First triple-wavelength lidar observations of depolarization and extinction-to-backscatter ratios of Saharan dust

Two layers of Saharan dust observed over Leipzig, Germany, in February and March 2021 were used to provide the first-ever lidar measurements of the dust lidar ratio (extinction-to-backscatter ratio) and linear depolarization ratio at all three classical lidar wavelengths (355, 532 and 1064 nm). The pure-dust conditions during the first event exhibit lidar ratios of 47 ± 8, 50 ± 5 and 69 ± 14 sr and particle linear depolarization ratios of 0.242 ± 0.024, 0.299 ± 0.018 and 0.206 ± 0.010 at wavelengths of 355, 532 and 1064 nm, respectively. The second, slightly polluted-dust case shows a similar spectral behavior of the lidar and depolarization ratio with values of the lidar ratio of 49 ± 4, 46 ± 5 and 57 ± 9 sr and the depolarization ratio of 0.174 ± 0.041, 0.298 ± 0.016 and 0.242 ± 0.007 at 355, 532 and 1064 nm, respectively. The results were compared with Aerosol Robotic Network (AERONET) version 3 (v3) inversion solutions and the Generalized Retrieval of Aerosol and Surface Properties (GRASP) at six and seven wavelengths. Both retrieval schemes make use of a spheroid shape model for mineral dust. The spectral slope of the lidar ratio from 532 to 1064 nm could be well reproduced by the AERONET and GRASP retrieval schemes. Higher lidar ratios in the UV were retrieved by AERONET and GRASP. The enhancement was probably caused by the influence of fine-mode pollution particles in the boundary layer which are included in the columnar photometer measurements. Significant differences between the measured and retrieved wavelength dependence of the particle linear depolarization ratio were found. The potential sources for these uncertainties are discussed.

Depolarization Ratio of the ν1 Raman Band of Pure CH4 and Perturbed by N2 and CO2

In this work, the effect of nitrogen and carbon dioxide on the depolarization ratio of the ν1 band of methane in the pressure range of 0.1–5 MPa is studied. A high-sensitivity single-pass Raman spectrometer was used to obtain accurate results. Moreover, we took into account the overlap of the ν1 band by the ν3 and ν2 + ν4 bands using the simulation of their spectra. The depolarization ratio of the ν1 band in pure methane is within 0–0.001, and the effect of nitrogen and carbon dioxide on this parameter is negligible in the indicated pressure range. The obtained results are useful for correct simulation of the Raman spectrum of methane at different pressures, which is necessary to improve the accuracy of gas analysis methods using Raman spectroscopy.

Microphysical process of precipitating hydrometeors from warm-front mid-level stratiform clouds revealed by ground-based lidar observations

Mid-level stratiform precipitations during the passage of warm fronts were detailedly observed on two occasions (light and moderate rain) by a 355 nm polarization lidar and water vapor Raman lidar, both equipped with waterproof transparent roof windows. The hours-long precipitation streaks shown in the lidar signal (X) and volume depolarization ratio (δv) reveal some ubiquitous features of the microphysical process of precipitating hydrometeors. We find that for the light-rain case precipitation that reaches the surface begins as ice-phase-dominant hydrometeors that fall out of a shallow liquid cloud layer at altitudes above the 0 ∘C isotherm level, and the depolarization ratio magnitude of falling hydrometeors increases from the liquid-water values (δv<0.09) to the ice/snow values (δv>0.20) during the first 100–200 m of their descent. Subsequently, the falling hydrometeors yield a dense layer with an ice/snow bright band occurring above and a liquid-water bright band occurring below (separated by a lidar dark band) as a result of crossing the 0 ∘C level. The ice/snow bright band might be a manifestation of local hydrometeor accumulation. Most falling raindrops shrink or vanish in the liquid-water bright band due to evaporation, whereas a few large raindrops fall out of the layer. We also find that a prominent δv peak (0.10–0.40) always occurs at an altitude of approximately 0.6 km when precipitation reaches the surface, reflecting the collision–coalescence growth of falling large raindrops and their subsequent spontaneous breakup. The microphysical process (at ice-bright-band altitudes and below) of moderate rain resembles that of the light-rain case, but more large-sized hydrometeors are involved.

Light Scattering by Pure Water and Seawater: Recent Development

Light scattering by pure water and seawater is a fundamental optical property that plays a critical role in ocean optics and ocean color studies. We briefly review the theory of molecular scattering in liquid and electrolyte solutions and focus on the recent developments in modeling the effect of pressure, extending to extreme environments, and evaluating the effect of salinity on the depolarization ratio. We demonstrate how the modeling of seawater scattering can be applied to better understand spectral absorption and attenuation of pure water and seawater. We recommend future efforts should be directed at measuring the polarized components of scattering by pure water over a greater range of wavelengths, temperature, salinity, and pressure to constrain and validate the model and to improve our knowledge of the water’s depolarization ratio.

Biomass burning events measured by lidars in EARLINET – Part 2: Optical properties investigation

Biomass burning episodes measured at 14 stations of the European Aerosol Research Lidar Network (EARLINET) over 2008–2017 were analysed using the methodology described in "Biomass burning events measured by lidars in EARLINET – Part 1: Data analysis methodology" (Adam et al., 2020, this issue). The smoke layers were identified in lidar optical properties profiles. A number of 795 layers for which we measured at least one intensive parameter was analysed. These layers were geographically distributed as follows: 399 layers observed in South-East Europe, 119 layers observed in South-West Europe, 243 layers observed in North-East Europe, and 34 layers observed in Central Europe. The mean layer intensive parameters are discussed following two research directions: (I) the long-range transport of smoke particles from North America, and (II) the smoke properties (fresh versus aged), separating the smoke events into four continental source regions (European, North American, African, Asian or a mixture of two), based on back trajectory analysis. The smoke detected in Central Europe (Cabauw, Leipzig, and Hohenpeißenberg) was mostly transported from North America (87 % of fires). In North-East Europe (Belsk, Minsk, Warsaw) smoke advected mostly from Eastern Europe (Ukraine and Russia), but there was a significant contribution (31 %) from North America. In South-West Europe (Barcelona, Evora, Granada) smoke originated mainly from the Iberian Peninsula and North Africa (while 9 % were originating in North America). In the South-East Europe (Athens, Bucharest, Potenza, Sofia, Thessaloniki) the origin of the smoke was mostly local (only 3 % represented North America smoke). The following features, correlated with the increased smoke travel time (corresponding to aging) were found: the colour ratio of the lidar ratio (i.e., the ratio of the lidar ratio at 532 nm to the lidar ratio at 355 nm) and the colour ratio of the backscatter Ångström exponent (i.e., the ratio of the backscatter-related Angstrom exponent for the pair 532 nm – 1064 nm to the one for the pair 355 nm – 532 nm) increase, while the extinction Ångström exponent and the colour ratio of the particle depolarization ratio (i.e., the ratio of the particle linear depolarization ratio at 532 nm to the particle depolarization ratio at 355 nm) decrease. The smoke originating from all continental regions can be characterized on average as aged smoke, with a very few exceptions. In general, the long range transported smoke shows higher lidar ratio and lower depolarization ratio compared to the local smoke.

Characterization of atmospheric pollen with active remote sensing

Atmospheric pollen is a well-known health threat causing allergy-related diseases. As a biogenic aerosol, pollen also affects the climate by directly absorbing and scattering solar radiation and by acting as cloud condensation or ice nuclei. A good understanding of pollen distribution and transport mechanisms is needed to evaluate the environmental and health impacts of pollen. However, pollen observations are usually performed close to ground and vertical information, which could be used to evaluate and improve pollen transport models, is widely missing. In this thesis, the applicability of lidar measurements to detect pollen in the atmosphere is investigated. For this purpose, measurements of the multiwavelength Raman polarization lidar PollyXT at the rural forest site in Vehmasmäki (Kuopio), Eastern Finland have been utilized. The depolarization ratio was identified to be the most valuable optical property for the detection of atmospheric pollen, as nonspherical pollen like pine and spruce pollen causes high depolarization ratios. However, detected depolarization ratios coincide with typical values for dusty mixtures and additional information such as backward trajectories need to be considered to ensure the absence of other depolarizing aerosols like dust. To separate pollen from background aerosol, a method to estimate the optical properties of pure pollen using lidar measurements was developed. Under the assumption that the Ångström exponent of pure pollen is zero, the depolarization ratio of pure pollen can be estimated. Depolarization ratios for birch and pine pollen at 355 and 532 nm were determined and suggested a wavelength dependence of the depolarization ratio. To further investigate this wavelength dependence, the possibility to use depolarization measurements of Halo Doppler lidars (1565 nm) was explored. In the lower troposphere, Halo Doppler lidars can provide reasonable depolarization values with comparable quality to PollyXT measurements. Finally, measurements of PollyXT and a Halo StreamLine Doppler lidar were used to determine the depolarization ratio at three wavelengths. A wavelength dependence of the particle depolarization ratio with maximum depolarization at 532 nm was found. This could be a characteristic feature of non-spherical pollen and the key to distinguish pollen from other depolarizing aerosol types.

Investigation of a Saharan dust plume in Western Europe by remote sensing and transport modelling

The evolution and the properties of a Saharan dust plume were studied near the city of Karlsruhe in south-west Germany (8.4298° E, 49.0953° N) from April 7 to 9, 2018 combining a scanning LIDAR (90°, 30°), a vertical LIDAR (90°), a sun photometer, and the transport model ICON-ART. The LIDAR measurements show that the dust particles had backscatter coefficients of 0.86 ± 0.14 Mm−1 Sr−1, an extinction coefficient of 40 ± 0.8 Mm−1, a LIDAR ratio of 46 ± 5 sr, and a particle depolarization ratio of 0.33 ± 0.07. These values are in good agreement with those obtained in previous studies of Saharan dust plumes in Western Europe. Compared to the remote sensing measurements, the model simulation predicts the plume arrival time, its layer height, and structure very well but overestimates the backscatter coefficient. In this manuscript, we discuss the complementarity and advantages of the different measurement methods as well model simulations to predict Saharan dust plumes. Main conclusions are that the ICON-ART model can predict the structure of Saharan dust plumes very well but overestimates the backscatter coefficients by a factor of 2.2 ± 0.16 at 355 nm and underestimates the aerosol optical depth (AOD) by a factor of 1.5 ± 0.11 at 340 nm for this Saharan dust plume event. Employing a scanning aerosol LIDAR allows determining backscatter coefficient, particle depolarization ratio and especially LIDAR ratio of Saharan dust both for daytime and nighttime independently. Combining LIDAR with sun photometer data allows constraining aerosol optical depth in different ways and determining column integrated LIDAR ratios. These comprehensive datasets allow for a better understanding of Saharan dust plumes in Western Europe.

First triple-wavelength lidar observations of depolarization and extinction-to-backscatter ratios of Saharan dust

Two Saharan dust layers observed over Leipzig in February and March 2021 were used to provide the first ever lidar measurements of the extinction coefficient at 1064 nm for desert dust. The advanced multiwavelength Raman polarization lidar was able to provide, for the first time, the lidar ratio (extinction-to-backscatter ratio) and particle linear depolarization ratio at all three classical lidar wavelengths (355, 532 and 1064 nm). The pure dust conditions during the first event exhibit lidar ratios of 47±8, 50±5 and 63±13 sr and particle linear depolarization ratios of 0.260±0.026, 0.298±0.017 and 0.214±0.025 at the wavelengths of 355, 532 and 1064 nm, respectively. The second, slightly polluted dust case shows a similar spectral behavior with values of the lidar ratio of 52±8, 47±5 and 61±10 sr and the depolarization ratio of 0.188±0.053, 0.270±0.017 and 0.242±0.007 at 355, 532 and 1064 nm, respectively. The results were compared to AERONET v3 inversions and GRASP retrievals at six and seven wavelengths, which could reproduce the spectral slope of the lidar ratio from 532 to 1064 nm. The spectral slope of the particle linear depolarization ratio could not be reproduced by the AERONET inversions, especially at 1064 nm.

Experimental assessment of a micro-pulse lidar system in comparison with reference lidar measurements for aerosol optical properties retrieval

Simultaneous observations of a polarized micro-pulse lidar (P-MPL) system and two reference European Aerosol Research Lidar Network lidars running at the Leipzig site Germany, 51.4∘ N, 12.4∘ E; 125 m a.s.l.) were performed during a comprehensive 2-month field intercomparison campaign in summer 2019. An experimental assessment regarding both the overlap (OVP) correction of the P-MPL signal profiles and the volume linear depolarization ratio (VLDR) analysis, together with its impact on the retrieval of the aerosol optical properties, is achieved; the experimental procedure used is also described. The optimal lidar-specific OVP function is experimentally determined, highlighting that the one delivered by the P-MPL manufacturer cannot be used long. Among the OVP functions examined, the averaged function between those obtained from the comparison of the P-MPL observations with those of the other two reference lidars seems to be the best proxy at both near- and far-field ranges. In addition, the impact of the OVP function on the accuracy of the retrieved profiles of the total particle backscatter coefficient (PBC) and the particle linear depolarization ratio (PLDR) is examined. The VLDR profile is obtained and compared with that derived from the reference lidar, showing that it needs to be corrected by a small offset value with good accuracy. Once P-MPL measurements are optimally (OVP, VLDR) corrected, both the PBC and PLDR profiles can be accurately derived and are in good agreement with reference aerosol retrievals. Overall, as a systematic requirement for lidar systems, an adequate OVP function determination and VLDR testing analysis needs to be performed on a regular basis to correct the P-MPL measurements in order to derive suitable aerosol products. A dust event observed in Leipzig in June 2019 is used for illustration.