Total atmospheric deposition was collected on a weekly basis over 3.5 years (March 2008–October 2011) at a remote coastal site on the west coast of Corsica. Deposition time series of macro- and micronutrients (N, P, Si, Fe) and trace metals (As, Cr, Cu, Mn, Ni, V, Zn) are investigated in terms of variability and source apportionment (from fluxes of proxies for aerosol sources (Al, Ti, Ca, Na, Mg, S, Sr, K, Pb)). The highest fluxes are recorded for Si, P, and Fe for nutrients and Zn and Mn for trace metals. For the majority of elements, data show some weeks with high episodic fluxes, except for N, Cr, and V, which present the lowest variability. A total of 12 intense mineral dust deposition events are identified during the sampling period. The contribution of these events to the fluxes of Fe and Si represents 52 % and 57 % of their total fluxes, respectively, confirming the important role of these sporadic dust events in the inputs of these elements in the Mediterranean. For N and P, the contribution of these intense dust deposition events is lower and reaches 10 % and 15 %, respectively. Out of these most intense events, positive matrix factorization (PMF) was applied to our total deposition database in order to identify the main sources of nutrients and trace metals deposited. Results show that P deposition is mainly associated with anthropogenic biomass burning inputs. For N deposition, inputs associated with marine sources (maybe associated with the reaction of anthropogenic N on NaCl particles) and anthropogenic sources are quasi-similar. A good correlation is obtained between N and S fluxes, supporting a common origin associated with inorganic secondary aerosol, i.e., ammonium sulfate. For trace metals, their origin is very variable: with a large contribution of natural dust sources for Ni or Mn and conversely of anthropogenic sources for V and Zn.
The Mediterranean Sea is a semi-enclosed basin situated at the interface
among contrasted continental areas of three continents, namely southern
Europe, northern Africa, and the Middle East, the coastal areas of which are heavily
populated. Thus, the Mediterranean Basin continuously receives anthropogenic
aerosols from industrial and domestic activities from all around the basin
and other parts of Europe (Sciare et al., 2008; Becagli et al., 2012). In
addition to deposition from this anthropogenic background, seasonal inputs
from biomass burning occur mainly during dry summers (Chester et al., 1996;
Guieu et al., 1997), and strong deposition pulses of mineral dust from the
Sahara are superimposed (Guerzoni et al., 1999a), with some extreme events
with dust deposition fluxes as high as 22 g m
A number of key elements for marine biota are associated with those inputs. Thus, several authors showed that the atmospheric deposition of aerosols constitutes the main source of major nutrients, such as N, P, or Fe, to the surface open waters of the Mediterranean Sea in the summer–fall period when surface water stratification prevents inputs from deep water by vertical mixing (Guerzoni et al., 1999a; Bonnet and Guieu, 2006; Krom et al., 2010; Pulido-Villena et al., 2010; Richon et al., 2018a, Violaki et al., 2018). In addition to the classical nutrients (N, P, and Fe), the aerosols also carry trace metals (hereafter called TMs) such as Cr, Cu, Ni, Mn, or Zn that are known to have a biological role, often as cofactors or part of cofactors, in enzymes and as structural elements in proteins (Morel and Price, 2003). The recent study of Ridame et al. (2011) suggests that the TMs released by Saharan dust could stimulate nitrogen fixation in summer in the Mediterranean Sea. This assumption is supported by the works of Tovar-Sánchez (2014), which show that the TM concentrations in the surface microlayer of the Mediterranean Sea are correlated with atmospheric deposition of mineral dust. However, it has also been suggested that the atmospheric deposition of particulate pollutants is responsible for the contamination of the Mediterranean waters in TMs (Bethoux et al., 1990; Guerzoni et al., 1999b). Gallisai et al. (2014) also show negative effects of dust deposition on chlorophyll, coinciding with regions under a large influence of aerosols from European origin.
Thus, the partitioning/mixing between anthropogenic and natural atmospheric inputs is critical to estimate and predict the role of the atmospheric deposition in the marine biosphere and associated services (Richon et al., 2018b). However, in the Mediterranean Sea, the existing database on atmospheric fluxes of nutrients and TMs remains quite limited. Most studies are focused on total deposition of dust and/or macronutrients such as P and N (e.g., Markaki et al., 2010). This approach does not include the variety of nutrients and does not enable one to distinguish the origin of nutrient-bearing particles. Moreover, the studies on TM deposition (Cd, Pb, etc.) often show an influence of local sources (Guieu et al., 2010), limiting the reliability of these data. Unlike atmospheric deposition, the source apportionment of suspended particles over the Mediterranean, from the positive matrix factorization (PMF) method, has been highly investigated in recent works and showed a large spatial variability in source contributions (Becagli et al., 2012, 2017; Calzolai et al., 2015; Amato et al., 2016; Diapouli et al., 2017). The signature of continental pollution sources was observed even in remote areas such as central Mediterranean islands (Calzolai et al., 2015). Yet, PM concentrations and sources are probably different than sources of deposited particles, which depend on aerosol size distribution and precipitation patterns, among other factors. Thus, in a context of anthropogenic changes, it is crucial to distinguish between anthropogenic and natural atmospheric inputs of nutrients in order to assess how the evolution of chemical atmospheric forcing will modify the marine nutrient cycling.
Here we show a 3.5-year-long continuous series of total deposition fluxes of macro- and micronutrients (N, P, Si, Fe), TMs (As, Cr, Cu, Mn, Ni, V, Zn), and source tracers (Al, Ti, Ca, Na, Mg, S, Sr, K, Pb) at a remote coastal site in Corsica. Between March 2008 and October 2011, a monitoring station was operated with a weekly sampling time step for total bulk deposition. In order to assess the contribution of sources in the fluxes of nutrients, work on the source apportionment of various nutrients and TMs was carried out from these data (PMF method). Specific attention was also given to the different types of extreme atmospheric events that are relevant regarding the biogeochemistry in the Mediterranean Sea. They include Saharan events and intense summer storms that trigger the washout of the atmosphere over an altitude of several thousands of meters in a short time.
Total bulk deposition (i.e., dry
The sampler is a 120 mm diameter PTFE Teflon® funnel
(collection aperture 0.0113 m
In the laboratory, bottles sampling total atmospheric deposition were
weighted. The amount of rainwater collected in the funnel was deduced by
subtracting added acid solution (i.e., 110 mL) from the sample total mass found
in the bottle. Each sample was shaken and then 15 mL was immediately
transferred into a PE sampling vial to measure the size distribution of the
particulate phase (not discussed here). The rest of the sample was filtered
before analysis with acid-washed Nuclepore® polycarbonate
filters (0.2
The weekly elemental deposition fluxes were calculated from concentrations
of all chemical species measured in dissolved and particulate samples by
considering the sampler area and the total liquid volume (preloading
The speciation between wet and dry deposition is a critical parameter to estimate the potential dissolved fluxes of nutrients. Precipitation (mm) was estimated at the site from the amount of water in the sample. The precipitation occurrences are in agreement with the rainfall records at Calvi airport, which is about 15 km away. Since they are more representative of local rainfall, precipitation estimated from our samples was used for the attribution of deposition fluxes to wet vs. dry deposition. Wet deposition was considered when rainfall was larger than 1 mm during the sampling period. The threshold value of 1 mm integrates the uncertainties on the weighing of samples in order to ascertain that the rainfall was real. Samples that present no precipitation or rainfall lower than 1 mm are considered dry deposition. In consequence, dry deposition is assimilated into wet deposition when happening the same week as a precipitation event. This method underestimates dry deposition and provides a lower estimate of deposition dry event number vs. total deposition event number.
Multivariate statistical methods, such as factor analysis, are widely used to identify source signatures and explore source–receptor relationships using the trace element compositions of atmospheric aerosols (e.g., Polissar et al., 2001; Calzolai et al., 2015) and precipitation (Keeler et al., 2006; Gratz et al., 2013). Since many sources emit characteristic relative amounts of certain trace elements, source–receptor techniques can be used with an understanding of these elemental signatures to identify the major sources influencing a given receptor site.
We applied EPA PMF v5.0 (Norris et al., 2014) to the matrices of tracers,
nutrients, and TM total deposition measurements. PMF is a multivariate
statistical technique that uses weighted least-squares factor analysis to
decouple the matrix of observed values (
The 3.5-year time series of weekly fluxes (195 samples) for nutrients, TMs, major source tracer elements (Al, Na, S, and K), and precipitation are presented in Fig. 1. Corresponding time series of other source tracer elements (Ti, Mg, Sr, Pb) are available in the Supplement with the total atmospheric flux data. The highest fluxes are recorded for Si, P, and Fe for major nutrients and Zn and Mn for TMs. A total of 51 % of the samples, i.e., 99 samples, sustained at least one event of precipitation during the week of sampling and are here referenced as wet deposition. In our set of 195 samples, 21 presented a rainfall higher than 20 mm and the highest weekly rainfall recorded is 29 mm. However, no systematic link is observed between the biggest rain event and the nutrients or metal fluxes.
Temporal variability in bulk weekly fluxes from March 2008 to October 2011 for main markers, nutrients, and trace metals and rainfall during the same period. The 10 most intense dust events are displayed in the boxes in orange (see details in Sect. 3.4).
The results emphasize large differences in timing of deposition fluxes
among the studied elements. But for all the elements, the data display some
weeks with high episodic fluxes. Due to the sporadic character of specific
events such as dust storms or forest fires, giving rise to high-deposition
events, it is known that the fluxes of elements associated with these sources
are often important in a short period. For example, for elements such as
aluminium associated with dust events, a half or more of the annual
deposition flux may occur in one event of a few days or even hours (Guieu et
al., 2010), and high-deposition events (
Monthly total and wet fluxes have been estimated to investigate the seasonal variability in the measured elements' inputs over the northwestern Mediterranean (Fig. 2). A large variability in the monthly deposition fluxes of all the elements is observed in agreement with the episodic pattern of weekly inputs. Nutrient deposition presents a clear seasonal pattern: P has major deposition fluxes in summer and N in winter, whereas the main fluxes are observed in spring for Fe, Si, Cr, Ni, and V. For As, excluding June, which shows its highest monthly mean flux due to the intense event of June 2010, the maximum of fluxes is recorded at the end of summer and beginning of fall. For Mn, no clear seasonality is observed. A monthly flux predominates in August and November for Zn and Cu, respectively, reaching at least twice the other monthly fluxes. For all the elements, the wet deposition predominates the total fluxes between October and April in agreement with the highest rainfall recording during this period, whereas dry deposition is the main path of input in May, July, and August. Our results are in agreement with the seasonal pattern observed in the 1980s for Si and Fe deposition at Capo Cavallo, 8 km further north on the Corsican coast (Bergametti et al., 1989). The maximum of deposition during spring is explained by the concomitance of rainfall and high dust concentrations, whereas Si and Fe atmospheric aerosol concentrations present their maximum in summer during the dry season. This emphasizes that the below-cloud scavenging of aerosol is the predominant process explaining atmospheric deposition of dust-related elements in this period. For the elements mainly associated with dry deposition, i.e., Zn, P, and Cr, Bergametti et al. (1989, 1992) observed that the highest deposition was typically associated with the period of their highest aerosol concentrations in summer. This is not the case for Cr in our results, which follows the Si and Fe behavior. Unlike our Corsica site, no clear seasonal variability is observed for the deposition fluxes recorded at Cap Ferrat, 170 km further NNE on the French continental coast, a site affected by the anthropogenic influences from continental Europe (Pasqueron de Fommervault et al., 2015). That could be the case for Mn atmospheric fluxes at our site.
The case of N deposition is specific since the N deposition flux corresponds mainly to total aerosol and wet gaseous deposition inputs in our samples. The general pattern for N with the highest fluxes in winter could be linked to the thermal instability of ammonium nitrate, which is the dominant form of N in aerosol particles associated with a decrease in rain events during the hot season and with extremely typical intense nitrate episodes recorded from November to March in the western Mediterranean Basin associated with maximum wet deposition (Querol et al., 2009). The highest N deposition flux is recorded in November 2010 (Fig. 1); this event is associated with wet deposition and is coincident with a deposition peak for Cu and K.
Temporal variation in monthly total (green bars) and wet (blue bars) deposition and precipitation during the sampling period March 2008–October 2011. Bars indicate standard deviations over the weekly values available over the period.
The average annual total deposition fluxes for the major nutrients and TMs during the 3.5 years of sampling are presented in Table 1. Among
major nutrients, the most abundant nutrients in bulk deposition are Si
followed by P and N, which have fluxes of the same order of magnitude. The
highest annual fluxes recorded for N in comparison to Fe are due to the
sporadic pattern of Fe fluxes in comparison to N that shows more regular
weekly fluxes. For TMs, the highest annual fluxes are observed for
Zn, Mn, and Cu, whereas the other TMs have fluxes smaller by 1
order of magnitude. Except for Ni, the standard deviations on the mean
fluxes are larger than 15 % and reach more than 50 % for P, As, and Cu,
meaning a large interannual variability in their deposition, in agreement
with the high recorded sporadic weekly fluxes for these elements. Our
results are compared with other fluxes in Corsica (Table 1) as reported in
the literature. Data show that for TMs, the recorded values are of
the same order of magnitude as previous measurements in Corsica. Conversely, for the major elements Fe, Si, and N, except for P, our
deposition flux values are much lower than the previous ones obtained in
Corsica (Table 1) and globally in the western Mediterranean (Bonnet and Guieu,
2006; Pasqueron de Fommervault et al., 2015). A net decrease in N deposition
is also observed between the 1990s and now in Europe (Waldner et al.,
2014). The only element with the highest deposition fluxes in comparison to the
literature is P, suggesting an increase in atmospheric fluxes for this
element. Keeping in mind that dry deposition events can be underestimated by
our method, the wet fluxes predominate the total deposition fluxes (
The left part shows the annual total and wet and dry deposition fluxes
(mg m
The annual deposition fluxes of soil dust have been estimated from Al
fluxes, considering an amount of Al of 7 % (Guieu et al., 2010). The
results show that the mean annual dust flux ranges from 1.39 to 1.94 g m
Time series of dust fluxes (g m
Over our sampling period (March 2008–October 2011), the average weekly dust
deposition is
The dust flux associated with these most intense dust deposition events
represents 56 % of the total dust flux in the 3.5 years of recording. The
contribution of dust events to the fluxes of Fe and Si represents 52 % and
57 % of their total fluxes, respectively. Our results confirm the important
role of these sporadic dust events in the inputs of these elements. In
agreement with previous observations, Si and Fe fluxes also present a good
correlation with Al fluxes (
For N and P, the contribution of the outlier dust events is lower and
reaches 10 % and 15 %, respectively, and even 11 % for P if the As-dust
mixed event is excluded. That means that sources other than soil dust
dominate the fallouts of these species (Fig. 1). However, a peak in N and
P fluxes is systematically observed during high dust events, showing at the
same time that intense dust deposition is also a source of these elements.
The reactivity between dust and nitric acid previously observed in the
Mediterranean (e.g., Puteaud et al., 2004) could explain the link between
dust fluxes and N fluxes. For TMs, the high-dust-deposition events
represent around
In order to perform a source apportionment with the PMF method, we excluded
the 12 samples corresponding to the events of high African dust deposition in
order to address background atmospheric deposition. We evaluated PMF
solutions with two to six factors. Finally, a solution with four factors has
been chosen since it is the optimum solution, coupling a good agreement with
our understanding of source identification and the indicator of PMF
optimization. The four-factor solution was the most stable, with a sharper
decrease in the
PMF-derived profiles of the four sources identified. From top to bottom:
In Fig. 6, we show the relative contribution from the identified sources
to the background deposition flux of nutrients and TMs. The results
show that the combustion sources (biomass burning or anthropogenic)
predominate in the background inputs of major nutrients and TMs, except Fe
and Si. Even for background deposition, the source apportionment of Fe and
Si is quasi-similar to Al (correlation coefficient is close to 1 for the
elemental fluxes even for intense events and the ratios of
Concerning major nutrients, P deposition is highly associated with biomass
burning inputs out of the most intense dust deposition events. Considering
that dust deposition accounts for 15 % of the total P deposition flux
(including intense dust deposition events
Relative contribution of each of the four identified factors (anthropogenic (Anthro), marine, dust, and biomass combustion (B comb)) to the “background” mass fluxes of nutrients and TMs (i.e., excluding the 12 most intense African dust deposition samples out of 195 samples).
For TMs, marine sources present the lowest contribution. The biomass burning/waste source is clearly predominant for Cu, Mn, and Zn, whereas atmospheric fluxes of Cr and Ni are largely linked to the anthropogenic source. Fu et al. (2017) show that the Cr deposition in Cape Corsica, even during intense dust events, originates from an anthropogenic source, suggesting a contamination by a local source. Even if Cape Corsica and our sites of deposition measurements are about 100 km apart, both suggest that Cr deposition is controlled by an anthropogenic source. For Zn, Guieu et al. (2010) also showed a large contribution of a non-dust source. Our work supports their conclusions and allows the identification of a biomass combustion source rather than a fossil fuel or industrial source. It appears that the deposition of Cu, Mn, Ni, and V is influenced, at least for 20 %, by dust deposition out of intense events. That means that for these TMs, the natural dust inputs can represent up to 50 % of annual fluxes.
Box plots of monthly molar
The typical
In our dataset, the yearly deposition mass fluxes measured for N and P are
quasi-equivalent (0.14–0.15 g m
In a context of anthropogenic changes, in order to assess how the evolution
of chemical atmospheric forcing will modify the marine nutrient cycling, it
is crucial to distinguish between anthropogenic and natural atmospheric
inputs of nutrients to oligotrophic Mediterranean surface waters. We
monitored elemental atmospheric deposition on a weekly basis over 3.5 years
(March 2008–October 2011) at a coastal site on the western coast of Corsica.
The contribution of four different source types to the fallout of nutrients and
TMs was determined using the statistical PMF method, namely desert dust,
sea salt, anthropogenic activities, and biomass combustion sources. The data
show that Si and Fe fluxes are typically related to African dust deposition,
with fluxes dominated by high-dust-deposition events. A typical
Atmospheric fluxes of Cu, Mn, Ni, and V are also associated by at least at 50 % with mineral dust deposition, whereas half of atmospheric fluxes are issued from biomass burning particle deposition (Cu and Mn), from fossil fuel combustion (V), or both (Ni). The anthropogenic/combustion sources govern the atmospheric fluxes of the major nutrients N and P, with a predominance of a biomass combustion source for P and secondary aerosols for N. Dust deposition contributes around 15 % of deposited P at the yearly timescale. Confirming recent model results that desert dust is not dominant in atmospheric P fluxes (Richon et al., 2018b), our results show that these combustion sources need to be considered in P deposition modeling. Finally, Zn or Cr deposition is very largely associated with continuous combustion sources.
This work is a first tentative assessment of the origin of nutrients and TMs deposited in the western Mediterranean. Of course, our study is not sufficient to apprehend the spatial variability in the influence of the identified source types over the basin. It needs to be supported by other studies of source apportionment in deposition samples in the region.
All the data used in this article are available in Supplement.
The supplement related to this article is available online at:
KD and EBN ensured the preparation and the transfer of the deposition samples. EBN, SC and ST conducted the sample analysis. KD and FD contributed to developing the scientific direction. KD directed this work and wrote the manuscript.
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.
The authors wish to warmly thank Pasquale Simeoni from Parc Naturel Régional de Corse, who made possible the weekly sample collection and the maintenance of the sampling site. This study received financial support from the French ANR through the project DUNE and from the MISTRALS program funded by INSU, ADEME, CEA, and Météo-France. This study contributes to WP5 on Atmospheric Deposition of the MISTRALS/ChArMEx project. The authors want to thank Rémi Losno for his involvement in the installation of deposition collectors and the staff of the Parc Naturel Régional de Corse for assistance in sampling.Edited by: Nikolaos Mihalopoulos Reviewed by: two anonymous referees