In order to measure the mass flux of atmospheric insoluble deposition and to constrain regional models of dust simulation, a network of automatic deposition collectors (CARAGA) has been installed throughout the western Mediterranean Basin. Weekly samples of the insoluble fraction of total atmospheric deposition were collected concurrently on filters at five sites including four on western Mediterranean islands (Frioul and Corsica, France; Mallorca, Spain; and Lampedusa, Italy) and one in the southern French Alps (Le Casset), and a weighing and ignition protocol was applied in order to quantify their mineral fraction. Atmospheric deposition is both a strong source of nutrients and metals for marine ecosystems in this area. However, there are few data on trace-metal deposition in the literature, since their deposition measurement is difficult to perform. In order to obtain more information from CARAGA atmospheric deposition samples, this study aimed to test their relevance in estimating elemental fluxes in addition to total mass fluxes. The elemental chemical analysis of ashed CARAGA filter samples was based on an acid digestion and an elemental analysis by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mass spectrometry (MS) in a clean room. The sampling and analytical protocols were tested to determine the elemental composition for mineral dust tracers (Al, Ca, K, Mg and Ti), nutrients (P and Fe) and trace metals (Cd, Co, Cr, Cu, Mn, Ni, V and Zn) from simulated wet deposition of dust analogues and traffic soot. The relative mass loss by dissolution in wet deposition was lower than 1 % for Al and Fe, and reached 13 % for P due to its larger solubility in water. For trace metals, this loss represented less than 3 % of the total mass concentration, except for Zn, Cu and Mn for which it could reach 10 %, especially in traffic soot. The chemical contamination during analysis was negligible for all the elements except for Cd, which has a very low concentration in dust. Tests allowed us to conclude that the CARAGA samples could be used to estimate the contents of nutrients and trace metals in the limits of loss by dissolution. Chemical characterization of CARAGA deposition samples corresponding to the most intense dust deposition events recorded between 2011 and 2013 has been performed and showed elemental mass ratios consistent with the ones found in the literature for Saharan dust. However, the chemical analysis of CARAGA samples revealed the presence of some anthropogenic signatures, for instance high Zn concentrations in some samples in Lampedusa, and also pointed out that mineral dust can be mixed with anthropogenic compounds in the deposition samples collected on Frioul. Results showed that the chemical analysis of CARAGA ashed samples can be used to trace the origins of elemental deposition. The elemental atmospheric fluxes estimated from these chemical analyses of samples from the CARAGA network of weekly deposition monitoring constitute the first assessment of mass deposition fluxes of trace metals and P during intense dust deposition events at the scale of the western Mediterranean Basin. The mass fluxes strongly depend on the distance from dust sources and the most intense events, while proximity from anthropogenic sources strongly impacted the masse fluxes of Zn and Cu at Lampedusa and Frioul.
The Mediterranean Basin is a receptor for the deposition of particles
emitted in surrounding coastal urban areas and continents. These deposited
particles can have both natural (e.g. Saharan dust and biogenic emissions)
and anthropogenic origins (e.g. Loÿe-Pilot and Martin, 1996; Kanakidou
et al., 2011). Atmospheric inputs supply as much nutrients as riverine
inputs in the Mediterranean and are less confined to coastal waters (e.g.
Moon et al., 2016; Richon et al., 2017). They play a significant role in
marine nutrient cycles during the Mediterranean water stratification period,
i.e. from May to September. This is the case for P and N macronutrients
(Loÿe-Pilot et al., 1990; Pulido-Villena et al., 2010) and Fe
micronutrients (Bonnet and Guieu, 2006). Saharan dust deposition is also
considered an input of trace metals, such as Co, Ni, Mo, Mn, Zn and Cd,
for the Mediterranean Sea, which play an essential role in phytoplanktonic
activity (Morel and Hudson, 1985). For instance, N
The ADIOS programme, based on a network of deposition measurements with a common protocol deployed over the same period at nine stations throughout the basin, provides key information for studying the spatial variability of nutrients and trace metals deposited in the Mediterranean area, but it was limited to only 1 year (June 2001–May 2002) with a monthly resolution that cannot isolate high-deposition events (Guieu et al., 2010). The measured elemental fluxes show a high spatial variability and suggest that the dust-derived Fe deposition is higher in the north-western basin. For P deposition, the measurements seem to indicate a longitudinal gradient with significantly lower values in the eastern Mediterranean Basin, attributed equally to dust and anthropogenic inputs. However, due to the high temporal variability of desert dust events transported over the Mediterranean Basin, it is difficult to conclude much from a 1-year data set of the chemical elements associated with dust. To do that, a long-term and large-scale deposition sampling network is required (Schulz et al., 2012). In order to obtain a long time-series of deposition mass fluxes at several sampling sites, a CARAGA (Collecteur Automatique de Retombées Atmosphériques insolubles à Grande Autonomie) collector has been developed (Laurent et al., 2015) and implemented at five sites in the western Mediterranean region (Vincent et al., 2016). This collector has up to 6 months autonomy without human intervention. To do that, only the insoluble fraction of total deposition is sampled on filters, excluding the soluble fraction of the deposition, which can be difficult to preserve over months.
Samples of atmospheric deposition have automatically been collected by CARAGA since 2011 at five sites: Le Casset (SE France), Corsica (France), Frioul (France), Mallorca (Spain) and Lampedusa (Italy). Measured total mass fluxes of atmospheric deposition and the weighing and ignition protocols used to quantify the mineral deposition are presented in Laurent et al. (2015) and Vincent et al. (2016). In the present study, we investigate the possibility of using the CARAGA ashed samples to estimate the elemental mass deposition flux of nutrients and trace metals. Firstly, we tested the elemental composition changes during the sampling and treatment protocol of deposition samples with aerosol analogues. Secondly, we estimated the nutrients and trace-metal fluxes of the most intense dust deposition events associated with dust events over the western Mediterranean Basin between 2011 and 2013.
Weekly deposition samples were collected between 2011 and 2013 with CARAGA
collectors at five stations in the western Mediterranean Basin presented in
Fig. 1. The site positions were selected to cover the western basin by
integrating east–west and north–south gradients:
Le Casset (44 Corsica (43 Frioul (43 Mallorca (39 Lampedusa (35
Location of the CARAGA sampling sites in the western Mediterranean Basin: Le Casset, Corsica, Frioul, Mallorca and Lampedusa.
In the CARAGA collectors, dry and wet deposition are collected by a funnel
(0.2 m
From 2011 to 2013, 108 samples corresponding to the most intense weekly dust deposition samples were studied by Vincent et al. (2016), and a Saharan origin was identified for 107 selected samples by using satellite data and air mass trajectories. The selected samples account for 84, 78 and 73 % in mass of the total deposition in Lampedusa (37 samples), Mallorca (21 samples) and Corsica (11 samples), and it contributes for around 50 % in Frioul (21 samples) and Le Casset (17 samples). A complete description of the treatment and selection of samples can be found in Vincent et al. (2016).
In order to further obtain their elemental composition, ashed CARAGA samples
were digested using a digestion protocol adapted from Heimburger et al. (2013). About 20 mg of ashed samples were weighed and transferred to a
Savillex™ PFA digestion vessels with 0.5 mL of ultrapure water
to avoid the loss of samples. The samples were then digested using 2.5 mL of
mixture of Suprapur® acid (1 mL HCl 30 %, 1 mL
HNO
Analyses were performed with a SPECTRO ARCOS (
In order to estimate the change in elemental composition during the CARAGA sampling and treatment protocol, tests have been carried on analogues of typical Saharan dust or soot particles, which were subject to the collection protocol and then ignition in the laboratory.
Wet deposition being the major pathway to solubilize nutrients and trace
metals, analogues of typical wet deposition CARAGA samples were made by
simulating rainwater containing two kinds of particles: dust and soot, which
are the main insoluble aerosols (Ault et al., 2011). Desert dust samples were
prepared from the fine fraction of soils (< 20
Simulations of rainwater were made by adding 50
In order to assess the potential change of the sample elemental composition
due to the calcination step, we simulated sampling of rainwater using the
Douz and Banizoumbou soils with the above-described protocol but by adding
90 mg of soils. After filtration and rinsing, filters were dried in the oven
at 40
Using the proposed protocol 107 selected samples were digested and analysed
for markers, nutrients and trace metals. In order to estimate the
contamination and limit of detection (LoD) of the analytical protocol,
nine blanks of CARAGA samples (filters exposed but not collected) were used
and processed. Even if the protocol of chemical analysis was tested for
20 mg of ashed samples, the mass of several CARAGA samples was inferior to
20 mg, especially the less intense deposition samples. In this case, the
totality of ashed samples was used for acid digestion. The minimum sample
mass was 2 mg. The weighing uncertainty was
For P, Cr, Cu and Zn, concentrations of blank values were below the limit of detection, so we considered that contamination could be neglected. For elements except Cd, blank concentrations were quantifiable and presented low variability but remained largely inferior to minimum concentrations in 2 mg of CARAGA samples. Cd concentration was inferior to LoD, so we could not estimate Cd content in samples with low mass. For calculations of mass fluxes, we subtracted these blanks values to elemental concentrations obtained in the CARAGA samples. We did not take into account Cd in the following study because of the large uncertainty on its concentrations.
Limit of detection (LoD; calculated with three times the standard deviation for nine blanks) and elemental concentrations in blanks and CARAGA samples (in ppb).
Elemental mass loss in soils of Douz and Banizoumbou, and traffic soot during the collection and ignition of wet deposition. Standard deviation (SD) results from triplicate experiments. Al, Ca, P, K, Na and Mg have been measured by ICP-AES and other elements by HR-ICP-MS.
Averages of elemental mass loss and the standard deviation of three replicates
for three types of particles (soils of Douz and Banizoumbou, and traffic soot)
during the sampling of wet deposition are presented in Table 2. Loss by
dissolution during collection and rinsing has been calculated as fractional
solubility:
The tests emphasized that the main change in chemical composition between deposited aerosols and ashed CARAGA samples was due to the loss by dissolution during wet deposition and by rinsing. The composition of nutrient and trace metals obtained from simulated CARAGA samples were representative of initial dust and soot analogues with a maximum of 13 % of underestimation for P, less than 10 % for Zn, Cu and Mn and less than 5 % for other elements. Tests were carried out for wet deposition. In the case of dry deposition, particles are rinsed and the contact time with water is less than for wet deposition. Thus, the underestimation due to dissolution would probably be lower in the case of dry deposition. The highest uncertainty of 13 % is inferior to the common discrepancy on the deposition fluxes estimated from different dry deposition samplers (Goosens and Rajot., 2008; López-García et al., 2013). To conclude, taking into account the underestimation of the soluble fraction, the CARAGA samples seem to be relevant for estimating the total deposition fluxes of Al, Fe, P, Co, Cr, Cu, Ni, Mn, Ti, V and Zn.
PCA factor loadings for 10 elements and for 107 samples. F is factor. Only loads larger than 0.3 (in absolute values) are reported. Loads larger than 0.6 are in bold.
After sample analysis, weekly elemental mass fluxes for the 107 deposition
samples were calculated from measured concentrations. These samples being
associated with deposition of Saharan dust, we checked their chemical
compositions with principal component analysis (PCA) and mass ratios X
Outlier samples identified by PCA for four sampling sites: Le Casset,
Corsica, Frioul and Lampedusa.
Principal component analysis was used to identify the possible sources of the deposition samples
from the 107 intense events. Correlation between variables and
factors for the elements Al, Fe, P, Ti, Co, Cr, Cu, Mn, Ni, V and Zn and
commutative variance for five sampling sites were presented in Table 3. For each
site, 2–4 possible sources were identified, and the first source always contributed
much more than half the variance of data, and the two first factors
were extracted as principal components that explained > 82 %
of the variance of the sample data. The lower variances of the first factor
explained 57.34 and 66.87 % at sites Lampedusa and Frioul respectively,
showing more influence from other sources at these two sampling sites. In the
studied 107 samples, all elements were globally issued from a similar source
for each site, but other sources (especially of Zn, Cu, Cr, Co and P)
influenced the sampling sites too, especially at sites Lampedusa and Frioul.
In order to identify the outlier samples, individual observations located on
plane factors of 1, 2 or 1, 3 are presented in Fig. 2, and the observations
located higher (> 1 %) on axis
In order to identify the main source at each site, Al contents and mass
ratios X
Al contents in deposition samples for five sites: Le Casset, Corsica, Mallorca, Lampedusa and Frioul.
It is also interesting to note that Al contents increased with a south to
north gradient, i.e. with the lowest values at Lampedusa, the closest site to
Saharan dust sources, and the highest values at Le Casset, the most distant
site from Sahara (Table 4). Vincent et al. (2016) emphasized that most of the
time the origin of the deposited dust changed from one sampling site to the
other: dust samples collected at Le Casset were mainly transported from the
western part of the Sahara, whereas dust arriving to Lampedusa generally came
from Tunisia and Libya. Nevertheless, the values of the Al content were
probably not only related to the provenance of dust, since the emission
sources providing dust deposition in Corsica and in Mallorca were generally
the same (Western Sahara/southern Morocco and Tunisia/eastern Algeria)
(Vincent et al., 2016) and yet the Al content was different. Desboeufs et
al. (2014b) observed an increase in the Al content in dust during its
settling in the water column after deposition to the ocean. This increase was
explained by the fact that the mass loss due to dissolution of the highest
soluble species during settling modifies the mass percentage of the less
soluble elements. The same effect could be observed in our samples due to the
dissolution of soluble species during wet deposition as Ca, whose solubility
can be increased by conversion of calcite to Ca sulfate or Ca nitrate during
long-range transport (Scheuvens et al., 2013). Vincent et al. (2016) showed
that 80, 63, 60 and 53 % of studied samples corresponded to wet
deposition events at Le Casset, Corsica, Lampedusa and Mallorca respectively.
The increasing contribution of wet deposition with distance to the dust
source is consistent with the obtained Al contents, since the farther from
source the site is, the more important the number of wet deposition samples
and the higher the Al content. The consistency of Al content values with the
dust references confirms the Saharan origin of deposition and supports the
reliability of CARAGA data. However, the variability of the soluble part with
distance makes it difficult to use CARAGA samples to estimate the elemental
content of deposition samples and hence to directly compare the composition
of the deposition at the different sites. In the literature, inter-elemental
mass ratios X
Inter-elemental ratio in samples collected at the different CARAGA sites and reference values. Rf1: reference values in soils of Banizoumbou and Douz. Rf2: reference values in wet deposition of dust near emitting area (Desboeufs et al., 2010) or in total deposition with dust signature in the western Mediterranean Basin (Ridame et al., 1999; Guieu et al., 2010). Rf3: reference values in transported Saharan dust (Formenti et al., 2003, 2008, 2011 and 2014) for the Saharan-emitting sources (PSA 1 to 3: northern Algeria/southern Tunisia, Mauritania/Morocco and northern Mali/southern Algeria respectively), corresponding with the identified provenance of our samples (Vincent et al., 2016).
The average of inter-elemental mass ratios X
At Lampedusa, the correlation are relatively low (
At Frioul, we already observed the low correlation of Al mass with total
mass, showing a contribution of other sources of particles to the deposited
mass. Indeed, Frioul is a rock/sand island in front of the
industrialized city Marseille, which could be affected by resuspended
mineral particles or by local anthropogenic sources. This conclusion is
confirmed by the values of correlation found between Al and trace metals, as
Cr, Cu or Zn (
At Le Casset, Cu and Zn present correlations with Al with
A high correlation of P and Al is obtained at Mallorca (
Mean weekly mass fluxes of nutrients and trace metals for 107 intense events samples, standard deviation (SD) and contribution of wet deposition on total fluxes for five studied sites.
To conclude on the validation of CARAGA samples to estimate origin of atmospheric deposition, the results showed that the atmospheric deposition of trace metals at Le Casset, Corsica, Mallorca and Lampedusa is mainly associated with dust fluxes during intense dust event, except Zn in Lampedusa and Cr in Corsica. The outlier samples at these sites represented less than 10 % of the deposited elemental mass. Thus the CARAGA samples can be used to estimate the atmospheric fluxes of these elements issued from dust deposition. On the contrary, the trace-metal composition of the CARAGA samples on Frioul was influenced by anthropogenic sources and the atmospheric fluxes are not representative of dust deposition even during intense dust events. This is also the case for P fluxes at the sites Le Casset, Frioul and Lampedusa, whereas P fluxes in Mallorca and Corsica can be used to estimate the dust-derived P deposition fluxes.
The weekly mass fluxes of P and trace metals and percentage of wet
deposition (wet only and mixed deposition according Vincent et al., 2016),
estimated for 107 samples covering the 2011–2013 sampling period, are
presented in Table 6. The trace-metal fluxes in Table 6 are presented in two categories: the fluxes mainly associated with dust deposition (sites of Le
Casset, Corsica (except Cr), Mallorca and Lampedusa
The estimation of atmospheric deposition mass fluxes of nutrients and trace
metals in the western Mediterranean Sea is critical to understand the role
of dust deposition on phytoplankton activity. Few deposition measurements
are available to document nutrient inputs for several years in the western
Mediterranean Sea. To estimate these nutrients and mass fluxes of trace metals,
we validated the possibility of using samples of mass measurements of atmospheric
deposition performed under the same conditions with CARAGA collectors at five sites
in the western part of the Mediterranean. A first step was to estimate
elemental mass loss during the in situ collection and ignition protocol used
for weighing samples. According to these tests with dust analogues and
traffic soot, solubility larger than 15 % was observed for major metals,
such as Ca, Mg and K, excluding the possibility to use the common Ca
Chemical analyses of 107 CARAGA samples corresponding to the most intense
dust deposition events observed between 2011 and 2013 in the western basin
were performed to validate using CARAGA samples in order to estimate the elemental
mass deposition flux of nutrients and trace metals. After checking the
chemical composition of samples using PCA, Al content and inter-elemental mass ratios
X
The atmospheric elemental fluxes estimated from CARAGA samples constitute
the first regional assessment of a mean weekly mass fluxes of trace metals
and P linked to intense dust deposition over the western Mediterranean Basin.
The mean mass fluxes were dependent on the distance to the source and on the
contribution of wet deposition. High averages and strong standard deviations
of Al (dust marker) mass fluxes were observed at Lampedusa and Mallorca due
to several intense dust events. Extreme Zn mass fluxes (122.63
To obtain the data from the CARAGA network, contact
Gilles Bergametti or Benoit Laurent at LISA
(
The authors declare that they have no conflict of interest.
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.
This study received financial support from the MISTRALS (Mediterranean Integrated Studies at Local and Regional Scales) and ChArMEx (Chemistry-Aerosol Mediterranean Experiment) programmes and ADEME. The authors also want to thank all those who operated the CARAGA sites. Edited by: Andrew Sayer Reviewed by: two anonymous referees