For the first time, a 355 nm backscatter N2-Raman
lidar has been deployed on the western part of the French Riviera to
investigate the vertical aerosol structure in the troposphere. This lidar
system, based at the AERONET site of Toulon–La Garde, performed continuous
measurements from 24 June to 17 July 2014, within the framework of the
multidisciplinary program Mediterranean Integrated Studies at the Regional
and Local Scales (MISTRALS). By coupling these observations with those of
the spaceborne instruments Cloud-Aerosol LIdar with Orthogonal Polarization
(CALIOP), Spinning Enhanced Visible and InfraRed Imager (SEVIRI), and
Moderate Resolution Imaging Spectroradiometers (MODIS), the spatial extents
of the aerosol structures are investigated. The origins of the aerosol
plumes are determined using back trajectories computed by the Hybrid Single
Particle Lagrangian Integrated Trajectory (HYSPLIT). This synergy allowed us to
highlight plumes of particulate pollutants moving in the low and medium free
troposphere (up to ∼5km above the mean sea level) towards
the French Riviera. This pollution originates from the Spanish coast,
more particularly from Costa Blanca (including Murcia) and Costa Brava–Costa
Daurada (including Barcelona). It is mainly due to traffic, but also to
petrochemical activities in these two regions. Desert aerosol plumes were
also sampled by the lidar. The sources of desert aerosols have been
identified as the Grand Erg Occidental and Grand Erg Oriental. During desert
dust events, we highlight significant differences in the optical
characteristics in terms of the backscatter-to-extinction ratio (BER, inverse
of the lidar ratio) between the planetary boundary layer, with 0.024 sr-1
(∼42sr), and the free troposphere, with 0.031 sr-1 (∼32sr). These differences are greatly reduced in
the case of pollution aerosol plume transport in the free troposphere (i.e.,
0.021 and 0.025 sr-1). Transported pollution aerosols appear to
have
similar BER to what is emitted locally. Moreover, using the correlation
matrix between lidar aerosol extinction profiles as a function of altitude,
we find that during transport events in the low free troposphere, aerosols
may be transferred into the planetary boundary layer. We also note that the
relative humidity, which is generally higher in the planetary boundary layer
(>80 %), is found to have no significant effect on the BER.
Introduction
The French Riviera region is the most densely populated area of southern
France with, as of 2018, about 4.2 million inhabitants in the Provence–Alpes –Côte d'Azur counties bordering the Mediterranean Sea and the
Principality of Monaco. The greater Côte d'Azur region is also the
first tourist destination in France after Paris, with 20 million tourists
generating over 130 million overnight stays every year, as well as 1.2 million
cruise passengers, increasing traffic pollutants, especially in
summer. Between 1990 and 2005, a marked increasing trend for PM10 ambient
concentrations was observed in this area, correlated with an increase in
airway diseases (Sicard et al.,
2010). The impact of local traffic is predominant in the summer season, but
the industries in the Bouche du Rhône area are also large contributors
(El Haddad et al.,
2013), and the impact of exogenous sources other than Saharan dust is not
negligible (Dimitriou and Kassomenos, 2018). In the coastal town
of Toulon, centrally located on the French Riviera,
Piazzola et al. (2012)
have shown during specific events that air masses could be impacted by
pollution transported over the Mediterranean. Yet, these source
apportionment studies are only based on surface chemical analyses and
back trajectories, which do not take into consideration the complex
meteorological environment of the coastline.
In the framework of the multidisciplinary program Mediterranean Integrated
Studies at the Regional and Local Scales (MISTRALS;
http://www.mistrals-home.org, last access: 15 March 2019), in particular for the Chemistry-Aerosol
Mediterranean Experiment (ChArMEx; http://charmex.lsce.ipsl.fr, last access: 15 March 2019)
(Mallet
et al., 2016), aerosols in the Mediterranean basin have been studied by
several authors, either via their chemical composition
(e.g., Cholakian et
al., 2018), their optical properties (e.g.,
Chazette et al.,
2016; Granados-Muñoz et al., 2016),
their radiative budget (e.g.,
Nabat et al., 2015;
Di Biagio et al.,
2016; Sicard
et al., 2016), or the identification of their sources (e.g.,
Ancellet et al., 2016;
Chrit et al., 2018). Among
these studies, few were conducted in the atmospheric column above the French
Mediterranean coast, which may be subject to aerosol loads of very different
origins and chemical compositions. These aerosols directly influence the air
quality (e.g., Knipping and Dabdub, 2003), as well as
the climate balance of the western Mediterranean Sea (e.g.,
IPCC, 2014;
Nabat et al., 2015), in
different ways depending on their nature and the surface albedo. For
instance, during the Hydrological Cycle in the Mediterranean Experiment
(HyMeX, also part of the MISTRALS program), the radiative effect of dust
aerosols has been shown to have little impact on the rainfall amounts and
location over the western Mediterranean basin
(Flamant et al.,
2015).
All of these studies were preceded by early campaigns such as that of the
European project Mediterranean Dust Experiment
(e.g., Hamonou et al., 1999) or even networked
observations such as those of the lidar Earlinet network (e.g.,
Balis et al., 2000;
Pappalardo et al., 2004;
Papayannis et al., 2008; and more
recently Granados-Muñoz et al.,
2016). Coupling in situ measurements and modeling, the vertical structure
of the planetary boundary layer under sea breeze conditions was also
investigated during the ExperimentS to COnstrain Models of atmospheric
Pollution and Transport of Emissions (ESCOMPTE;
Cros
et al., 2004) over the Marseille–Berre area, ∼40km west of
the French Riviera.
Little information exists describing the transboundary transport
of aerosols within the free troposphere over the French Riviera. For this
reason, a ground-based N2-Raman lidar was installed in this region
between 24 June and 16 July 2014. The lidar has combined more than 500 h
of continuous operation and has made it possible to carry out a
significant study of aerosol types and origins in synergy with
spaceborne observations and back trajectory modeling. Works performed
using lidar measurements over the Balearic island of Menorca
(Chazette et al., 2016) have already highlighted the long-range
transport of aerosols coming from forest fires in North America (see also
Ancellet et al., 2016), from
deserts, and, to a lesser extent, from pollution sources located on the Costa
Brava (Barcelona). During the summer period, we were not able to establish a
clear link between the polluted air masses passing over Menorca and those
reaching the French Riviera, suggesting another pathway and/or other aerosol
sources. Moreover, previous studies carried out on the French Riviera were
only based on surface observations
(e.g., Piazzola et al.,
2012) and have not offered the possibility of clearly identifying the origin
of aerosols, which will be shown hereafter.
The experimental strategy is developed in Sect. 2, where the lidar, the
signal processing, and the main uncertainty sources are presented. The
temporal evolution of aerosol optical properties and the vertical
atmospheric structure over the French Riviera, observed by the ground-based
lidar, are discussed in Sect. 3. Section 4 is devoted to the meteorological
conditions during the field campaign. The long-range transport of aerosol
plumes highlighted from the lidar measurements is described in Sect. 5,
using the coupling with both spaceborne measurements and back trajectory
studies. Section 6 summarizes and concludes.
Strategy
The perimeter of the western Mediterranean is composed of mountains with
elevations generally greater than 1000 m above mean sea level (a.m.s.l.). This
specific morphology facilitates the recirculation of air masses, and
therefore aerosols, via the sea breeze–land breeze cycle, the alternation
between the katabatic and anabatic winds, and the guiding of the air mass
circulation in the valleys and along the seashore. Sea breeze is an
effective means of exchange between the marine or continental boundary layer
and the free troposphere, especially around the Mediterranean basin
(Bouchlaghem
et al., 2007; Lasry et al., 2005; Levy et al., 2008; Millán et al.,
1991). The aerosols trapped in the low and medium free troposphere will then
be transported over long distances and may arrive over the Mediterranean
coasts where they will reach the surface via free or forced convection
processes generated by the mountains. This transport often happens in thin
sheet-like plumes whose vertical limits are generally marked by
discontinuities in the potential temperature gradient (e.g.,
Dalaudier et al., 1994;
Chazette et al., 2001). The
observation of such layers requires a profiler with a high vertical
resolution and justified the deployment of an N2-Raman lidar on the
French Riviera, close to the AERONET (AErosol RObotic NETwork) site of
Toulon–La Garde (43.13556∘ N, 6.00944∘ E; 50 m
elevation), which is representative of peri-urban conditions. The location
of the site is given in Fig. 1a and b. The lidar performed automatic
measurements from 24 June to 17 July 2014. It was remote-controlled from
the Paris area. Lidar measurements are investigated together with active and
passive remote sensing spaceborne observations, as well as by back
trajectory studies.
(a) Map of the western Mediterranean basin and (b) crop showing the
location of the lidar at the Toulon–La Garde AERONET site on the French
Riviera (elevation data from the GTopo30 DEM, courtesy of USGS). (c) Lidar in
its confinement during the field experiment.
The automatic N2-Raman lidar
The N2-Raman lidar LAASURS (Lidar Automatic for Atmospheric Surveys
using Raman Scattering) is a research instrument composed of three channels
for parallel and perpendicular polarizations, with respect to the laser
emission and the inelastic nitrogen vibrational Raman line of the laser-induced atmospheric backscattered signal
(Royer et al., 2011; Chazette et
al., 2016). The lidar in its confinement is shown in
Fig. 1c. The emission energy was 16 mJ at the wavelength of 355 nm and
fulfilled eye-safety requirements. The overlap function of the lidar is
equal to 1 at distances between 150 and 250 m from the emission. The
emission comes from an Ultra® Nd:YAG laser manufactured by
Quantel, delivering 6 ns width pulses at a repetition rate of 20 Hz. The
detection is carried out by photomultiplier tubes and narrowband filters
with a bandwidth of 0.2 nm. The signal acquisition is performed using
PXI technology, manufactured by the National Instruments™ company
(http://www.ni.com, last access: 15 March 2019), and using an acquisition board with a sampling rate of
200 MHz corresponding to a lidar native vertical sampling of 0.75 m. Note
that the acquisition uses a pre-trigger for the correction of the sky
background. This avoids effects related to potential variability in the
baseline of analog-to-digital converters.
Inversion scheme
In order to retrieve the optical properties of aerosols transported in the
free troposphere, we used an N2-Raman lidar system coupled with the
sun photometer (e.g., Royer
et al., 2011; Chazette et al., 2016) of the Toulon–La Garde AERONET
(http://aeronet.gsfc.nasa.gov/, last access: 15 March 2019; level 2 data of version 3) site. Indeed, the
signal-to-noise ratio (SNR) of the daytime lidar profiles is insufficient to
use the N2-Raman channel as it presents a low emitted energy.
Contrariwise, the N2-Raman channel was used during nighttime for the
retrieval of the cumulative aerosol optical thickness (AOT). Combined with
the elastic channel, this leads to the retrieval of the aerosol backscatter
and extinction coefficients (ABC and AEC) and their ratio
(Chazette et
al., 2016). The backscatter-to-extinction ratio (BER) is equal to the
product of the single-scattering albedo and of the probability of a photon
being backscattered after an interaction between the laser flux and the
atmospheric scatterers. It is the inverse of the lidar ratio (LR) often used
in the literature. We prefer to consider the BER hereafter, which has a more
direct physical meaning. The linear particle depolarization ratio (PDR) is
also retrieved as in Chazette
et al. (2012). Calculations are performed with a temporal resolution of at
least 30 min and a vertical resolution of 30 m to improve the SNR in the
middle troposphere (between 6 and 7 km a.m.s.l.).
Temporal evolution (UTC) between 24 June and 7 July of (a) the
backscatter-to-extinction ratio (BER; mean value as a black line and error
bar in orange), as well as the aerosol optical thicknesses (AOT), at
355 nm
derived from the sun photometer (red circles) and lidar measurements (blue
triangle), (b) the vertical profile of the aerosol extinction coefficient
(AEC) at 355 nm, and (c) the vertical profile of the linear particulate
depolarization ratio (PDR) at 355 nm. White time stripes correspond to
periods of low and middle clouds. The origins of the main aerosol plumes
trapped in the free troposphere are indicated.
Same as Fig. 2 for the period between 8 and 17 July.
Compared to the inversion scheme proposed in
Chazette et
al. (2016), in which an equivalent BER for the entire aerosol column is
justified, the frequent presence of two aerosol layers, one in the marine
boundary layer and the other in the free troposphere, led us to develop a
multilayer inversion to independently evaluate the BERs of each aerosol
layer and verify the relevance of the first approach, leading to an
equivalent BER. This method builds on the case of a single aerosol layer
described above. We begin by inverting the upper layer and determining its BER
or LR, then we apply the same approach to the lower layer, retaining the LR
of the higher layer. The constraint is given by the partial AOTs
(Dieudonné et al.,
2015) calculated from the N2-Raman channel for each of the aerosol
layers previously located in altitude. The transition altitude between the
layers is determined manually and the continuity is ensured by a sigmoid
function on a thickness of about 1 km between the two layers. Such an
approach is possible, just like in the case of a single aerosol layer, if
the SNR is larger than 10. This leads us to produce nighttime profiles with
a time average of 5 h between 23:00 and 04:00 universal time count (UTC).
Uncertainties
The main uncertainty sources are discussed in
Royer et al. (2011). The
relative uncertainty in the N2-Raman-derived cumulative AOT is less
than 2 % for SNR >10. The uncertainty in the determination of
the equivalent BER is in the range of 4–6×10-3sr-1 (10–15 sr in
terms of LR). Such a value is very dependent on the SNR, which limits the
exploitable range of the lidar profile, as shown in Table 2 of
Dieudonné et al. (2017). The
relative uncertainties in the PDR are close to 10 % for the AOTs
encountered at 355 nm (AOT >0.2).
Temporal evolutions (UTC), from 4 June to 17 July 2014, of thermodynamic
temperature and relative humidity at 2 m above the ground level.
Lidar observationsTemporal evolution
The temporal evolution of the vertical profiles of optical parameters derived
from lidar observations is shown in Fig. 2 and 3, separated into two periods:
before and after 7 July. Several days show anomalies of the aerosol load in
the free troposphere, whereas others are more common, with a marked
signature of the boundary layer cycle. The aerosol layers above
1.5 km a.m.s.l. are generally linked to long-range transport, especially on 24 and 28 June and 1–4 and 7 July
(high values of AEC in Fig. 2b). Over the coastal site,
aerosols within the planetary boundary layer (PBL) are mainly from local
sources, either sea-spray aerosols generated by breaking waves
(e.g., Yoon et
al., 2007) or continental components arising from both natural and
anthropogenic sources. The relative influence of these aerosol types is
modulated by the land–sea breeze cycle
(e.g., Piazzola et al.,
2012).
The values of PDR are very variable, ranging between ∼1 %
and more than 20 %. These highest values are observed in the free
troposphere on 24 June and 3–4 July and may be associated with plumes of
terrigenous aerosol with nonspherical shapes. In the PBL, we note the
existence of vertical streaks, which are the signature of thermal updrafts
developing during the day. They can lift local terrigenous particles and
even pollen to the PBL top. The PDR is greater, but not very high inside
these structures (2 %–3 %). Indeed, these aerosols are most probably mixed
with a significant quantity of spherical hygroscopic particles from the sea
or from local pollution.
Relatively high AOTs are measured over much of the observation period, with
values exceeding 0.2 at 355 nm and peak values greater than 0.5. AOT values
below 0.2 correspond to undisturbed periods, i.e., without the presence of
aerosol layers in the free troposphere. Hereafter, days are tagged as
disturbed with an aerosol charge anomaly in the free troposphere or as
undisturbed in the contrary case. It should be noted that, during the second
period of the measurement campaign, there is no significant aerosol load in
the free troposphere. During the first period (24 June to 7 July), it is
rather the opposite, except from 30 June to 1 July. These two days are
grouped together with those of the second period in our analysis. Moreover,
these two days show a very strong variation in the BER along time as is
the case in the beginning of the second period, from 8 to 10 July
(0.020 sr-1 during nighttime and 0.035 sr-1 during daytime).
These days are associated with low relative humidity at the ground level:
below 50 %, as shown in Fig. 4. The liquefaction point of the soluble
compounds trapped on the aerosol was therefore probably not reached (e.g.,
Randriamiarisoa et al., 2006). The strong
variations in BER are certainly attributable to the local breeze regime with
a stronger marine contribution during the day and therefore larger aerosols.
The daytime value is similar to that found by
Flamant et al. (1998), which was
∼0.040sr-1 for marine boundary layer aerosols close to
the Azores.
Vertical profiles of the cumulative aerosol optical thickness
(AOT) for six different nights. The lidar profiles are time-averaged between
23:00 UTC the previous day and 04:00 UTC the next day. The inflection levels between
the two main aerosol layers are highlighted by a horizontal blue line for
each night. The backscatter-to-extinction ratios (BERs) are given in the
legends in parentheses for the lower (BERdl) and upper (BERul)
aerosol layers.
Variability along the altitude
Before proceeding further, we will check whether or not the hypothesis of a
constant BER in the whole atmospheric column is justified. An easy way to
verify this is to compare the results for the profiles of the cumulative AOT
derived from the inversion of the elastic channel and the one from the
N2-Raman channel as in
Chazette et al. (2017):
the selected BER value is close to the real one if the two profiles
coincide.
Here, the two-layer method described in Sect. 2.2 is applied for nighttime lidar profile inversion. The profiles of cumulative AOT for the main
cases of the campaign are given in Fig. 5. We noticed that BER values in the
lowermost layers (0.024 sr-1) are only significantly different to the
ones found in the upper aerosol layers (0.031 sr-1) in the case of 24 June.
On the same day, the retrieved column-equivalent BER ranges between
0.017 and 0.033 sr-1, with a mean value of 0.027 sr-1. Taking
uncertainties into account, these values overlap. We can, however, expect an
error in total AOT of about 15 % to 20 %. For the other nights, the
hypothesis of one column-equivalent BER value is reasonable, although the
nature of aerosols can be different between layers. In our case, there is a
strong anthropic component associated with the marine aerosols over the
measurement site
(Piazzola et al., 2012).
These aerosols being hydrophilic, both their size and their complex
refraction index can change significantly between the PBL and the free
troposphere (Randriamiarisoa et al., 2006).
Nevertheless, the work of Raut and
Chazette (2008a) has shown that a variation in relative humidity does not
significantly modify the value of BER for traffic aerosols. Here, the low
vertical variability in BER supports these findings, given that
the relative humidity in the PBL was mainly above 80 %, whereas it was
below 50 % in the free troposphere.
Lidar-derived correlation matrices for (a) the entire measurement
period, between 24 June and 17 July 2014, and (b) the undisturbed period
(without aerosol load anomaly in the free troposphere). These matrices are
symmetric by construction.
In addition, the study of correlation matrices computed from the AEC
profiles between altitudes for (i) the whole duration of the campaign and
(ii) for the undisturbed period shows that there are correlated aerosol
plumes between the PBL and the free troposphere, which can be due to
the transfer of aerosols between these two layers. Figure 6 gives a graphical
representation of the magnitude of the coefficients in the matrices of both
the entire measurement period, between 24 June and 17 July 2014, and the
non-perturbated period. For undisturbed cases (Fig. 6a), the correlation
distance, calculated as the distance over which the correlation coefficient
is greater than 0.6, does not exceed ∼2km, whereas it
largely exceeds 3 km when averaged over the whole duration of the campaign.
This argues for the conclusion that during transport events in the low free
troposphere, aerosols may be transferred into the PBL. Simultaneous
transport between the marine boundary layer and the free troposphere is
unlikely over the large distance separating the emission and the arrival of
the aerosol plumes over the French Riviera. The most relevant hypothesis is
that the recirculation of air masses along the seashore due to relief and a
modification of average winds in the PBL between undisturbed and disturbed
cases explain these transfers. Furthermore, large correlation distances are
found in the middle free troposphere (Fig. 6b), where aerosol plumes have
been transported over a long distance. For undisturbed situations above 5.5 km a.m.s.l.,
the patchiness of the data is due to the relatively high noise when
aerosol scatterer is almost absent.
Meteorological conditions
Wind conditions control the transport of aerosols above the French Riviera.
To analyze the meteorological conditions during the field campaign over our
observation site of Toulon, we use the ERA5 reanalyses
(https://www.ecmwf.int/en/forecasts/datasets/archive-datasets/reanalysis-datasets/era5, last access: 15 March 2019)
with a horizontal resolution of 0.25 or 0.30∘. This dataset is
provided by the European Centre for Medium-Range Weather
Forecasts integrated forecast system (ECMWF), developed through the
Copernicus Climate Change Service (https://climate.copernicus.eu/, last access: 15 March 2019).
Figure 7 shows the wind direction distribution, computed from ERA5 data for
the measurement site, at the 975 hPa level (∼0.4km a.m.s.l.)
during the disturbed (Fig. 7a) and undisturbed cases (Fig. 7b). For the
undisturbed cases, the majority of winds come from the northwest, and there
is little marine contribution. We therefore do not observe a marked see–land
breeze cycle at the model resolution. In this configuration, one can expect
the influence of pollution sources to be linked to road traffic, which is
intense in the summer season just north of the measurement site, or of
possible biomass fires that are very frequent in the backcountry at this period. For
disturbed cases, the origin of winds near the surface is much more diverse
and reflects the breeze cycle with a significant marine contribution from
the south sector during the day and dominant winds from the east or west
along the coast. The northwest component is still present in the disturbed
cases but much less frequent. In the lower free troposphere, there are also
strong differences between disturbed and undisturbed situations. Figure 8
presents wind distributions at the 700 hPa level (∼3km a.m.s.l.).
For undisturbed cases (Fig. 8b), winds are predominantly from the
north-northwest. It is much less distinct during disturbed cases, and
presumably multiple contributions are observed from the west-northwest,
west-southwest, and south-southwest. The different potential
contributions are explained hereafter.
Frequency of counts (%) by wind speed direction at
975 hPa
(∼0.4km a.m.s.l.) for (a) disturbed cases, which are days with aerosol
anomaly within the free troposphere, and (b) undisturbed cases, which are days without
anomaly. The hourly data are from the ERA5 reanalyzes at 0.25∘ of
horizontal resolution and at the closest grid point from the station.
Same as Fig. 7 for the pressure level of 700 hPa (∼3km a.m.s.l.).
On a larger scale, the circulation of air masses advected over the western
Mediterranean mostly depends on the relative positions of the Azores and
Siberian highs. It is strongly modulated by lows traveling east over midlatitudes. Depending on the position of these lows, air masses from the
Atlantic (over Gibraltar) and from the Sahara can reach the western
Mediterranean coast. Figure 9 gives an illustration of the two different
configurations predominantly encountered during the measurement campaign.
In Fig. 9a, the meteorological situation of 24 June is presented for level
700 hPa in the ERA5 reanalyses. It corresponds to elevated values of PDR in the
lower free troposphere (Fig. 2c). Tropical air masses are channeled by the
presence of two highs, one over the North Atlantic (Azores high) and the other
over the Sahara, as well as a weak low over the Iberian Peninsula and a
strong low over Scandinavia. This configuration favors the transport of
desert dust aerosols over the Mediterranean Sea
(Hamonou et al., 1999). It is repeated on 3–4 July,
when strong PDR values are also observed. Concerning the aerosol plumes
observed on 1, 2, and 4 July (Fig. 2b), we found a displacement towards the
British Isles, a strengthening of the low near Iceland (Fig. 9a), and a
weakening of the low near Scandinavia. Tropical air masses are then deviated
towards Sardinia, and air masses from the eastern Spanish coast are more
often advected above the marine boundary layer, which explains the
significant PDR decrease in the observed aerosol layers.
Geopotential altitude from the ERA5 reanalyses given for the
pressure levels of 700 hPa at 12:00 UTC on (a) 24 and (b) 28 June 2014. The
wind field is also shown in each panel. The horizontal resolution is
0.30∘.
The second typical meteorological situation is illustrated in Fig. 9b for 28 June.
It shows an Atlantic circulation mostly driven by the location of the
low over the British Isles and Scandinavia. This circulation favors the
arrival of air masses from the Spanish coast over the French Riviera area.
The situation is very similar on 7 July, with a deepening of the low on the
British Isles and a high in the Baltic countries.
Aerosol transport in the free troposphere
Two types of aerosol transport are described using a joint approach between
lidar measurements, spatial observations, and back trajectory modeling. To
complete and generalize the ground-based lidar measurements, version 4.10
of level 2 products of the Cloud-Aerosol LIdar with Orthogonal
Polarization (CALIOP; Winker et al., 2007) is
used (https://www-calipso.larc.nasa.gov/products/, last access: 15 March 2019). The horizontal extent of
the events over the western Mediterranean basin is studied using the
coupling between the level 2 AOT version 1.03 of the Spinning Enhanced
Visible and InfraRed Imager (SEVIRI; https://wdc.dlr.de/sensors/seviri/, last access: 15 March 2019,
Bennouna et al., 2009) aboard Meteosat Second
Generation and the level 2 AOT version 4 of the Moderate Resolution Imaging
Spectroradiometers (MODIS; http://modis-atmos.gsfc.nasa.gov, last
access: 15 March 2019;
Salmonson et al., 1989;
Levy
et al., 2013) onboard the Aqua and Terra platforms. The back trajectories
are computed using the Hybrid Single Particle Lagrangian Integrated
Trajectory (HYSPLIT) model
(e.g., Stein et al., 2015).
The model is initialized using the wind fields of the Global Data
Assimilation System (http://www.ncep.noaa.gov/, last access: 15 March 2019) at 0.5∘ horizontal
resolution and works in its ensemble mode; i.e., 27 back trajectories are
computed for each end location. The end points of the back trajectories are
defined using the lidar profiles in Fig. 2 or 3 to determine both their
temporal and altitude locations above the lidar. The origin of the aerosol
plumes trapped in the free troposphere is highlighted in Fig. 8a and
discussed hereafter.
2-day back trajectories from the French Riviera (Toulon;
43.13556∘ N, 6.00944∘ E). The altitudes of the end
locations of the air mass trajectories are in the main pollution plumes
detected by the ground-based N2-Raman lidar on (a) 28 June 2014 for an
end location of 2 km, (b) 28 June 2014 for an end location of 3.5 km, (c) 7 July 2014
for end locations of 1.7 and 2.5 km, and (d) 7 July 2014 for end
locations of 3 and 3.8 km. The HYSPLIT model worked in its ensemble mode,
i.e., 27 back trajectories computed for each end location.
Spain's contribution
It is between 28 June and 7 July that aerosol plumes with low depolarization
have been observed in the free troposphere (see Fig. 2), mostly on 28–29 June and
6–7 July. The study of back trajectories shows that most of these
plumes originate from the Spanish Mediterranean coast (Fig. 10). They take
about 1 day to reach the western part of the French Riviera. We have
identified two main contributing regions on the Spanish coast: the region of
Barcelona (Costa Brava and Costa Daurada) and the region of Murcia (Costa
Blanca). The origins of the plumes are indicated at 700 hPa on the wind rose
in Fig. 8a and Fig. 2b and also take into account pollution plumes of
weaker magnitude. The contributions of the different sources observed above
the French Riviera are not systematically mixed when arriving above the
lidar site but are frequently separated over different altitudes. For this
reason, back trajectories starting at different altitudes have been
performed for the same day. On 28 June, the aerosol plume in the lower free
troposphere, around 2 km a.m.s.l., is predominantly from the Murcia region
(Fig. 10a). AOT values above 0.4 were recorded at 355 nm wavelength by
the local AERONET station of Murcia 2 days prior. The aerosol plumes
originating from the region of Barcelona are mostly observed at higher
altitude, around 3.5 km (Fig. 10b), with similar local AOTs. The visible
Ångström exponent is characteristic of pollution aerosols with
values over 1.5. The aerosol plume on 7 July originates from the same
regions, but with a contribution from Barcelona in the lower free
troposphere (Fig. 10c), with local AOTs at 355 nm around 0.3 2 days prior,
and Ångström exponent values close to those of the previous case.
The contribution of the Murcia region starts a day earlier and at a higher
altitude (Fig. 10d), with similar AOTs and Ångström exponents.
The AOT fields at 550 nm derived from MODIS observations are presented in
Fig. 11 for the two main dates with aerosol plumes in the free troposphere.
The main contributing areas are located around Barcelona and specifically
around Murcia, where AOTs above 0.4 are observed. Note that no major forest
fire could be identified in the Iberian Peninsula from the MODIS fire
product (https://ladsweb.modaps.eosdis.nasa.gov/search/, last access: 15 March 2019) during the whole measurement
period. In Barcelona, road traffic is the foremost cause of pollution, with
an urban conglomerate of more than 1.6 million inhabitants
(Dall'Osto et al.,
2012). Emissions linked to the automobile industry, petrochemistry, and
shipping activity also contribute in this area. As for Murcia, it is a
conglomerate with more than 700 000 inhabitants, on which traffic also has
a non-negligible impact, but cannot single-handedly explain the strong
signature seen on the MODIS AOT map. It is also a region with intense
agriculture, which has been dubbed “Europe's orchard”; nonetheless, the
June period is not suitable for slash-and-burn agriculture or muck spreading. However,
the locality of Escombreras, near Cartagena in the southeast of Murcia,
includes a gigantic seaside oil processing complex with a refinery and a
harbor. This area can therefore be a strong emitter of aerosol precursors.
Without considering the desert dust episodes, we confirmed that MODIS
observations often show AOT values above 0.6 in the south of Murcia during
summertime (https://worldview.earthdata.nasa.gov, last access: 15 March 2019). A mix of traffic and
industrial emissions can certainly explain the plume observed over Murcia,
part of which is transported towards the French Riviera.
AOT at 550 nm derived from the composition of MODIS over
land and SEVIRI over sea observations: (a) on 28 June 2014 and (b) on 6 July 2014.
There are no CALIOP observations of the Barcelona area. Conversely, there
are daytime and nighttime orbits overflying the region of Murcia, yet none
of these orbits are exploitable during the campaign, as they are mostly
associated with desert dust episodes, which are very frequent in the region. Local
aerosols therefore cannot be isolated with good accuracy. In order to obtain
the CALIOP classification of the aerosols emitted in the Murcia region
(Burton et al., 2013), we have thus extended our
search for a coincident nighttime orbit until August. The choice of a nighttime orbit is motivated by the need for a
higher signal-to-noise ratio. The
only day in summer 2014 when CALIOP overflew Murcia and local aerosols were
recorded is 25 August. Figure 12 presents the MODIS image and the
corresponding CALIOP orbit. The two observations are 12 h apart. The
MODIS-derived AOT values are close to those in Fig. 11. The CALIOP aerosol
classification scheme indicates mostly polluted dust with a BER of 0.018 sr-1
(LR=55sr), matching the values identified in the aerosol
plumes from the ground-based lidar (Fig. 5).
(a) AOT at 550 nm derived from the composition of MODIS over
land and SEVIRI over sea observations on 25 August 2014. The nearest nighttime (∼02:15 UTC) ground tracks
of CALIOP are given as
dark grey continuous lines. (b) CALIOP-derived aerosol typing (version 4.10)
as observed on August 2014. DEM: digital elevation model.
The pollution plumes from Barcelona and Murcia can be injected into the free
troposphere when the warm continental air mass is advected over the colder
water of the Mediterranean Sea. This process can lead to the injection of
pollution aerosols up to altitudes exceeding 4 km a.m.s.l., as observed in Fig. 2b.
Such plumes can then be seamlessly transported towards and above the
French Riviera. These pollution particles are finally eliminated mostly by
rainfall and will reach the surface and water streams. The probable presence
of black carbon, as identified by
Chrit et al. (2018), will
favor the trapping of solar energy in the aerosol layer and induce local
heating, which is known to modify the balance of the low and middle
troposphere (e.g., Raut and Chazette, 2008b).
3-day back trajectories from the French Riviera (Toulon;
43.13556∘ N, 6.00944∘ E). The altitudes of the end
locations of the air mass trajectories are in the main dust plumes detected
by the ground-based N2-Raman lidar on (a) 24 June 2014 for end locations
of 2.6, 3, and 4.5 km, (b) 3 July 2014 for end locations of 3, 5, and 6 km. The
HYSPLIT model worked in its ensemble mode, i.e., 27 back trajectories
computed for each end location.
Northern Africa contribution
Desert dust aerosol transport is rather frequent during May and June above
France and decreases in frequency over July, while remaining probable
(Israelevich et al., 2012). In itself, it
is not extraordinary to observe such events during a field campaign with a
duration of about 1 month. The lifting zones located in the Sahara
are variable, depending on the west–east travel of lows over the
Mediterranean basin (Hamonou et al., 1999). Here, we
describe the two events sampled by the ground-based lidar, as highlighted by
the high PDR values in Fig. 2c. This will allow us, among other aspects, to evaluate
the degree of coherence between the observations of the spaceborne lidar
CALIOP and the ground-based lidar in the French Riviera.
AOT at 550 nm derived from the composition of MODIS over
land and SEVIRI over sea observations: (a) on 24 June 2014 and (b) on 3 July 2014.
The nearest nighttime (∼02:00 UTC) ground tracks
of CALIOP are given as dark grey continuous lines.
CALIOP-derived aerosol typing (version 4.10) for the nighttime
orbit of (a) 24 June 2014 and (b) 3 July 2014. The latitudinal location of the
nearest latitude of the ground-based lidar is indicated by the vertical
black line.
The 3-day back trajectories in Fig. 13 highlight the probable sources of
terrigenous aerosols, which are the Grand Erg Occidental (between Morocco
and Algeria) and the Grand Erg Oriental (southwest of Tunisia). The
contributions of these two sources can be superimposed in altitude
(Hamonou et al., 1999). The plumes partly sampled by
the ground-based lidar are also shown on MODIS and SEVIRI AOT observations
in Fig. 14. They are intense events, with AOTs at 550 nm exceeding 1 along
the African coast. Dispersion will decrease the AOT during transport,
leading to values around 0.5 to 0.6 when reaching the French Riviera. These
values match the sun photometer measurements of the AERONET station of
Toulon–La Garde on 24 June. The comparison is difficult for the second plume
happening during the night of 3–4 July; however, there could be an
overestimation of AOT of about 0.15 by the satellite compared to the
sun photometer. During nighttime, the calculation of AOT is possible with
the lidar thanks to its N2-Raman channel and yields AOT values of
∼0.5 at 550 nm, when assuming a 0.8 Ångström exponent
for the spectral variation of the AEC.
We have at our disposal, for each of these two dust aerosol events, a
single nighttime CALIPSO orbit passing close to the lidar site. These
orbits are traced in Fig. 14. The aerosol classification given by version 4.1
of the CALIOP operational algorithm is shown in Fig. 15. Note that the
plume must have moved between the observations of CALIOP and MODIS since
they are 12 to 14 h apart. However, desert dust aerosols are indeed
identified, and the summit altitude of the layers is compatible with the
ground-based lidar observations. The CALIOP products from version 4.10 give BER
values around 0.023 sr-1 (LR=44sr) at 532 nm. We have found
BERs (LRs) between 0.025 and 0.031 sr-1 (32 and 40 sr) at 355 nm from
the ground-based lidar profiles, which compares well with CALIOP product,
remaining within the error bars of the two lidars.
Incidentally, Haarig et
al. (2017) have not found significant differences between LR values at 355
and 532 nm for desert dust aerosols transported over Barbados, but caution
is due since neither the activated sources nor the transport durations are
similar. Over the Balearic Islands,
Chazette et
al. (2016) report values of BER (LR) between 0.020 and 0.025 sr-1
(40 and 50 sr) for the same type of activated sources. In the synthesis
table (Table 1) presented in
Dieudonné et al. (2015), we note that BER (LR) values range from 0.017 to 0.029 sr-1 (34
to 58 sr) for pure dust. Thus, there is a wide range of plausible values,
which warrants a measurement of BERs (or LRs) as often as possible so as to
properly invert lidar profiles.
Other contributions
The French Mediterranean coast is a densely populated area generating traffic
and industrial emissions, identified by the “local” indicator in Figs. 2b
and 3b, but also with a frequent occurrence of forest fires
(Guieu et al., 2005). Aerosol plumes are observed on 12–13 and 15–17 July in
Fig. 3b above 1.5 km a.m.s.l. Back trajectories show
northwestern contributions, which could be the result of wildfires. Indeed,
their occurrence is high during this summer period in the hinterland, whose
dry soils are covered mainly by garrigue and populations of cork and holm oaks,
interspersed with some coniferous and palm trees. The equivalent BER of this
plume is of the order of 0.022 sr-1 and could correspond to a mixture
of aerosols from biomass burning and terrigenous sources, as has been observed over the
Mediterranean; see Fig. 2b by
Chazette et
al. (2016). Nevertheless, as for the dust aerosols, the likely BER values
for biomass burning aerosols are spread over a very wide range and depend
on the type of fuel, the nature of the soil, and the intensity of the fires, all
modulated by weather situations. The forest fires and bushfires are fortunately
mastered quickly in this region, which makes it more difficult to detect
them via MODIS.
Conclusion
For the first time, a backscatter N2-Raman lidar was implemented on the
French Riviera and operated continuously during 3.5 weeks in
June–July 2014. Coupled with a ground-based sun photometer, passive (MODIS and
SEVIRI) and active (CALIPSO) spaceborne observations, and back
trajectory modeling, this instrument made it possible to identify
pollution aerosol transport in the low–medium free troposphere from the
Mediterranean Spanish coast to the French Riviera. Two desert aerosol
transport events have also been sampled by the lidar. The likely sources of
aerosol plumes trapped in the free troposphere have thus been located. To
our knowledge, the literature does not report the contribution to the AOT of
pollution plumes from the eastern Spanish coast. These can represent nearly
two-thirds of the total AOT and, given their anthropogenic origins, may have
a significant effect on the vertical stability of the atmosphere in the
coastal area. The air masses that contain them may also subside and
recirculate along the coast, which is lined with mountains.
Desert aerosols sampled by the N2-Raman lidar come from the two major
sources known in northwestern Africa: the Grand Erg Occidental and Grand
Erg Oriental. There is no noticeable difference in optical properties
retrieved from the lidar measurement between these two aerosol
contributions. The backscatter-to-extinction ratios (lidar ratios) are
similar, with values of ∼0.027sr-1 (∼37sr) at
355 nm. They are close to that derived from the CALIOP observations
(BER=0.023sr-1 or LR=44sr at 532 nm). Pollution aerosols
encountered in the free troposphere above the measurement site came from the
Barcelona region (Costa Brava and Costa Daurada) as well as from the Murcia
region (Costa Blanca). This second origin of pollution aerosol plumes was
unexpected to this extent. These two sources can be mixed before reaching
the western French Riviera, but they are also separately identifiable
according to altitude. Transport altitudes are close to those of desert
aerosol plumes, suggesting a similar injection process related to the large
temperature difference between the sea surface and the continental air
masses. Except for desert aerosols, an important point is the similarity of
the BERs in the PBL and in the free troposphere. It is certainly related to
the fact that locally produced aerosols, mainly due to traffic, have similar
characteristics as those transported from Spain. The low sensitivity of
the BER to relative humidity also probably contributes to this effect, and a
likely air mass recirculation in the presence of mountain ranges close to
the French Riviera coast may also tend to mix boundary layer and lower free
tropospheric aerosols.
This study relies on a time-limited dataset (∼1 month, 500 h
of lidar measurements), but it raises questions as to the origin of the
pollution aerosols that are sampled on the coast by the air quality
stations, since the pollution may not be only local and also seems to be
imported over the sea at the scale of the larger western Mediterranean
basin. It would be interesting to implement lidar systems more densely and
for a longer period in the French Riviera and further along
Mediterranean coastlines, which are under very strong anthropic pressure.
The interest in such an approach has been shown in previous works, in which it
has been found that few lidar stations were required for an air quality
forecasting similar to those constrained only by ground-based in situ
measurements (Wang et al., 2013). A first
test was also conducted within the framework of ChArMEx using the Earlinet
network
(Wang
et al., 2014), which we may have to complete in the upcoming years to also
meet the needs of operational meteorology to improve the forecasting of
extreme events.
Data availability
Data can be downloaded from
http://mistrals.sedoo.fr/ChArMEx/ (last access: 15 March 2019) upon request to the first author of the
paper.
Author contributions
PC performed the experiment, analyzed the
data, and wrote the paper; JT performed the experiment and participated in
the paper editing; XS participated in the data analysis and the paper editing.
Competing interests
The authors declare that they have 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
The campaign was supported by the CNRS/INSU
through the MISTRALS/ChArMEx and HyMeX programs. We especially thank François Dulac
for his help in implementing the instrumental site and Jacques Piazzola for
his welcome and the availability of the measurement site provided by the
Mediterranean Institute of Oceanography. This work was supported by the
Commissariat à l'Energie Atomique et aux énergies alternatives
(CEA). The Centre National d'Etude Spatial (CNES) helped maintain the
Raman–lidar instrument. The authors would like to thank the AERONET network
for sun photometer products (https://aeronet.gsfc.nasa.gov/, last access: 15 March 2019). The authors
acknowledge the MODIS Science, Processing and Data Support Teams for
producing and providing MODIS data (https://modis.gsfc.nasa.gov/data/dataprod/, last access: 15 March 2019) and the Atmospheric Science
Data Center (ASDC) at NASA Langley Research Center (LaRC) for the data
processing and distribution of CALIPSO products (level 4.10;
https://www-calipso.larc.nasa.gov/products/, last access: 15 March 2019). The
authors would like to thank the entire MSG/SEVIRI team from ESA, Alcatel
Space Industries, and Matra Marconi Space. SEVIRI data have been downloaded
from the ICARE Data and Services Centre (http://www.icare.univ-lille1.fr/, last access: 15 March 2019).
The NOAA Air Resources Laboratory (ARL) is acknowledged for the provision of
the HYSPLIT transport and dispersion model and READY website
(http://www.ready.noaa.gov, last access: 15 March 2019) used in this publication. ECMWF data used in
this study have been obtained from the ESPRI/IPSL data server.
Review statement
This paper was edited by Oleg Dubovik and reviewed by two anonymous referees.
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