We investigate the potential of polarization lidar to provide vertical profiles of aerosol parameters from which cloud condensation nucleus (CCN) and ice nucleating particle (INP) number ...concentrations can be estimated. We show that height profiles of particle number concentrations n50, dry considering dry aerosol particles with radius > 50 nm (reservoir of CCN in the case of marine and continental non-desert aerosols), n100, dry (particles with dry radius > 100 nm, reservoir of desert dust CCN), and of n250, dry (particles with dry radius > 250 nm, reservoir of favorable INP), as well as profiles of the particle surface area concentration sdry (used in INP parameterizations) can be retrieved from lidar-derived aerosol extinction coefficients σ with relative uncertainties of a factor of 1.5–2 in the case of n50, dry and n100, dry and of about 25–50 % in the case of n250, dry and sdry. Of key importance is the potential of polarization lidar to distinguish and separate the optical properties of desert aerosols from non-desert aerosol such as continental and marine particles. We investigate the relationship between σ, measured at ambient atmospheric conditions, and n50, dry for marine and continental aerosols, n100, dry for desert dust particles, and n250, dry and sdry for three aerosol types (desert, non-desert continental, marine) and for the main lidar wavelengths of 355, 532, and 1064 nm. Our study is based on multiyear Aerosol Robotic Network (AERONET) photometer observations of aerosol optical thickness and column-integrated particle size distribution at Leipzig, Germany, and Limassol, Cyprus, which cover all realistic aerosol mixtures. We further include AERONET data from field campaigns in Morocco, Cabo Verde, and Barbados, which provide pure dust and pure marine aerosol scenarios. By means of a simple CCN parameterization (with n50, dry or n100, dry as input) and available INP parameterization schemes (with n250, dry and sdry as input) we finally compute profiles of the CCN-relevant particle number concentration nCCN and the INP number concentration nINP. We apply the method to a lidar observation of a heavy dust outbreak crossing Cyprus and a case dominated by continental aerosol pollution.
We applied the recently introduced polarization lidar–photometer networking (POLIPHON) technique for the first time to triple-wavelength polarization lidar measurements at 355, 532, and 1064 nm. The ...lidar observations were performed at Barbados during the Saharan Aerosol Long-Range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE) in the summer of 2014. The POLIPHON method comprises the traditional lidar technique to separate mineral dust and non-dust backscatter contributions and the new, extended approach to separate even the fine and coarse dust backscatter fractions. We show that the traditional and the advanced method are compatible and lead to a consistent set of dust and non-dust profiles at simplified, less complex aerosol layering and mixing conditions as is the case over the remote tropical Atlantic. To derive dust mass concentration profiles from the lidar observations, trustworthy extinction-to-volume conversion factors for fine, coarse, and total dust are needed and obtained from an updated, extended Aerosol Robotic Network sun photometer data analysis of the correlation between the fine, coarse and total dust volume concentration and the respective fine, coarse, and total dust extinction coefficient for all three laser wavelengths. Conversion factors (total volume to extinction) for pure marine aerosol conditions and continental anthropogenic aerosol situations are presented in addition. As a new feature of the POLIPHON data analysis, the Raman lidar method for particle extinction profiling is used to identify the aerosol type (marine or anthropogenic) of the non-dust aerosol fraction. The full POLIPHON methodology was successfully applied to a SALTRACE case and the results are discussed. We conclude that the 532 nm polarization lidar technique has many advantages in comparison to 355 and 1064 nm polarization lidar approaches and leads to the most robust and accurate POLIPHON products.
We present spectrally resolved optical and microphysical properties of
western Canadian wildfire smoke observed in a tropospheric layer from
5–6.5 km height and in a stratospheric layer from 15–16 km ...height during
a record-breaking smoke event on 22 August 2017. Three polarization/Raman
lidars were run at the European Aerosol Research Lidar Network (EARLINET)
station of Leipzig, Germany, after sunset on 22 August. For the first time,
the linear depolarization ratio and extinction-to-backscatter ratio (lidar
ratio) of aged smoke particles were measured at all three important lidar
wavelengths of 355, 532, and 1064 nm. Very different particle depolarization
ratios were found in the troposphere and in the stratosphere. The obviously
compact and spherical tropospheric smoke particles caused almost no
depolarization of backscattered laser radiation at all three wavelengths
(<3 %), whereas the dry irregularly shaped soot particles in the
stratosphere lead to high depolarization ratios of 22 % at 355 nm and
18 % at 532 nm and a comparably low value of 4 % at 1064 nm. The lidar
ratios were 40–45 sr (355 nm), 65–80 sr (532 nm), and 80–95 sr
(1064 nm) in both the tropospheric and stratospheric smoke layers
indicating similar scattering and absorption properties. The strong
wavelength dependence of the stratospheric depolarization ratio was probably
caused by the absence of a particle coarse mode (particle mode consisting of
particles with radius >500 nm). The stratospheric smoke particles
formed a pronounced accumulation mode (in terms of particle volume or mass)
centered at a particle radius of 350–400 nm. The effective particle radius
was 0.32 µm. The tropospheric smoke particles were much smaller
(effective radius of 0.17 µm). Mass concentrations were of the
order of 5.5 µg m−3 (tropospheric layer) and
40 µg m−3 (stratospheric layer) in the night of
22 August 2017. The single scattering albedo of the stratospheric particles
was estimated to be 0.74, 0.8, and 0.83 at 355, 532, and 1064 nm,
respectively.
An analysis of the Cloudnet data set collected at Leipzig, Germany, with special focus on mixed-phase layered clouds is presented. We derive liquid- and ice-water content together with vertical ...motions of ice particles falling through cloud base. The ice mass flux is calculated by combining measurements of ice-water content and particle Doppler velocity. The efficiency of heterogeneous ice formation and its impact on cloud lifetime is estimated for different cloud-top temperatures by relating the ice mass flux and the liquid-water content at cloud top. Cloud radar measurements of polarization and Doppler velocity indicate that ice crystals formed in mixed-phase cloud layers with a geometrical thickness of less than 350 m are mostly pristine when they fall out of the cloud.
Light extinction coefficients of 500 Mm−1, about 20 times higher than
after the Pinatubo volcanic eruptions in 1991, were observed by European
Aerosol Research Lidar Network (EARLINET) lidars in the ...stratosphere over
central Europe on 21–22 August 2017. Pronounced smoke layers with a 1–2 km
vertical extent were found 2–5 km above the local tropopause. Optically
dense layers of Canadian wildfire smoke reached central Europe 10 days after
their injection into the upper troposphere and lower stratosphere which was
caused by rather strong pyrocumulonimbus activity over western Canada. The
smoke-related aerosol optical thickness (AOT) identified by lidar was close
to 1.0 at 532 nm over Leipzig during the noon hours on 22 August 2017.
Smoke particles were found throughout the free troposphere (AOT
of 0.3) and in the pronounced 2 km thick stratospheric smoke layer at an
altitude of 14–16 km (AOT of 0.6). The lidar
observations indicated peak mass concentrations of
70–100 µg m−3 in the stratosphere. In addition to the lidar
profiles, we analyzed Moderate Resolution Imaging Spectroradiometer (MODIS)
fire radiative power (FRP) over Canada, and the distribution of MODIS AOT and
Ozone Monitoring Instrument (OMI) aerosol index across the North Atlantic.
These instruments showed a similar pattern and a clear link between the
western Canadian fires and the aerosol load over Europe. In this paper, we
also present Aerosol Robotic Network (AERONET) sun photometer observations,
compare photometer and lidar-derived AOT, and discuss an obvious bias (the
smoke AOT is too low) in the photometer observations. Finally, we compare the
strength of this record-breaking smoke event (in terms of the particle
extinction coefficient and AOT) with major and moderate volcanic events
observed over the northern midlatitudes.
Triple-wavelength lidar observations of the depolarization ratio and the backscatter coefficient of marine aerosol as a function of relative humidity (RH) are presented with a 5 min time resolution. ...The measurements were performed at Barbados (13° N, 59° W) during the Saharan Aerosol Long-range Transport and Aerosol-Cloud interaction Experiment (SALTRACE) winter campaign in February 2014. The phase transition from spherical sea salt particles to cubic-like sea salt crystals was observed with a polarization lidar. The radiosonde and water-vapor Raman lidar observations show a drop in RH below 50 % in the marine aerosol layer simultaneously with a strong increase in particle linear depolarization ratio, which reaches values up to 0.12 ± 0.08 (at 355 nm), 0.15 ± 0.03 (at 532 nm), and 0.10 ± 0.01 (at 1064 nm). The lidar ratio (extinction-to-backscatter ratio) increased from 19 and 23 sr for spherical sea salt particles to 27 and 25 sr (at 355 and 532 nm, respectively) for cubic-like particle ensembles. Furthermore the scattering enhancement due to hygroscopic growth of the marine aerosol particles under atmospheric conditions was measured. Extinction enhancement factors from 40 to 80 % RH of 1.94 ± 0.94 at 355 nm, 3.70 ± 1.14 at 532 nm, and 5.37 ± 1.66 at 1064 nm were found. The enhanced depolarization ratios and lidar ratios were compared to modeling studies of cubic sea salt particles.
We present particle optical properties of stratospheric smoke layers observed with multiwavelength polarization Raman lidar over Punta Arenas (53.2∘ S, 70.9∘ W), Chile, at the southernmost tip of ...South America in January 2020. The smoke originated from the record-breaking bushfires in Australia. The stratospheric aerosol optical thickness reached values up to 0.85 at 532 nm in mid-January 2020. The main goal of this rapid communication letter is to provide first stratospheric measurements of smoke extinction-to-backscatter ratios (lidar ratios) and particle linear depolarization ratios at 355 and 532 nm wavelengths. These aerosol parameters are important input parameters in the analysis of spaceborne CALIPSO and Aeolus lidar observations of the Australian smoke spreading over large parts of the Southern Hemisphere in January and February 2020 up to heights of around 30 km. Lidar and depolarization ratios, simultaneously measured at 355 and 532 nm, are of key importance regarding the homogenization of the overall Aeolus (355 nm wavelength) and CALIPSO (532 nm wavelength) lidar data sets documenting the spread of the smoke and the decay of the stratospheric perturbation, which will be observable over the entire year of 2020. We found typical values and spectral dependencies of the lidar ratio and linear depolarization ratio for aged stratospheric smoke. At 355 nm, the lidar ratio and depolarization ratio ranged from 53 to 97 sr (mean 71 sr) and 0.2 to 0.26 (mean 0.23), respectively. At 532 nm, the lidar ratios were higher (75–112 sr, mean 97 sr) and the depolarization ratios were lower with values of 0.14–0.22 (mean 0.18). The determined depolarization ratios for aged Australian smoke are in very good agreement with respective ones for aged Canadian smoke, observed with lidar in stratospheric smoke layers over central Europe in the summer of 2017. The much higher 532 nm lidar ratios, however, indicate stronger absorption by the Australian smoke particles.
In this study we use a new dust product developed using CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) observations and EARLINET (European Aerosol Research Lidar Network) ...measurements and methods to provide a 3-D multiyear analysis on the evolution of Saharan dust over North Africa and Europe. The product uses a CALIPSO L2 backscatter product corrected with a depolarization-based method to separate pure dust in external aerosol mixtures and a Saharan dust lidar ratio (LR) based on long-term EARLINET measurements to calculate the dust extinction profiles. The methodology is applied on a 9-year CALIPSO dataset (2007–2015) and the results are analyzed here to reveal for the first time the 3-D dust evolution and the seasonal patterns of dust over its transportation paths from the Sahara towards the Mediterranean and Continental Europe. During spring, the spatial distribution of dust shows a uniform pattern over the Sahara desert. The dust transport over the Mediterranean Sea results in mean dust optical depth (DOD) values up to 0.1. During summer, the dust activity is mostly shifted to the western part of the desert where mean DOD near the source is up to 0.6. Elevated dust plumes with mean extinction values between 10 and 75 Mm−1 are observed throughout the year at various heights between 2 and 6 km, extending up to latitudes of 40° N. Dust advection is identified even at latitudes of about 60° N, but this is due to rare events of episodic nature. Dust plumes of high DOD are also observed above the Balkans during the winter period and above northwest Europe during autumn at heights between 2 and 4 km, reaching mean extinction values up to 50 Mm−1. The dataset is considered unique with respect to its potential applications, including the evaluation of dust transport models and the estimation of cloud condensation nuclei (CCN) and ice nuclei (IN) concentration profiles. Finally, the product can be used to study dust dynamics during transportation, since it is capable of revealing even fine dynamical features such as the particle uplifting and deposition on European mountainous ridges such as the Alps and Carpathian Mountains.
In the Arctic summer of 2017 (1 June to 16 July) measurements with the
OCEANET-Atmosphere facility were performed during the Polarstern cruise
PS106. OCEANET comprises amongst other instruments the ...multiwavelength
polarization lidar PollyXT_OCEANET and for PS106 was complemented
with a vertically pointed 35 GHz cloud radar. In the scope of the
presented study, the influence of cloud height and surface coupling on the
probability of clouds to contain and form ice is investigated. Polarimetric
lidar data were used for the detection of the cloud base and the identification
of the thermodynamic phase. Both radar and lidar were used to detect cloud
top. Radiosonde data were used to derive the thermodynamic structure of the
atmosphere and the clouds. The analyzed data set shows a significant impact of
the surface-coupling state on the probability of ice
formation. Surface-coupled clouds were identified by a quasi-constant
potential temperature profile from the surface up to liquid layer base. Within
the same minimum cloud temperature range, ice-containing clouds have been
observed more frequently than surface-decoupled clouds by a factor of up to 6
(temperature intervals between −7.5 and −5 ∘C, 164 vs. 27
analyzed intervals of 30 min). The frequency of occurrence of
surface-coupled ice-containing clouds was found to be 2–3 times higher
(e.g., 82 % vs. 35 % between −7.5 and
−5 ∘C). These findings provide evidence that above
−10 ∘C heterogeneous ice formation in Arctic mixed-phase
clouds occurs by a factor of 2–6 more often when the cloud layer is coupled
to the surface. In turn, for minimum cloud temperatures below
−15 ∘C, the frequency of ice-containing clouds for coupled
and decoupled conditions approached the respective curve for the
central European site of Leipzig, Germany (51∘ N,
12∘ E). This corroborates the hypothesis that the free-tropospheric ice
nucleating particle (INP) reservoir over the Arctic is controlled by
continental aerosol. Two sensitivity studies, also using the cloud radar for
detection of ice particles and applying a modified coupling state detection,
both confirmed the findings, albeit with a lower magnitude. Possible
explanations for the observations are discussed by considering recent in situ
measurements of INP in the Arctic, of which much higher concentrations were
found in the surface-coupled atmosphere in close vicinity to the ice shore.
For the first time, a dense data set of particle extinction-to-backscatter ratios (lidar ratios), linear depolarization ratios, and backscatter- and extinction-related Ångström exponents for a ...Central Asian site are presented. The observations were performed with a continuously running multiwavelength polarization Raman lidar at Dushanbe, Tajikistan, during an 18-month campaign (March 2015 to August 2016). The presented seasonally resolved observations fill an important gap in the database of aerosol optical properties used in aerosol typing efforts with spaceborne lidars and ground-based lidar networks. Lidar ratios and depolarization ratios are also basic input parameters in spaceborne lidar data analyses and in efforts to harmonize long-term observations with different space lidar systems operated at either 355 or 532 nm. As a general result, the found optical properties reflect the large range of occurring aerosol mixtures consisting of long-range-transported dust (from the Middle East and the Sahara), regional desert, soil, and salt dust, and anthropogenic pollution. The full range from highly polluted to pure dust situations could be observed. Typical dust depolarization ratios of 0.23–0.29 (355 nm) and 0.30–0.35 (532 nm) were observed. In contrast, comparably low lidar ratios were found. Dust lidar ratios at 532 nm accumulated around 35–40 sr and were even lower for regional background dust conditions (20–30 sr).
Detailed correlation studies (e.g., lidar ratio vs. depolarization ratios, Ångström exponent vs. lidar ratio and vs. depolarization ratio) are presented to illuminate the complex relationships between the observed optical properties and to identify the contributions of anthropogenic haze, dust, and background aerosol to the overall aerosol mixtures found within the 18-month campaign.
The observation of 532 nm lidar ratios (<25 sr) and depolarization ratios (around 15 %–20 %) in layers with very low particle extinction coefficient (<30 sr) suggests that direct emission and emission of resuspended salt dust (initially originated from numerous desiccating lakes and the Aralkum desert) have a sensitive impact on the aerosol background optical properties over Dushanbe.
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