Polarization Lidar Research Articles

The vertical aerosol type distribution above Israel – 2 years of lidar observations at the coastal city of Haifa

Abstract. For the first time, vertically resolved long-term lidar measurements of the aerosol distribution were conducted in Haifa, Israel. The measurements were performed by a PollyXT multi–wavelength Raman and polarization lidar. The lidar was measuring continuously over a 2-year period from March 2017 to May 2019. The resulting data set is a series of manually evaluated lidar optical property profiles. To identify the aerosol types in the observed layers, a novel aerosol typing method that was developed at TROPOS is used. This method applies optimal estimation to a combination of lidar-derived intensive aerosol properties to determine the statistically most-likely contribution per aerosol component in terms of relative volume. A case study that shows several elevated aerosol layers illustrates this method and shows, for example, that coarse dust particles are observed up to 5 km height over Israel. From the whole data set, the seasonal distribution of the observed aerosol components over Israel is derived. Throughout all seasons, coarse spherical particles like sea salt and hygroscopically grown continental aerosol were observed. These particles originate from continental Europe and were transported over the Mediterranean Sea. Sea-salt particles were observed frequently due to the coastal site of Haifa. The highest contributions of coarse spherical particles are present in summer, autumn, and winter. During spring, mostly coarse non-spherical particles that are attributed to desert dust were observed. This is consistent with the distinct dust season in spring in Israel. An automated time–height-resolved air mass source attribution method identifies the origin of the dust in the Sahara and the Arabian deserts. Fine-mode spherical particles contribute significantly to the observed aerosol mixture during all seasons. These particles originate mainly from the industrial region at the bay of Haifa.

Polarization Lidar Measurements of Dust Optical Properties at the Junction of the Taklimakan Desert–Tibetan Plateau

Previous studies have shown that dust aerosols may accelerate the melting of snow and glaciers over the Tibetan Plateau. To investigate the vertical structure of dust aerosols, we conducted a ground-based observation by using multi-wavelength polarization lidar which is designed for continuous network measurements. In this study, we used the lidar observation from September to October 2020 at the Ruoqiang site (39.0°N, 88.2°E; 894 m ASL), located at the junction of the Taklimakan Desert–Tibetan Plateau. Our results showed that dust aerosols can be lifted up to 5 km from the ground, which is comparable with the elevation of the Tibetan Plateau in autumn with a mass concentration of 400–900 μg m−3. Moreover, the particle depolarization ratio (PDR) of the lifted dust aerosols at 532 nm and 355 nm are 0.34 ± 0.03 and 0.25 ± 0.04, respectively, indicating the high degree of non-sphericity in shape. In addition, extinction-related Ångström exponents are very small (0.11 ± 0.24), implying the large values in size. Based on ground-based lidar observation, this study proved that coarse non-spherical Taklimakan dust with high concentration can be transported to the Tibetan Plateau, suggesting its possible impacts on the regional climate and ecosystem.

Polarization Aberration Correction for Telescopic System in Polarization Lidar

Polarization Aberration Correction for Telescopic System in Polarization Lidar

Polarization lidar for detecting dust orientation: system design and calibration

Abstract. Dust orientation has been an ongoing investigation in recent years. Its potential proof will be a paradigm shift for dust remote sensing, invalidating the currently used simplifications of randomly oriented particles. Vertically resolved measurements of dust orientation can be acquired with a polarization lidar designed to target the off-diagonal elements of the backscatter matrix which are nonzero only when the particles are oriented. Building on previous studies, we constructed a lidar system emitting linearly and elliptically polarized light at 1064 nm and detecting the linear and circular polarization of the backscattered light. Its measurements provide direct flags of dust orientation, as well as more detailed information of the particle microphysics. The system also has the capability to acquire measurements at varying viewing angles. Moreover, in order to achieve good signal-to-noise ratio in short measurement times, the system is equipped with two laser sources emitting in an interleaved fashion and two telescopes for detecting the backscattered light from both lasers. Herein we provide a description of the optical and mechanical parts of this new lidar system, the scientific and technical objectives of its design, and the calibration methodologies tailored for the measurements of oriented dust particles. We also provide the first, preliminary measurements of the system during a dust-free day. The work presented does not include the detection of oriented dust (or other oriented particles), and therefore the instrument has not been tested fully in this objective.

Mass concentration estimates of long-range-transported Canadian biomass burning aerosols from a multi-wavelength Raman polarization lidar and a ceilometer in Finland

Abstract. A quantitative comparison study for Raman lidar and ceilometer observations, and for model simulations of mass concentration estimates of smoke particles is presented. Layers of biomass burning aerosol particles were observed in the lower troposphere, at 2 to 5 km height on 4 to 6 June 2019, over Kuopio, Finland. These long-range-transported smoke particles originated from a Canadian wildfire event. The most pronounced smoke plume detected on 5 June was intensively investigated. Optical properties were retrieved from the multi-wavelength Raman polarization lidar PollyXT. Particle linear depolarization ratios (PDRs) of this plume were measured to be 0.08±0.02 at 355 nm and 0.05±0.01 at 532 nm, suggesting the presence of partly coated soot particles or particles that have mixed with a small amount of dust or other non-spherical aerosol type. The layer-mean PDR at 355 nm (532 nm) decreased during the day from ∼0.11 (0.06) in the morning to ∼0.05 (0.04) in the evening; this decrease with time could be linked to the particle aging and related changes in the smoke particle shape properties. Lidar ratios were derived as 47±5 sr at 355 nm and 71±5 sr at 532 nm. A complete ceilometer data processing for a Vaisala CL51 ceilometer is presented from a sensor-provided attenuated backscatter coefficient to particle mass concentration (including the water vapor correction for high latitude for the first time). Aerosol backscatter coefficients (BSCs) were measured at four wavelengths (355, 532, 1064 nm from PollyXT and 910 nm from CL51). Two methods, based on a combined lidar and sun-photometer approach, are applied for mass concentration estimations from both PollyXT and the ceilometer CL51 observations. In the first method, no. 1, we used converted BSCs at 532 nm (from measured BSCs) by corresponding measured backscatter-related Ångström exponents, whereas in the second method, no. 2, we used measured BSCs at each wavelength independently. A difference of ∼12 % or ∼36 % was found between PollyXT and CL51 estimated mass concentrations using method no. 1 or no. 2, showing the potential of mass concentration estimates from a ceilometer. Ceilometer estimations have an uncertainty of ∼50 % in the mass retrieval, but the potential of the data lies in the great spatial coverage of these instruments. The mass retrievals were compared with the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) meteorological and aerosol reanalysis. The inclusion of dust (as indicated by MERRA-2 data) in the retrieved mass concentration is negligible considering the uncertainties, which also shows that ceilometer observations for mass retrievals can be used even without exact knowledge of the composition of the smoke-dominated aerosol plume in the troposphere.

Wildfire smoke, Arctic haze, and aerosol effects on mixed-phase and cirrus clouds over the North Pole region during MOSAiC: an introduction

Abstract. An advanced multiwavelength polarization Raman lidar was operated aboard the icebreaker Polarstern during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition to continuously monitor aerosol and cloud layers in the central Arctic up to 30 km height. The expedition lasted from September 2019 to October 2020 and measurements were mostly taken between 85 and 88.5∘ N. The lidar was integrated into a complex remote-sensing infrastructure aboard the Polarstern. In this article, novel lidar techniques, innovative concepts to study aerosol–cloud interaction in the Arctic, and unique MOSAiC findings will be presented. The highlight of the lidar measurements was the detection of a 10 km deep wildfire smoke layer over the North Pole region between 7–8 km and 17–18 km height with an aerosol optical thickness (AOT) at 532 nm of around 0.1 (in October–November 2019) and 0.05 from December to March. The dual-wavelength Raman lidar technique allowed us to unambiguously identify smoke as the dominating aerosol type in the aerosol layer in the upper troposphere and lower stratosphere (UTLS). An additional contribution to the 532 nm AOT by volcanic sulfate aerosol (Raikoke eruption) was estimated to always be lower than 15 %. The optical and microphysical properties of the UTLS smoke layer are presented in an accompanying paper (Ohneiser et al., 2021). This smoke event offered the unique opportunity to study the influence of organic aerosol particles (serving as ice-nucleating particles, INPs) on cirrus formation in the upper troposphere. An example of a closure study is presented to explain our concept of investigating aerosol–cloud interaction in this field. The smoke particles were obviously able to control the evolution of the cirrus system and caused low ice crystal number concentration. After the discussion of two typical Arctic haze events, we present a case study of the evolution of a long-lasting mixed-phase cloud layer embedded in Arctic haze in the free troposphere. The recently introduced dual-field-of-view polarization lidar technique was applied, for the first time, to mixed-phase cloud observations in order to determine the microphysical properties of the water droplets. The mixed-phase cloud closure experiment (based on combined lidar and radar observations) indicated that the observed aerosol levels controlled the number concentrations of nucleated droplets and ice crystals.

Retrievals of dust-related particle mass and ice-nucleating particle concentration profiles with ground-based polarization lidar and sun photometer over a megacity in central China

Abstract. The POLIPHON (polarization lidar photometer networking) method is a powerful pathway to retrieve the height profiles of dust-related particle mass and ice-nucleating particle (INP) concentrations. The conversion factors fitted from the sun photometer observation data are the major part of the POLIPHON computations, which can convert the polarization-lidar-derived dust extinction coefficients into dust-related particle mass and INP concentrations. For the central Chinese megacity of Wuhan (30.5∘ N, 114.4∘ E), located at the downstream area several thousands of kilometers far away from the source regions of Asian dust, dust particles always mix with other aerosols from local emissions. Therefore, very few dust case data sets can be available when using the column-integrated Ångström exponent (for 440–870 nm) <0.3 and aerosol optical depth (at 532 nm) >0.1 recorded by a sun photometer as the filtering criteria. Instead, we present another dust case data set screening scheme that applies the simultaneous polarization lidar observation to verify the occurrence of dust. Based on the 33 dust-intrusion days identified during 2011–2013, the extinction-to-volume (cv,d) and extinction-to-large particle (with radius >250 nm) number concentration (c250,d) conversion factors are determined to be (0.52±0.12)×10-12Mmm3m-3 and 0.19±0.05 Mm cm−3, respectively. The c250,d for Wuhan is 27 % larger than that observed at Lanzhou SACOL (36.0∘ N, 104.1∘ E), a site closer to the Gobi Desert, and tends to be closer to those observed in North Africa and the Middle East, indicating dust aerosols from these two sources are also possibly involved in the dust events observed over Wuhan. As a comparison, the conversion factor c290,c of 0.11±0.02Mmcm-3 for continental aerosol is much smaller than c250,d, indicating that there is no significant influence of urban aerosols on the retrievals of dust-related conversion factor over Wuhan. The conversion factors are applied in a dust event in Wuhan to reveal the typical dust-related immersion-mode INP concentration over East Asian cities. The proposed dust case data set screening scheme may potentially be extended to the other polluted city sites that are more influenced by mixed dust.

A prolonged and widespread thin mid-level liquid cloud layer as observed by ground-based lidars, radiosonde and space-borne instruments

A ~ 50-h-lasting mid-level thin liquid cloud layer was present over south and central China on December 13–15, 2018. Its characteristics were revealed by the observations from a polarization lidar and a water vapor Raman lidar at a mid-latitude site together with two geostationary satellites and the space-borne lidar CALIOP, as well as the conventional radiosondes. This layer was optically thick with cloud top at ~3.5-km altitude and a thickness of ~0.7 km. It resided on a warm-front-related inversion layer with the cloud top extending slightly above the inversion top and having a temperature of ~ − 4 to 1 °C. We found that the relative humidity over water peaked (~82–98%) near the liquid cloud center where the inversion layer had the temperature minima (−7 to −5 °C). This suggests that the moisture condensation into liquid droplets proceeded around the temperature minima of the inversion layer. The drizzle-sized water drop virga (no ice) occurred beneath the thin liquid cloud layer only during the first 7 h of its presence over our lidar site. The warm cloud top temperature (−3 °C) and strong wind shear at the cloud altitudes might be responsible for its formation. The Raman lidar observed a discernible increase in the subcloud water vapor mixing ratio due to the evaporation of the water drop virga. Satellite data revealed that the thin cloud layer covered a huge area over southern and central China. We found that its cloud top altitude increased overall with increasing latitude. This is consistent with the well-known warm-front cloud structure. It is shown through examining ECMWF reanalysis data that in the presence of the thin liquid cloud layer, a tropical cyclone located in the Bengal bay was uplifting continuously a large amount of moisture from the Indian Ocean. The resultant mid-level warm moist air masses were transported to China by the Indian monsoon, and then dispersed to the entire southern and central China under the influence of the southern branch of the Westerlies. Our findings provide a new insight into the formation and persistence of thin mid-level supercooled liquid cloud layers.

Classification of atmospheric aerosols and clouds by use of dual-polarization lidar measurements.

Accurate identification of aerosols and cloud from remote sensing observations is of importance for quantitatively evaluating their radiative forcing and related impacts. Even though polarization lidar has exhibited a unique advantage of classifying atmospheric aerosols and clouds over the past several decades, polarization measurements are often achieved at one wavelength (UV or VIS) using laser remote sensing. To better identify the types of aerosols and clouds, we developed a ground-based dual-polarization lidar system that can simultaneously detect polarization measurements at wavelengths of 355 nm and 532 nm. Our results show that the volume depolarization ratios (VDRs) at 355 nm and 532 nm markedly differ for typical types of aerosols and clouds in the atmosphere. For non-spherical particles, the ratio of VDRs at 532 nm and 355 nm are 2.87 ± 1.35 for ice cloud and 1.51 ± 0.29 for dust-dominated aerosols, respectively. However, for spherical particles, the ratios are 0.43 ± 0.26 for water cloud and 0.56 ± 0.05 for air pollutants. Consequently, we proposed a simple reliable method for classifying atmospheric aerosols and clouds from polarization measurements observed by the developed lidar system. The proposed method first distinguishes clouds from aerosols using a combination of the color ratio (CR, 532 nm/355 nm) and attenuated backscattering coefficients (ABC) at 532 nm. Then, subtypes of clouds and aerosols are identified based on the ratio of VDRs at 532 nm and 355 nm. The results showed that dual-polarization lidar measurements can remarkably improve the classification of atmospheric aerosols and clouds, compared with results using a traditional method. This study illustrates that more information on atmospheric aerosols and clouds can be obtained from polarization measurements at multiple wavelengths by active remote sensing.

Long-term variations of aerosol optical properties over Wuhan with polarization lidar

Aerosols greatly impact the global and regional climate system and public health. We conducted lidar measurements at Wuhan (30.53°N, 114.37°E) from October 2010 to June 2020 and analyzed the massive dataset with an automatic lidar processing algorithm, to investigate the long-term variations of aerosols. Lidar-derived total aerosol optical depth (AOD) at 532 nm shows an interannual decrease of −0.049 since 2010. Compared with AODs in other megacities worldwide, the AOD for Wuhan shows the largest decrease rate. After separating AOD contributions from boundary layer (BL) and free troposphere (FT), different evolution patterns are revealed with AOD decrease rates of −0.009 and −0.044 per year for FT and BL, respectively, which indicates more than 80% of AOD decrease are associated with reductions of local emissions. In addition, BL AOD was always larger than FT AOD before COVID-19 lockdown. However, this relation was conversed thereafter, with the lowest value of 0.125 for BL AOD since 2010, which is only the half of FT AOD within the same period. This feature corroborates strong influences of COVID-19 lockdown on human activities and local emissions. These findings can improve our understandings of regional aerosol responses to pollution controls and aerosol direct impacts on regional climate.

Polarization aberrations corrections and polarization-dependent imaging quality analysis in polarization lidars

The polarization aberration in lidar system can reduce the imaging quality and causes the atmospheric polarization measurement error. In order to accurate polarization signal measurement, the polarization aberration correction of system is necessary. In this paper, the physical properties of Jones pupil function are analyzed and the polarization-dependent point spread function is derived for the polarization lidar. A high-reflection dielectric multilayer coating with low polarization effect is designed to correct the polarization aberration of Cassegrain telescope in lidar. The simulation shows that the polarization aberration of telescope coated with multilayer coatings are significantly smaller than that coated with aluminum coating. Furthermore, the imaging quality performance of telescope coated with multilayer coatings is analyzed, and the relative error of atmosphere parameter is calibrated based on the imaging information of two receiver channels in linear or circular polarization lidar. The correction method can reduce the polarization aberration and improve the measurement accuracy in polarization lidar.

Observation of clouds, aerosols, and precipitation by multiple-field-of-view multiple-scattering polarization lidar at 355 nm

We developed a multiple-field-of-view multiple-scattering polarization lidar at 355 nm (MFMSPL-355) to study the microphysics of clouds and aerosols, and understand the multiple scattering effects on space lidar measurements. The MFMSPL-355 is used to simulate and interpret multiple scattering signals by the 355 nm lidar installed on the Earth Clouds, Aerosol and Radiation Explorer satellite. This is the first multiple scattering lidar developed at 355 nm in the world. The system has five receiver modules and can observe both parallel and perpendicular attenuated backscatter coefficients for the on-beam direction and four off-beam directions with different tilting angles. Thus, the MFMSPL-355 can measure the single scattering signal and multiple scattering signal independently; accordingly, it can provide microphysical properties, such as the effective radius, of clouds and aerosols. We demonstrated that the MFMSPL-355 could observe multiple scattering signals for water clouds, ice clouds, aerosols, and drizzle. The observed features of the water clouds and drizzle were consistent with those observed by a previously developed 532-nm MFMSPL. Furthermore, we found that aerosol signals and their multiple scattering contributions were generally more pronounced at 355 nm, which is the advantage of performing measurements at 355 nm. Off-beam channels can offer a unique opportunity to study aerosol microphysics.

Dual-Wavelength Polarimetric Lidar Observations of the Volcanic Ash Cloud Produced during the 2016 Etna Eruption

Lidar observations are very useful to analyse dispersed volcanic clouds in the troposphere mainly because of their high range resolution, providing morphological as well as microphysical (size and mass) properties. In this work, we analyse the volcanic cloud of 18 May 2016 at Mt. Etna, in Italy, retrieved by polarimetric dual-wavelength Lidar measurements. We use the AMPLE (Aerosol Multi-Wavelength Polarization Lidar Experiment) system, located in Catania, about 25 km from the Etna summit craters, pointing at a thin volcanic cloud layer, clearly visible and dispersed from the summit craters at the altitude between 2 and 4 km and 6 and 7 km above the sea level. Both the backscattering and linear depolarization profiles at 355 nm (UV, ultraviolet) and 532 nm (VIS, visible) wavelengths, respectively, were obtained using different angles at 20°, 30°, 40° and 90°. The proposed approach inverts the Lidar measurements with a physically based inversion methodology named Volcanic Ash Lidar Retrieval (VALR), based on Maximum-Likelihood (ML). VALRML can provide estimates of volcanic ash mean size and mass concentration at a resolution of few tens of meters. We also compared those results with two methods: Single-variate Regression (SR) and Multi-variate Regression (MR). SR uses the backscattering coefficient or backscattering and depolarization coefficients of one wavelength (UV or VIS in our cases). The MR method uses the backscattering coefficient of both wavelengths (UV and VIS). In absence of in situ airborne validation data, the discrepancy among the different retrieval techniques is estimated with respect to the VALR ML algorithm. The VALR ML analysis provides ash concentrations between about 0.1 μg/m3 and 1 mg/m3 and particle mean sizes of 0.1 μm and 6 μm, respectively. Results show that, for the SR method differences are less than <10%, using the backscattering coefficient only and backscattering and depolarization coefficients. Moreover, we find differences of 20–30% respect to VALR ML, considering well-known parametric retrieval methods. VALR algorithms show how a physics-based inversion approaches can effectively exploit the spectral-polarimetric Lidar AMPLE capability.

Convective Boundary Layer Clouds as Observed with Ground-Based Lidar at a Mid-Latitude Plain Site

A total of 3047 individual shallow cumuli were identified from 9 years of polarization lidar measurements (2011–2019) at Wuhan, China (30.5°N, 114.4°E). These fair-weather shallow cumuli occurred at the top edge of the convective boundary layer between April and October with the maximum occurrence in July over the 30°N plain site. They persisted mostly (>92%) for a short period of ~1–10 min and had a geometrical thickness of ~50–600 m (a mean of 209 ± 138 m). The majority (>94%) of the cloud bases of these cumuli were found to appear ~50–560 m (a mean of 308 ± 254 m) above the lifting condensation level (LCL). In this height range from the LCL to the cloud base, the lidar volume depolarization ratio (δδV) slightly decreased with increasing height, showing gradually increasing condensation in this sub-cloud region due to penetrative thermals. Most of the observed shallow cumuli (79%) formed under the conditions of high near-surface air temperature (>30 °C) and water vapor mixing ratio (>15 g kg−1).

Horizontally oriented ice crystals observed by the synergy of zenith- and slant-pointed polarization lidar over Wuhan (30.5°N, 114.4°E), China

From November 2019 to June 2020, horizontally oriented ice crystal (HOIC) layers were observed on 32 occasions using the synergy of zenith- and slant-pointed polarization lidar over Wuhan (30.5°N, 114.4°E), China. The HOIC layers appeared at altitudes of 2.5–10.5 km. 20 out of 32 cases showed a specific period that HOIC layers maintained in the atmosphere without an evident sign of orientation breakup, revealing their potential non-negligible impact on the radiation budget. The duration time ranged from 0.3 to 8.5 h with a mean of 3.5 h. HOIC layers occurred mostly at temperatures ranging from -25 to -5 °C and rarely presented at temperatures lower than -30 °C. Meanwhile, they always existed accompanying with light wind of 0–20 m/s. According to the lidar-derived crystal fall streaks, the terminal velocities and corresponding crystal diameters were estimated for 12 cases to be 0.28–1.0 m/s and 464–1305 µm, respectively. Associated Reynolds numbers for the HOIC layers concentrated in a range of 4–58. The observed events can be classified into two categories: (1) stratiform HOIC layer (22 cases), associated with the pristine crystals falling out from the liquid top of mixed-phase clouds which has been widely known; (2) continuously descending HOIC layer (10 cases), which is seldomly reported to our knowledge. For the latter pattern, once the crystals drop to the altitudes with more favorable aerodynamic conditions (i.e. appropriate temperature and adequately low wind speed), these early-formed ice crystals can also be preferred to align horizontally.

Impacts of laser beam divergence on lidar multiple scattering polarization returns from water clouds

Theoretically, laser beam divergence can redistribute multiple scattering lights by spreading laser illumination and thus alter lidar polarization measurements. To study multiple scattering effects of laser divergence on lidar polarization signals, we apply the Monte Carlo polarized radiative transfer model MSCART to simulate lidar Stokes vector signals from a uniform water cloud with and without taking account of laser divergences. The comparison analysis shows that for lidar receiver footprints with roughly the same sizes of the multiple scattering regimes, laser divergence has almost no effect on ground-based lidar signals, but can have significantly large and much complex polarization effects on spaceborne lidar signals. An increase in laser divergence on one hand can greatly enhance spaceborne lidar multiple scattering depolarization when the divergence is larger than the FOV, and on the other hand can also weaken it slightly when the divergence less than the FOV. The weakest multiple scattering depolarization occurs at the divergence equal to the FOV. Furthermore, laser divergence can significantly reduce the sensitivity of FOV-resolved polarization measurement of spaceborne lidar to non-diagonal elements of phase matrix and make the measurement at different receiving polar angles only sensitive to diagonal elements of phase matrix at almost the same scattering angles. The spaceborne MFOV and CCD polarized lidar thus might not provide more information about phase matrix than the single FOV lidar does.

Measurement report: characteristics of clear-day convective boundary layer and associated entrainment zone as observed by a ground-based polarization lidar over Wuhan (30.5° N, 114.4° E)

Abstract. Knowledge of the convective boundary layer (CBL) and associated entrainment zone (EZ) is important for understanding land–atmosphere interactions and assessing the living conditions in the biosphere. A tilted 532 nm polarization lidar (30∘ off zenith) has been used for the routine atmospheric measurements with 10 s time and 6.5 m height resolution over Wuhan (30.5∘ N, 114.4∘ E). From lidar-retrieved aerosol backscatter, instantaneous atmospheric boundary layer (ABL) depths are obtained using the logarithm gradient method and Harr wavelet transform method, while hourly mean ABL depths are obtained using the variance method. A new approach utilizing the full width at half maximum of the variance profile of aerosol backscatter ratio fluctuations is proposed to determine the entrainment zone thickness (EZT). Four typical clear-day observational cases in different seasons are presented. The CBL evolution is described and studied in four developing stages (formation, growth, quasi-stationary and decay); the instantaneous CBL depths exhibited different fluctuation magnitudes in the four stages and fluctuations at the growth stage were generally larger. The EZT is investigated for the same statistical time interval of 09:00–19:00 LT. It is found that the winter and late autumn cases had an overall smaller mean (mean) and standard deviation (SD) of EZT data compared to those of the late spring and early autumn cases. This statistical conclusion was also true for each of the four developing stages. In addition, compared to those of the late spring and early autumn cases, the winter and late autumn cases had larger percentages of EZT falling into the subranges of 0–50 m but smaller percentages of EZT falling into the subranges of > 150 m. It seems that both the EZT statistics (mean and SD) and percentage of larger EZT values provide measures of entrainment intensity. Common statistical characteristics also existed. All four cases showed moderate variations of the mean of the EZT from stage to stage. The growth stage always had the largest mean and SD of the EZT and the quasi-stationary stage usually the smallest SD of the EZT. For all four stages, most EZT values fell into the 50–150 m subrange; the overall percentage of the EZT falling into the 50–150 m subrange between 09:00 and 19:00 LT was > 67 % for all four cases. We believe that the lidar-derived characteristics of the clear-day CBL and associated EZ can contribute to improving our understanding of the structures and variations of the CBL as well as providing a quantitatively observational basis for EZ parameterization in numerical models.

Evolution of Aerosols in the Atmospheric Boundary Layer and Elevated Layers during a Severe, Persistent Haze Episode in a Central China Megacity

Aerosol vertical profiling is crucial to understand the formation mechanism and evolution processes of haze, which have not yet been comprehensively clarified. In this study, we investigated a severe, persistent haze event in Wuhan (30.5° N, 114.4° E), China during 5–18 January 2013 by the use of a polarization lidar, a Cimel sun photometer, meteorological datasets, and the hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) model, focusing on the time–height evolution of aerosols in both the atmospheric boundary layer (ABL) and elevated layers. During the haze period, the integrated particle depolarization ratio was 0.05 ± 0.02, and the fine mode fraction reached 0.91 ± 0.03, indicating haze particles were rather spherical and predominately submicron, that is, of anthropogenic nature. Compared with the clear period, columnar aerosol optical depth at 500 nm tripled to 1.32 ± 0.31, and the strongest enhancement in aerosol concentration occurred from near the ground to an altitude of 1.2 km during the haze period. The daytime evolution of aerosol vertical distribution in the ABL exhibited a distinct pattern under haze weather. Abundant particles accumulated below 0.5 km in the morning hours due to stable meteorological conditions, including a strong surface-based inversion (4.4–8.1 °C), late development (from 1000–1100 LT) of the convective boundary layer, and weak wind (<4 m∙s−1) in the lowermost troposphere. In the afternoon, improved ventilation delivered an overall reduction in boundary layer aerosols but was insufficient to eliminate haze. Particularly, the morning residual layer had an optical depth of 0.29–0.56. It influenced air quality indirectly by weakening convective activities in the morning and directly through the fumigation process around noon, suggesting it may be an important element in aerosol–ABL interactions during consecutive days with haze. Our lidar also captured the presence of the elevated aerosol layers (EALs) embodying regional/long-range transport. Most of the EALs were observed to subside to <1.2 km and exacerbate the pollution level. Backward trajectory analysis and lidar data revealed the EALs originated from the transport of anthropogenic pollutants from the Sichuan Basin, China, and of dust from the deserts in the northwest. They were estimated to contribute ~19% of columnar aerosol-loading, pointing to a non-negligible role of transport during the intense pollution episode. The results could benefit the complete understanding of aerosol–ABL interactions under haze weather and air quality forecasting and control in Wuhan.

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.

Observational study of the vertical aerosol and meteorological factor distributions with respect to particulate pollution in Xi'an

To understand the characteristics of atmospheric pollution in Xi'an, China, the vertical structure of aerosols and meteorological data (temperature and relative humidity) during atmospheric pollution processes were obtained and studied. Lidar and radiosonde data were used for analyses of atmospheric pollution. Polarization lidar was used to study the vertical distribution and optical properties of aerosols during haze and dust processes that occurred in 2015–2016. The planetary boundary layer height (PBLH) and types of aerosol particles were retrieved from polarization lidar. A case study shows that a low PBLH and complex vertical distributions of aerosols, temperature and relative humidity (RH) were the main structural characteristics of haze days. The backscatter coefficient exceeded 0.015 km−1·Sr−1, while the linear particle depolarization ratio was relatively small. A large depolarization ratio above the haze layer, which induces the existence of floating dust, was observed by polarization lidar. Compared with those of haze processes, the vertical aerosol and temperature distributions of dust processes were different. Statistical analyses of the PBLH, temperature inversion, RH and near-surface aerosol concentration were also presented. A lower PBLH, lower average lapse rate of temperature within the PBL, and higher RH were found when haze pollution occurred. An inversion at the top of the boundary layer aggravates ground pollution, while there is not always a temperature inversion when haze occurs in Xi'an. Together, a lower PBLH and lower average lapse rate of temperature weaken the vertical diffusion of pollutants. Decreased vertical diffusion should be the main reason for the formation and persistence of pollution in Xi'an. Increased RH was the catalyst for haze pollution. The aggravation of haze was usually accompanied by an increase in RH, and the decline of haze processes was also accompanied by a decrease or increase (>90%) in RH.