An unprecedented scientific flight was conducted over the
Strait of Gibraltar to study the optical properties of the atmospheric
aerosols from the sea surface to the lower free troposphere within the
framework of the southern Spain experiment for spaceborne mission validation
(SUSIE). A Rayleigh–Mie lidar was installed on an ultralight aircraft (ULA)
for vertical (nadir) and horizontal line-of-sight measurements. This
experiment took place on 13 August 2011 in parallel with continuous observations
with a N2-Raman lidar from the coastal site of San Pedro Alcantara
(∼ 50 km north-east of Gibraltar). Significant differences
were observed between the optical properties of the aerosol layers sampled
over the Strait of Gibraltar and San Pedro Alcantara. These differences are
related to the surface–atmosphere interface in the planetary boundary layer
and the origins and transport processes in the lower free
troposphere. A significant contribution of terrigenous aerosols originating
from the Iberian Peninsula is highlighted over the two areas. These polluted
dusts are identified with lidar ratios (LRs) ∼45±8 sr
higher than those of Saharan aerosols sampled during the same period
(<34 sr) at 355 nm. Furthermore, the particle depolarization ratio
is derived with values of ∼ 10 %–15 % for the polluted dust
and >20 % for the Saharan dusts. The difference in LRs is the
opposite of what is usually assumed for these two types of aerosols and
highlights the need to update the classification of aerosols in terms of LR
to be used in the inversion of vertical profiles from future spaceborne
missions embedding a lidar operating at 355 nm.
Introduction
Very little data exist on the aerosol characterization above the Strait of
Gibraltar and its surrounding area where the Atlantic Ocean and the
Mediterranean Sea meet. The temperature difference between these two water
surfaces inevitably induces a specific atmospheric circulation within the
lower troposphere associated with the well-known low-pressure corridor from
the Atlantic Ocean to the Mediterranean Sea. The vertical distribution of
aerosols can therefore be very heterogeneous against time and space in this
region and is an exciting source of study. In addition to this, the
Mediterranean region is identified as one of the “hotspots” in projections
of future climate change (Giorgi and
Lionello, 2008), in which direct and semi-direct effects of aerosol are not
properly taken into account in global climate change simulations
(IPCC, 2014). Indeed, the presence
of aerosols in the lower and middle troposphere have a significant impact on
sea surface temperature, evaporation, and precipitation at the regional scale
(e.g. Nabat et al., 2015).
This impact is mainly felt through a probable positive feedback on the trend
for future dryer and thus more atmospherically turbid Mediterranean summers.
In order to characterize the vertical distribution of aerosols over a long
period, from mid-June to the end of August 2011, a ground-based remote
sensing station was therefore installed in southern Spain in the
municipality of San Pedro Alcantara (36∘29′11′′ N, 4∘59′33′′ W) near Marbella in Andalusia. This installation was one of the
components of the Fennec programme which was conducted from June to July 2011
(Ryder
et al., 2013) and was specifically extended by the southern Spain experiment
for spaceborne mission validation (SUSIE) to support an airborne experiment
planned in August 2011 over the Strait of Gibraltar. This airborne
experiment was funded by the Centre National d'Etude Spatiales (CNES) and
the Commissariat à l'Énergie Atomique et aux Énergies Alternatives
(CEA). Its main goal was the preparation of the validation campaign for
355 nm wavelength Earth observation space missions such as the Atmospheric
Dynamics Mission Aeolus (ADM-AEOLUS), which was launched in August 2018
(Stoffelen et al., 2005; Andersson et al., 2008).
It is also a powerful tool for the preparation of the future Earth Clouds,
Aerosols and Radiation Explorer mission (EarthCARE;
Illingworth
et al., 2015), which is part of the Living Planet Programme of the European
Space Agency (ESA) and for which upstream studies based on simulations
(e.g. Chazette et al., 2001,
1998) have been conducted in the past to prepare the ATmospheric LIDar (ATLID)
planned to be embedded on board the EarthCARE satellite. Therefore, it
appeared necessary to rely on actual observations of different
scientifically relevant atmospheric environments to build a robust database
of vertical lidar profiles at 355 nm in order to conduct further realistic link budget studies. The SUSIE experiment is part of this
objective and preceded the CHemistry and AeRosols Mediterranean EXperiments
(ChArMEx; http://charmex.lsce.ipsl.fr, last access: 14 August 2020) which took place in the western Mediterranean in 2013–2014
(Mallet et al., 2016) over the Balearic Islands (Ancellet
et al., 2016; Chazette et al., 2016), Lampedusa island (e.g. Meloni et al.,
2018), and the French Riviera (Chazette et al.,
2019). It differs from ChArMEx by its location which is at the far west of
the Mediterranean Sea in connection with the Atlantic Ocean.
The choice of the Strait of Gibraltar and Andalusia to conduct this field
campaign was dictated by the high variability in optical thicknesses and
aerosol types that can be encountered over this geographical area, as shown
by Rodríguez et al. (2001) via ground measurements. This variability is
closely linked to the diversity of sources but also to highly variable
meteorological conditions over time (Gallero et al., 2006). The objective of
this paper is to present the original results obtained from this field
experiment. It brings a piece of information towards the understanding of
the variability in the lower troposphere of both the optical properties and
origins of observed aerosols. We will see that this is not as predictable as
one might expect.
In Sect. 2, we present the instrumental configuration. The methods used to
derive the optical properties of aerosols from the lidar profiles are
explained in Sect. 3. The analysis of the observations is carried out in
Sect. 4, and Sect. 5 presents the origin of the aerosols observed over
Gibraltar during the airborne experiment. In Sect. 6, we summarize and
conclude.
Instrumental set-up and strategy
Two observational platforms are used in this work, one is airborne and the
other one is positioned at ground level. Hereafter, we present the
instruments which compose those platforms.
Airborne measurements
Airborne measurements over the Strait of Gibraltar were performed using an
ultralight aircraft (ULA) equipped with an active remote sensing device. In
this subsection, the ULA scientific payload is presented as the flight plane
used during the experiment.
Payload
The ULA/Tanarg-installed eye-safe lidar system ULICE (Ultraviolet LIdar for
Canopy Experiment) (Shang et al., 2016) was used
to study the lower troposphere (between ∼ 0.15 and 3 km above
mean sea level, a.m.s.l.) over the Strait of Gibraltar. The lidar and the
ULA's flights close to Gibraltar are represented in Fig. 1. The Tanarg 912
XS was built by the company Air Création (http://www.aircreation.fr/, last access: 14 August 2020)
and offers a maximum total payload of ∼ 250 kg including a
scientific payload of ∼ 120 kg with a maximum autonomy of
∼ 3 h. The cruise speed is around 85–90 km h-1 and
the maximum flight altitude is ∼ 6 km a.m.s.l.
(Chazette
et al., 2007). The ULA is also equipped with a global positioning system
(GPS) and an attitude and heading reference system (AHRS), which are part of
the MTi-G components from Xsens (https://www.xsens.com/, last access: 14 August 2020).
Ultralight aircraft (ULA) above the Strait of Gibraltar. The lidar is located on the
left side of the ULA in nadir-shooting position. The aerosol layer can be
seen in ochre on the horizon of the photo.
The data acquisition was performed by averaging 1000 laser shots at 100 Hz
pulse repetition frequency, leading to a temporal sampling close to 10 s.
The lidar is controlled by a custom LabVIEW software on a PCI eXtensions for
Instrumentation (PXI) computer (National Instruments, http://www.ni.com, last access: 14 August 2020).
The ULA payload is autonomous with power supplied by an alternator
associated with the propeller. It can deliver the required 600–700 W.
Flight plan
The flight plan allowed a maximum number of lidar profiles during the flight
over the Strait of Gibraltar. There were two phases during the flight: a first
phase where the lidar line of sight was horizontal and a second phase where
the lidar shots were made with nadir sighting
(Fig. 2). It was during the ascent and descent
that the lidar's line of sight was horizontal to allow the vertical profile
of the aerosol extinction coefficient to be reconstructed without
assumptions about aerosol types, as shown by
Chazette
et al. (2007). After take-off from a private field aerodrome (Tahavilla,
Spain; 36∘11′17.8′′ N, 5∘45′23.2′′ W), the ceiling for
the flight was reached between 2.5 and 3 km a.m.s.l. in agreement with the
Spanish and British aviation authorities. The flight remained confined to
Spanish and British airspaces.
Flight plan of the ULA above the Strait of Gibraltar on 13 August
2011. The colour bar represents the angle of the line of sight (with respect
to the true horizon). The broken blue line corresponds to both the ascent
and the descent and the broken yellow line to the ULA's flight ceiling. The
view from the ULA in Fig. 1 is also highlighted. DEM stands for digital elevation model.
As the ULA is not an aircraft, it cannot take off in strong winds or rainy
conditions. It was therefore necessary to wait until the weather conditions
were suitable for take-off, i.e. with winds between 10 and 15 m s-1 at
ground level. The wind conditions also had to be associated with the
presence of significant aerosol layers above the area of Gibraltar that was
the subject of this airborne experiment. Between 1 and 22 August, when the
ULA was available, only Saturday 13 August in the late afternoon (19:50–21:20 local time, LT) met these conditions.
Ground-based tools
The sea shore site of San Pedro Alcantara (see
Fig. 2; ∼ 50 km north-east of
Gibraltar) was equipped with the first version of Lidar for Automatic
Atmospheric Surveys using Raman Scattering (LAASURS;
Chazette et al., 2017, 2019), described in
Royer et al. (2011). LAASURS
is an N2-Raman lidar dedicated to research activities. It comprises
three channels for the parallel and perpendicular polarizations with respect
to the laser emission and the inelastic nitrogen vibrational Raman line of
the laser-induced atmospheric backscattered signal. The version used during
this field campaign includes a Nd:YAG laser (Ultra®
manufactured by Lumibird/Quantel) emitting 16 mJ at 355 nm collimated to
fulfil eye-safety requirements. On the same site, a sun photometer was
installed and linked to the AErosol RObotic NETwork (AERONET) that
distributes the data (https://aeronet.gsfc.nasa.gov/, last access: 14 August 2020).
Temporal evolution in local time (LT) of both the aerosol optical
thickness (AOT) and Ångström exponent between 380 and 500 nm. The
flight period is highlighted by the grey area.
The ground-based site was operational from 25 June to 23 August 2011. Within
the framework of this study, which focuses on the ULA flight, the interest
is on the period from 11 to 14 August when significant aerosol optical
thicknesses (AOTs) were observed, as shown in Fig. 3. These AOTs may correspond to a mix of different aerosol types throughout the
time period when considering the Ångström exponent range
(∼ 0.3 to 1.3) also given in Fig. 3.
Data processing for lidar measurements
Whether for airborne observations (horizontal or nadir sightings) or from
ground level, after the background correction, the range-corrected lidar
signal, also called the apparent backscatter coefficient (ABC), S is written
for the distance s from the emitter as (Ansmann et al., 1992)
S(s)=C⋅βπ(s)⋅F(s)⋅exp-∫0s1+λRλE-4⋅αm(s′)+1+λRλE-A(s′)⋅LR(s′)⋅βπa(s′)⋅ds′.C is the system constant and F is the overlap function. The
backscatter coefficient βπ is the sum of the molecular and
aerosol βπa backscatter coefficients for the emitted
wavelength λE or the Raman backscatter coefficient for the
N2-Raman wavelength λR. The molecular extinction αm is given at the emitted wavelength λE. It is worth noting
that for the airborne lidar and the elastic channel of the N2-Raman
lidar, λR equals λE, whereas for the N2-Channel of
LAASURS, the wavelengths are different and it is necessary to take into
account the Ångström exponent A derived from the sun photometer. The
extinction to backscatter ratio (lidar ratio, LR) is given at the wavelength
λE, and it can depend on the altitude.
Airborne lidar
The inversion of airborne lidar data is performed in two successive steps
using the flight plan configuration that mixes horizontal and nadir sights.
This approach was first used by
Chazette
et al. (2007), where it is discussed. As a reminder, the aerosol extinction
coefficient αa (AEC) is calculated using horizontal sighting
measurements (the angle of the line of sight with respect to the true
horizon is here at ±10∘), assuming that the aerosol
optical properties do not change along the line of sight between two
distances s. During this flight, the lowest variability is obtained for s
between 0.7 and 1.2 km, which allows us to get as far as possible from a
residual effect of the overlap factor. Hence, for a flight altitude
zf, the AEC is given by
αazf=12∂LnSs,zf∂s-αmzf.
An estimate of the LR can then be calculated by adjusting the inversion
(Klett, 1981) of the nadir-looking lidar profiles to
the AEC profile. There are several independent measurements to differentiate
two aerosol layers against the altitude and to evaluate their respective LRs.
The separation altitude z0 between the two layers is also evaluated, and
the continuity of the LR is ensured via a sigmoid function defined against
the altitude z expressed in kilometres as
1LR(z)=1LRl+1LRu-1LRl/1+ez-z02.5.
LRl and LRu correspond to the LRs for the lower and upper aerosol
layers, respectively. The ULICE system also allows us to evaluate the volume
depolarization ratio (VDR) and then the linear particle depolarization ratio
(PDR), as presented in
Chazette et al. (2012).
Lidar profiles derived from ULICE on board the ULA above the Strait
of Gibraltar: (a) the apparent backscatter coefficient and (b) the volume
depolarization ratio (VDR). DEM stands for digital elevation model.
Figure 4 shows the vertical profiles in nadir
sighting for the ABC and VDR. They are obtained in the middle of the Strait
of Gibraltar. They show the presence of the two aerosol layers between the
sea surface and z0∼1 km a.m.s.l., a transition to a less
scattering and also low depolarizing layer, and then a second layer at an
altitude above 2 km a.m.s.l. The ULA flew within the last aerosol layer, and
we do not see its vertical extension that stretches beyond 3 km a.m.s.l. This
layer appears more marked because the associated backscattered signal is
less attenuated being close to the flight altitude. The red layer in
Fig. 4, close to the sea level (∼ 200 m a.m.s.l.), corresponds to the marine boundary layer (MBL). The VDR is lower
due to spherical aerosol presence. Unlike the evolution of VDR profiles
along the ULA path, the layered structure of the ABC evolves significantly.
The upper aerosol layer is more intense in the western part than in the
eastern part of the strait of Gibraltar. There also appears to be more
aerosol around 1 km a.m.s.l. on the west side.
N2-Raman lidar
The retrieval of aerosol optical properties from the N2-Raman lidar is
based on the approach previously presented in various papers and used both
the synergy with the sun photometer and the coupling between the elastic and
N2-Raman channels of LAASURS
(e.g.
Chazette et al., 2016; Royer et al., 2011). The signal-to-noise ratio (SNR)
of the daytime lidar profiles is insufficient to use the N2-Raman
channel to reach the upper aerosol layers above 3 km a.m.s.l. and makes it
difficult to identify different LR values. For this study, the inversions
are performed with 30 min time averages in order to evaluate an
equivalent LR value between 0.4 (excluding the residual effect of the
overlap factor) and 3 km a.m.s.l. During the day, photometric data are also
used assuming a constant extinction value between the ground and 0.4 km a.m.s.l. For special cases, a “two-layers” type inversion is performed as in
Dieudonné et al. (2017) in order
to verify that an equivalent LR does not induce significant uncertainties on
the other optical parameters. As for airborne measurements, the PDR is also
calculated for each lidar profile following
Chazette et al. (2012).
The main sources of uncertainty for this ground-based lidar are discussed in
Royer et al. (2011). The
uncertainty in the determination of the equivalent LRs is in the range of
10–15 sr. The SNR limits the exploitable range of the lidar profile, as
shown in Table 2 of Dieudonné et
al. (2017). The relative uncertainties on the PDR are close to 10 % for
the AOTs encountered at 355 nm (AOT >0.2;
Fig. 3).
Cross-calibration
Apart from the overlap factor, which is determined based on horizontal
sighting, it is necessary to check the calibration of the parallel and
perpendicular (via the VDR) channels between the two lidars so that the
derived products can be compared. VDR calibrations are carried out
independently of each other according to the procedure presented in
Chazette et al. (2012). They should result in identical vertical profiles
for both instruments. The cross-comparison experiment was performed at the
San Pedro Alcantara site on 6 August 2011 a few days before the flight. The
results are shown in Fig. 5. The different
vertical structures are coherent. The ABC profiles
(Fig. 5a) match very well with good agreement
considering the molecular signal above the aerosol layer (>4 km a.m.s.l.). The absolute deviation on the VDR is less than 0.2 %
(Fig. 5b), leading to an absolute error of less
than 2 % on the PDR for the aerosol layers encountered during the
experiment.
Vertical profiles of the (a) apparent backscatter coefficient (ABC) and (b) volume depolarization ratio (VDR) derived from the ground-based N2-Raman lidar LAASURS and the lidar ULICE placed at ground level in
zenith sighting.
Vertical profiles of aerosol optical properties
The lidar observations will be analysed in two stages. In the first stage,
the aerosol layers over the Strait of Gibraltar and their associated optical
properties will be studied. The link with ground-based lidar measurements at
San Pedro Alcantara will be discussed in the second stage.
Aerosol optical properties from the airborne lidar
In Fig. 6a, the vertical profile of the AEC
retrieved from horizontal sighting is shown for the flight between 19:50 and
21:20 LT on 13 August 2011. A very close profile has been derived using nadir
sighting when considering an LR ∼26±2 sr below 1 km a.m.s.l. and an LR ∼45±10 sr above this altitude. The first
corresponds to what is expected for marine aerosols
(e.g. Chazette et al., 2019; Flamant et al., 2000), while the second corresponds more to Saharan dust aerosols (e.g.
Papayannis et al., 2008; Soupiona et al., 2018). Using both the LRs and AEC,
Fig. 6b shows the corresponding vertical profile
of the PDR. The two profiles match very well with a lower PDR value
(∼ 5 %) within the MBL, as expected. The MBL presents a
higher variability of the AEC, which may not be linked only to the
heterogeneity of the wind field in the Strait of Gibraltar but also to strong maritime activity with the presence of numerous tankers. The PDR at
355 nm is between 10 % and 15 % within the dust layer, which appears low for
a potential layer of Saharan aerosols for which one would expect values
between 20 % and 30 % (e.g.
Freudenthaler et al., 2009). Note that lower values have been reported by
Papayannis et al. (2008) and
Chazette et al. (2016) (∼10 %–27 %), as well as by Soupiona
et al. (2019) (11±1 %–34±2 %).
Vertical profiles of (a) the aerosol extinction coefficient (AEC) derived from the airborne lidar for horizontal and nadir sightings, (b) the particle depolarization ratio (PDR) derived from the airborne lidar for horizontal and nadir sightings, (c) the AEC derived from the ground-based lidar, and (d) the PDR derived from the ground-based lidar. The ground-based
profiles correspond to an average between 19:00 and 22:00 LT on 13 August 2011. The aerosol optical thickness (AOT) is also given in Fig. 6c. In the figure, rms stands for root mean square.
Link with the ground-based lidar
Only ground-based lidar observations between 11 and 15 August will be
considered here. This period allows the framing of the airborne measurements by
showing the atmospheric conditions before and after the flight.
Figure 7 shows the temporal evolution of the AEC and
PDR profiles, the equivalent LR in the aerosol column, and the AOT. A first
event can be identified on 11 August with AOTs ∼ 0.5 at 355 nm.
It appears to diminish on 12 August and to start again on 13 August. The
presence of depolarizing particles is shown from the PDR to ∼ 6 km a.m.s.l. on 11 and 13 August. It is this presence, with a favourable
weather forecast, that triggered the flight of 13 August in the late
afternoon with an AOT ∼ 0.35 at 355 nm.
Ground-based lidar-derived temporal evolution of the vertical
profiles of (a) the aerosol extinction coefficient (AEC), the particle depolarization ratio (PDR), and (c) the lidar ratio (LR). In panel (c), the aerosol optical thickness (AOT), as derived from both the ground-based lidar and sun photometer, are also shown. The time location of the ULA flight over the Strait of Gibraltar is highlighted by the light-red area and is around 20:30 LT on 13 August 2011.
The mean AEC and PDR profiles derived from the ground-based lidar during the
flight period are shown in Fig. 6c and
d, respectively. Even if the shape of the
AEC profile is similar to that retrieved from the airborne measurements
between 0 and 3 km a.m.s.l., the amplitude is lower by a factor of more than 2.
The PDR nevertheless appears consistent between the airborne and
ground-based measurements in the dust-like layers with values close to
15 % in the layer between 2 and 3 km a.m.s.l. Near the surface, high PDR
values (>15 %) are observed over San Pedro Alcantara. These
values may be associated with local uprisings of dust aerosols (reported by
visual observations at the site), which were less present before the afternoon of 13 August (Fig. 7b). In the late afternoon of 13 August, the LR value is intermediate (36±5 sr) compared to those of the two
layers identified from airborne measurements (from 26±2 to 45±8 sr). Figure 8 shows the inversion of the average
profile in Fig. 6c using a two-layer distribution
of the LR similar to that considered for vertical profiles from ULICE. The
adjustment leads to similar LR values for the upper layer (34±4 sr)
compared to an inversion with a constant LR. The discrepancy with the
vertical profiles retrieved above the Strait of Gibraltar is mainly in the
lower layer, where the value of 45±6 sr is more in favour of the
presence of dust-like aerosols than marine aerosols for San Pedro Alcantara.
Nevertheless, we note the presence of aerosols of marine origin below 1 km a.m.s.l. before noon on 13 August. During the day on 12 August, low-layer
structures are observed in Fig. 7b, which can
suggest mixtures between marine and dust aerosols. The values of the LR can
then go below 35 sr and can drop to ∼ 25 sr.
Mean vertical profiles derived from the ground-based lidar between
19:00 and 22:00 LT on 13 August 2011: aerosol extinction coefficient (AEC),
particle depolarization ratio (PDR), and lidar ratio (LR).
Since the two lidars are consistent when measuring at the same site, the
observed differences are therefore related to different local emissions for
the lower layers and different transport processes for the upper layers,
although the distance between the two measurement points is only about 50 km. Nevertheless, it is worth noting that the range of LR values found in
the literature for dusts is quite wide, ranging from 28 sr (Soupiona
et al., 2019) to 80 sr (Papayannis
et al., 2008). Intermediate values are reported in the Aegean Sea by
Giannakaki et al. (2010) (52±18 sr) and Siomos et al. (2018) (Spring
∼47±13 sr, Summer ∼60±17 sr,
Autumn ∼47±15 sr). Over the Iberian Peninsula,
Fernández
et al. (2019) report LRs between 44 and 55 sr corresponding to an extreme
Saharan dust event intrusion. Chazette
et al. (2007) reported values in the order of 40 sr for pure dust aerosols
above Niger. Such variability may make the LR indiscriminate in the
identification of atmospheric aerosols. Veselovskii et al. (2020) explained
that the lidar ratio of dust aerosols at 355 nm above Senegal strongly depends
on the imaginary part of the refractive index and that such low values of LRs
observed in this work may indicate an imaginary part that is too low.
Origin of aerosols observed over Gibraltar
Desert dust from northern Africa is one of the main sources of aerosols over
the Gibraltar area. Their transport is linked to large-scale meteorological
conditions. On the decadal time scale, it has already been shown that the
North Atlantic Oscillation (NAO) index can play a significant role in the
occurrences of desert dust transport over the western Mediterranean
(Moulin et al., 1997) with higher mean
optical thicknesses during periods of positive NAO. The NAO index was
negative over the period of the experiment, as shown in
Fig. 9 plotted from data recorded at the site
https://climatedataguide.ucar.edu/climate-data/hurrell-north-atlantic-oscillation-nao-index-station-based
(last access: 14 August 2020; Hurrell et al.,
2004; Hurrell and Deser, 2010). Nonetheless, it is still somewhat higher
than those encountered during most summer situations in the decade
including the period of the experiment. This intermediate regime therefore
does not provide a clear view of the specificity of summer 2011 in terms of
the occurrence of dust transport events over southern Spain.
Temporal evolution of the monthly average North Atlantic
Oscillation (NAO) index. The data are those of the site
https://climatedataguide.ucar.edu/climate-data/hurrell-north-atlantic-oscillation-nao-index-station-based.
The grey area highlights summer 2011 when the SUSIE field campaign took
place.
From a local perspective, the particles influencing mainly the planetary
boundary layer (PBL) close to the Gibraltar area are related to marine
emissions but also to industrial activities, as shown by
Gallero et al. (2006). The region of
Gibraltar, near Algeciras, is indeed significantly industrialized with a
refinery, a petrochemical factory, a steel factory, a coal power plant, a
heavy fuel oil power plant, and a paper factory, all of which emit particles
and aerosol precursors.
Meteorological situation
During the period of the airborne experiment, the Azores high was moving
strongly northward (Fig. 10) leading to a blockage
in the flow from the African coast and even the establishment of a
north–south circulation at 850 hPa over the Strait of Gibraltar. On 13 August 2011 (Fig. 10a), the air masses arriving
over the Strait of Gibraltar were coming from the north-west, passing over
the Iberian Peninsula. This type of circulation is obviously not favourable
for the transport of dust from a Saharan origin. It was different on 11 August when a small depression was present west of the Strait of
Gibraltar, facilitating the transport of aerosols from Morocco
(Fig. 10b). These two contrasting weather patterns
may explain the evolution of the LR in the upper aerosol layer between 11
and 13 August.
Geopotential altitude at 850 hPa on (a) 13 August 2011 at 18:00 UTC and (b) 11 August 2011 at 18:00 UTC. The wind fields at the same pressure level are superimposed. The ERA5 reanalyses (https://www.ecmwf.int/en/forecasts/datasets/archive-datasets/reanalysis-datasets/era5, last access: 14 August 2020) with a
horizontal resolution of 0.25∘ are used.
Aerosol optical thickness (AOT) at 550 nm derived from MODIS on
(a) 13 August 2011 and (b) 11 August 2011.
Spaceborne observations
The location of aerosol plumes can be highlighted by the Moderate Resolution
Imaging Spectroradiometer (MODIS) on board the polar-orbiting platforms Terra
and Aqua
(King
et al., 1992; Salmonson et al., 1989). The level-2 products are provided
with a spatial horizontal resolution of 10 km×10 km
(http://modis.gsfc.nasa.gov, last access: 14 August 2020). The uncertainty on the AOT is ±0.15±0.05 AOT over land and
±0.05±0.03 AOT over ocean (Chu et al., 2002). A combination of the
AOT at 550 nm derived from the two satellites is
given in Fig. 11. On 13 August 2011
(Fig. 11a), a significant contrast is observed
between the west and east of the Strait of Gibraltar with a higher AOT
(∼ 0.4) at 550 nm to the west. This is consistent with what
was inferred from the airborne lidar observations compared to those made
from the ground-based lidar at San Pedro Alcantara. In contrast, on 11 August, similar AOTs (>0.6) are observed over the Atlantic Ocean
and the Mediterranean Sea (Fig. 11b). The higher
AOTs are consistent with those retrieved from the ground-based lidar. The
continuity of the aerosol plume between the Moroccan coast and the Strait of
Gibraltar is more pronounced than on 13 August.
CALIOP-derived (a) aerosol extinction coefficient at 532 nm and (b) aerosol typing. Two orbits are plotted on 13 August 2011, the first one
during nighttime at ∼ 02:40 UTC (westernmost over northern Africa)
and the second one during daytime at ∼ 13:40 UTC (easternmost
over northern Africa). DEM stands for digital elevation model.
Complementing the data from MODIS, the vertical profiles of the
aerosol layers are derived from the Cloud-Aerosol LIdar with Orthogonal
Polarization (CALIOP) on board Cloud-Aerosol Lidar and Infrared Pathfinder
Satellite Observation (CALIPSO; http://www-calipso.larc.nasa.gov, last access: 14 August 2020; Winker et al., 2007). The 4.10 version of
CALIOP level-2 data is used, whose aerosol typing has been improved
(Burton et al.,
2015). A night orbit (∼ 02:40 UTC) over the Atlantic and a day
orbit (∼ 12:40 UTC) just over the Strait of Gibraltar were used
for 13 August. They both show aerosol layers up to ∼ 4 km a.m.s.l.
for the studied area (Fig. 12a). These layers are
mainly identified as dusts and polluted dusts (Fig. 12b). Over Gibraltar (daytime orbit), polluted dusts are predominant. They
are also preponderant over the Iberian Peninsula with even elevated smoke
over land. This is therefore consistent with what has been found as a
vertical structure and LR via the airborne lidar. At this stage, the upper
aerosol layer that was sampled by the airborne lidar does not appear to be
pure dusts but a mix that can be associated with air masses of different
origins.
Air mass origins
In order to better identify the origin of the aerosols observed during the
field campaign, back trajectory studies were performed. For this, the Single
Particle Lagrangian Integrated Trajectory (HYSPLIT;
Stein et al., 2015) model was
used. It was initialized according to lidar observations over the Strait of
Gibraltar and San Pedro Alcantara. The wind fields used were taken from the
Global Data Assimilation System (GDAS; http://www.ncep.noaa.gov/, last access: 14 August 2020) at the
horizontal resolution of 0.5∘. HYSPLIT operated in ensemble mode;
i.e. 27 back trajectories are computed for each end location and for
different altitude ranges with a vertical sampling of 250 m.
Bi-dimensional histogram derived from back trajectories computed
using HYSPLIT on (a) 13 August 21:00 UTC over the flight location, (b) 13 August 03:00 UTC at the central position of CALIOP ground track off Gibraltar, (c) 13 August 21:00 UTC over San Pedro Alcantara, and (d) 12 August 00:00 UTC over San Pedro Alcantara. The aerosol transport altitudes are indicated for each main trajectory.
Most of the aerosol layers observed by the airborne lidar and daytime orbit
of CALIOP are located below 2.5 km a.m.s.l. As shown in Fig. 13a, the air
mass comes very clearly from the Iberian Peninsula for these altitudes. The
aerosol type is reminiscent of local uptakes of terrigenous dusts that may
be mixed with pollution aerosols. Above 2.5 km a.m.s.l., the air mass comes from
the tropical Atlantic and may have trapped Saharan dust aerosols. For CALIOP's night orbit on 13 August (∼ 02:40 UTC), although the
trajectories are significantly different, the origin is also the Iberian
Peninsula (Fig. 13b), and the same type of aerosol is likely to be
observed over the Atlantic off Gibraltar until 4 km a.m.s.l. Further north, a
contribution from forest fires cannot be excluded, but no satellite
observations clearly identify them. Above San Pedro Alcantara, at 21:00 UTC
(Fig. 13c), similar trajectories are observed as the ones over Gibraltar.
The LR is nevertheless quite different because its calculation integrates
layers not accessible to the airborne experiment above 2.5 km a.m.s.l. It
should be noted that the back trajectories below 500 m a.m.s.l. are not taken
into consideration as they are not significant with regard to the topography
of the Spanish coast. The back trajectories in Fig. 13d are calculated on
11 August and show that the differences in LR and PDR observed between 11
and 13 August 2011 over San Pedro Alcantara are explained by very different
origins of the air masses. On 11 August, the probable source of the aerosols
is located in Morocco. This conclusion can be supported using the brightness
temperature anomaly (BTA) calculated over the month of August 2011 from the
10.8 µm channel of the Spinning Enhanced Visible and InfraRed Imager
(SEVIRI; Schmetz et al., 2002) following an
approach similar to that proposed by Legrand et al. (1992). The BTA on 11 August is given in Fig. 14. It reveals very clearly
the presence of dust aerosols north-east of Agadir, Morocco, in the way the back
trajectories are plotted in Fig. 13d. It should be noted that no active source
is detectable by the same approach on 13 August 2011.
Brightness temperature anomaly (BTA) on 11 August 2011 at 12:00 UTC. The cloud mask has been applied in grey.
Conclusion
The western end of the Mediterranean, which connects with the Atlantic
Ocean, is one of the areas most subject to the transport of terrigenous
aerosols. This can be explained by the passage of lows, which, together with
the Azores high, favours the transport of desert aerosols through the Strait
of Gibraltar towards the Iberian Peninsula and along an inverse path towards
north-west Africa. Nevertheless, there have been few studies on the
characterization of the vertical distribution of aerosols over this region.
Although punctual in time, like many other studies, the SUSIE experiment has
provided useful information on vertical profiles of aerosol optical
properties. The profiles were obtained from airborne lidar observations over
Gibraltar and ground-based N2-Raman lidar measurements near
Gibraltar at San Pedro Alcantara located ∼ 50 km east of
Gibraltar. Over the Strait of Gibraltar, the aerosol extinction coefficient
(AEC), particle depolarization ratio (PDR), and lidar ratio (LR) were derived
from a flight plan that allowed separate retrievals of the aerosol
extinction and backscatter properties. Although the distance between the two
measurement sites is small, the optical properties of the aerosols proved to
be significantly different. The planetary boundary layer was more influenced
by terrigenous aerosols over San Pedro Alcantara, whereas the marine
aerosols were dominant over Gibraltar. In the lower free troposphere, the
difference between the LRs, 45±8 sr for Gibraltar and 34±4 sr
for San Pedro Alcantara, is somewhat less noticeable and can be attributed
to a higher weighting of the upper atmospheric layers as sampled by the
ground-based lidar. The back trajectories show a dichotomy between the air
masses below and above 2.5 km a.m.s.l. and thus a possible mix between
continental and Saharan aerosols, respectively. This would lead one to think
that the continental terrigenous aerosols would have an LR of about 45 sr,
while the Saharan aerosols would have an LR of about 34 sr. This is also what
is found in Fig. 7 during the Saharan aerosol event of Moroccan origin on
11 August 2011. For such low values, there may be a mixture of marine
particles in the upper aerosol layer. It may be generated by a recirculation
at altitude above the PBL top of a certain quantity of marine aerosols above
the coastal site
(e.g. Chazette
et al., 2019), but since no strong argument is available to claim this,
that statement therefore remains speculative. As a result, we can infer that a mixture
of different types of particles is likely, to which pollution or biomass
burning aerosols in varying quantities may be added. All this can explain
the range of variation of the LR at 355 nm that is deduced from the
scientific literature.
The use of LR look-up tables for the inversion of satellite lidar
measurements can therefore lead to biased results in situations such as
those encountered during SUSIE. Using the CALIOP classification in the
context of this work, polluted dusts should be classified with an LR ∼ 45 sr and Saharan dusts with an LR ∼ 34 sr for
the wavelength of 355 nm. It is worth noting that there are numerous other
Raman lidar measurements in Africa bringing an LR at 355 nm well above 40 sr
(e.g. the experiments SAMUM, Ansmann et al., 2011, and SHADOW, Veselovskii et al., 2020). Here, the LR variation is inversed compared to what is
considered for CALIOP (44 sr for dusts and 55 sr for polluted dust at 532 nm) and even for the Cloud-Aerosol Transport System (CATS;
Yorks et al., 2016), for which the LR equals 45 sr for dusts and 35 sr for polluted dusts at 532 nm. It
therefore appears important to update the classifications in the perspective
of the analysis of lidar profiles from the ADM-AEOLUS mission and also from
the future EarthCARE mission.
Data availability
The sun photometer products were provided by the AERONET network (https://aeronet.gsfc.nasa.gov/, last access:
14 August 2020, Giles et al., 2019). MODIS data were provided by NASA Langley Research Center Atmospheric Sciences Data Center (https://modis.gsfc.nasa.gov/data/dataprod/, last access: 14 August 2020, e.g. Levy and Hsu, 2015a, b). CALIPSO data were provided by NASA's Earth Observing System Data and Information System (EOSDIS; https://www-calipso.larc.nasa.gov/products/, last access: 14 August 2020, Vaughan et al., 2004). The ERA5 dataset is provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) integrated forecast system developed through the Copernicus Climate Change Service (https://climate.copernicus.eu/, last access: 14 August 2020, Copernicus Climate Change Service, 2018).
Competing interests
The author declares that there is no conflict of interest.
Special issue statement
This article is part of the special issue “CHemistry and AeRosols Mediterranean EXperiments (ChArMEx) (ACP/AMT inter-journal SI)”. It is not associated with a conference.
Acknowledgements
Joseph Sanak is
thanked for his help during the field experiment. The ULA flights were
performed by Santi Font. The author would like to thank the AERONET network
for sun photometer products (at https://aeronet.gsfc.nasa.gov/, last access: 14 August 2020), the MODIS
Science, Processing and Data Support Teams for producing and providing level 2 MODIS
data (at https://modis.gsfc.nasa.gov/data/dataprod/, last access: 14 August 2020), and the NASA Langley
Research Center Atmospheric Sciences Data Center for the data processing and
distribution of CALIPSO products (level 4.10, at
https://www-calipso.larc.nasa.gov/products/, last access: 14 August 2020). The
NOAA Air Resources Laboratory (ARL) is thanked for the provision of the
HYSPLIT transport and dispersion model and READY website
(http://www.ready.noaa.gov, last access: 14 August 2020) used in this paper. The European Centre for Medium-Range Weather Forecasts (ECMWF) is thanked for the provision of the ERA5 dataset.
Financial support
This research has been supported by the Centre National d'Etude Spatiales (CNES) and the Commissariat à l'Énergie Atomique et aux Énergies Alternatives (CEA).
Review statement
This paper was edited by Oleg Dubovik and reviewed by two anonymous referees.
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