Mineral dust particles emitted from the Sahara are a factor in excessive daily air particles concentrations of PM10 (particulate matter less than 10 µm in diameter) observed in the Mediterranean Basin and southern Europe (Pey et al., 2013). In order to estimate these Saharan dust outbreaks over this region, chemistry-transport models (CTM) could simulate continuous and forecasted dust concentration fields. Simulations of mineral dust are validated by comparing the simulated dust load with numerous available data sets, for instance direct aerosol concentration measurements and aerosol optical depth (AOD) derived from ground-based and/or satellite observations. However, this remains insufficient to ensure the consistency of regional or global dust simulations, because, due to the lack of quantitative measurements, emission and deposition can be adjusted quite freely to allow models to match observed atmospheric dust concentrations. This means that at least one additional term, emission or deposition mass flux, has to be measured to correctly constrain the simulated dust mass concentration.

Large uncertainties remain, for instance, on how dry and wet dust deposition processes are modelled (Zhao et al., 2003; Textor et al., 2006; Jung and Shao, 2006; Bergametti and Forêt, 2014). Few experimental field measurements of dust deposition have been performed recently (Guieu et al., 2010; López-Garcia et al., 2013; Heimburger et al., 2013; Osada et al., 2014). Accurate measurements of dust mass fluxes remain scarce whereas they should be conducted continuously and homogenously over regions impacted by mineral dust in order to constrain model dust simulations. One of the reasons why such a small number of field studies has been performed is the heavy workload that both sampling and measurement of dust deposition represent over long periods of time.

This paper presents a new device designed to perform continuous dust deposition measurements over long periods and a weighing and ignition protocol to determine the total insoluble mineral dust mass deposited. This automatic collector, named CARAGA (Collecteur Automatique de Retombées Atmosphériques insolubles à Grande Autonomie), has been specially developed to sample total insoluble atmospheric particles deposition in remote areas and to insure robust automatic sampling with a large autonomy and a minimum need of man power. Since mid-2010, a collector sampling total (dry and wet) deposition has be installed on Frioul Island (43.27∘ N; 5.29∘ E), which is located in the Gulf of Lions in the north-western Mediterranean Sea, where Saharan dust outbreaks can be observed (see for example Ridame et al., 1999). In the following sections, the in situ deposition sampling strategy is presented as well as the lab protocol established to quantify mineral dust deposition. Almost 2 years of mineral deposition measurements on Frioul Island are discussed and open up the possibility to develop a CARAGA sampling network of total insoluble deposition in remote areas.

Dry deposition mainly depends on sedimentation, interception, impaction and Brownian diffusion processes (Slinn and Slinn, 1980; Wesely, 1989; Venkatram and Pleim, 1999). Wet deposition processes correspond to the capture of particles by droplets either inside or below the clouds (Dana and Hales, 1976; Slinn, 1984; Garcia Nieto et al., 1994). These deposition processes of atmospheric particles are supposed to be well understood. However, most of the theoretical understanding and parameterizations of deposition were based on studies and measurements performed under controlled conditions, for instance in wind tunnels (Chamberlain, 1967; Goossens, 2008) or towers and laboratories (Wang and Pruppacher, 1977; Leong et al., 1982; Barlow and Latham, 1983; Pranesha and Kamra, 1996).

In situ dust deposition measurements of atmospheric particles are technically difficult and have to be adapted depending on the aim of the study: dry, wet or total deposition; soluble, insoluble or bulk deposition; short- or long-term sampling, etc. Quantitative estimates of dust deposition remain challenging (see for example Wiggs et al., 2002; Goossens and Rajot, 2008). Various techniques have been proposed to directly measure or to estimate (from atmospheric concentrations) dry deposition of dust on surfaces (Seinfeld and Pandis, 1998; Etyemezian et al., 2003; Goossens, 2005; Sow et al., 2006). Experimental studies usually show a wide range in dry deposition values, depending on the sampling device (Goossens and Rajot, 2008). Even if wet deposition measurements are easier to perform, correctly sampling the first millimetres of a precipitation event is crucial to precisely measure wet deposition (Claassen and Halm, 1995). In the framework of this study, the sampling of total (dry and wet) Saharan dust deposition is investigated.

Up to now, Saharan dust deposition sampling has required frequent human intervention to be carried out. Most of the time, deposition collectors consist of simple passive collecting systems: funnel capped bottles (Markaki et al., 2010; Prospero et al., 2010), polyethylene bottles (Bonnet and Guieu, 2006; Markaki et al., 2010), bags (Galy-Lacaux et al., 2009), buckets (Prospero et al., 2010) or surfaces covered with glass marbles (Kouvarakis et al., 2001; Sow et al., 2006). Similar collectors have been used to sample mineral dust in other regions, for example in Asia by Osada et al. (2014) or the Kerguelen islands in the Southern Ocean (Heimburger et al., 2013). None of these collecting systems allows the automatical sampling of dust deposition with sufficient autonomy in order to limit human intervention after each sampling.

In the Mediterranean Basin, Saharan dust deposition was sampled in Capo Cavallo (NW Corsica) using a CRAPAL sampler (Bergametti, 1987; Remoudaki et al., 1991), which is a hemispheric plexiglass collection device (0.1 m2) with a 10 cm high neck covered by a 1 mm nylon mesh (Lambert and Nezami, 1965). Its base is connected to an acid-cleaned polyethylene bucket in which the atmospheric deposition is collected during precipitation and by manual acid flushing at the end of a 1-week sampling period. Loÿe-Pilot and Martin (1996) also collected Saharan dust deposition in Corsica during an 11-year period (1984–1994) using a bulk plastic collector (Standard Rain Gauge) which has a 400 cm2 aperture. Deposition measurements were also performed on the two sides of the Ligurian Sea at the Cap Ferrat site in 2004 and 2006 (Bonnet and Guieu, 2006; Pulido-Villena et al., 2008) and in Corsica in 2003 and 2005 (Ternon et al., 2010) using the same bulk plastic collector or a funnel and a polyethylene bottle. The most comprehensive field study to assess the magnitude and the composition of atmospheric deposition in the Mediterranean Basin was the ADIOS program, during which atmospheric deposition was collected at 10 sampling sites (Guieu et al., 2010; Markaki et al., 2010). The mass of deposited Saharan dust was estimated from the measured Al amount. The sampling device collected bulk samples (dry and wet deposition) using a 1 gallon Nalgene high-density polyethylene bottle with a polyethylene funnel (0.011 m2) attached at its top. The sampling duration was 1 month for each sample and it required human intervention.

Total dust deposition was also collected in the Sahelian region using a CAPYR sampler (Orange et al., 1990; Herrmann, 1996; Rajot, 2001). The CAPYR is a 40 cm high funnel-shaped sampler with a 0.25 m2 horizontal inlet opening. It was used to collect total dust deposition in Niger from 1996 to 1998 (Rajot, 2001). In Niger, a Frisbee sampler, which consists of a circular stainless steel collecting bowl (0.07 m2 and 3.6 cm deep) surrounded by an aerodynamically shaped aluminum deflector ring (Wiggs et al., 2002), was also used to collect total dust deposition (Sow et al., 2006). This device requires manual rinsing to collect total deposition.

To collect dry and wet Saharan dust deposition separately, ARS MTX or Aerochem Metrics model 301 samplers were used on the Eastern and Western Mediterranean coasts (Ozsoy, 2003; Morales-Baquero et al., 2013) and on Canary Islands (López-Garcia et al., 2013). To perform successive samples, the collect systems have to be manually replaced. Wet deposition was also collected with wet-only collectors in the Eastern Mediterranean (Theodosi et al., 2010), in Niger (Galy-Lacaux et al., 2009) and Florida (Prospero et al., 2010). The conservation of the deposition soluble fraction over time strongly limits the autonomy of the sampling device.

These various approaches (and associated samplers) have technical drawbacks on the field that have limited their development in terms of sampling network expansion or duration. This is especially due to a short autonomy, as human intervention is required to replace the collect system after each sampling period. Therefore, a robust automatic system is necessary to achieve low costs in the long term and a wide area network survey system.

A CARAGA collector was installed mid-2010 on Frioul Island (43.27∘ N; 5.29∘ E), situated in the Gulf of Lions off Marseille. One of the main constraints with Saharan dust sampling is that their transport from source regions towards the Mediterranean Sea and Europe is sporadic. The sampling time needs to be short enough to catch dust transport events individually but long enough to provide sufficient autonomy to the CARAGA. Dust fallout events are brief and usually less than 3 days long (Loÿe-Pilot and Martin, 1996). A sampling duration of 1 week is adapted for collecting mineral particles corresponding to dust transport and deposition over the Mediterranean Basin. This sampling frequency also guaranties a large autonomy of the collector (up to 25 weeks depending on the desired number of in situ blank filters).

In order to collect the deposited particles on the filter, the funnel vibrates and is rinsed with 100 mL of pure water twice (2 and 1 h before the filter change). To bring back the samples to the laboratory, a suitcase designed to keep 25 filter holders with their lids in an upright position is used. Figure S2 visualizes the filters set collected on Frioul Island between July and December 2011. The different colours of the filters illustrate the variation in mass and the nature of the deposition from one week to another. Insects, vegetal debris, pollens or other organic matters collected on the filters are manually removed only if this manipulation does not affect the sample. If the removal of these elements could damage the sample, we leave them on the filter and the ignition of the samples at 550 ∘C eliminates these organic matters. The mineral mass fluxes of the weekly total (dry and wet) insoluble deposition for samples collected from February 2011 to October 2012 are reported in Fig. 2. The precipitation rates measured on Frioul Island are also presented in Fig. 2 in order to point out the potential wet deposition occurring during precipitation events.

The insoluble mineral deposition measured on Frioul Island is 2.45 g m-2 for February to December 2011 and 3.16 g m-2 for January to October 2012, which corresponds to low deposition amounts without very strong dust events in this area of the Mediterranean Basin. Loÿe-Pilot and Martin (1996) measured an average annual dust deposition flux of 12.5 g m-2 in Corsica for an 11-year period between 1984 and 1994, with annual deposition fluxes varying from 4.0 to 26.2 g m-2 yr-1. They mentioned that high magnitude events drive the variability of dust fallout at an annual and interannual scale. They also showed that most of Saharan events in Corsica are associated to wet deposition. Ternon et al. (2010) found an average annual deposition value of 11.4 g m-2 at Cap Ferrat and Corsican sites between 2003 and 2006. Their measurements showed a high range of frequency and intensity of Saharan dust deposition events: very low (5 10-2 g m-2) dust flux events occurred relatively often (27 events in 4 years), and there was only one extreme event of 22 g m-2 representing almost 90 % of the deposition in 2004.

The weekly total insoluble deposition fluxes of mineral dust at the Frioul site also exhibit a large variability ranging over 2 orders of magnitude, from no noticeable deposition (in the range of the uncertainty) to 3.3 × 10-1 g m-2 (Fig. 2). About 60 % of the measured deposition fluxes at the Frioul site are lower than 5 × 10-2 g m-2. This indicates that, most of the time, mineral deposition on Frioul Island can be attributed to low deposition due to atmospheric particles background. Nine major events with mineral deposition fluxes ranging from 1.5 × 10-1 to 3.3 × 10-1 g m-2 have been recorded on this site between (a) 07 and 14 July 2011, (b) 20 and 27 October 2011, (c) 26 April and 03 May 2012, (d) 17 and 24 May 2012, (e) 14 and 21 June 2012, (f) 28 June and 5 July 2012, (g) 23 and 30 August 2012, (h) 30 August and 06 September 2012 and (i) 27 September and 04 October 2012. In 2011 and 2012, major deposition events occurred more frequently in late spring and summer. Precipitations ranging from 1.0 to 80.8 mm were measured on Frioul Island for seven of these event periods (a, b, c, d, g, h and i), precipitation data being totally or partly missing for two event periods (e and f respectively).

The monthly average insoluble deposition fluxes measured at the Frioul site from February 2011 to October 2012 are presented in Fig. 3. The range of these fluxes (from 100 to 470 mg m-2) is comparable to measurements previously performed in Cap Ferrat (Bonnet and Guieu, 2006; Pulido-Villena et al., 2008) in 2004 and 2006 and Corsica in 2003 and 2005 on both sides of the Ligurian Sea (see more details in Ternon et al., 2010). The measurements performed in Corsica between 1984 and 1994 by Loÿe-Pilot and Martin (1996) show higher monthly deposition amounts ranging from about 200 to 2500 mg m-2. The monthly deposition measurements on Frioul Island show a maximum in early spring (April–May) in agreement with the measurements of Loÿe-Pilot and Martin (1996) in Corsica. Ternon et al. (2010) showed maximum deposition in February 2004 due to a huge dust event and in late spring (June). These seasonal differences can be explained by the large dust deposition variability caused by intense sporadic events observed in the northern Mediterranean Basin (Loÿe-Pilot and Martin, 1996; Ternon et al., 2010).

For the major deposition events measured at the Frioul site, AOD from MODIS AQUA and TERRA observations ranged from 0.3 to 0.7, suggesting significant atmospheric loads in particulate matter. The origins of the air masses reaching the Frioul site for the nine main deposition events measured were analyzed using the HYSPLIT model backward trajectories (https://ready.arl.noaa.gov/HYSPLIT.php) (Fig. 4). The HYSPLIT trajectories computed for the major mineral deposition events (a, b, c, d, e, f, g and i) indicate air masses originating from the southern Mediterranean Basin and North Africa. For one event (h) the air mass trajectories suggest a stagnant meteorological situation over the Gulf of Lions and the Western Mediterranean Basin.

An automatic collector (CARAGA) has been specially developed to sample total (dry and wet) insoluble atmospheric deposition, and a protocol for the treatment of the collected samples implying ignition and weighing has been tested. This protocol enabled us to focus on the mineral fraction of the deposition, most of the organic particles being volatilized or oxidized at 550 ∘C.

The present study on the estimates of Saharan dust deposition on Frioul Island, a site located in the Gulf of Lions, illustrates the use of a CARAGA. The collector has worked continuously for almost 2 years and provided weekly total deposition samples. Insoluble mineral deposition measured on Frioul Island is 2.45 g m-2 for February to December 2011 and 3.16 g m-2 for January to October 2012, which corresponds to low deposition amounts without very strong dust deposition events in the north-western Mediterranean Basin. The weekly deposits of insoluble mineral particles range over 2 orders of magnitude. Nine major events with mineral deposition ranging from 1.5 × 10-1 to 3.3 × 10-1 g m-2 were recorded on Frioul Island. During this period, the major deposition events occurred more frequently during spring and summer. Precipitations, ranging from 1.0 to 80.8 mm, are associated to seven of these dust event periods, precipitation data having not totally or partly being recorded for the two other dust event periods. Nine main mineral deposition events are measured in periods during which MODIS aerosol optical depth is high. Air masses passing over the Frioul site during these periods came from the southern Mediterranean Basin and North Africa. These elements strongly indicate that the higher deposition events measured on Frioul Island are due to Saharan dust transport associated to local precipitation.

In order to improve Saharan dust deposition monitoring in the Mediterranean Basin and the south of France, CARAGA collectors are now deployed over eight stations located in France, Spain, Italy and Tunisia: Frioul (43.27∘ N; 5.29∘ E), Le Casset (44.99∘ N; 6.47∘ E), Montandon (47.28∘ N; 6.82∘ E), Cap Corse (43.00∘ N; 9.36∘ E), Mallorca (39.27∘ N; 3.05∘ E), Sierra Nevada (36.95∘ N; 3.43∘ W), Lampedusa (35.52∘ N; 12.63∘ E) and Medenine (33.50∘ N; 10.64∘ E). These constitute an operating and standardized network of total insoluble dust deposition. As such, it should provide us with a promising basis to document the multiannual spatial and temporal variability of mineral dust deposition and to constrain CTM dust simulation over the Western Mediterranean region.