify the temporal variations in aerosol loadings and sizes in East Asia based on a method that uses the linear particle depolarization ratio (δP) and coarse-mode fraction of volume concentration (CMFvc) from AERONET Sun/Sky radiometer data. East Asia is the polluted region. Pollution levels continue to increase as a result of population growth and increasing industrialization. The main pollution sources are derived from anthropogenic activities and Asian dust. The data were used to separate the total aerosol optical depths (τ) of mixed aerosol plumes into optical depths contributed by dust (τD), coarse- (τPC), and fine-mode pollution (τPF). We used data taken from 1999 to 2021 in East Asia (Beijing, Xianghe on the west, Seoul, Yonsei, Anmyon, and Gwangju in the middle, Osaka and Shirahama in the east, and Taipei in the south). We identified the regional differences; the contribution of τD and τPC to the total τ of Chinese sites was almost twice that of other East Asian sites. We analyzed the monthly, seasonal and annual values each of the three aerosol types contributed to total optical depth. The annual variations in τD and τPC from 1999 to 2021 showed a decreasing trend. In contrast, τPF either increased or became stagnant during that time. We conclude that aerosol types in East Asia may have changed. The Ångström exponent of fine-mode particles increased in most regions (0.0008–0.0103 yr−1 for the wavelength range 440–870 nm), which means the mean size of the fine-mode pollution particles tended to decrease, even though τPF increased. The change in particle size might be caused by the decrease in primary emission sources due to the strong air pollution reduction policies in China, Korea, and Japan. Therefore, we need to focus on the change in aerosol properties not just on aerosol loadings.

DeLiAn – a growing collection of depolarization ratio, lidar ratio and Ångström exponent for different aerosol types and mixtures from ground-based lidar observations

Abstract. This paper presents a collection of lidar-derived aerosol intensive optical properties for several aerosol types, namely the particle linear depolarization ratio, the extinction-to-backscatter ratio (lidar ratio) and the Ångström exponent. The data collection, named DeLiAn, is based on globally distributed, long-term, ground-based, multiwavelength, Raman and polarization lidar measurements, conducted mainly with lidars that have been developed at the Leibniz Institute for Tropospheric Research. The intensive optical properties are presented at two wavelengths, 355 and 532 nm, for 13 aerosol categories. The categories cover the basic aerosol types (i.e., marine, pollution, continental European background, volcanic ash, smoke, mineral dust), as well as the most frequently observed mixtures they form. This extensive collection also incorporates more peculiar aerosol categories, including dried marine aerosol that, compared to marine aerosol, exhibits a significantly enhanced depolarization ratio (up to 15 %). Besides Saharan dust, additional mineral dust types related to their source region were identified due to their lower lidar ratios (Central Asian and Middle Eastern dust). In addition, extreme wildfire events (such as in north America and Australia) emitted smoke into the stratosphere showing significantly different optical properties, i.e., high depolarization values (up to 25 %), compared to tropospheric smoke. The data collection reflects and underlines the variety of aerosol mixtures in the atmosphere and can be used for the development of aerosol-typing schemes. The paper contains the most up-to-date and comprehensive overview of optical properties from aerosol lidar measurements and, therefore, provides a solid basis for future aerosol retrievals in the frame of both spaceborne and ground-based lidars. Furthermore, DeLiAn can assist the efforts for the harmonization of satellite records of aerosol properties performed at different wavelengths.

Case study of biomass-burning aerosol events over Sofia, Bulgaria, based on AERONET sun photometer data

In the present work, the peculiarities were analyzed of aerosol events over Sofia, Bulgaria, characterized by a daily-mean aerosol optical depth (AOD) exceeding 0.3 and Ångström exponent (AE) above unity. Such aerosol situations arise most frequently in summer and early autumn, and sometimes in late autumn and winter. The analysis was based on AERONET sun photometer data and showed that the corresponding aerosol ensembles consisted mainly of fine-fraction biomass-burning (BB) smoke particles due to summer wildfires or domestic heating in winter, with a high sphericity factor (SF) that reached unity and a low linear particle depolarization ratio (LDR) of the order of thousandths. In the cases of aged BB smoke particles, the values of AE decrease without essential changes in SF and LDR. The presence or the periodic appearance of desert dust and/or marine aerosols during the day will also lead to a lower AE, accompanied however by a decreasing SF and an increasing LDR.

Lidar Profiling of Aerosol Vertical Distribution in the Urbanized French Alpine Valley of Annecy and Impact of a Saharan Dust Transport Event

The vertical aerosol layering of the troposphere is poorly documented in mountainous regions, particularly in the Alpine valleys, which are influenced by valley and mountain winds. To improve our knowledge of particulate matter trapped in the Annecy valley, synergetic measurements performed by a ground-based meteorological Raman lidar and a Rayleigh-Mie lidar aboard an ultralight aircraft were implemented as part of the Lacustrine-Water vApor Isotope inVentory Experiment (L-WAIVE) over Lake Annecy. These observations were complemented by satellite observations and Lagrangian modeling. The vertical profiles of aerosol optical properties (e.g., aerosol extinction coefficient (AEC), lidar ratio (LR), particle linear depolarization ratio (PDR)) are derived from lidar measurements at 355 nm during the period between 13 and 22 June 2019. The background aerosol content with an aerosol optical thickness (AOT) of 0.10 ± 0.05, corresponding to local–regional conditions influenced by anthropogenic pollution, has been characterized over the entirety of Lake Annecy thanks to the mobile ultralight payload. The aerosol optical properties are shown to be particularly variable over time in the atmospheric column, with mean LRs (PDRs) varying between 40 ± 8 and 115 ± 15 sr (2 ± 1 and 35 ± 2%). Those conditions can be disturbed by air masses that have recirculated over the valley, as well as by contributions from neighboring valleys. We have observed an important disruption in the atmospheric aerosol profiles by the arrival of an exceptionally dry air mass (RH ~ 30%), containing aerosols identified as coming from the Great Western Erg (AOT ~ 0.5, LR = 65 ± 10 sr, PDR = 20–35%) in the Sahara. These desert dust particles are shown to influence the entire atmospheric column in the Annecy valley. Such an experimental approach, coupling upward and downward lidar and spaceborne observation/Lagrangian modelling, was shown to be of significant interest for the long-term monitoring of the evolution of aerosol loads over deep valleys. It allows a better understanding of the influence of dust storms in the presence of severe convective weather processes.

Simultaneous profiling of dust aerosol mass concentration and optical properties with polarized high-spectral-resolution lidar

Dust particles originating from arid desert regions can be transported over long distances, presenting severe risks to climate, environment, social economics, and human health at the source and downwind regions. However, there has been a dearth of continuous diurnal observations of vertically resolved mass concentration and optical properties of dust aerosols, which hinders our understanding of aerosol mixing, stratification, aerosol-cloud interactions, and their impacts on the environment. To fill the gap of the insufficient observations, to the best of our knowledge, this work presents the first high-spectral-resolution lidar (HSRL) observation providing days of continuous profiles of the mass concentration, along with particle linear depolarization ratio (PLDR), backscattering coefficient, extinction coefficient and lidar ratio (LR), simultaneously. We present the results of two strong dust events observed by HSRL over Beijing in 2021. The maximum particle mass concentrations reached (1.52 ± 3.5) x103 μg/m3 and (19.48 ± 0.36) x103 μg/m3 for the two dust events, respectively. The retrieved particle mass concentrations and aerosol optical depth (AOD) agree well with the observation from the surface PM10 concentrations and sun photometer with correlation coefficients of 0.90 and 0.95, respectively. The intensive properties of PLDR and LR of the dust aerosols are 0.31 ± 0.02 and 39 ± 7 sr at 532 nm, respectively, which are generally close to those obtained from observations in the downwind areas. Moreover, inspired by the observations from HSRL, a universal analytical relationship is discovered to evaluate the proportion of dust aerosol backscattering, extinction, AOD, and mass concentration using PLDR. The universal analytical relationship reveals that PLDR can directly quantify dust aerosol contribution, which is expected to further expand the application of polarization technology in dust detection. These valuable observations and findings further our understanding of the contribution of dust aerosol to the environment and help supplement dust aerosol databases.

Satellite observations of seasonality and long-term trends in cirrus cloud properties over Europe: investigation of possible aviation impacts

Abstract. Linear contrails and contrail cirrus induced by global aviation have long been known to contribute to climate change by warming the atmosphere. Besides increasing global cirrus cloudiness, aviation may also alter the properties of natural cirrus clouds by soot emissions which lead to more heterogeneous freezing. During the first COVID-19 lockdown in Europe, changes in the properties and occurrence of cirrus clouds were determined with the lidar measurements of CALIPSO, which are presumed to be caused by the corresponding reduction in civil aviation. In the 10 years before the COVID-19 outbreak, however, aviation grew strongly in terms of CO2 emissions and flight densities in Europe. In this study, 10-year lidar measurements with CALIPSO are analysed to determine the seasonality and long-term trends in cirrus clouds as well as their correlations with the ambient temperatures and air traffic. The results show that there is a distinct seasonal cycle in the occurrence rates (ORs) and particle linear depolarization ratio (PLDR) of cirrus clouds. In addition, cirrus clouds appear within a broader altitude range in winter than in summer and they are characterized by larger OR and PLDR values in winter than in summer. The monthly medians of PLDR as well as their deseasonalized time series in the 10-year period before COVID-19 both show positive trends, which are statistically significant according to the Mann–Kendall (MK) significance test. However, the ORs of cirrus clouds show a negative trend, which might be connected with the background meteorological conditions. Since the cirrus PLDR strongly depends on the ambient temperatures, the contributions induced by temperature are further removed from the cirrus PLDR with a simple linear regression model. The derived residuals show significant positive trends according to the MK test. To compare the cirrus PLDR and air traffic (with the CO2 emissions from aviation as a proxy), the deseasonalization of both datasets were previously conducted since the seasonal cycles in both are not consistent. The deseasonalized time series determined for the cirrus PLDR and CO2 emissions from aviation both show increasing trends and their correlation coefficient is r=0.54 at the confidence level above 99.5 %. Finally, comparisons between the cirrus PLDR and aviation in every season were made and revealed a strong correlation in other seasons than in summer.

High-spectral-resolution LIDAR based on a few-longitudinal mode laser for aerosol and cloud characteristics detection.

A novel implementation of high-spectral-resolution LIDAR based on a passively Q-switched few-longitudinal mode laser (PQFLM-HSRL) is proposed, and the prototype is built for detecting aerosol and cloud characteristics. The spatial-temporal distributions of the aerosol and cloud are continuously observed by the PQFLM-HSRL for the first time, to the best of our knowledge. Based on observation, we present the retrieval results of backscatter coefficient, particle linear depolarization ratio, and LIDAR ratio, and these intensive parameters are used to classify the aerosol and cloud into different types. Particularly, we have observed mix-phased clouds. The resulting aerosol optical depths (AODs) are highly consistent with CE-318, the Sun photometer measurements of the local National Meteorological Station (NMS), which verify the retrieval accuracy and the system stability. In addition, the retrieved AODs also characterize the ambient air quality, which show a high correlation with the measured PM2.5 concentrations. The implementation of the PQFLM-HSRL provides a new method for atmospheric feature detection, which shows superior scientific potential for further study on climate change and environmental health.

Identification of typical dust sources in Tarim Basin based on multi-wavelength Raman polarization lidar

Asian mineral dust is one of the main aerosol sources in the Earth–atmosphere system, which generates significant effect on air quality, human health, and climate change. Meanwhile, knowledge of vertical optical properties of dust aerosol is crucially needed for identification of the dust source and improved understanding of radiative effect in climate model. In the study, triple-wavelength polarization Raman lidar observation combined with photometer, radiosonde and simultaneous model data was performed at Kashi in the northwestern of Tarim Basin. Taklimakan desert, located in the center of Tarim Basin, is the largest desert of Asia. Base on the measurement and model, two typical dust sources in different altitude were identified in the study, namely Taklimakan desert (East Road), Central Asia desert and Middle East desert (West Road). Particle size distribution of photometer shows that these cases were all coarse-mode-dominated with effective radius larger than 1.7 μm. The lidar observations revealed particle linear depolarization ratios (PLDR) of the Taklimakan dust ranged from 0.28 to 0.34 at 355 nm, 0.33 to 0.35 at 532 nm and 0.29–0.35 at 1064 nm, while lidar ratios (LR) ranged from 47 to 54 sr at 355 nm and from 42 to 51 sr at 532 nm wavelength. Spectral variation of LR and PLDR for Asian dust and Saharan dust was analyzed. All observed Asian dust present the consistent spectral variation that lidar ratio at 355 nm is higher than that of 532 nm, which however is not the case for Saharan dust. Both Saharan and Asian dust measurement PLDR at 532 nm is larger than that of 355 nm and 1064 nm. The measured dust properties provide particularly valuable information for dust simulation and dust climate model for different dust source.

Investigation of Aerosol Types and Vertical Distributions Using Polarization Raman Lidar over Vipava Valley

Aerosol direct radiative forcing is strongly dependent on aerosol distributions and aerosol types. A detailed understanding of such information is still missing at the Alpine region, which currently undergoes amplified climate warming. Our goal was to study the vertical variability of aerosol types within and above the Vipava valley (45.87°N, 13.90°E, 125 m a.s.l.) to reveal the vertical impact of each particular aerosol type on this region, a representative complex terrain in the Alpine region which often suffers from air pollution in the wintertime. This investigation was performed using the entire dataset of a dual-wavelength polarization Raman lidar system, which covers 33 nights from September to December 2017. The lidar provides measurements from midnight to early morning (typically from 00:00 to 06:00 CET) to provide aerosol-type dependent properties, which include particle linear depolarization ratio, lidar ratio at 355 nm and the aerosol backscatter Ångström exponent between 355 nm and 1064 nm. These aerosol properties were compared with similar studies, and the aerosol types were identified by the measured aerosol optical properties. Primary anthropogenic aerosols within the valley are mainly emitted from two sources: individual domestic heating systems, which mostly use biomass fuel, and traffic emissions. Natural aerosols, such as mineral dust and sea salt, are mostly transported over large distances. A mixture of two or more aerosol types was generally found. The aerosol characterization and statistical properties of vertical aerosol distributions were performed up to 3 km.

Australian Bushfires (2019–2020): Aerosol Optical Properties and Radiative Forcing

In the present study, we present the aerosol optical properties and radiative forcing (RF) of the tropospheric and stratospheric smoke layers, observed by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, during the extraordinary Australian biomass burning (BB) event in 2019–2020. These BB layers were studied and analyzed within the longitude range 140° E–20° W and the latitude band 20°–60° S, as they were gradually transported from the Australian banks to the South American continent. These layers were found to be trapped within the Andes circulation, staying for longer time periods in the same longitude region. The BB aerosols reached altitudes even up to 22 km amsl., and regarding their optical properties, they were found to be nearly spherical (particle linear depolarization ratio (PLDR) < 0.10) in the troposphere; while, in the stratosphere, they were more depolarizing with PLDR values reaching up to 0.20. Fine and ultrafine smoke particles were dominant in the stratosphere, according to the observed Ångström exponent, related to the backscatter coefficients obtained by the pair of wavelengths 532 and 1064 nm (Åb up to 3), in contrast to the Åb values in the troposphere (Åb < 1) indicative of the presence of coarser particles. As the aerosols fend off the source, towards North America, a slightly descending trend was observed in the tropospheric Åb values, while the stratospheric ones were lightly increased. A maximum aerosol optical depth (AOD) value of 0.54 was recorded in the lower troposphere over the fire spots, while, in the stratosphere, AOD values up to 0.29 were observed. Sharp changes of carbon monoxide (CO) and ozone (O3) concentrations were also recorded by the Copernicus Atmosphere Monitoring Service (CAMS) in various atmospheric heights over the study region, associated with fire smoke emissions. The tropospheric smoke layers were found to have a negative mean radiative effect, ranging from −12.83 W/m2 at the top of the atmosphere (TOA), to −32.22 W/m2 on the surface (SRF), while the radiative effect of the stratospheric smoke was estimated between −7.36 at the TOA to −18.51 W/m2 at the SRF.

The characterization of long-range transported North American biomass burning plumes: what can a multi-wavelength Mie–Raman-polarization-fluorescence lidar provide?

Abstract. This article presents a study of long-range transported biomass burning aerosols (BBA) originated from the North American wildfires in September 2020. The BBA plumes presented in this study were in the troposphere and underwent 1–2 weeks of aging before arriving at the lidar station ATOLL (ATmospheric Observatory of LiLle) in northern France. A novel lidar-derived dataset, 2α+3β+3δ+ϕ (α: extinction coefficient; β: backscatter coefficient; δ: particle linear depolarization ratio, i.e., PLDR; ϕ: fluorescence capacity), is provided for the characterization of BBA. The fluorescence capacity is an intensive aerosol parameter describing the ability of aerosols in producing fluorescence when exposed to UV excitation. In our BBA observations, obvious variations in aerosol intensive parameters, reflecting the variability of BBA properties, were detected. The PLDRs varied from less than 0.03 at all wavelengths to 0.15–0.22 and 0.12–0.16, respectively, at 355 and 532 nm. The extinction related Angström exponent was within the range of −0.3 to 1.0 and the fluorescence capacity was 1.0 × 10−4–4.0 × 10−4. Lidar ratio as low as 24 ± 4 sr (50 ± 8 sr) was observed in the BBA plumes at 355 (532) nm on 17–18 September, which was lower than most previously observed aged BBAs. These variations are likely correlated with the combustion process, the lifting of BBA plumes and the conditions (temperature, humidities, etc.) in the aging process. In addition, our results indicate BBA could act as ice nucleating particles in tropospheric conditions. The lidar fluorescence channel proves to be an important added value in aerosol characterization and aerosol–cloud interactions studies, due to its high sensitivity. With the increase in wildfire occurrence and intensity, BBAs become a more and more important atmospheric component. In this context, we show the potential of our novel lidar-derived dataset for aged BBA particles' characterization and for the understanding of their role in cloud processes.

Pollen observations at four EARLINET stations during the ACTRIS-COVID-19 campaign

Abstract. Lidar observations were analysed to characterize atmospheric pollen at four EARLINET (European Aerosol Research Lidar Network) stations (Hohenpeißenberg, Germany; Kuopio, Finland; Leipzig, Germany; and Warsaw, Poland) during the ACTRIS (Aerosol, Clouds and Trace Gases Research Infrastructure) COVID-19 campaign in May 2020. The reanalysis (fully quality-assured) lidar data products, after the centralized and automatic data processing with the Single Calculus Chain (SCC), were used in this study, focusing on particle backscatter coefficients at 355 and 532 nm and particle linear depolarization ratios (PDRs) at 532 nm. A novel method for the characterization of the pure pollen depolarization ratio was presented, based on the non-linear least square regression fitting using lidar-derived backscatter-related Ångström exponents (BAEs) and PDRs. Under the assumption that the BAE between 355 and 532 nm should be zero (±0.5) for pure pollen, the pollen depolarization ratios were estimated: for Kuopio and Warsaw stations, the pollen depolarization ratios at 532 nm were of 0.24 (0.19–0.28) during the birch-dominant pollen periods, whereas for Hohenpeißenberg and Leipzig stations, the pollen depolarization ratios of 0.21 (0.15–0.27) and 0.20 (0.15–0.25) were observed for periods of mixture of birch and grass pollen. The method was also applied for the aerosol classification, using two case examples from the campaign periods; the different pollen types (or pollen mixtures) were identified at Warsaw station, and dust and pollen were classified at Hohenpeißenberg station.