ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-14199-2017Dry versus wet marine particle optical properties: RH dependence of depolarization ratio, backscatter, and extinction
from multiwavelength lidar measurements during SALTRACEHaarigMoritzhaarig@tropos.dehttps://orcid.org/0000-0002-5533-2112AnsmannAlbertGasteigerJosefhttps://orcid.org/0000-0002-4401-0118KandlerKonradAlthausenDietrichBaarsHolgerhttps://orcid.org/0000-0002-2316-8960RadenzMartinhttps://orcid.org/0000-0002-7771-033XFarrellDavid A.https://orcid.org/0000-0003-0644-7164Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, GermanyFaculty of Physics, University of Vienna, Vienna, AustriaTechnische Universität Darmstadt, Darmstadt, GermanyCaribbean Institute for Meteorology and Hydrology, Bridgetown, BarbadosMoritz Haarig (haarig@tropos.de)30November20171723141991421712June201719June201723September20177October2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/17/14199/2017/acp-17-14199-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/14199/2017/acp-17-14199-2017.pdf
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.
Introduction
Since more than 70 % of the Earth is covered with water, the
optical properties of marine particles must be carefully considered in
radiative transfer schemes in global atmospheric models. This includes marine
conditions with relative humidity (RH) < 50 %, so marine particles get
increasingly dry and change their shape and thus their optical properties, as we
will demonstrate in this paper. The shape of sea salt particles strongly
depends on RH. At typical values of RH > 80 % in the marine boundary layer (MBL),
sea salt particles are liquid solution drops and thus spherical in shape.
When RH decreases below 45 %, they crystallize and become mostly cubic-like
in shape , but they will not become absolutely dry. The
change in shape leads to different optical properties, especially to changes
in the linear depolarization ratio. Spheres have a linear depolarization
ratio of ideally zero, while nonspherical particles exhibit higher values
e.g.,. The different optical properties of dry and
humid sea salt have to be considered in various applications.
Satellite passive remote sensing as well as ground-based passive remote sensing AERONET; e.g.,for marine environments
may sometimes be significantly affected by dry marine particles in marine environments (coastal regions during sea breeze effects).
Cubic-like particles have a different scattering phase function than spherical particles. Analogous to the mixture of Saharan dust
(assumed to be spheroidal in shape) and spherical anthropogenic particles, in the case of marine particles one would need an
analysis scheme which considers cubic particles (and related scattering phase function) besides the spherical ones. The same
should be considered in lidar inversion methods e.g.,, when inverting microphysical
properties over the oceans and coastal areas. Nonspherical particles can have a sensitive impact on the retrieval products,
and thus particle shape has to be carefully considered .
Aerosol classification from active remote sensing based on the depolarization ratio will be
misleading if dried marine aerosol with a high depolarization ratio is present. The polarization lidar observations used to
separate dust from non-dust e.g., rely on the assumption that
non-dust aerosol always produces depolarization ratios around 0.05 or less and significant depolarization is only caused by
dry irregularly shaped dust particles. The cubic-like sea salt particles will consequently be misinterpreted as dust, leading
to an overestimation of the dust concentration.
Active remote sensing from satellites (CALIOP and EarthCARE) use the
polarization technique to separate aerosol types. Derived optical
properties like the extinction coefficient depend on the detection of the
correct aerosol type to choose the appropriate lidar ratio .
Dry sea salt particles may not be detected as sea salt but rather misinterpreted as
a different aerosol type with a higher lidar ratio, which leads to an
overestimation of the extinction coefficient.
Sodium chloride deliquescence at 75 % RH observed at laboratory conditions (at 4.9 ∘C) in an
environmental scanning electron microscope. The dry cubic particle with sharp edges at RH of 46 % becomes
surrounded by a liquid sphere when RH increases to 77 %.
The study aims to show the change of the optical properties with RH, i.e.,
how spherical and cubic-like sea salt particles influence backscatter, extinction, and light depolarization. The change in shape
characteristics of marine aerosol can be easily observed with polarization
lidar . When equipped with water
vapor and nitrogen Raman channels the lidar delivers profiles of specific
humidity. Combined with regularly available temperature profiles from
radiosondes or models, our Raman lidar observations provide RH together with
the depolarization ratio and can thus carefully measure changing particle
shape effects with RH, as a function of height. The potential of using Raman
lidar to study the hygroscopic growth of aerosol particles was demonstrated
by for summer haze on the east coast of the United
States. observed the decrease of depolarization
ratio of marine aerosol mixtures with increasing RH over southern Spain.
Sea salt crystallization and deliquescence was observed with a
triple-wavelength polarization lidar in the marine aerosol
layer (MAL) over the remote tropical Atlantic in the absence of any
disturbing anthropogenic impact and lofted Saharan dust layer. In this way,
lidar can provide valuable information on the state of the MAL from the point of view of optical properties. The results are presented
and discussed in Sect. .
By performing triple-wavelength polarization lidar observations, we provide
combined information on the shape and size characteristics of marine aerosol
ensembles for the modeling community dealing with the optical properties of
irregularly shaped mineral dust and sea salt particles . Besides the depolarization ratio of the
particles, we also deliver extinction-to-backscatter ratios (lidar ratios), which are also sensitive to changing particle properties. The results
are presented in Sect. .
RH ranged from 40 to more than 80 % in the MAL. Scattering
enhancement factors with RH are measured under these atmospheric conditions.
The enhanced backscatter coefficient depicts the hygroscopic growth of the
particles in terms of changing optical properties. For pure marine aerosol,
the hygroscopic growth factors and thus the backscatter and extinction
enhancement are larger compared to cases with mixtures of marine and
continental particles . This will be presented in Sect. .
At the beginning (in Sect. 2), we will give an introduction to sea salt
aerosol under dry and humid conditions and show examples of sea salt
particles collected above Barbados. The lidar measurements in the framework
of the Saharan Aerosol Long-range Transport and Aerosol-Cloud interaction
Experiment (SALTRACE; ) will be described briefly in
Sect. 3. Then we will present our observations with a special focus on the
phase transition from spherical solution droplets to crystalline sea salt
particles and the scattering enhancement factor (Sect. ). In the discussion (Sect. ), we
compare our results to model calculations.
Sea salt under dry and humid conditions
The oceans are the source of marine aerosol, which consists mainly of sea
salt and organic compounds from the sea surface . Marine
aerosol from sea spray acts as cloud condensation nuclei and ice nucleating
particles over the ocean where other types of particles are rare
. Marine aerosols are found in a wide size
range from nanometer (10-9 m) up to several micrometer (10-5 m; ). The Aitken mode (diameter < 70–80 nm) is
dominated by organic material emitted from the ocean. New particle formation
dominates the accumulation mode (80–300 nm) and the larger particles are
mostly sea salt e.g.,. The sea salt particles may contain
some organics on its surface . Size-resolved studies of marine aerosol (up to
2.5 µm) have been performed on Barbados by . They found
that the sea spray mode (> 300 nm) contributes 4–10 % to the total number.
The number size distribution is dominated by the Aitken mode. But the surface
area and the volume size distribution are dominated by sea spray (90 % of the
total mass and volume) with negligible contribution of the Aitken mode,
resulting in a bimodal size distribution (, and personal
communication with H. Wex, 2017). For the radiation studies as for lidar
measurements the surface area and its bimodal distribution is decisive. In
this study, we will focus on sea salt and its different shapes, which mainly
forms the coarse or sea spray mode. The accumulation mode (later on called
fine mode) consists of sea salt particles and newly formed organic particles.
Sea salt is the
dominant component of the coarse mode and dominates the optical properties.
Therefore most of the following investigations concern the sea salt and its
changes with RH.
Crystalline sodium chloride has a cubic shape, while sulfates form frequently
needle-like shapes . With different compounds present in
sea-salt, mixed particle geometries can occur . With
increasing RH the hygroscopic material takes up water, deliquesces, and forms
a spherical solution droplet. In humid marine environments, sea salt
particles are of spherical shape. Consequently sea salt exists in two shape
modes, spherical and nonspherical crystalline.
The shape and thus the depolarization ratio are dependent not only on RH but
also on the chemical composition and the rate at which the particles have
dried , leading to different crystalline shapes. NaCl is the
major component of sea salt, but other salts such as Na2SO4, MgCl2, MgSO4, and possibly some organics are part of atmospheric sea salt
. These components prevent the perfectly cubic shape of dried
sea salt .
In Fig. , the deliquescence behavior of a pure sodium chloride
particle is shown. It was observed with a scanning electron microscope.
The cubic sodium chloride particle grows with increasing RH. Once the
deliquescence point (in this case at RH of approximately 75 %) is reached, it
turns into a droplet. The studies of found the deliquescence
point for sea salt at 70–74 %, depending on the composition of the sea salt.
The particles keep the spherical shape until RH decreases to 45–48 %
. As a result of the hysteresis effect, sea salt particles
may exist in both shape modes between approximately 50 and 70 % RH,
depending on their individual history. These conditions may occur quite often
over the oceans and coastal areas, as illustrated by .
The hysteresis behavior has been previously studied .
Images of dry atmospheric sea salt particles (red arrows) surrounded by Saharan dust particles,
collected during the summer transport regime (summer 2013), whereas the later-presented profile measurements were perfomed in the absence of dust layers in February 2014. The particles were sampled aboard the Falcon research
aircraft at different heights over Barbados on (a) 21 June, (b) 10 July, (c) 26 June and
(d) 30 June 2013 during SALTRACE-1. Sampling altitude and conditions are (a) 2560 m a.s.l., 15 ∘C, 29 % RH;
(b) 3550 m a.s.l., 8 ∘C, 28 % RH; (c) 3570 m a.s.l., 7 ∘C, 39 % RH;
(d) 3230 m a.s.l., 8 ∘C, 34 % RH. For the sea
salt particles in (b) and (d) the XRF images are included showing that chloride (Cl) and sodium (Na) are the
main components. The sulfate (S) component is negligible for particle (b) but significant for particle
(d), which exhibits a more spherical shape. Panel (c) shows an outline of a former droplet (green arrows),
indicating a still (partial) deliquesced state during collection. The white bar in the bottom right corner indicates 2 µm.
Atmospheric samples of dry salt particles (Fig. ) were
collected during the SALTRACE-1 campaign (Barbados, summer 2013), whereas the
case studies shown later in this publication are for pristine marine
conditions observed during February 2014. The samples in Fig.
are taken in the dust layer (2–4 km a.s.l.), which was present during the
summer months but not the winter months, when our observations of dry
marine particles took place. The samples in Fig. were taken in
the free troposphere to ensure that the particles have been dried in airborne
state to be representative for atmospheric aerosol, in contrast to wet
collection followed by drying on a substrate, which might lead to substrate
effects. The four dry marine particles shown in Fig. were
collected at RH between 28 and 39 %. The X-ray fluorescence spectroscopy
(XRF) images reveal that particle (b) with a shape closer to a cube has a
negligible contribution of sulfate, whereas particle (d) is more spherical in
shape and has a larger contribution of sulfate. Also, organics of marine
origin might be the reason for the more spherical shape .
Particle (c) shows the outline of a spherical marine aerosol droplet, so this
one was probably still deliquesced at the time of collection. Overall we see
that sea salt particles have a nonspherical shape that could be approximated
by a cube for RH below 40 %. But the shape is not perfectly cubic as for pure
sodium chloride (see Fig. ). The edges of the sea salt
particles dried in the atmosphere are smoother. In this publication we will
call the shape of crystalline sea salt “cubic-like” to separate it from the
spherical sea salt droplets under humid conditions. Compared to model results
of perfectly cubic particles or pure NaCl salt particles investigated in the
laboratory, we should measure lower depolarization ratios for dried marine
aerosol in the atmosphere because of the smoothed cubic-like shape.
For spherical marine particles a low particle linear depolarization ratio (PLDR) of
0.03 ± 0.01 at 532 nm prevails . The range of
depolarization values given in the mentioned publication is 0.01–0.11. This
indicates that not all cases classified as marine aerosol consisted of
spherical sea salt particles.
Field studies on dried marine particles are very rare. First evidence of an
enhanced depolarization ratio for dry marine particles was reported by
based on lidar measurements in Tokyo. A clear separation
from a potential dust influence was not possible. They observed a peak in the
PLDR of 0.1 at 532 nm during sea breeze.
measured RH and the depolarization ratio over Nagoya,
Japan, from 1994 to 1997 with a Raman lidar. They found 532 nm PLDRs between 0.1 and 0.2 for 25–45 % RH at heights from
2 to 4 km in the free troposphere. Backward trajectories indicated pure marine
conditions. These values are in good agreement with our observations.
However, it cannot be excluded that continental particles were present as
well and contributed to the light depolarization.
In a laboratory study, measured the PLDR of spherical sea salt particles of 0.01 ± 0.001
at 532 nm. For crystalline sea salt particles, they found a PLDR of
0.08 ± 0.01. Pure crystalline NaCl has a significantly higher
depolarization ratio (PLDR = 0.21 ± 0.02) than atmospheric sea salt
. In a laboratory study, observed pure
NaCl particles with a depolarization ratio of 0.25.
Additionally to the change in particle shape, the size of sea salt aerosols
changes with RH. Due to water uptake sea salt aerosols are much larger under
humid conditions and smaller under dry conditions. The process is known as
hygroscopic growth e.g.,. The change in
optical properties (backscatter coefficient, extinction coefficient, lidar
ratio) with varying RH can be measured with a Raman lidar. Results are shown
in Sect. .
Early discrete dipole approximation (DDA) modeling attempts of spherical and
cubic sea salt particles have been done by . For cubic
particles larger than 0.8 µm a PLDR of 0.08–0.22 at 532 nm was predicted.
used a T-matrix approach for cubic sea salt particles to
model the depolarization ratio (approx. 0.16 in the visible and UV) and the
lidar ratio (19 sr in UV and 20 sr in the visible). The DDA approach for
cubic particles including surface roughness leads to a
PLDR 0.1–0.2 and a lidar ratio of 15–20 sr for the particle radius equal to
the wavelength (size parameter = 6). Our observations will also be compared
with recently performed model calculations (Sect. ).
MethodsThe SALTRACE project
The three SALTRACE field campaigns in 2013 and 2014 are the final observational efforts of the long-term SAMUM-SALTRACE
project . During SALTRACE, we investigated the Saharan dust properties
after an atmospheric travel over 5–15 days and 5000–8000 km . In the
summer seasons of 2013 and 2014 (SALTRACE-1 and SALTRACE-3 in June–July), aged dust layers were observed. To investigate
aged mixtures of dust and biomass burning smoke after long-range transport, we performed an additional campaign in
February–March 2014 (SALTRACE-2, winter transport regime). In February 2014 there was a period without aerosol transport
from Africa, resulting in very clean marine conditions over Barbados. The SALTRACE lidar activities were complemented
by shipborne lidar observations along the main Sahara dust transport route over the tropical North Atlantic in April–May
2013 .
The ground-based remote sensing station was deployed at the Caribbean
Institute for Meteorology and Hydrology (CIMH) in Husbands, north of the
capital Bridgetown at the west coast of Barbados (13.15∘ N, 59.62∘ W; 110 m a.s.l.). The BERTHA (Backscatter Extinction lidar Ratio
Temperature Humidity profiling Apparatus) lidar system, an AERONET sun photometer (see
AERONET web page https://aeronet.gsfc.nasa.gov/, Barbados_SALTRACE site),
and a Vaisala radiosonde station (RS92 for profiling of pressure,
temperature, RH, and the vector of the horizontal wind component) were
operated at the field site. A second AERONET station (Ragged Point) is
located at the east coast of Barbados, approximately 20 km away from the
CIMH.
Triple-wavelength lidar BERTHA
The multi-wavelength polarization Raman lidar BERTHA of the Leibniz Institute
for Tropospheric Research (TROPOS) is a container-based mobile lidar system.
As a unique feature, it enables the measurement of the depolarization ratio
at three wavelengths (355, 532, and 1064 nm) simultaneously. A more detailed
description of the lidar system and the polarization characteristics can be
found in . Currently it operates as a 3+2+3 lidar system
(three backscatter coefficients, two extinction coefficients, and three depolarization
ratios) with an additional water vapor channel (407 nm) and a
high-spectral-resolution channel at 532 nm. It has been used in a 3+3+2
configuration in . The signals are detected with a range
resolution of 7.5 m and a time resolution of 10 s.
The particle backscatter coefficient gives information about the aerosol
layers. For particles with sizes comparable to the wavelength or larger, it
is in first approximation proportional to the surface area of the bulk of
particles. The extinction coefficient is determined from the transmission of
the laser beam through the atmosphere. Both are calculated independently from
the lidar signals via the Raman lidar method . The
backscatter-to-extinction ratio, also called lidar ratio, contains
information about the particle size and shape, as well as about the
refractive index. Therefore it is used together with the particle
depolarization ratio to classify aerosol types .
The 532 nm channels reach full overlap at 800–1000 m height. The 355 nm
channels reach full overlap at 2500–3000 m. Therefore, they have to be
overlap-corrected according to . The uncertainty
especially of the extinction and the lidar ratio is therefore larger in the
UV.
The PLDR is a measure of the depolarization caused by the scattering of linear-polarized light
(defined as parallel) at atmospheric particles. It is defined as the ratio of cross-polarized to parallel-polarized light scattered back
from aerosol particles. As an intensive parameter it is characteristic for a certain aerosol type. For spherical particles (droplets,
wet marine particles) the PLDR is < 0.03, whereas nonspherical particles have a higher PLDR (dust approx. 0.3 (at 532 nm), ice
crystals approx. 0.5; ). To ensure the good quality of the depolarization measurements, a
Δ90∘
calibration was performed for each measurement. In the UV the systematic uncertainties are quite high (0.01
in the volume linear depolarization ratio). The calculation of the depolarization ratio follows and is
described in , where a detailed error estimation can be found in addition.
For the calculation of RH, the temperature profile of the radiosonde is used.
BERTHA measures the pure rotational Raman signals from nitrogen and oxygen
from the 532 nm-emission wavelength to retrieve the temperature profile, but
the uncertainty is too large to retrieve RH with a reasonable uncertainty
. During the SALTRACE campaign, a Vaisala RS92 radiosonde
was launched for each measurement. The water vapor mixing ratio of the
radiosonde is used to calibrate the water vapor mixing ratio derived by the
ratio of the hydrogen (407 nm) and nitrogen (387 nm) Raman signal of the
lidar . Due to the weak 407 nm signal, the technique can
be used at nighttime only. By using the water vapor mixing ratio profiles of
the lidar and the temperature and pressure profile of the radiosonde the
temporal and vertical evolution of RH can be derived. The relative error of
the water vapor mixing ratio caused by calibration and signal noise was < 5 %
at all heights within the aerosol layer (in these cases up to 2 km). The
temperature of the radiosonde is used for the 2 h of measurement, so an
uncertainty of 1 K is reasonable. These errors lead to a maximal relative
uncertainty in RH of 12 %, resulting in a dry RH of 40 ± 5 %. A detailed
error estimation for RH derived with a Raman lidar can be found in
.
The DDA model for cubic sodium chloride
We simulate optical properties of dry sea salt particles as cubes with the
refractive index m of sodium chloride, provided by ,
i.e., with values m=1.582 at λ=355 nm, m=1.549 at
λ=532 nm, and m=1.531 at λ=1064 nm. The size distribution
is taken from the corresponding AERONET measurement. The version 2 inversions
for spherical particles are used .
A range of volume-equivalent particle radii up to 2 µm is covered by
modeling with the DDA code ADDA
with logarithmically equidistant size steps of a factor of 1.1. We use the
DDA formulation “filtered coupled dipoles” included in
ADDA, which was also used for example by , and use eight
dipoles (dpl) per wavelength. To simulate random particle orientation, DDA
runs for 100 orientations were carried out for each particle. The weighted
distributions of particle orientations were selected according to those
presented by . For each DDA run, the optical properties were
averaged over 64 scattering planes rotated around the incident light
direction. The single particle properties are used in a subsequent step for
the calculation of the bulk optical properties (see Sect. 5).
To estimate the accuracy of these model simulations, the scattering problems
were modeled in addition with settings associated with higher accuracy, i.e.,
one case with increased number of dipoles per wavelength (dpl =12 instead of
8) and another case where we increased the number of orientations from 100 to
225.
The uncertainty is estimated for the lidar ratio of single randomly oriented
particles to be on the order of ±10 % and for the linear depolarization
ratios about ±0.02. A similar uncertainty is estimated for the final
optical properties of the bulk sea salt aerosol. The estimate for the bulk
properties is based on the uncertainty being, on the one hand,
reduced by averaging over the size distribution or, on the other hand,
increased due to the fact that the assumed size distribution, particle shape,
and refractive index are given with an uncertainty.
Three days of lidar observations (23–25 February 2014) of layers with dried marine particles
in the marine aerosol layer (MAL). On 23 February (left panel), a vertically extended layer of dried sea
salt particles (red area) occurred on top of the convective boundary layer. A continuously 100–200 m
thick layer with dried marine particles (yellow layer at MAL top) was present over the whole day on 24 February
2014;
this layer was still present on 25 February. The convectively active MBL reaches about 1 km on all 3 days and
permanently pushed marine aerosol upward. The shown range-corrected 1064 nm backscatter signal
(cross-polarized channel, 10 s temporal, 7.5 m vertical resolution) is most sensitive to enhanced
light depolarization produced by dry marine aerosol. LT: local time.
Marine aerosol layer (MAL) over Barbados on 23 February 2014, 19:38–21:39 local time, with the
cloud-free MBL reaching to 0.8–1 km height (indicated by a low depolarization ratio and high RH) and an
extended layer between about 1 and 2.1 km height with dried marine sea salt particles causing enhanced
light depolarization (b yellow–red areas) at low RH of < 50 % (c, bluish areas). The cross-polarized
1064 nm signal (a 10 s temporal, 7.5 m vertical resolution) highlights the layer with dried marine
particles (yellow areas). Panels (b) and (c) are based on depolarization ratio and RH profile values averaged
over 30 s and vertically smoothed over 22 m (b) and 52 m (c).
Same as Fig. , except for 24 February 2014, 18:12–20:27 local time.
The MAL top is again close to 2 km height. The convective MBL reaches to about 800 m height and
permanently pushes sea salt particles into the upper part of the MAL. RH is high throughout the
MAL (and thus depolarization ratio caused by spherical, wet marine particles is low). Only at MAL
top do dried marine particles cause a thin layer of enhanced cross-polarized signal (a 10 s temporal,
7.5 m vertical resolution) and depolarization ratio (b). Daytime noise is visible in the RH (c) in
the first half hour after sunset at 22:06 UTC. Panels (b) and (c) are based on the depolarization ratio and RH profile values
averaged over 30 s and vertically smoothed over 22 m (b) and 52 m (c).
Observations
The island of Barbados is ideal to observe pure marine conditions. It is the
eastern-most island of the Caribbean and located in the trade wind zone with
predominant wind direction from the east. In winter, the inner tropical
convergence zone is shifted to the Southern Hemisphere, so the air masses
originating from the African continent are transported to South America
e.g.,, leaving the Caribbean under marine influence.
The MAL extends up to the strong trade wind inversion at around
2 km height. The MAL is defined by the
predominance of marine aerosol. It includes the convective MBL and another residual layer of marine particles.
The dust removal process in the MAL is
very efficient as shown in . As there was no dust or
other continental aerosol in the free troposphere above the MAL, it is very
unlikely that dust reached Barbados during February 2014.
Ensemble of 7-day backward trajectories (HYSPLIT, 2017) for (a) 24 February 2014, 00:00 UTC,
and (b) for 24 February 2014, 23:00 UTC, arriving at 1200 m over Barbados.
During the SALTRACE-2 campaign (16 February–8 March 2014), a layer of
enhanced cross-polarized signal was observed for several days. Figure shows the period from 23 to 25 February 2014. A strong decrease
in RH from 80 to less than 10 % at the trade wind inversion height (MAL
top) was continuously observed between 20 and 25 February 2014. The enhanced
depolarization ratio corresponding to this RH decrease was found to be
between 0.04 and 0.12 (at 532 nm). The two night measurements of 23 and
24 February 2014 will be discussed in the following section to demonstrate
how the decrease in RH leads to an increase of the linear depolarization
ratio due to the change in the shape properties of the sea salt particles.
Firstly, the observations during the two night measurements will be
described. Then, the changes in shape and size with RH will be discussed by
using the PLDR and the particle backscatter
coefficient, respectively.
Relative residence time of the air masses over ground within the 10 days prior to their arrival over
Barbados (24 February 2014, 00:00 UTC). For each height level (500 m) an ensemble of 27 HYSPLIT trajectories
was calculated. The numbers on the right-hand side indicate the total amount of hours that the ensemble of
10-day backward trajectories at each height level spend close to the ground (below 2 km height) and could be
possibly laden with aerosols. The color bar indicates the portion of different surface areas, where blue
stands for ocean and green for different types of continental land cover (forest, grass, etc.). The land
cover is taken from MODIS. Between 2.5 and 4.0 km height, the trajectories did not come close to the ground.
The air masses in the MAL (up to 2 km height) are purely marine. The same result was found on 25 February 2014, 00:00 UTC (not shown).
AERONET retrieval products for 21–28 February 2014: (a) Aerosol optical thickness (AOT) at 500 nm; (b) Ångström
exponent (440–870 nm); (c) fine mode fraction (FMF). AERONET level 2.0 data
from Ragged Point and Barbados_SALTRACE are shown. The gray area highlights the time period
of lidar observations discussed in this section.
Profile measurements of backscatter, depolarization, lidar ratio, and RH
An overview of the observations on 23 and 24 February 2014 is given in
Figs. and . On 23 February 2014,
the MAL was not well mixed. RH decreased steadily from 80 % at 250 m height
to 35 % at 1000 m height. Above the 1000 m height level, RH increased again
up to 80 % at 1800 m height. Then a fast decrease of RH (from 80 to less
than 10 %) occurred at the trade wind inversion height between 1850 and
2150 m height. The strong decrease of RH at MAL top was observed for most of
the measurements under clean marine conditions in February 2014. The
time–height display of RH is shown in Fig. c. The
increased signal in the cross-polarized channel and the volume depolarization
ratio at 1064 nm are shown in Fig. a and b. In parts with low
RH, mostly between 1000 and 1600 m, the volume depolarization ratio is high,
indicating nonspherical particles.
On 24 February 2014, only a thin layer of dried marine particles was
observed. The MAL reached up to 2 km height (Fig. ). The
feature of interest is the enhanced 1064 nm cross-polarized signal in the
upper 200 m of the aerosol layer. The radiosonde launched at 23:07 UTC (19:07
local time; LT) shows a strong temperature inversion of 4 K within 200 m
around 2000 m height (trade wind inversion height). RH was about 65 to 80 %
throughout the MAL and decreased to values of 5 % 200 m above the MAL.
RH (Fig. c) indicates that the environmental conditions
remained unchanged during the 2 h average (18:11–20:20 LT). Marine
particles lost their spherical shape at the top of the marine aerosol layer
(efflorescence). This caused an enhanced depolarization ratio, as can be seen
in Fig. b.
For the further discussion it is important to show that predominately marine
aerosol particles were present over Barbados in the lowest 2000 m. HYSPLIT
backward trajectories and AERONET
observations are used to demonstrate that
pure marine conditions were given.
The ensemble of 7-day backward trajectories for 23 February (Fig. a) indicates marine sources over the Atlantic for the
air mass, with only a rather small chance of aerosol uptake over Western Sahara.
On 24 February the ensemble of 7-day backward trajectories (Fig. b) shows only marine aerosol sources for the MAL.
In Fig. , only one height level (1200 m) is considered
for the MAL. A more sophisticated analysis of the ensembles of trajectories
at different heights is shown in Fig. . To estimate the
contributions of marine and continental aerosol sources to the observed air
masses, ensembles of HYSPLIT backward trajectories were
calculated in vertical steps of 500 m. Each ensemble consists of 27 single
trajectories which are initialized with a small spatial offset. An air parcel
is assumed to get laden with aerosols over a given marine or continental
source region when the parcel is at heights below 2 km above this region. The
colors in Fig. indicate the contribution of different land
cover categories, taken from MODIS .
Considering the ensembles of trajectories at different heights below 2000 m
for both nights (Fig. , only the night of 23 February shown),
none of the 10-day back trajectories crossed a continental site at heights
below 2 km. This analysis clearly indicates that our lidar observations were
performed at pure marine aerosol conditions.
The measurements of the AERONET sun photometers at Ragged Point and at the
field site (Barbados_SALTRACE) are shown in Fig. . The aerosol optical
thickness (AOT at 500 nm) is low and accumulates around 0.05. Very low Ångström exponents (AE) were found and indicate the dominance of coarse mode
particles. In our cases the AE at the closest AERONET
observations (after 20:00 UTC) are between 0.1 and 0.2. The fine mode
fraction (FMF) is also typical for coarse-mode-dominated particle ensembles.
The transport of dust over 5000–8000 km below the trade wind inversion is
extremely unlikely. The dust particles will get lost due to turbulent
downward mixing or gravitational sedimentation or wet deposition. The
observations of support the conceptual model
, which implies a very efficient removal of dust from
the MAL. Dust is transported over long distances in the Saharan air layer
(SAL) above the trade wind inversion (above the MAL). But in late February
2014, the SAL was not present over the MAL. In conclusion, all air mass back
trajectory studies and the AERONET observations indicate clean marine
conditions over Barbados during 23 and 24 February 2014.
The vertical profiles of the particle backscatter coefficient and the PLDR
for the three wavelengths (355, 532, and 1064 nm) of the BERTHA lidar system
are shown in Figs. and . A good
agreement is achieved between RH measured with the radiosonde and the
30 min (2 h) mean RH profile retrieved from the lidar observations. The
profile of RH in combination with the profiles of the backscatter coefficient
and the PLDR indicate the sensitive changes in the marine optical properties
with changing RH. The crystallization point for sea salt of 45–48 % RH
is marked with a dashed line. The depolarization ratio
increases strongly when RH drops below this point. The change of the
backscatter coefficient with RH is less pronounced.
Thirty-minute mean profiles of particle backscatter coefficient (a) and particle linear
depolarization ratio (c) at three wavelengths together with radiosonde RH (indicating the MAL up
to 2 km height). The lidar observation was performed on 23 February 2014, 19:45–20:15 local time
(23:45–00:15 UTC). The atmospheric variability was low during the signal averaging period (Fig. ).
Panel (b) shows the potential temperature (Tpot, radiosonde, launch at 00:00 UTC) and RH from radiosonde and
from lidar (30 min average). Note the sharp drop in RH from > 70 % (at 1850 m height) to < 10 %
(at 2100 m height). The dashed line marks the sea salt efflorescence point (45 % RH). Error bars indicate
the lidar retrieval uncertainty. The vertical smoothing window length is 50 m for the backscatter and RH (lidar) and 100 m for the depolarization ratio.
When RH decreases below the crystallization point at 810 m height, the
PLDR starts to increase and takes its maximum
of 0.148 ± 0.035 at 532 nm at around 1150 m height. RH increases again
between 1000 and 1800 m and reaches 70–74 % (deliquescence point of sea
salt) at 1780 m. The PLDR decreases below 0.02 (532 nm) at this height. This
behavior will be further discussed in Sect. . The second
decrease in RH at the trade wind inversion height leads to a less pronounced
peak in the PLDR (value ± systematic uncertainty) of 0.069 ± 0.161 at
355 nm, 0.079 ± 0.036 at 532 nm, and 0.063 ± 0.018 at 1064 nm around
2000 m height.
In the 2 h mean profiles of the 24 February 2014 (Fig. , 100 m vertical smoothing is applied), maximum
PLDRs
of 0.055 ± 0.109 (355 nm), 0.068 ± 0.035 (532 nm), and 0.038 ± 0.010
(1064 nm) are reached in the thin layer of dried marine aerosol at MAL top. In
the humid MAL below, the PLDR is below 0.02 and thus clearly
indicates the spherical shape of the sea salt particles.
Same as Figure but for 24 February 2014, 18:12–20:20 local time
(22:12–00:20 UTC). In panel (b), the respective 2 h RH profile (from lidar) is shown
together with the radiosonde profiles (launch at 23:07 UTC). The MAL was entirely humid on this day.
The Raman lidar method allows us to derive the extinction coefficient
independently of the backscatter coefficient and therefore to measure the
lidar ratio, shown in Fig. . In
the humid MAL on 24 February the lidar ratio is 19 ± 5 sr at 355 nm and
23 ± 2 sr at 532 nm, which is typical for spherical marine particles
. For the aerosol layer with
dried marine particles on 23 February (1000–1600 m height), we obtained a
lidar ratio of 27 ± 6 sr (at 355 nm) and 25 ± 3 sr (at 532 nm) for the
cubic-like sea salt particles. The increase in the lidar ratio may be
explained by the reduction in particle size, as the sidewards scattering
(extinction without absorption) increases for smaller particles. The results
are summarized in Table for dry and humid conditions.
There, the results of our modeling efforts presented in Sect. are shown as well for comparison. For cubic sea salt
particles we simulated lidar ratios of 13, 20, and 36 sr at 355, 532, and
1064 nm, respectively. simulated with the T-matrix approach
a lidar ratio of 19 sr and 20 sr at 355 and 532 nm for cubic sea salt
particles.
Observation of the phase transition in the depolarization ratio
A unique opportunity is provided to study the relation between marine
particle shape and RH with high temporal (5 min) and vertical (50 m)
resolution under atmospheric conditions. The correlation between PLDR and RH
for 23 February 2014 is shown in Fig. d–f. Only the
decrease in RH is depicted here (375–1100 m height range). The
crystallization point (45–48 % RH) is more important than the deliquescence
point (70–74 % RH) when the drying process is highlighted. Inorganic sea
salt has multiple crystallization points due to its complex composition
. The drying follows the upper branch of the
hysteresis as fully deliquesced sea salt particles are drying. From 80 to
50 % RH the PLDR increases slightly with decreasing RH but remains ≤
0.02 (532, 1064 nm) and ≤ 0.03 (355 nm, due to higher noise level). This
observation is in line with typical depolarization values for marine aerosol
as used in aerosol classification schemes e.g.,. At
around 50 % RH the PLDR increases drastically indicating a significant change
in particle shape from spherical to cubic-like shape. The PLDR reaches
maximum values (with systematic uncertainty) of about 0.12 ± 0.08 (355 nm),
0.15 ± 0.03 (532 nm), and 0.10 ± 0.01 (at 1064 nm) at RH of around 40 %.
After the phase transition from spherical sea salt droplets to cubic-like sea
salt crystals, the depolarization ratio remains at the high level even for RH
close to 35 %. A further drying was not observed in the atmosphere yet, so we
can only speculate about the depolarization ratio for sea salt particles
under very dry conditions (0–35 % RH).
Furthermore, the measurement of 23 February 2014 contains information
about the humidification process. Between 1100 and 1800 m height RH is
increasing again up to 80 %. Figure contrasts the PLDR
dependence on RH during humidification and drying. The sea salt keeps a
cubic-like shape, causing an enhanced depolarization ratio even at RH close to
60 %. The depolarization values then slowly decrease to values below 0.02
typical for spherical sea salt particles (above around 65–70 % RH). The
hysteresis effect leads to the existence of both shape modes between 50 and
70 % RH.
Two-hour mean profiles of the lidar ratio for the 23 February (a 500 m vertical smoothing) and
the 24 February 2014 (b 750 m vertical smoothing): 532 (green line) without overlap correction,
full overlap at 800–1000 m, and 355 (blue line) with overlap correction. The RH profile of the radiosonde
(black line) indicates the different layers.
Correlation of the particle backscatter coefficient (a–c) and particle linear depolarization
ratio (d–f) with RH for the three wavelengths 355, 532, and 1064 nm. The BERTHA measurements of
23 February 2014, 23:38–01:08 UTC, at 375–1100 m height are used (5 min temporal and 50 m vertical
resolution). The dashed line marks the sea salt efflorescence point (45 % RH).
Correlation of the PLDR at 1064 nm and RH for the same settings as in Fig. ,
but for a different height interval (375–1800 m). Above 1100 m (purple stars) RH increases again,
up to 80 %, as can be seen in Fig. . The depolarization ratio decreases with
increasing RH but keeps higher values. The hysteresis effect between crystallization (45–48 % RH,
dashed line) and deliquescence (70–74 % RH, dashed line) can be seen.
Measurement and simulation of the lidar ratio and the particle linear depolarization ratio for wet and dry marine
particles. The depolarization ratio (100 m vertical smoothing) and the lidar ratio (500 m vertical smoothing) are
measured at the indicated height, to which the given RH value belongs. The systematic uncertainties are given.
The lidar ratio at 1064 nm could not be measured in this configuration of BERTHA see. For the
DDA simulation of spherical and cubic particles the AERONET size distribution (SD) from Ragged Point at 12:31 UTC on
23 February 2014 is taken. The effective radius was 1.033 µm. In order to mimic dry marine particles, the
radius of the size distribution was divided by a factor of 2. For the wet marine particles a mixture of 7:1 parts of
water to salt was used. The model uncertainties are described in Sect. . The T-matrix
results for a typical size distribution of sea salt are taken from . The modeled
uncertainties are extremely small, less than 1 sr for the lidar ratio.
Scattering enhancement factors of pure marine aerosol
Beside the change in particle shape, the particle size is changing with decreasing RH. Sea salt particles grow by water
uptake when RH increases. The particle backscatter coefficient is proportional to the surface area of the scattering
particles and therefore a good indicator for particle growth. The AE (not shown) increase
monotonically between 1.0 and 1.5 km (extinction AE, lower limit due to overlap) and between 0.4 and 1.5 km
(backscatter 532 to 1064 nm AE).
This is a clear indication for the decrease in particles size. In Fig. a-c the particle backscatter coefficient β
is plotted against RH for the height interval 375–1100 m for the three wavelengths (λ=355, 532, 1064 nm), measured with a
vertical resolution of 50 m and temporal resolution of 5 min on 23 February 2014.
The backscatter enhancement factor fβ(RH,λ) is calculated:
fβ(RH,λ)=β(RH,λ)β(40%,λ).
Under atmospheric conditions it is hardly possible to observe completely dry
marine particles (RH < 10 %). Therefore the backscatter coefficient at 40 % RH
was chosen as reference value to normalize the data, as is common in the
literature see discussion in. Lower values than 35 % RH
were not accessible under the measurement conditions over Barbados. RH of
40 % is below the crystallization point, so sea salt particles should not be
affected by the hysteresis any more; i.e., significant shrinking at further
decreasing RH should not be the case. Nevertheless, the sea salt particles
are not completely dry at 40 % RH. Laboratory studies show an increase of the enhancement factor of 20–30 % between
0 % RH (f=1.0) and 40 % RH (f≈ 1.2–1.3). Other lidar-based
studies used 40 % or 60 % as reference RH
since it was the lowest value
found in their atmospheric measurements. In the following we will use the
expression “dry” when we refer to RH of 40 %, keeping in mind that sea salt
is not completely dry but below the crystallization point.
Backscatter enhancement factor for a dry value at 40 % RH (a) on a linear scale and (b) on a
log–log scale. The three wavelengths are fitted separately by Eq. ().
To parameterize the backscatter enhancement factor, we follow
and :
fβ(RH,λ)=A*(1-RH/100)-γ.
The parameter A gives the extrapolated value at 0 % RH and the exponent
γ describes the hygroscopic behavior of the particles. This
parameterization (sometimes with A=1 for starting at 0 % RH) has been used
by various investigators .
Retrieving values of A<1 (Table ) for the upper branch of
the hysteresis clearly indicates that the reference RH of 40 % RH for the
backscatter coefficient is too high, so that the sea salt particles are not
completely dry.
Simulation of the lidar ratio and the particle linear depolarization ratio for sea salt
cubes depending on volume equivalent radius. The three lidar wavelengths are treated separately.
The model simulations with increased number of dipoles (blue line) and orientations (orange line)
are given as an estimation of the uncertainty. A dashed line marks the effective radius
(reff=1.033µm) of the AERONET size distribution given in Fig. .
For the simulations of the cubic sea salt particles, the radius of the particles was reduced by a factor of 2. Details are given in the text.
AERONET particle volume size distribution for 23 February 2014, 12:31 UTC, at Ragged Point,
version 1.5 .
The backscatter enhancement factors are shown in Fig.
with the parameterization according to Eq. (). The fit is
shown on a linear scale (Fig. a) and on a log–log scale
(Fig. b). The log–log plot shows that the
parameterization does not hold for the lowest RH. The fit parameters are
listed in Table . RH has fairly reached 80 % in the used
height interval. fβ(75–80 %) is the averaged value between 75 % and
80 % RH. Additionally, fβ(80 %) was calculated by extrapolating the
fit. Both values of the backscatter enhancement factor with respect to the
reference RH of 40 % can be found in Table . By taking a
dry value at 40 % RH, we underestimate the scattering enhancement factor by
approximately 25 % . The extrapolated values are a lot
higher than the measured values at 75–80 % RH. In the following discussion we
will only use the measured enhancement factors. The error of the backscatter
enhancement factor results from the standard deviation of the mean values at
40 and 75–80 % RH and the uncertainty introduced by the error in RH.
The fit parameters for the backscatter enhancement factor according to Eq. (). The
backscatter enhancement factor fβ(80 %) calculated with these fit parameters is compared to the measured
factor fβ(75–80 %) between 75 and 80 % RH. The extinction enhancement factor fα(75–80 % RH)
was derived by Eq. () to compare with the literature.
For a better comparison to reported literature values, we convert our
backscatter enhancement factors fβ to extinction enhancement factors
fα by means of the backscatter-to-extinction ratio (lidar ratio
S), which was measured for wet and dry marine particles (see Table and and Fig. ).
fα(RH,λ)=SwetSdryfβ(RH,λ)
The extinction enhancement factor is commonly used in studies of the
dependence of particle optical properties on RH . For 1064 nm the simulated lidar ratios (Table ) have to be used. The errors of the lidar ratios are
included in the error of fα by Gaussian error propagation. Overall,
the relative error of extinction enhancement factor is approximately 50 % in
the UV and 30 % for the other wavelengths.
The measured extinction enhancement factors of pure marine aerosol range from
1.94 ± 0.94 (at 355 nm) to 3.70 ± 1.14 (at 532 nm) and 5.37 ± 1.66 (at
1064 nm). A clear wavelength dependence is given. Qualitatively the same
wavelength dependence of fα was observed by
for the wavelengths 450, 550, and 700 nm.
By assuming a high single scattering albedo (ratio of scatter to extinction
coefficient) for marine particle , the
scattering and extinction enhancement factors should be almost equal.
The scattering enhancement factors in the present study are in the upper range of reported literature values for marine aerosol.
This probably reflects the almost ideal marine conditions over Barbados during the winter season, when dust and pollution aerosol
from Africa are at the lowest level and 5000 km upwind is only ocean. reported mean scattering enhancement
factors f(85 %, 550 nm) for sea salt of 2.28 at Mace Head, Ireland, 2.86 at Ny-Ålesund, Svalbard, and 3.38 at Cabauw, Netherlands.
A review of mostly nephelometer-derived scattering enhancement factors at 550 nm is given in , where the values
for marine environments range between 1.5 and 3.5, depending on the amount of pollution.
The hygroscopic growth of pure marine particles is strong compared to
continental aerosol. The 7-year measurements in the Southern Great Plains,
continental USA (900 km away from the ocean), yield a scattering enhancement
factor f(85 %, 550 nm) of 1.78 compared to a dry value of 40 % RH
. found in a 4-year measurement
period over Leipzig, central Germany (400 km away from the ocean), an
extinction enhancement factor f(80 %, 550 nm) of 1.75 ± 0.4 compared to a
dry value of 40 % RH. For northwesterly wind directions (from the North Sea)
the enhancement factor was slightly higher at Leipzig (1.95 ± 0.5).
Comparison with optical modeling of sea salt cubes
The optical properties of cubic sodium chloride particles were modeled by
using the DDA. The model settings are
described in Sect. . The lidar ratio and PLDR for
each wavelength were modeled as a function of particle size (Fig. ).
If the particle diameter is close to the laser wavelength
(λ) and smaller, the lidar ratio is high (up to more than 100 sr).
For particle diameters >2λ, the lidar ratio is 20 ± 10 sr. The
lidar ratio decreases with further increasing particle size, e.g., down to
3.6 sr for the radius r=2µm at 355 nm. If the particle diameter is
smaller than the wavelength, the PLDR is very small. A significant
depolarization is produced for particle diameters ≥λ. Then,
values up to 0.27 are reached, although they vary strongly with particle size. In
the atmosphere, we have always a distribution of particles sizes and so
extreme optical effects are widely smoothed out.
We consider the size distribution inverted by the AERONET algorithm
from measurements at Ragged Point on 23 February 2014 at
12:31 UTC (Fig. ). It is version 1.5 only, but the
inversion of that measurement resulted in the lowest residual error of the
sky radiance on that day. We assume that the optical depth during that
measurement was dominated by wet marine particles. To calculate the optical
properties for dry marine particles we assume that their size is a factor of
2 smaller compared to the size obtained by AERONET and assume that the lidar
ratio and the linear depolarization ratio for particles with r>2µm is
the same as for r=2µm particles. The results are given in Table .
In the UV the simulated PLDR for the reduced radius agrees
with the measurements. At 532 nm the model underestimates the PLDR, but it is
still within the uncertainties of the lidar system. Whereas at 1064 nm, the
model overestimate the PLDR. By using the dry radius size distribution (a
factor of 2 lower radii) the agreement of the spectral slope of PLDR is much better than using the original AERONET distribution.
The T-matrix results for cubic sea salt are given for
comparison (Table ). They agree with the maximum PLDR in the
UV and visible (see Fig. d and e), whereas the 30 min mean
PLDRs
(Table ) are smaller. The simulations of cubic particles by
are limited by the maximum size parameter (x=10).
Taking the effective radius (1.033 µm) from the AERONET size distribution
(Fig. ) into account, the lidar ratio (18 ± 2 sr) and the
PLDR (0.15 ± 0.05) can be determined at 1064 nm only.
showed in a modeling study that a cubic shape
assumption (DDA model) is necessary to reproduce the backscatter of dry
marine particles. Their modeled scattering coefficient could not reproduce the
measurements no matter which shape is assumed.
The modeled lidar ratios are smaller than the measured ones. It is evident
that the low modeled lidar ratios at 355 nm (Table ) are due
to the very low lidar ratios of particles r> 1 µm (see Fig. a). Thus, the deviation between measurements and the models is
an indication that our applied size distributions contain too many large
particles. The uncertainty in the lidar ratio for dried marine particles in
the UV is high. Nevertheless, there is no large difference between wet and
dry marine lidar ratios.
Conclusions
The phase transition between spherical sea salt droplets and cubic-like sea
salt crystals has been observed under pure marine conditions over Barbados.
The particle linear depolarization ratio, measured with a triple-wavelength
polarization lidar, significantly increased when RH dropped below 50 %. The
combination of polarization and water vapor measurements with lidar offered
the unique opportunity to study this behavior with high vertical and temporal
resolution. The particle linear depolarization ratio in these dried marine
layers was enhanced on 23 and 24 February 2014 (0.05–0.12 at 355 nm,
0.07–0.15 at 532 nm, and 0.04–0.10 at 1064 nm). The systematic
investigations of the depolarization ratio for dry marine particles showed
maximum values (with systematic uncertainty) of 0.12 ± 0.08 (at 355 nm),
0.15 ± 0.03 (at 532 nm), and 0.10 ± 0.01 at 1064 nm.
We compared the optical properties for dry and wet marine particles at three
wavelengths at 40 and 80 % RH, respectively. Complete dry (0 % RH) sea salt
particles could not be found under atmospheric conditions. The extinction
enhancement factor for the range 40 to 80 % RH is 1.94 ± 0.94 (at 355 nm),
3.70 ± 1.14 (at 532 nm), and 5.37 ± 1.66 (at 1064 nm). These results are
given in Tables and .
A layer of dried marine aerosol observed over Barbados in February 2014
probably often exists at the MAL top when dry free-tropospheric air mixes
with humid air in the uppermost part of the MAL. Extended layers with dried
marine particles, as observed on 23 February, may also occur frequently when
the residual layer in the MAL is isolated from the convectively active MBL,
and this residual layer entrains dry air from the free troposphere.
Satellite-based studies, for example with CALIPSO or EarthCARE, would be
helpful to assess the global occurrence of dried MALs. These
changes in particle shape may have an impact on the Earth's radiative budget
over the oceans and therefore should be studied with global atmospheric
models, although the impact of thin layers of dry marine particles may be low
compared to the thicker layers of humid marine particles below.
Assuming generally spheroidal shape for nonspherical particles causes errors
in the case of marine observations. Inversion algorithms as used in AERONET
may be affected as well. For dry marine cases, we
suggest that a cubic model could be included.
An enhanced depolarization ratio for dry sea salt particles (up to 0.15 at
532 nm) leads to an overestimation of dust in aerosol classification and
separation schemes , so one should be careful in marine environments (decrease in RH in upper part of MAL)
and at coasts (sea breeze effects and decrease in RH over land). Marine
particles can be injected into the SAL by convective
cumulus convection, where RH is typically below 40 % and thus marine particles
have a cubic-like shape. Mistyping of aerosol layers will lead to wrong
results in further retrieved products, such as the extinction coefficient,
the mass concentration, or the estimates of cloud condensation nuclei and ice
nucleating particles .
The existence of cubic-like and spherical salt particles has been known for a
long time, but this study points out the atmospheric relevance. Cubic-like
sea salt has been measured under atmospheric conditions. The two shape modes
of sea salt (spherical and cubic-like) exist under atmospheric conditions
over the ocean and should be considered in future aerosol studies.
The data sets are available at the Leibniz Institute for Tropospheric Research and can be obtained upon request.
The authors declare that they have no conflict of interest.
This article is part of the special issue “The Saharan Aerosol Long-range Transport and
Aerosol-Cloud interaction Experiment (SALTRACE) (ACP/AMT inter-journal SI)”. It does not belong to a
conference.
Acknowledgements
Konrad Kandler acknowledges support from the Deutsche Forschungsgemeinschaft (grant KA 2280/2).
Josef Gasteiger has received funding from the European Research Council
(ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 640458, A-LIFE).
The logistical support of the Caribbean Institute for Meteorology and Hydrology (CIMH), Husbands, Barbados, should be acknowledged. The authors want
to express their gratitude to the three anonymous referees and Paul Zieger for carefully reading the manuscript and providing
helpful comments to improve it.
Edited by: Joshua Schwarz
Reviewed by: three anonymous referees
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