Emmanuel Kouakou Kouadio

Mathieu Goriaux

Philippe Laguionie

Véronique Ruban

Metal particle seeding on urban surface samples

In order to estimate the resuspension of the particles empirically, it is necessary to carry out a homogeneous distribution of the particles on the tested surfaces. Thus, in many studies, seeding or deposition in experimental chambers is performed to quantify initial concentrations for subsequent resuspension experiments. The current study was carried out to assess metal particle seeding efficiency on four types of urban surfaces (slate, facade coating, tile, and glass) in a test chamber. To achieve this objective, we compared firstly different solubilization techniques of silver polydisperse particles (1.3-3.2 μm and 0.5-1.0 μm) and gold polydisperse particles (Ø˂5 μm) for chemical quantification by ICP-MS. The result showed better yields in the case of gold for all solubilization techniques studied (82% ± 5% to 98% ± 2% for gold versus 23% ± 18% to 84% ± 12% for silver). Based on this result, four seeding tests were carried out with the gold particles (distribution in chamber centered on 1μm). The concentrations seeded on urban surfaces (mean ± SD) varied from 10,900 ± 1,900 μg.m -2 (facade coating sample) to 1900 ± 390 μg.m -2 (glass sample). The relative standard deviation of the measured concentrations equaled 9.5% (tested for aluminum foils), which was less than the measurement uncertainty of the recording equipment (≈14%) and reflected good seeding homogeneity. Observations by scanning electron microscopy coupled to microanalysis (SEM-EDX) were in agreement with these conclusions.

Introduction

Urban environments contain high concentrations of atmospheric metal particles due to their population density, types of activities, and the nature of their component surfaces [START_REF] Laidlaw | Re-Suspension of lead contaminated urban soil as a dominant source of atmospheric lead in birmingham, Chicago, Detroit and Pittsburgh, USA[END_REF][START_REF] Sternbeck | Metal emissions from road traffic and the influence of resuspension -results from two tunnel studies[END_REF][START_REF] Thorpe | Sources and properties of non-exhaust particulate matter from road traffic: a review[END_REF]. Such an environment is distinguished by both a high proportion of impervious surfaces and the specific nature of these surfaces (asphalt roads, glass, building uses). In their scientific expertise report on soil artificialization, [START_REF] Béchet | Sols artificialisés et processus d'artificialisation des sols[END_REF] established the importance of taking surface composition into account for a full urban environment assessment (e.g., heat islands). They illustrated that the cumulative total of roofs and impermeable soils accounts for more than 50% of all surfaces in large cities [START_REF] Bhatti | Built-up area extraction using Landsat 8 OLI imagery[END_REF][START_REF] Carlson | Satellite remote sensing of land use changes in and around San Jose, Costa Rica[END_REF][START_REF] Li | Effects of spatial resolution of remotely sensed data on estimating urban impervious surfaces[END_REF][START_REF] Liu | Landscape metrics for analysing urbanization-induced land use and land cover changes[END_REF]. Many studies have been dedicated to the presence of metal particles in the air of cities [START_REF] Sobhanardakani | Human health risk assessment of potentially toxic heavy metals in the atmospheric dust of city of Hamedan, West of Iran[END_REF][START_REF] Sobhanardakani | Ecological and human health risk assessment of heavy metal content of atmospheric dry deposition, a case study: Kermanshah, Iran[END_REF]. The metal elements identified, especially within residential areas, seem most often to display a distribution centered on a submicronic or fine fraction [START_REF] Percot | Contribution des retombées Atmosphériques aux flux de polluants issus d'un petit bassin versant urbain: cas du Pin Sec à Nantes[END_REF]. In addition, according to [START_REF] Allen | Size distributions of trace metals in atmospheric aerosols in the United Kingdom[END_REF], most metal particles in urban air are associated with an accumulation mode (i.e., of anthropogenic origin). [START_REF] Mbengue | Size-distributed metallic elements in submicronic and ultrafine atmospheric particles from urban and industrial areas in Northern France[END_REF] compared the particle size distribution and chemical composition of airborne particles downwind of an urban industrial area at a site near Dunkirk (France) during winter 2012. Most elements measured (Sb, Cd, As, Mo, Pb, Zn, Cu, Ni, Cr, Mn, and V) were of anthropogenic origin. The authors found that, as opposed to coarse particles, the mass concentrations of submicron (PM1) and ultrafine (PM0.29) particles were higher downwind of the urban area. The mean concentration of metal elements measured was 1.7 × 10 3 ng•m -3 . In other European cities, authors have measured, for example, 2.5×10 3 ng•m -3 in Augsburg, Germany [START_REF] Gu | Source apportionment of ambient particles: comparison of positive matrix factorization analysis applied to particle size distribution and chemical composition data[END_REF], 783 ng•m -3 in Palermo, Italy [START_REF] Dongarrà | Study on ambient concentrations of PM10, PM10-2.5, PM2.5 and gaseous pollutants. Trace Elements and Chemical Speciation of Atmospheric Particulates[END_REF], 2.0 × 10 3 ng•m -3 in Zurich, Switzerland (Richard et al. 2011), and 200 ng•m -3 in London, UK [START_REF] Visser | Kerb and urban increment of highly time-resolved trace elements in PM10, PM2.5 and PM1.0 winter aerosol in London during ClearfLo 2012[END_REF]. In Asia, [START_REF] Lyu | atmospheric size-resolved trace elements in a city affected by non-ferrous metal smelting: indications of respiratory deposition and health risk[END_REF] sampled metal particles, with aerodynamic diameters between 0.43 and 9 μm, present in Zhuzhou's (China) ambient air during a 2-month campaign. Measured trace element concentrations ranged from 10 -2 to 10 4 ng•m -3 , with Cu, Zn, As, Se, Ag, Cd, TI, and Pb all revealing a unimodal distribution in the fine particle size range (Ø < 2.1 μm).

In general, particle resuspension is not quantified in the current estimates of dry deposition and source identification [START_REF] Lengweiler | Which parameters are important deposition and[END_REF][START_REF] Percot | Contribution des retombées Atmosphériques aux flux de polluants issus d'un petit bassin versant urbain: cas du Pin Sec à Nantes[END_REF]. In cities, this phenomenon is mainly studied in the road traffic context (tire abrasion, road debris, sand particles) [START_REF] Amato | Effect of rain events on the mobility of road dust load in two Dutch and Spanish roads[END_REF][START_REF] Amato | Traffic induced particle resuspension in Paris: emission factors and source contributions[END_REF][START_REF] Escrig | simple estimates of vehicle-induced resuspension rates[END_REF][START_REF] Ferm | Concentrations and emission factors for PM2.5 and PM10 from Road Traffic in Sweden[END_REF][START_REF] Gehrig | Mobile load simulators -a tool to distinguish between the emissions due to abrasion and resuspension of PM10 from Road Surfaces[END_REF][START_REF] Martuzevicius | Resuspension of particulate matter and pahs from street dust[END_REF]Nicholson 1988;[START_REF] Sabin | Dry deposition and resuspension of particle-associated metals near a freeway in Los Angeles[END_REF][START_REF] Thorpe | estimation of particle resuspension source strength on a major London road[END_REF][START_REF] Weinbruch | A quantitative estimation of the exhaust, abrasion and resuspension components of particulate traffic emissions using electron microscopy[END_REF]. Resuspended particles are thus most often associated with the coarse mode [START_REF] Damay | Détermination expérimentale de la vitesse de dépôt sec des aérosols submicroniques en milieu naturel: influence de la granulométrie, des paramètres micrométéorologiques et du couvert INSA de Rouen[END_REF][START_REF] Percot | Contribution des retombées Atmosphériques aux flux de polluants issus d'un petit bassin versant urbain: cas du Pin Sec à Nantes[END_REF]. By failing to take resuspensions into account, the uses of such quantification in urban spaces can greatly overestimate the level of deposition. Several papers have reported that particle resuspension contributes significantly to the airborne emission of metals, such as zinc (Zn) and lead (Pb) [START_REF] Damay | Détermination expérimentale de la vitesse de dépôt sec des aérosols submicroniques en milieu naturel: influence de la granulométrie, des paramètres micrométéorologiques et du couvert INSA de Rouen[END_REF][START_REF] Laschober | Particulate emissions from on-road vehicles in the Kaisermühlen-Tunnel (Vienna, Austria)[END_REF][START_REF] Percot | Contribution des retombées Atmosphériques aux flux de polluants issus d'un petit bassin versant urbain: cas du Pin Sec à Nantes[END_REF]Nicholson 1988;Nicholson 2009). However, resuspension remains a rather complex concept to accurately measure (Nicholson 2009). Various approaches have been developed to quantify resuspended elements. Generally speaking, particle resuspension is often expressed as either a resuspension factor K (in m -1 ) or a resuspension rate Λ (in s -1 ) (Nicholson 1988;[START_REF] Sehmel | Particle and gas dry deposition: a review[END_REF]. According to the authors, significant limitations constrain the application of these concepts under ambient conditions (Nicholson 1988;[START_REF] Sehmel | Particle and gas dry deposition: a review[END_REF]. The application of K or Λ requires an assessment of the deposit available for resuspension, i.e., the thickness of deposit suitable for resuspension [START_REF] Nicholson | Wind tunnel experiments on the resuspension of particulate material[END_REF]. In addition, it is difficult to determine the concentration of particles in the air directly above the considered surface; this concentration is used to calculate the resuspension factor or rate and depends on the height at which sampling is performed. For uniform areas (in terms of roughness height), the use of numerical models may be an option, but less so for highly heterogeneous environments like cities (Nicholson 1988). In practice, the application of K under actual conditions assumes that all airborne particles solely originate via the surfaces studied (Nicholson 1988). Emissions (whether natural or anthropogenic) then become difficult to separate from re-emitted elements.

Very few studies have been dedicated to resuspension without the use of K or Λ (Boor et al. 2013a(Boor et al. , 2013b;;[START_REF] Lengweiler | Which parameters are important deposition and[END_REF]. [START_REF] Lengweiler | Which parameters are important deposition and[END_REF] studied the influence of various parameters on the deposition and resuspension tested simultaneously in a confined space. Their tests were carried out with talcum dust on painted wood fiber boards in the aim of evaluating the dust load, i.e., the amount of dust accumulated at the time of equilibrium between dust deposition (g•m -2 •h -1 ) and resuspension (g•m -2 •h -1 ). They concluded that, at least as far as the influence of turbulence is concerned, deposition and resuspension should be examined separately. Boor et al. (2013aBoor et al. ( , 2013b) ) developed two experimental chambers to study the aerodynamic resuspension of deposits of fluorescent particles on indoor environment surfaces. The first chamber was used to perform particle seeding on surface samples, while the second was used for resuspension tests under different wind speeds. Their approach refers to the term "absolute resuspension fraction" φ, which is defined as the change in seeding density following a windy episode. In short, whether encountering difficulties in applying the resuspension factor and rate in the field or implementing other approaches from the literature, it seems necessary to separately experiment with the two processes: the seeding (or deposition) phase and the seeded particle resuspension phase. Knowing the quantities of deposited particles assigned to the resuspension experiments would appear to be most critical. Hence, the experimental chambers are used for the particle seeding (or deposition) step on typical surfaces (Boor et al. 2013a(Boor et al. , 2013b;;Hussein et al. 2009aHussein et al. , 2009b;;[START_REF] Ould-Dada | Resuspension of small particles from tree surfaces[END_REF][START_REF] Thatcher | Particle deposition from natural convection enclosure flow onto smooth surfaces[END_REF]Qian and Ferro 2008). These chambers offer several advantages, including the choice of particle diameter and type of test surface. In most studies however, they have been built to test surfaces representative of indoor environments, e.g., offices or bedrooms [START_REF] Byrne | Stable tracer aerosol deposition measurements in a test chamber[END_REF][START_REF] Thatcher | Particle deposition from natural convection enclosure flow onto smooth surfaces[END_REF][START_REF] Lai | Investigation of electrostatic forces on particle deposition in a test chamber[END_REF]Hussein et al. 2009aHussein et al. , 2009b)). Moreover, due to the type of particles being studied, the quantifications of particles seeded (or deposited) were essentially based on optical microscopy, fluorescence measurement, or weighing methods [START_REF] Friess | A generic model for the resuspension of multilayer aerosol deposits by turbulent flow[END_REF][START_REF] Lengweiler | Which parameters are important deposition and[END_REF][START_REF] Matsusaka | particle reentrainment from a fine powder layer in a turbulent air flow[END_REF]. During the experiments conducted by [START_REF] Lengweiler | Which parameters are important deposition and[END_REF], the dust load was measured by vacuuming surface samples with a cleaning head on fiberglass filters. The amount of dust sampled was determined by differentiating the weight of the filter before and after sampling. In addition, because it was difficult to recover small particles by suction, the authors relied on a sticky sheet to collect the particles from the surfaces, and the quantity of these particles was obtained by counting them under a microscope. In Boor et al. (2013aBoor et al. ( , 2013b)), experimentation and quantification were carried out by means of stereomicroscopy combined with an image processing software. Nevertheless, this technique required working on very small test surfaces (i.e., 4.5 × 4.5 cm) and scanning the entire test surface during observation, or else assuming that the sampled particle distribution was perfectly uniform. All these conditions favor high measurement uncertainty, capable of influencing measurement of the part to be resuspended.

Finally, in his work, [START_REF] Fromentin | Particle resuspension from a multi-layer deposit by turbulent flow PSI[END_REF] indicates the importance of the effect of the homogeneity of the deposits (i.e., the different adhesion layers) on the resuspension flow. It seems that the conditions during aerosol deposition, i.e., during the first stage of the experiment, are key parameters for the resuspension process. Thus, a significant difference in the resuspension results was found as the inhomogeneity of the deposit increases (by two orders of magnitude over 10min). Thus, the effect of inhomogeneity has to be seriously taken into account; it influences the resuspension flow substantially. However, in most of the studies (Table 1), the question of deposit homogeneity (initial concentration in the case of resuspension experiments) is not or not clearly studied. In addition, measurement uncertainties related to the quantification techniques used are not mentioned (e.g., optical counting methods, fluorescence measurements or weighing….).

Table 1: Experimental conditions for studies involving particle seeding (or deposition) chambers

This paper presents the first step of a general framework for quantifying the resuspension of metal particles under dry conditions (i.e., no rainfall) and in the urban outdoor environment. Since the resuspension phase requires homogeneous seeding and particles adapted to the ambient environment, a methodology for effective particle recovery and quantification is developed.

The main goals of the current investigation can be summarized as follows:

• To seed urban surfaces with PM2.5 metal particles (micro-particles of silver (Ag) and gold (Au))

• To validate the choice of particle (analytical method) • To develop a methodology for effective particle recovery and quantification by ICP-MS • To evaluate the seedings obtained (homogeneity of concentrations by surface type)

Materials and methods

Description of the chamber

Seeding experiments were conducted in a 1 m -3 cubic lined plywood chamber. The plywood was chosen to minimize the effect of electrostatic charges between particles and chamber walls. Chamber sealing was ensured by cleats, a silicone gasket, and EPDM (ethylene propylene diene monomer) foam tape. The device is shown in Fig. 1 and was installed at the Water and Environment Laboratory (Gustave Eiffel University, Nantes); it consists of an air filter-drying system, a particle generator, and the chamber itself. Metal particles were generated using a 3jet Collison nebulizer supplied with compressed air. In order to improve the quality of the compressed air supplied to the nebulizer, this air was dried and filtered by a filter-drying module. Three Festo MS4 pneumatic filters connected in series (5 μm, 1 μm, and 0.01 μm), associated with an MS4 membrane dehydrator, formed this filtration module. This serial system allowed for ultrafiltration and drying of the compressed air (residual particles < 0.1 μm, residual oil < 0.1 mg•m -3 ). Two adjustable digital flowmeters (SMC flowmeter, model PFM7) were installed in the device, downstream of the filter-drying module. Each flowmeter provided an air flow rate of 10 L•min -1 : one injected the correct air flow into the nebulizer, while the other supplied dilution air for the input aerosols at the nebulizer outlet. Mixing and turbulent flow were induced by a small 3000-rpm fan (12 V), mounted in the center of the ceiling 13.5 cm below the top. The characteristics and location of the fan were determined based on the work of [START_REF] Lai | Supermicron particle deposition from turbulent chamber flow onto smooth and rough vertical surfaces[END_REF], who studied deposits of fluorescein particles. Most studies have introduced such devices to induce aerosol mixing and turbulence in the test chamber [START_REF] Byrne | Stable tracer aerosol deposition measurements in a test chamber[END_REF][START_REF] Lai | Supermicron particle deposition from turbulent chamber flow onto smooth and rough vertical surfaces[END_REF]Hussein et al. 2009aHussein et al. , 2009b)). However, no specific recommendations regarding fan speeds were found in the literature, given that chamber configurations and objectives differ from one author to another. System checkpoints were also monitored throughout our trials. A humidity, temperature, and pressure sensor was installed in the chamber during the entire seeding process; it was a KISTOCK KPA 320 automatic recorder. Moreover, a water bubbler connected to a low-flow suction pump (≈ 6.4 L•min -1 ) induced discharge from the system.

Fig. 1: Diagram of the seeding system used in these tests

In practice, each particle generation started by cleaning the chamber with compressed air (vacuum nebulizer). The optimal cleaning time was measured during preliminary tests using a GRIMM 1.108 particle counter; it was estimated at 60 min for a minimum pressure of 2 bars. The particle concentration in the nebulized solution equaled 20 g•L -1 , and it remained limited to this quantity in order to comply with the manufacturer's specifications while generating a large mass of particles. The solution was a 50/50 mixture of ultrapure water and ethanol. Particle generation lasted 60 min, i.e., a time period corresponding to the aerosolization of approx. 20 mL of the mixture with a compressed air pressure of 3 bars. Once seeding had been completed, the suction was interrupted. The chamber was then sealed for 16 h, at the end of which it was opened, and the various seeded surfaces tested were covered with aluminum foil to facilitate their transport and avoid any lift-off.

Urban surface test sample arrangement during seeding

Four types of seeding surfaces were tested: slate, facade coating, tile (flat), and glass; all test samples were dimensioned 20 cm × 20 cm and selected on the basis of their representativeness in the urban environment. During testing, the samples were placed on the chamber floor. [START_REF] Byrne | Stable tracer aerosol deposition measurements in a test chamber[END_REF] determined the vertical (wall) and horizontal (floor) deposition fluxes in a test chamber for four aerosol particle size distributions (0.7 μm, 2.5 μm, 4.5 μm, and 5.4 μm). For each one, more than 50% of the generated material was deposited on the chamber floor. In practice, for our experiments, 16 test samples were deposited in the chamber during a given seeding test (4 surface samples per type of surface). As shown in Figs. 2 and3, the test samples were arranged in two configuration modes, labeled 1 and 2. Mode 1 was a vertical line arrangement of four test samples of the same type (slate or facade coating or tile or glass). For mode 2, also arranged in a vertical line, test samples were alternated two-by-two according to their type. These arrangements were driven by technical choices. It was useful to seed as many different types of test samples as possible at the same time, in addition to assessing whether or not their location on the chamber floor significantly affected seeding homogeneity. The level of roughness by type of test piece had not been defined, so it was also considered to be homogeneous over the entire surface for each test piece. Yet the test samples were identified by codes in order to identify any uniqueness in seeding results.

Particulate matter investigated

Selection of study particles

For subsequent ambient (outdoor) resuspension tests (subject to relative humidity, temperature, urban pollution, etc.), the abundance of the selected metals in urban ambient air flows had to be low, while their chemical stability needed to be at a good level. In addition, these metals had to differ from the elements being remobilized by test sample washing.

A blank wash was carried out to identify the chemical elements capable of being released from the test samples. The washing technique employed was identical to that applied during all experiments reported in this paper (see "Particle recovery after seeding"). Metals such as copper (Cu) and zinc (Zn) were present in high concentrations. Gold and silver were selected for further work based on the blank results (zero/low concentration in the test sample blanks). Gold (Au) was selected for both its low abundance in ambient air and chemical stability. Silver (Ag) was selected for its very low concentrations (close to the limit of quantitation) measured in the blanks. Both are transition metals; they are not highly reactive with water at ambient temperature. The test particles (Ag and Au) were spherical and 99.9% pure polydisperse powder (Alfa Aesar/ThermoFischer). A logistical limit was imposed on the use of metal micro-particles as we were unable to find monodisperse size powders on the market. The ranges supplied by the distributor and used in this study were thus powders with diameters varying between 1.3-3.2 and 0.5-1.0 μm for silver and a powder with diameters smaller than 5 μm for gold. The size distribution of the particles generated in the chamber during seeding was centered on 1 μm (particle counter MINI-WRAS 1371, GRIMM).

Effectiveness of analytical methods associated with the selected particles

In order to be quantified by means of ICP-MS (inductively coupled plasma mass spectrometry), the metals had to be solubilized. After selecting the test metals, the first operation consisted of evaluating the solubilization efficiency of each particle type as well as the efficiency of their quantification by ICP-MS (8900 Triple Quadrupole). All methods tested were based on LEE's internal protocols for the dissolution of metals present in all types of water (as adapted from Standard NF EN ISO 11885 (2009)). To determine the most effective method, yields were deduced for each type of particle according to the method applied. Solubilization was performed on volumes of acidified ultrapure water (pH=1, HNO3: nitric acid) doped with silver or gold particles. The concentration of the doped solutions was 200 μg•L -1 regardless of the element. This concentration corresponded to the highest point on the ICP-MS calibration curve. The mixtures were mechanically homogenized before sampling. Two analytical methods were compared: metal particle solubilization in total fractions and solubilization in extractable fractions. The total fraction is the sum of both the dissolved and particulate fractions. The dissolved/particulate separation was achieved by membrane filtration (MF-Millipore, pore diameter: 0.45 μm). Subsequently, dissolved fractions were directly analyzed by ICP-MS; also, particulate fractions, referred to here as "filters" (filters = MF-Millipore membrane + particles Ø > 0.45 μm), were mineralized for measurement by ICP-MS as well. The filters were mineralized in various ways, depending on the nature of the tested particles. In the case of silver particles, two mineralization processes were performed, namely, total hydrofluoric and perchloric acid attacks and hot filter mineralization with concentrated nitric acid. The gold filters were exclusively mineralized using the hot acid mineralization process, which was performed with aqua regia. The metals in the extractable fractions were solubilized directly on the doped samples, without separating out the dissolved and particulate fractions. The solutions were thus acidified (HNO3 for silver and aqua regia for gold) and then evaporated. The residues obtained after evaporation were once again recovered, with the various acids and ultrapure water, and heated for 1 h. The preparations were ultimately diluted with ultrapure water before conducting the ICP-MS measurements.

Particle recovery after seeding

A washing method was developed as part of this study to recover the seeded particles from test samples. Various authors have used washing techniques to estimate the quantity of particles deposited on test surfaces [START_REF] Maro | Étude Du Dépôt Sec Des Aérosols En Milieu Urbain[END_REF][START_REF] Percot | Contribution des retombées Atmosphériques aux flux de polluants issus d'un petit bassin versant urbain: cas du Pin Sec à Nantes[END_REF][START_REF] Roupsard | Etude phénoménologique du d´dépôt sec d'aérosols en milieu urbain : Influence des propriétés des surfaces, de la turbulence et des conditions météorologiques[END_REF]. After seeding, each test sample was therefore washed with 600 mL of an ultrapure water solution acidified to pH=1 with nitric acid. Washings took place in polyethylene tanks using a laboratory wash bottle. This technique was the same for all test samples: 500 mL of washing solution was introduced to thoroughly rinse the test sample and then 100 mL to rinse the tray and recover any residual particles. The acidity (pH=1) of the wash solution allowed for chemical stabilization of the recovered solutions while maintaining test sample structural integrity throughout the testing campaign. Particle concentrations measured as a result of washing were correlated with the test sample surface area (μg•m -2 ).

One of the main objectives of the washing technique developed was to achieve efficient recovery (i.e., recovering a maximum quantity of seeded particles, saving time and limiting the volume of solution used). Based on previous studies [START_REF] Maro | Étude Du Dépôt Sec Des Aérosols En Milieu Urbain[END_REF][START_REF] Roupsard | Etude phénoménologique du d´dépôt sec d'aérosols en milieu urbain : Influence des propriétés des surfaces, de la turbulence et des conditions météorologiques[END_REF], we estimated that washing as is would not have been effective in relying on a single operation. Therefore, in order to carry out just a single washing operation without underestimating the exact seeded concentrations, experimental correction factors were defined. For this purpose, a preliminary test of successive washes was run on the test samples until obtaining concentrations close to or below the ICP-MS limit of quantification. Four washes (4 × 600 mL of wash solution) were necessary to reach these values. This test was conducted after validating the study particles and analytical method. We were able to establish experimental correction factors for each type of test sample when only one washing operation was performed.

Results and discussion

Particles investigated

Silver particles

Table 2 shows the yields obtained for silver particles by the various solubilization methods employed. Regardless of size range and method, average yields were all below 50%, except for hot mineralization in the 0.5-1.0 μm range, where the values equaled 51% and 84% for extractable and total fractions, respectively. The low yields observed could be due to silver sorption phenomena on the containers used for sample preparation (polyethylene tubes). They might also be due to the chemical stability of silver in the doped samples before solubilization. We could not explain the best yields being obtained for the 0.5-1.0 μm range; however, even though these yields are higher than the others, they still all lie below 90%. In addition, the silver particles tested seem to resist remaining in suspension in the doped solutions as thin films of particles, especially silver, were observed on the sample surface. We concluded that, in our study context, silver particles were not adequate for future resuspension experiments. The difficulty involved in avoiding particle losses during pretreatment (sorption, particle film, or other phenomena) and associated errors could strongly influence resuspension quantification.

Table 2 : Yields of the various silver particle solubilization methods

Gold particles

Table 3 presents the yields resulting from the various gold solubilization methods. The concentrations measured in the dissolved and particulate fractions (hot mineralization) have an average yield of 82% ± 5%; this figure was 98% ± 2% for the extractable fraction method. The solubilization of gold in the extractable fraction had yielded nearly twice as much as all methods applied to silver particles. The relative standard deviations of these measurements were smaller for the tests with gold particles than those with silver (2% to 5% for gold, 2% to 32% for silver).

Table 3 : Yields of the various gold particle solubilization methods

On the basis of these results, gold particles were accepted as model particles for the next step. The gold solubilization method in the extractable fraction was selected as the most appropriate pretreatment method for analysis of these particles according to ICP-MS. In addition, the chemical stability of gold (in both air and water) will provide a great advantage during outdoor resuspension testing. Table 4 presents the experimental correction factors obtained for 3 successive washings of the test samples (recovery of gold particles). A 4th washing was also performed. The recovery rates were not taken into account due to the low concentration values obtained after measurement (i.e., concentrations below or equal to the ICP-MS limit of quantification). In the remainder of this work, these factors were applied to the concentrations measured on urban surface samples after each gold particle seeding test.

Quantification of seeded gold particle concentrations

Seeding conditions

The pressure values measured within the chamber during testing ranged from 995 to 1039 hPa. The average temperatures measured, between 19 and 21°C, were close to those of the ambient environment (± 1°C). The relative humidity (RH) observed throughout the tests varied from 42 to 64% with a mean of 53%; it was important to ensure this value remained below 67% [START_REF] Kim | Source term models for fine particle resuspension from indoor surfaces[END_REF]). An RH below this threshold makes it possible to neglect the particle-surface capillary forces [START_REF] Kim | Source term models for fine particle resuspension from indoor surfaces[END_REF], which are suspected to influence the resuspension phenomenon. Scanning electron microscopy (SEM, model Hitachi SU5000, combined with an EDX-Xflash Brucker detector) observations were also carried out on both carbon scotch pieces (Ø = 6 mm) and small slate test samples (Ø ≈ 10 mm). The objective of this observation was to verify the conformity of the particle sizes indicated by the supplier and their distribution on the surfaces (homogeneity, possible aggregate formation, monolayer or multilayer condition). The carbon scotch and slate samples were placed at different points in the chamber and seeded with gold particles. The particles observed by SEM and the sizes were indeed well within the range indicated by the supplier. The gold particles on both the carbon scotch and slate samples were individually and rather homogeneously distributed over the observed surfaces. The seeding was monolayer, and no aggregates could be observed.

Seeding homogeneity and measured concentrations

In order to evaluate the robustness of the homogeneity of gold particle seeding, an experiment was conducted with aluminum foil. Nine aluminum squares approximately 20 cm × 20 cm (equal to a test sample area) were distributed on the test chamber floor. The quantities of gold seeded were measured for each foil (with replicates). The relative standard deviation of the measured concentrations equaled 9.5%; this value was less than the ICP-MS measurement uncertainty (≈14%). This result allowed concluding that gold particles were homogeneously distributed over the various points of the chamber floor during seeding tests. In their work with fluorescein particles, [START_REF] Lai | Supermicron particle deposition from turbulent chamber flow onto smooth and rough vertical surfaces[END_REF] obtained a maximum relative difference of less than 15% among the various points in their chamber. Their tests were performed on eight 2 cm × 2 cm copper sheets, and they concluded that for this standard deviation (SD < 15%), the concentration in the chamber was assumed to be "reasonably" uniform. Four gold seeding tests were performed on the surface test samples. Figures 4,5,6,and 7 show the mean surface concentrations from these tests (tests 1 and 2: mode 1 configuration; tests 3 and 4: mode 2 configuration). The average (mean ± SD) maximum and minimum concentrations measured across all tests were 10,900 μg•m -2 (facade coating, test 2) and 1900 μg•m -2 (glass, test 3). For test 1, the mean concentrations by test sample type were 6200 ± 900 μg•m -2 , 9000 ± 1000 μg•m -2 , 3800 ± 500 μg•m -2 , and 7400 ± 1000 μg•m -2 , for slate, facade coating, tile and glass, respectively. Measurements during tests 2, 3, and 4 reported 4847 ± 679 μg•m -2 , 4930 ± 690 μg•m -2 , and 3152 ± 441 μg•m -2 on the slate; 7700 ± 1100 μg•m -2 , 3900 ± 500 μg•m -2 , and 4300 ± 600 μg•m -2 on façade coating; 7700 ± 1100 μg•m -2 , 4000 ± 600 μg•m -2 , and 5400 ± 800 μg•m -2 on tile; and 5900 ± 800 μg•m -2 , 2800 ± 400 μg•m -2 , and 3500 ± 500 μg•m -2 on glass. As these figures attest, the concentration values within each test are of the same order of magnitude for each type of surface. Nevertheless, a decrease in the quantities measured between test 1 and the other tests is noticeable. We have attributed this decline to the nebulizer. A gradual clogging of the probe spray nozzle orifices would have occurred over time, leading to a reduction in the quantity of particles generated in the chamber and thus received on the surfaces. Table 5 summarizes the relative standard deviations derived from the measured gold concentrations; the test sample values ranged from 3 to 32% for slate, 3 to 13% for facade coating, 15 to 21% for tile, and 13 to 34% for glass. However, these deviations did include the ICP-MS measurement uncertainty, which at the time of testing was ≈14%. Except for slight variations in temperature, humidity, and pressure within the chamber, all test parameters remained unchanged. No real constant is observed in the deviation values. With regard to the impact of clogging, it was found that the data from test 1 (before nebulizer clogging) varied from 13 to 23%, with the largest differences being observed for test 3. In test 4, a reduction in the deviations can be observed. Prior to this test (test 4), the nebulizer had been unclogged using ultrasound. Thus, test 1 was carried out before the clogging and test 4 after the unclogging procedure. Tests 2 and 3, on the other hand, were carried out, while the nebulizer was clogging. Even without taking into account the impact of nebulizer clogging, the differences obtained lie within the ranges found in the literature. [START_REF] Lai | Supermicron particle deposition from turbulent chamber flow onto smooth and rough vertical surfaces[END_REF] and Qian and Ferro (2008) have clearly estimated the homogeneity of seeding on the surface. The coefficients of variation between seedings (for the same type of surface) were 15% (preliminary test on copper foil) and 20%, respectively. Qian and Ferro (2008) also performed a homogeneity check in the chamber using sulfur hexafluoride (SF6) at six different locations in the chamber. The conclusions obtained with this gas were not in agreement with the estimates made directly from the deposits. The differences in concentrations between a control point and the other 5 points ranged from -7 ± 4 to 43 ± 12%. It therefore appears that the verification technique plays an important role in the validation of homogeneity. This indicates the interest of carrying out these homogeneity tests as close as possible to the method used for deposition (same type of particle, for example). It can be assumed that the assessments carried out directly from the deposits present results closer to reality. In Table 6, we have also estimated the coefficients of variation from the results presented by different authors. For example, in the work of [START_REF] Rosati | Resuspension of Particulate matter from carpet due to human activity[END_REF], these estimated coefficients varied from 6 to 44%, while in the case of [START_REF] Kassab | High resolution study of micrometer particle detachment on different surfaces[END_REF], they varied from 33 to 43%. Boor et al. (2013b) worked with 3μm and 10μm spherical polystyrene particle monolayer deposits (d = 1060 kg•m -3 ) for resuspension testing; they obtained seeding densities of 67 ± 19 particles/mm 2 and 0.73 ± 0.16 particles/mm 2 , respectively. For these values, the RSD correspond to 28% of the measured quantities for the 3 μm diameters and 22% for the 10μm diameters. Their seeding was carried out on test surfaces twenty times smaller than those of our tests, and their seeded particles were quantified by optical measurement. Compared to the results of Boor et al. (2013aBoor et al. ( , 2013b)), due to the size of the surfaces tested in our study (nearly 20 times larger), our deviations can be considered an improvement. Moreover, compared to optical counting, a chemical method should ensure greater robustness in the quantification of deposits. The relative standard deviations of our measurements lie above the 9.5% obtained with aluminum foil; however, it is important to note that the test sample roughness was not measured. Hypotheses formulated during the experimental set-up suggested neglecting the potential impact of this parameter. For one thing, the roughness was considered uniform over the entire test sample surface. Also, the urban surface test samples used in this work, similar to those encountered in actual urban environments, could hardly observe such uniformity. Lastly, the roughness was considered to be identical, e.g., between two test samples of the same type. These assumptions could explain the higher relative standard deviation values than those observed for aluminum foil.

Conclusions

Apart from experiments conducted in a chamber or wind tunnel, characterizing the resuspension of particles in an outdoor environment can be very complex. The objective of this paper was to evaluate the seeding step of metal micro-particles on urban surface samples intended for resuspension tests. An experimental approach was developed involving a seeding chamber along with a methodology for recovering and quantifying the deposited materials. This study is original by virtue of using metal particles for the seeding. Results showed that for our experimental context, gold particles are the most suitable, for analytical reasons, for both seeding and future resuspension work in urban outdoor environments. The test chamber made it possible to carry out monolayer seeding of individual gold micro-particles on urban surface samples. The relative standard deviations of concentrations among surface samples ranged from 3 to 34%. The originality of this method prevented drawing any comparisons with previous works. However, the results obtained with aluminum foils, to account for homogeneous roughness (RSD = 9.5%), did show the robustness of particle seeding homogeneity in our chamber. As a matter of fact, this difference was smaller than that found in the literature; future adjustments will aim to reduce it even further.

Fig. 2 :Fig. 3 :

 23 Fig. 2 : Arrangement of the test samples in the chamber -Mode 1

Fig. 4 :Fig. 5 :Fig. 6 :

 456 Fig. 4 : Gold particle concentrations measured for seeding test 1

Fig. 7 :

 7 Fig. 7 : Gold particle concentrations measured for seeding test 4

Table 4 :

 4 Experimental correction factors obtained for gold particles

Table 5 :

 5 Relative standard deviations deduced from the seeding testsAs shown in Table6, in the literature, only

Table 6 : Comparison of deposit homogeneity in the literature

 6

Acknowledgements

Special thanks to the members of the Water and Environment laboratory, Nadège Caubrière, Martin Guillon and Dominique Demare, for their precious help during the analyses.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.