Previous studies have provided some insight into the Saharan dust deposition
at a few specific locations from observations over long time periods or
intensive field campaigns. However, no assessment of the dust deposition
temporal variability in connection with its regional spatial distribution has
been achieved so far from network observations over more than 1 year. To
investigate dust deposition dynamics at the regional scale, five automatic
deposition collectors named CARAGA (Collecteur Automatique de Retombées
Atmosphériques insolubles à Grande Autonomie in French) have been
deployed in the western Mediterranean region during 1 to 3 years
depending on the station. The sites include, from south to north, Lampedusa, Majorca, Corsica, Frioul and Le Casset (southern French
Alps). Deposition measurements are performed on a common weekly period at the
five sites. The mean dust deposition fluxes are higher close to the northern
African coasts and decrease following a south–north gradient, with values
from 7.4 g m
A reliable estimation of the dust content in the atmosphere and of its variability in space and time is needed to assess desert dust impacts on the Earth system. The most convenient tools to conduct this assessment are atmospheric dust models in which the atmospheric cycle of mineral dust is represented: dust emissions by wind erosion on arid and semiarid regions; atmospheric transport, which is strongly controlled by the meteorological situations; and deposition of dust along their atmospheric path by dry or wet processes. A validation of the closure of the dust budget in atmospheric dust models needs to quantify precisely the amount of emitted dust, the atmospheric dust load and the dry and wet deposited dust mass (Bergametti and Fôret, 2014).
Significant progress has been made on dust emission modelling during the last 2 decades (Shao et al., 1993; Marticorena and Bergametti, 1995; Alfaro and Gomes, 2001; Shao, 2004; Marticorena, 2014) and on the dust source monitoring, especially by using satellite observations (Brooks and Legrand, 2000; Prospero et al., 2002; Washington et al., 2003; Schepanski et al., 2012). However, quantitative estimates of dust emissions in atmospheric models are still affected by large uncertainties (Zender et al., 2004; Textor et al., 2006; Huneeus et al., 2011), mainly because a direct and quantitative validation of soil dust emissions at a large scale remains not possible. The spatial distribution and temporal variability of atmospheric dust content has also been significantly improved through the development of aerosol products from spaceborne (e.g. Moulin et al., 1997; Torres et al., 2002; Shi and Cressie, 2007; Remer et al., 2008; Nabat et al., 2013), ground-based (Holben et al., 2001) and ship-borne (Smirnov et al., 2011), remote-sensing instruments. Presently, large available data sets of aerosol optical depth (AOD) have been widely and mostly used to validate dust atmospheric contents simulated by 3-D models at global (e.g. Chin et al., 2002; Ginoux and Torres, 2003; Huneeus et al., 2011) or regional scales (e.g. Cautenet et al., 2000).
Dust deposition fluxes measured in the Mediterranean basin and
northern Africa.
Atmospheric dust particles are removed from the atmosphere by dry and wet depositions processes (Duce and Tindale, 1991; Schulz et al., 2012). These two sinks, which counterbalance dust emissions on the global scale, control the atmospheric lifetime of dust particles (Bergametti and Fôret, 2014). However, rather little attention has been paid to dust deposition and few experiments were dedicated to test dust deposition schemes against in situ data. There is an urgent need for further research and measurements of dust deposition. Dust models are mainly validated against proxies for the atmospheric dust load, e.g. AOD, concentrations, dust vertical profiles or combinations of these. However, at least two of the terms emissions, dust load and deposition, need to be documented to close the dust mass budget.
In this paper, we will present the results over a 3-year period concerning atmospheric mass deposition measurements associated with Saharan dust transport over the western Mediterranean Basin. The main goal of the sampling strategy is to provide data that can be used directly to test the dust mass budget in dust transport model. This data set of Saharan dust deposition in the western Mediterranean region can also be used to identify the transport patterns and the provenance of the dust. To do that, deposition measurements are coupled with satellite observations and air mass trajectories.
Atmospheric deposition fluxes have been measured in the Mediterranean region during the last 50 years. Table 1 gathers the direct deposition measurements performed both close to Saharan dust source areas and far away on both sides of the Mediterranean basin. Most of these deposition mass fluxes were obtained directly by weighting the deposited mass, the others being derived from aluminum deposition measurements assuming that this element contributes about 7 to 8 % of the total dust mass (e.g. Guieu et al., 2002). Note that dust deposition can be also estimated indirectly based on the measurements of atmospheric aerosol concentrations and assuming dust dry deposition velocity and scavenging ratio (e.g. Le Bolloch et al., 1996).
Dust deposition measurements close to the North African dust sources are
rare: dust deposition fluxes were only measured in Morocco, Tunisia and
Libya. The deposition samples from Tunisia and Morocco (Guieu et al., 2010)
were collected at sampling sites located along the Mediterranean coasts and
indicate deposition fluxes ranging from 7 to 23 g m
In the Mediterranean basin itself, dust deposition fluxes exhibit a large
spatial variability ranging over more than 1 order of magnitude, from 2 to
more than 27 g m
These measurements provide a picture of the dust deposition in the Mediterranean basin, the heterogeneity of data does not allow one to precisely investigate the spatial and temporal variabilities of the dust deposition fluxes. Indeed, most of these dust deposition measurements were performed either in one site during a long time period (e.g. data sets obtained in Corsica between 1984 and 1994 by Loÿe-Pilot and Martin, 1996, or in Crete between 1988 and 1994 by Pye, 1992, and Mattson and Nihlén, 1994), or on a network of several stations but during a much shorter time period. Moreover, the dust deposition fluxes were generally not measured with the same devices. This complicates the comparison of the data collected at the different sites.
Thus, a better understanding of the dust deposition in the Mediterranean basin, and especially of its spatial and temporal variability, requires measurements performed simultaneously at several stations and with similar devices during a long-time period. With this objective, a new deposition sampler was developed for use at remote sites in full autonomy over several months (Laurent et al., 2015).
In order to be able to maintain over a long time period a network to measure
dust mass deposition fluxes, our strategy was to sample only the insoluble
deposition. This greatly simplifies the design of the collector and the on-site
operations. More than 80 % of the total Saharan dust deposition mass in
the Mediterranean basin occurs in the form of insoluble material (Guerzoni et
al., 1993; Avila et al., 2007). As a consequence, an automatic collector
called CARAGA (Collecteur Automatique de Retombées Atmosphériques
insolubles à Grande Autonomie) was developed in collaboration between
the ICARE Ingénierie Company and the Laboratoire Interuniversitaire des
Systèmes Atmosphériques (Fig. 1). The CARAGA was designed to require
limited human intervention and to be produced in small series to develop a
standardised deposition network in remote areas (Laurent et al., 2015). A
collecting open funnel (0.2 m
The CARAGA collector operating on Frioul.
The deposition network in the western Mediterranean basin is constituted of
five CARAGA instruments installed mainly on Mediterranean island coasts
(Fig. 2). This network is thought to allow for dust deposition sampling along a
south–north transect, from near the North African coast to the south-east
of France, covering about 1050 km from south to north and 870 km from west
to east. The network is constituted of four island sites and one continental
site. The first CARAGA was installed in October 2010 on the small elongated
Pomègues Island, which is part of the Frioul islands, a 2 km
Location of the CARAGA samplers constituting the deposition network deployed in the western Mediterranean basin and southern of France.
Atmospheric total (wet
Atmospheric particulate concentrations measured in the Mediterranean suggest
that more than 90 % of dust particles mass is in the coarse mode
(diameter larger than 1.2
Air mass trajectories and satellite observations can be jointly analysed to point out the provenance of the dust deposition measured at the stations.
Weekly insoluble mineral deposition fluxes (orange bars) and precipitation amount (blue line) for Lampedusa, Majorca, Corsica, Frioul and Le Casset from January 2011 to December 2013. The grey areas correspond to periods without sampling. The numbers of most intense dust deposition for each station as described in Sect. 4.3 are indicated by black bars above the deposition flux values: 34 in Lampedusa, 20 in Majorca, 11 in Corsica, 18 in Frioul and 15 in Le Casset.
Air masses trajectories computed using the HYSPLIT model (Draxler and Rolph,
2003;
Since the transport of Saharan air masses towards the western Mediterranean basin can occur at various altitudes in the troposphere (Bergametti et al., 1989; Martin et al., 1990; Hamonou et al., 1999; di Sarra et al., 2001; Israelevich et al., 2002; Meloni et al., 2008; di Iorio et al., 2009), 4-day backward air mass trajectories starting every day at 12:00 UTC were computed starting at 0, 500, 2000, 3000, 4000 and 5000 m a.g.l. for each of the five sampling stations. When the circulation of the model air masses slows over the western Mediterranean basin, the duration of the backward trajectories was extended to 6 days.
These air mass trajectories were combined with aerosol optical depth (AOD)
products of the Moderate Imaging Spectrometer (MODIS;
Once the dust provenance is identified, the HYSPLIT model was also used to compute forward air mass trajectories in ensemble mode (i.e. multiple trajectories from the selected starting location by offsetting the meteorological data) or in matrix mode (i.e. starting from the borders of these identified dust sources). These forward air mass trajectories at 00:00 and 12:00 UTC were computed starting from the dust provenance area at an altitude of 500 m a.g.l., which can be considered as a common dust entrainment altitude (Meloni et al., 2008). We checked the coherence between the backward and forward air mass trajectories computed for studied dust cases. We also checked for each forward trajectory the altitude of the air mass and if precipitation occurred during transport.
The weekly fluxes of insoluble mineral deposition measured at the five sites are
reported in Fig. 3. The highest weekly deposition fluxes are recorded at the two stations nearest
to the Sahara desert: 3.2 and 2.7 g m
In the late 1980s and 2000s, the mean annual Saharan dust deposition measured
in Corsica by Loÿe-Pilot et al. (1986), Loÿe-Pilot and Martin (1996)
and Ternon et al. (2010) were of 14.0, 12.5 and 11.4 g m
Illustration of the data used jointly to identify a dust transport
event and its origin, which leads to high deposition in Lampedusa between
April 12 and 13 2012.
The annual deposition flux we measured in Corsica corresponds to a low dust
deposition year. This is also supported by the fact that none of the weekly
deposition fluxes we measured exceeds 1 g m
The previous studies mentioned above indicated that the largest deposition events observed in the western Mediterranean basin are strongly associated with Saharan dust transports (Bergametti et al., 1989; Loÿe-Pilot and Martin, 1996; Guerzoni et al., 1999). We decided to give particular attention to the most intense weekly deposition (MID) flux measured at each station and to verify if these high deposition events were linked to Saharan dust transport towards the Mediterranean Sea. We cannot exclude that local mineral contribution, especially during high wind speed periods at the station, may affect some samples, in particular those for which the deposition due to long-range transported dust is the lowest. Moreover, for the station located the furthest from the African coasts, such as Frioul or Le Casset, the anthropogenic background in refractive material may also contribute in a limited way to the insoluble mineral deposition. Among the MID, the most intense Saharan dust deposition (MIDD) samples and the corresponding dust deposition events (DDE) will be discussed.
Number of weekly deposition fluxes measured, number (and relative proportion in %) of the most intense weekly deposition fluxes recorded (MID), number of MID events with identified Saharan provenance area (MIDD), and their respective contribution (% in mass) to the deposition fluxes measured, at each station of the network.
To select the MID, we first merged all the weekly samples collected at the
different stations (537 samples) and then selected the weekly deposition
greater than the last sextile of the data set, i.e. the 16.67 % highest
deposition values (i.e. 90 samples). This leads to a threshold weekly
deposition flux of
9.3
The MID events being selected, we verified whether these deposition events were associated or not with air masses originating from the Saharan desert. To point out the provenances and the main transport pathways of deposition events at each station, HYSPLIT air mass trajectories and satellite aerosol observations were jointly analysed as presented in Sect. 3.4. Among the 108 MID samples, only one sample collected at Le Casset was not associated with at least one air mass trajectory having crossed northern Africa during the sampling week.
For each MID, we identified by using MODIS AOD or MSG satellite observations where dust is coming from. Figure 4 illustrates the different satellite observations and air mass trajectories used to identify the dust provenance area and transport pattern associated with dust deposition in the western Mediterranean basin. In all, 98 samples among the 107 MID present an air mass coming from North African areas with high AOD and reaching the stations. Hereafter, these 98 most intense Saharan dust deposition samples are called MIDD. For the remaining nine cases, the absence of matching between high AOD observed from satellite images and air mass trajectories linking the dust provenance region to the sampling stations does not necessary mean that these cases are not cases of Saharan dust deposition. The MIDD accounts for 84, 78 and 73 % of the deposition in Lampedusa, Majorca and Corsica, respectively, while it contributes around 50 % in Frioul and Le Casset (Table 2).
Weighted number of occurrence of MIDD per month over the whole sampling period at each site.
To look at the seasonality of the MIDD occurrence for the studied period, the
fact that each month of the year has not been sampled with the same frequency
at each sampling station has to be taken into account. We computed the number
of weeks which have been sampled for each month and for each station. A
weighting coefficient
Figure 5 reports the number of MIDD occurrence per month at each site. Most
of the MIDD occurred during spring (March–June): 53 % in Le Casset,
49 % in Frioul, 81 % in Corsica, 38 % in Majorca and 55 % in
Lampedusa. A second maximum is observed in autumn in Majorca and Lampedusa,
and at the end of summer and early autumn in Frioul, Corsica and Le Casset.
From their long-term data set in Corsica, Loÿe-Pilot and Martin (1996)
also observed the most frequent and intense dust events in spring and autumn.
According to Bergametti et al. (1989), the frequency of Saharan inputs in
Corsica seems to be the highest during spring and summer, 80 % of the
events being observed between March and October 1985. Ternon et al. (2010)
observed high deposition in spring and summer with a maximum in June from
their measurements performed in the Ligurian area between 2003 and 2007.
Avila et al. (1997) showed that the occurrence of red rain episodes in
north-eastern Spain between 1983 and 1994 were higher in autumn and spring.
The dust deposition seasonality cannot be directly compared with other
atmospheric observations. For instance in Lampedusa, the seasonality of
atmospheric dust content and dust deposition are different. Maximum AOD
indicate the highest dust atmospheric content in summer in 2001–2005 (Meloni et
al., 2004, 2008), whereas the crustal aerosol contribution to PM
Frequency of dust provenance areas identified using MODIS AOD and
HYSPLIT air mass trajectories for the DDE contributing to the MIDD recorded
at
Our results point out that the stations of the network are not systematically concerned by dust deposition at the same period. To fully understand the variability of Saharan dust deposition in the western Mediterranean basin, several sampling sites are required to perform direct deposition measurements.
The number of stations operating when a MIDD was recorded is given in Table 3. In all, 98 MIDD have been collected during 75 different weeks of sampling and 82 % of these MIDD were recorded when at least four stations were simultaneously operating. However, only 17 of these MIDD have affected more than one station during the same sampling week (12 at two stations, 4 at three stations, and 1 at four stations). Furthermore, 75 % of the cases were affected when at least two stations recorded a MIDD associated the Majorca stations or the Lampedusa station with northern stations, whereas only two cases associated at least both Lampedusa and Majorca stations. The stations the most often associated with a given MIDD are (i) Frioul and le Casset (six cases for which at least these two stations are associated), (ii) Majorca and Corsica (five cases for which at least these two stations are associated). This suggests that, in the western Mediterranean basin, the MIDD are associated with different dust provenance and transport pathways, and/or the dust plumes are washed out by precipitation during their transport over the basin.
Number of stations operating and number of stations recording a MIDD during the same sampling week.
The joint analysis of the HYSPLIT air mass trajectories and MODIS AOD allows us to identify where dust deposited at the stations for the MIDD is likely coming from. During a sampling week, several DDE can be identified and contribute to the weekly deposition flux. A dust event contributing to dust deposition during several days at a station is considered as a single DDE. A MIDD can be a combination of several DDE originating from different dust areas. We identified 132 DDE for the studied period: 50 reached Lampedusa, 27 Majorca, 22 Frioul, 15 Corsica and 18 Le Casset. The number of events contributing to the dust deposition is greater for the stations close to the North African dust sources.
For each DDE, the localisation of the highest AOD southernmost along the modelled air mass trajectory defined a rough region where dust comes from. As mentioned by Meloni et al. (2008), due to the low resolution of the model meteorological fields and transport model intrinsic errors, the dust location can be relatively wide. It should also be kept in mind that other sources located along the pathway of the dust plume can also contributed to the dust uplifts. Thus, we defined seven large dust provenance areas (DPA) by grouping together the closest dust localisations (Fig. 6): Niger and Chad (DPA1), northern Mali and southern Mauritania (DPA2), Western Sahara and southern Morocco (DPA3), central Algeria (DPA4), Libya (DPA5), Tunisia and eastern Algeria (DPA6), and northern Morocco and north-western Algeria (DPA7).
Typical forward air mass trajectories computed with the HYSPLIT model and corresponding to the different Saharan deposition events collected over the western Mediterranean basin. The number in brackets indicates the relative occurrence frequency for each of the six cases (3.7 % are unclassified).
The number of DDE at each station (weighted as in Sect. 3.3 and expressed in
%) originating from the seven areas are reported in Fig. 6; 73 % of DDE in
Frioul and 69 % of DDE in Le Casset come from the western part of the
Sahara (DPA2, 3 and 7). Western Sahara (DPA3) and Tunisia (DPA6) are the
most frequent provenance of DDE reaching Majorca. Dust deposited during the
DDE in Lampedusa generally come from the Tunisian (DPA6) and Libyan (DPA5)
regions and central Algeria (DPA4). DDE in Corsica generally come from
the Western Sahara and southern Morocco (DPA3), Tunisia and eastern Algeria (DPA6)
and Libya (DPA5), and the same level of similarity can be observed between
the dust provenance areas affecting Majorca and Corsica than between Corsica
and Lampedusa. We also noted that provenance areas, even south of
20
These results confirm that the different parts of the western Mediterranean basin are not affected in the same proportion by Saharan dust coming from different regions. It is nevertheless important to keep in mind that what was tracked here is the southernmost occurrence of dust along the trajectory associated with intense dust deposition events. Hamonou et al. (1999) showed that dust layers of different origins can also be present concurrently over a given station in the northern part of the Mediterranean.
The main transport routes associated with the 132 DDE in the western Mediterranean Sea were investigated. We classified the forward air mass trajectories computed for each DDE depending on their pathway. The six most frequent types of trajectories (representing 96.3 % of all trajectories) are illustrated in Fig. 7. Note that four cases among them were classified as “others”, each of them corresponding to a trajectory observed only one time during the studied period. The air mass trajectories over the western Mediterranean basin are often transported in high altitude (Escudero et al., 2005; Querol et al., 2009). Low level transport of dust are mostly observed at Lampedusa. Trajectory types (a), (c) and (d) are the most frequent transport ways of Saharan dust towards the western Mediterranean basin, since they all together account for almost 70 % of all trajectories. Trajectory type (a) corresponds to a straight transport of dust emitted from sources located in Tunisia and/or Libya towards Lampedusa and the eastern part of western Mediterranean basin. This type of trajectory is the dominant transport in spring (Fig. 8). Trajectory type (c) corresponds to transport from sources located in western Algeria/Morocco and Mauritania/Mali towards the western part of the basin and type (d) to transport in a west to east flow from sources located in western and central Sahara and mainly towards the south-western Mediterranean Basin. They are the dominant Saharan dust transport pathways during summer (Fig. 8). These trajectories have already been mentioned in previous studies as major transport ways for Saharan dust over the Mediterranean Sea (Bergametti et al., 1989; Guerzoni et al., 1997; Moulin et al., 1998; Israelevich, 2003; Meloni et al., 2008; Marconi et al., 2014). Even if they are less frequent, Saharan dust transport trajectories of type (b) (straight transport towards the westernmost part of the Mediterranean basin from sources located in northern Morocco and western Algeria) and (e) (stagnant air masses and cyclonic flow centred over the Atlas and southern Mediterranean) represent 12.1 and 10.6 % of all trajectories, and often occur in spring (Fig. 8).
Seasonal occurrence of the different Saharan dust trajectories (see text for details).
HYSPLIT model trajectories and precipitation were used to examine whether the
different transport trajectories to the western Mediterranean Basin were
systematically associated or not with precipitation during transport. We
consider a trajectory with an occurrence of precipitation along its path
between the Saharan dust provenance area and a given station as a “wet
transport case”, whatever the rainfall rate. Main uncertainties are due to
the low spatial resolution of the meteorological data set, which prevents, for
example, accounting for the summer precipitation due to convective cells
(Meloni et al., 2008). Moreover, the precipitation is precipitation rate at
the grid cell where the trajectory is located and does not take into account
the air mass altitude transport. Figure 9 presents the proportion of Saharan
dust trajectories coming from each dust provenance area (identified in
Sect. 3.4) and for which precipitation during transport have been computed by
the HYSPLIT model. Dust air masses from the Western Sahara have the highest
probability (
Proportion of HYSLPIT trajectories for the DDE (in %) with precipitation during their transport between the source regions and the western Mediterranean basin.
Results on dust provenance and transport pathways of the DDE suggest that different parts of the western Mediterranean basin are affected by dust deposition events at different periods and from different dust source regions. Dust masses follow different transport trajectories to reach the Mediterranean, some of them being probably washed by precipitation during their transport. This means that Saharan dust inputs to the different parts of the western Mediterranean Basin do not occur at the same time, and can differ in intensity and in composition.
MIDD during which DDE occur only by dry, by wet or by mixed (wet
In this study, no direct measurements of dry-only and wet-only deposition are
performed. However, in order to provide information on the relative
importance of dry and wet deposition to the MIDD, the daily precipitation
measured were analysed in combination with forward dust air mass trajectories
starting from the identified provenance areas and reaching the sampling
sites. When no precipitation is recorded, we consider that dust deposition is
driven by dry deposition processes. As mentioned by Löye-Pilot and
Martin (1996), significant deposits can occur in almost “dry conditions”,
i.e. very low and short rain events and/or fog periods that classical
meteorological rain gauges cannot detect. As a consequence, in these cases,
the deposition is considered as dry and this leads to a possible
overestimation of the contribution of the dry-only deposition to the total
deposited flux. The air mass trajectories provide for each identified dust
event a theoretical date of its arrival at a given sampling station. As
mentioned above, there are uncertainties on the computed trajectories due to
both the model and the resolution of the meteorological fields. Moreover, in
most cases, the starting date of the trajectory from the source regions (as
determined by looking at the satellite images) is known with a precision not
better than
Table 4 reports, for each sampling station, estimated proportions of the
wet-only, dry-only and mixed DDE to MIDD (in terms of occurrences and mass
fluxes). Between 36 and 82 % of MIDD (depending on the sampling stations)
occur in only wet conditions. The deposition in the northern stations is
dominated by wet deposition (77 to 82 % of the total mass deposition in
Le Casset, 61 to 66 % in Frioul, 69 to 74 % in Corsica). For the
southern stations, Lampedusa and Majorca, wet deposition could contribute to
51 and 36 to 41 % of total deposition mass, respectively. Even if the wet
or dry deposition events can be roughly classified following our approach,
the occurrence and the intensity of dry deposition events are far to be
negligible. In terms of mass, dry deposition represents between 10 and
46 % of the deposited mass, the lower contribution being observed in the
remote island sites of Majorca (10 to 15 %) and Corsica (10 to
15 %). The highest MIDD measured in Lampedusa
(2.7 g m
A network of five sampling sites was deployed from 2011 on the western Mediterranean region to measure the insoluble atmospheric deposition fluxes. It included from south to north an Italian site (Lampedusa), a Spanish site (Majorca) and three French sites (Corsica, Frioul, and Le Casset in south-eastern France). The data recovery rate varied between 77 and 91 % depending on the station. The deposition data set acquired between 2011 and 2013 include 537 weekly samples. It allowed us to investigate Saharan dust deposition events in this region.
At the three northern stations of the network, Le Casset
(44
We selected the 98 most intense dust deposition events (MIDD) for the investigated period. They occurred preferentially in spring whatever the sampling station. However, the southern stations of the network (Lampedusa and Majorca) exhibit a second maximum in autumn while the northern stations (Corsica, Frioul, Le Casset) exhibit this second maximum in summer. Few dust deposition events were recorded simultaneously on several stations, suggesting that different dust events contribute to the deposition measured in different parts of the western Mediterranean.
By matching satellite observations of MODIS AOD and HYSPLIT air mass trajectories, we defined seven large Saharan dust provenance areas and discussed how they contribute to dust deposition in the western Mediterranean region. The western Sahara is by far the most frequent dust provenance of the intense dust deposition measured in the northern and western part of the western Mediterranean basin. The central and the south-eastern parts of the western basin are equally affected by dust transported from western and eastern Saharan regions. In the same way, we also discussed the main dust transport trajectories leading to the Saharan dust deposition in the different parts of the western Mediterranean. We identified six major dust transport routes. Three of them, corresponding to 70 % of all trajectories, are dominant in spring (one trajectory type, a) and in summer (the other two, c and d).
Finally, we showed that several dust deposition events (DDE) can contribute to high weekly dust deposition fluxes measured in the stations of the network. The daily precipitation measured at the station allowed us to discuss the relative contribution of wet and dry dust deposition to the weekly deposition fluxes. Even if the procedure we used only allows one to roughly estimating wet vs. dry deposition occurrences, the dry deposition can contribute significantly for the highest deposition fluxes (MIDD) to the total deposition, from 10 and 46 % of the total deposited mass depending on the station.
The results show that dust deposition in the western Mediterranean region is far from being homogeneous. A high spatial and temporal variability of the deposition is observed. A south–north decrease of the intensity of the deposition fluxes is noticed. Moreover, during the investigated period, different source regions contribute to the dust deposition in different locations of the central and western Mediterranean in relation with different dust transport pathways. Our results suggest a seasonal pattern of the Saharan high dust deposition within the western Mediterranean basin for the investigated period, which could be refined with longer time series of deposition measurements. This unique data set will be used to test the dust deposition in atmospheric transport models in complement to other aerosols measurements available at the stations.
To obtain the data of the CARAGA network, contact Gilles Bergametti or Benoit Laurent at
LISA
(
This study was funded by the PRIMEQUAL-ADEME programme on “Pollution atmosphérique longue distance” through the research project “Mesure du dépôt atmosphérique et validation de sa représentation dans les modèles régionaux” (DEMO project, contract no. 0962c0067). This project was also funded by the MISTRALS (Mediterranean Integrated Studies at Local and Regional Scales) programme as part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx) and by the Spanish Government project ChArMEx: aerosols deposition ref: CTM2011-14036-E. The development of the CARAGA collector was supported both by the Chemistry Faculty of the Paris Diderot University and the PRIMEQUAL-ADEME DEMO project. The authors would like to thank M.-D. Loÿe-Pilot and two anonymous reviewers for their insightful and helpful comments on the manuscript. Terra-and Aqua-MODIS AOD used in this study were produced with the Giovanni online data system, developed and maintained by the NASA Goddard Earth Sciences (GES) Data and Information Services Center (DISC). We thank the HYSPLIT teams for making the backward and forward air mass trajectories available, and EUMETSAT and ICARE for the MSG/SEVIRI products. Edited by: N. Mihalopoulos Reviewed by: M.-D. Loÿe-Pilot and two anonymous referees