A Pasko
email: pasko@satie.ens-cachan.fr

M Lobue

E Fazakas

L K Varga

F Mazaleyrat

Spark plasma sintering of Mn-Al-C hard magnets

Keywords: numbers: 75.50.Cc, 75.50.Ww, 75.60.Ej, 81.40.Rs

Structural and magnetic characterization of isotropic Mn-Al-C bulk samples obtained by spark plasma sintering (SPS) is reported. This technique, to our knowledge, has not been used for preparation of Mn-Al-based permanent magnets yet. Transformation from the parent ǫ-phase to the ferromagnetic τphase occurred on heating in the process of sintering. The phase constitution of melt-spun precursors and consolidated samples was determined by X-ray diffraction. Magnetic hysteresis loops were recorded using a vibrating sample magnetometer. The compositional dependence of coercivity, magnetization and density of sintered materials is analyzed. To combine good magnetic properties with proper densification, further optimization of the production parameters is necessary.

Introduction

Mn-Al alloys are attractive candidates for permanent magnet applications due to reasonable magnetic properties, good technological properties (strength, workability, corrosion resistance) and low cost (no rare earth elements). With proper optimization, they would be capable of filling the gap between hard ferrites and Nd-Fe-B magnets [START_REF] Coey | Permanent magnets: Plugging the gap[END_REF]. The ferromagnetic tetragonal τ -phase with strong uniaxial magnetocrystalline anisotropy [START_REF] Kono | On the ferromagnetic phase in manganese-aluminum system[END_REF][START_REF] Koch | New material for permanent magnets on a base of Mn and Al[END_REF] forms from the non-magnetic hexagonal ǫ-phase by annealing a quenched Mn 50+x Al 50-x alloy (x = 0 . . . 10) at 400-700 °C. Both phases are metastable at room temperature and therefore need special thermal treatment. Carbon addition stabilizes τ -phase, increases coercivity, and facilitates production of enhanced anisotropic materials by hot deformation [START_REF] Ohtani | Magnetic properties of Mn-Al-C permanent magnet alloys[END_REF]. Magnetic hysteresis of Mn-Al-C alloys is sensitive to the microstructure and presence of defects developed during τ -phase formation, comprising martensitic or massive transformation mechanisms [START_REF] Wiezorek | Grain boundary mediated displacive-diffusional formation of τ -phase MnAl[END_REF], and is therefore strongly influenced by the production route.

Various manufacturing techniques have been employed for preparation of homogeneous Mn-Al-C alloys: melt-spinning [START_REF] Sakka | Rapid quenching and properties of hard magnetic materials in MnAl-X (X = Ti, Cu, Ni, C, B) systems[END_REF][START_REF] Fazakas | Preparation of nanocrystalline Mn-Al-C magnets by melt spinning and subsequent heat treatments[END_REF][START_REF] Liu | Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C, B, and rare earth elements[END_REF], levitation melting with splat quenching [START_REF] Kim | Formation of a metastable ferromagnetic τ phase during containerless melt processing and rapid quenching in Mn-Al-C alloys[END_REF], spark erosion [START_REF] Berkowitz | Properties of Mn-Al-C magnets prepared by spark erosion and other rapid solidification techniques[END_REF], ball milling [START_REF] Zeng | Structural and magnetic properties of nanostructured Mn-Al-C magnetic materials[END_REF][START_REF] Lee | Effect of mechanical milling and heat treatment on the structure and magnetic properties of gas atomized Mn-Al alloy powders[END_REF], mechanical grinding [START_REF] Saito | Magnetic properties of Mn-Al-C alloy powders produced by mechanical grinding[END_REF]. Depending on the solidification rate or size reduction degree, fine-grained materials with improved functional properties can be obtained. In this paper we report on the structural and magnetic characterization of isotropic Mn-Al-C bulk samples produced by spark plasma sintering (SPS). This technique, to our knowledge, has not been used for preparation of Mn-Al-based permanent magnets yet. During SPS the heat is generated by a pulsed electric current passing directly through the powder compacted by a simultaneously applied uniaxial pressure [START_REF] Munir | The effect of electric field and pressure on the synthesis and consolidation of materials: A review of the spark plasma sintering method[END_REF]. The process is very fast and ensures full densification of nanosized powders at moderate temperatures without significant coarsening. Melt-spinning of the cast alloys is employed to make homogeneous ǫ-phase precursors with fine grains [START_REF] Pasko | Hard magnetic properties of melt-spun Mn-Al-C alloys[END_REF]. Transformation to the ferromagnetic τ -phase occurs on heating in the process of sintering. The goal is to investigate a correlation between the production route, phase constitution and resulting hard magnetic properties.

Experimental

Binary alloy Mn 55 Al 45 and ternary compositions Mn 49 Al 49 C 2 and Mn 54 Al 44 C 2 doped with carbon were selected for the present study. The ingots prepared by induction melting were cast into ∼3 mm wide and ∼25 µm thick ribbons using a melt-spinning technique [START_REF] Fazakas | Preparation of nanocrystalline Mn-Al-C magnets by melt spinning and subsequent heat treatments[END_REF]. Brittle ribbons were subsequently milled by hand with pestle and mortar. The obtained powders were compacted in a graphite die with internal diameter of 8 mm and sintered in a Sumitomo Dr. Sinter Lab SPS machine under vacuum with applied pressure of 70 MPa. Thermal conditions were chosen as follows: maximum temperature from 650 to 950 °C, heating/cooling rate 100 K/min, dwell time 5 min (temperature was measured by a thermocouple inside the die). For comparison, the ribbons were also annealed either in a vacuum furnace at 550 °C for 10 min with ramping rate of 10 K/min or directly in the magnetometer cell. To ensure better densification, we increased the sintering temperature of ǫ-phase precursors. Crystal structures were examined in Co-K α radiation by a PANalytical X'Pert Pro X-ray diffractometer (XRD) equipped with an X'Celerator linear detector. Full-profile analysis of XRD spectra was performed using a Rietveld-based software MAUD [START_REF] Lutterotti | Maud: a Rietveld analysis program designed for the internet and experiment integration[END_REF]. The density of sintered samples was measured by Archimedes method. Magnetization curves were recorded by a Lake Shore 7400 Series vibrating sample magnetometer (VSM) with a variable temperature assembly.

Results and discussion

Spark plasma sintering

The idea behind spark plasma sintering is simultaneous application of heat and pressure for more efficient consolidation of powders. Unlike hot pressing, SPS uses a resistive heating by a pulsed electric current passing directly through the die and (for conducting materials) the sample. This gives a number of advantages as the current can play an intrinsic role contributing to mass transport, and much higher heating rates can be achieved [START_REF] Munir | The effect of electric field and pressure on the synthesis and consolidation of materials: A review of the spark plasma sintering method[END_REF]. Materials are typically sintered in a matter of minutes as opposed to hours for conventional methods, and the temperatures needed for consolidation to full density are significantly lower, which open the possibility to create materials with nanoscale features [START_REF] Hulbert | The absence of plasma in "spark plasma sintering[END_REF]. In addition to rapid Joule heating, various mechanisms of observed enhanced sintering were proposed: local melting and evaporation, plasma formation between particles, cleaning effect due to removal of oxide films and adsorbed gases from the particle surface, enhanced diffusion at Sintering is completed at ∼800 °C.

forming particle necks [START_REF] Orrù | Consolidation/synthesis of materials by electric current activated/assisted sintering[END_REF]. However, for many of these phenomena (especially those that invoke the presence of plasma) no unambiguous experimental evidence has been reported [START_REF] Hulbert | The absence of plasma in "spark plasma sintering[END_REF].

As applied to synthesis of Mn-Al-C magnets, SPS technique can be beneficial to combine in one step the heat treatment necessary for τ -phase formation and the powder sintering itself, while avoiding excessive grain coarsening and decomposition of metastable phases in the intermediate temperature region. We tried two thermal regimes: at lower temperatures (650 °C) sintering is accompanied by ǫ → τ phase transition; at higher temperatures (850-950 °C) the material is sintered in the stable ǫ-phase state, and transformation to τ -phase occurs on cooling.

The SPS machine allows to record several important parameters controlling the process of sintering. Figure 1 shows the temperature dependence of the volume occupied by the powder in a die caused by application of heat and pressure. A sharp increase of the punch displacement on heating indicates the temperature at which sintering begins. Vertical segments of the curves correspond to the isothermal stage of sintering. The negative slope on cooling reflects a thermal contraction of densified samples. In Mn 55 Al 45 alloy the powder consolidation starts at ∼450 °C and practically finishes before the isothermal stage. On the other hand, in Mn 49 Al 49 C 2 and Mn 54 Al 44 C 2 alloys the powder consolidation starts at ∼500 °C and continues during the isothermal stage. Among the three samples, Mn 54 Al 44 C 2 shows the longest isothermal sintering and the smallest total deformation. This behaviour can be explained by increasing stability of ǫ-phase in the sequence Mn 55 Al 45 → Mn 49 Al 49 C 2 → Mn 54 Al 44 C 2 to decomposition, as described below, and higher sintering ability of the equilibrium γ 2 -phase. Figure 2 shows similar curves with different maximum temperatures. We can estimate that sintering of Mn 49 Al 49 C 2 alloy completes at ∼800 °C.

Phase analysis

Phase compositions of melt-spun ribbons (as-quenched and annealed) and sintered bulk samples have been determined by XRD. According to the phase diagram of Al-Mn system [START_REF] Shukla | Thermodynamic assessment of the Al-Mn and Mg-Al-Mn systems[END_REF], formation of two equilibrium phases is expected: Mn-rich disordered cubic β-phase and Al-rich ordered trigonal γ 2 -phase. In addition, two metastable phases may appear in non-equilibrium conditions: disordered hexagonal ǫ-phase and ordered tetragonal τ -phase. Stable at higher temperatures, ǫ-phase can be preserved on quenching and transformed into ferromagnetic τ -phase by a composition-invariant (involving only short-range diffusion) structural transition. Depending on the initial chemical composition and thermal history, all these phases were observed in different combinations. Figure 3 presents XRD spectra from the precursor powders, whereas figure 4 shows them after spark plasma sintering at 550 °C (some weak unreferenced peaks belong to the identified phases).

Some results of quantitative phase analysis based on Rietveld refinement of XRD data are presented in table 1. Weight fractions and lattice parameters of observed crystal structures have been determined. The melt-spun Mn 55 Al 45 and Mn 54 Al 44 C 2 alloys enriched in Mn are composed entirely of ǫ-phase, whereas Mn 49 Al 49 C 2 composition contains also significant amount of γ 2 -phase. The effect of subsequent heat treatment depends on the presence of carbon. In the binary Mn 55 Al 45 alloy ǫ-phase decomposes completely into a pair of equilibrium γ 2 -phase and β-phase. In the ternary Mn 49 Al 49 C 2 and Mn 54 Al 44 C 2 alloys ǫ-phase transforms on heating into ferromagnetic τ -phase. However, certain quantity of γ 2 -phase is always detected (the rest is Mn 3 AlC not given in the table). On the other hand, we obtain 100 % τ -phase in Mn 54 Al 44 C 2 melt-spun sample after furnace annealing at 550 °C [START_REF] Pasko | Hard magnetic properties of melt-spun Mn-Al-C alloys[END_REF]. This may indicate that, despite high heating/cooling rate, the elevated sintering temperatures of 650-950 °C are not optimal for the structural ǫ → τ transition to occur without decomposition. Addition of carbon slightly increases both lattice parameters of ǫ-phase, whereas in τ -phase it leads to decrease of a and pronounced (∼1.5 %) increase of c, the unit cell volume also increases [START_REF] Pasko | Hard magnetic properties of melt-spun Mn-Al-C alloys[END_REF]. Though C has been reported to occupy ( 1 2 , 1 2 , 1 2 ) sites together with Al [START_REF] Yang | Neutron diffraction study of hard magnetic alloy MnAlC[END_REF], this observation provides a new evidence for presence of carbon atoms in the interstitial positions (0, 0, 1 2 ) and ( 1 2 , 1 2 , 0) of tetragonal τ -phase [START_REF] Zeng | Structural and magnetic properties of nanostructured Mn-Al-C magnetic materials[END_REF][START_REF] Lee | Phase and lattice parameter relationships in rapidly solidified and heat-treated (Mn 0.53 Al 0.47 ) 100-x Cx pseudo-binary alloys[END_REF] similar to Fe-C martensite.

The mean size of reflecting crystallites determined from line broadening is 80-160 nm for most phases. Exception is ǫ-phase having larger grains in as-quenched melt-spun samples due to a columnar microstructure. Lattice parameters and volume fraction of each phase have been used for calculation of a theoretical density to compare with the macroscopic density measured directly. The values of relative density in table 1 Moreover, there is a correlation between the weight fraction of τ -phase determined from XRD and the saturation magnetization value extracted from VSM measurements. The Mn 54 Al 44 C 2 alloy with major τphase shows higher magnetization and lower coercivity of 0.19 T. Two-phase alloy Mn 49 Al 49 C 2 with minor τ -phase shows lower magnetization and higher coercivity of 0.31 T. Our values of coercivity are higher than 0.17 T [START_REF] Fazakas | Preparation of nanocrystalline Mn-Al-C magnets by melt spinning and subsequent heat treatments[END_REF] and 0.15 T [START_REF] Liu | Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C, B, and rare earth elements[END_REF] reported for rapid solidification attempts, but lower compared to 0.48 T [START_REF] Zeng | Structural and magnetic properties of nanostructured Mn-Al-C magnetic materials[END_REF] achieved in a ball milled (not sintered) powder. Sintering at 650 °C gives better coercivity because of larger fraction of non-magnetic γ 2 -phase, finer grains and increased porosity, which leads to formation of numerous pinning centres. Sintering at 850-950 °C results in higher density, more homogeneous phase distribution and less complex microstructure. Figure 7 shows the magnetization of as-quenched Mn 54 Al 44 C 2 sample as a function of temperature in a constant magnetic field of 1.4 T. Thermal ramps were performed between 27 °C and 627 °C at 5 K/min rate. The precursor ǫ-phase is antiferromagnetic at low temperatures [START_REF] Wys Locki | Magnetic properties of the non-oriented ǫ-phase in Mn-Al-C permanent magnet[END_REF] and paramagnetic under normal conditions. On heating, ǫ-phase transforms to τ -phase which is also paramagnetic at the transition temperature ∼513 °C (see the inset). The structural transition is detected due to a difference in magnetic susceptibility of the phases involved. On cooling, the magnetization is increased drastically below the Curie temperature of τ -phase as the reverse transition to ǫ-phase does not occur, probably, due to kinetic reasons. In Mn-Al-C alloys the Curie temperature of τ -phase decreases with carbon doping, however available data largely vary [START_REF] Fazakas | Preparation of nanocrystalline Mn-Al-C magnets by melt spinning and subsequent heat treatments[END_REF][START_REF] Liu | Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C, B, and rare earth elements[END_REF][START_REF] Zeng | Structural and magnetic properties of nanostructured Mn-Al-C magnetic materials[END_REF]. Taking into account this dependence, we find a good agreement of our T c measurements with results of Zeng et al. [START_REF] Zeng | Structural and magnetic properties of nanostructured Mn-Al-C magnetic materials[END_REF]. The coercivity of Mn 54 Al 44 C 2 sample subjected to in-situ annealing is 0.19 T, close to the values received after furnace annealing and low temperature SPS (Table 2).

Additional information on magnetization processes can be obtained from fielddependent remanence measurements. According to the Stoner-Wohlfarth theory, for a system of non-interacting single-domain particles the following relation holds true The dependence of magnetic properties on heat treatment conditions is less pronounced compared to Mn 49 Al 49 C 2 alloy due to higher stability of τ -phase.

[23]: Here m r (H) ≡ M r (H)/M r (∞) is the reduced initial remanence obtained by applying a magnetic field to the virgin state (IRM), m d (H) ≡ M d (H)/M r (∞) is the reduced demagnetization remanence obtained in a reverse field after saturating the magnetization (DCD). Deviation parameter ∆m(H) serves as a measure of interaction between particles in real systems [START_REF] Kelly | Switching mechanisms in cobaltphosphorus thin films[END_REF]. Positive values of ∆m(H) are due to interactions promoting the magnetized state, whereas negative values of ∆m(H) are caused by interactions tending to assist magnetization reversal. Similar relations between remanence curves can also be derived using the Preisach model of hysteresis [START_REF] Basso | Interpretation of hysteresis curves and Henkel plots by the Preisach model[END_REF].

∆m(H) ≡ 2m r (H) + m d (H) -1 = 0. ( 1
For Mn 49 Al 49 C 2 and Mn 54 Al 44 C 2 alloys sintered at 650 °C, remanence curves m r (H) and m d (H) are presented in figure 8, deviation parameter ∆m(H) is plotted in figure 9. Two samples show similar initial remanence shape, whereas their demagnetization behaviour differs. The Mn 54 Al 44 C 2 alloy with high content of ferromagnetic τ -phase reveals regions of positive and negative ∆m(H) values, which can be explained by a competition of different dipolar or exchange interactions. The Mn 49 Al 49 C 2 alloy, which contains significant amount of non-magnetic phase and probably has a specific microstructure, exhibits much stronger magnetic interactions and positive ∆m(H) values. This behaviour can be attributed to higher density of pinning centres in the inhomogeneous material and is related to its higher coercivity.

Conclusion

Spark plasma sintering technique has been employed to produce solid Mn-Al-C permanent magnets from melt-spun ribbons. Simultaneous application of heat and pressure in the process of consolidation enabled us to eliminate the separate stage of annealing usually required for transformation of the parent ǫ-phase into the ferromagnetic τ -phase. However, further optimization of the production parameters is necessary, including the choice of particle size and thermal regimes of sintering.

Figure 1 .

 1 Figure 1. Displacement of the punch (expressed per powder mass) as a function of temperature during SPS. Isothermal sintering behaviour at 650 °C depends on the alloy composition.

Figure 2 .

 2 Figure 2. Displacement of the punch (expressed per powder mass) as a function of temperature during SPS for Mn 49 Al 49 C 2 alloy with isothermal stages indicated.Sintering is completed at ∼800 °C.

Figure 3 .

 3 Figure 3. XRD patterns of melt-spun powders without thermal treatment. These precursors are mostly composed of ǫ-phase.

Figure 5 .Figure 6 .

 56 Figure 5. Magnetic hysteresis loops of Mn 49 Al 49 C 2 alloy at room temperature.The sintered at 650 °C sample shows higher coercivity but lower magnetization than the annealed at 550 °C ribbon due to a smaller content of τ -phase.

Figure 7 .

 7 Figure 7. Magnetization as a function of temperature under applied magnetic field for as-quenched Mn 54 Al 44 C 2 sample. Inset: a magnified fragment of the curve showing the temperature induced ǫ → τ structural phase transition.

Figure 8 .Figure 9 .

 89 Figure 8. Reduced initial remanence mr (IRM) and reduced demagnetization remanence m d (DCD) as functions of applied magnetic field. The reversal behaviour differs for two compositions.

Table 1 .

 1 confirm the analysis of SPS curves. Bulk Mn 55 Al 45 sample (containing no τ -phase) is fully densified, whereas in Mn 49 Al 49 C 2 and Mn 54 Al 44 C 2 samples (which experienced ǫ → τ transformation during SPS at 650 °C) the density is only ∼90 %. Thus, further investigation of the sintering mechanism in the presence of structural phase transformations is needed. A possible solution to this problem can be found by introducing a ball milling stage before SPS. Nominal composition of the studied alloys, XRD phase analysis (weight fractions) of as-quenched (AQ), annealed at 550 °C and sintered at 650-950 °C samples, and relative density (measured directly / calculated from XRD data).

			Mn 55 Al 45	τ-phase γ 2 -phase β-phase carbide
	Intensity (arb. unit)		Mn 54 Al 44 C 2
			Mn 49 Al 49 C 2
	40	50	60	70	80	90
		Diffraction angle, 2θ (°)
	Composition State	Phases (wt.%)		Density
	(at.%)		ǫ	τ	γ 2	β	(%)
	Mn 49 Al 49 C 2 AQ	59 -	41 -	-
		550	-	70	30 -	-
		650	-	25	67 -	92
		850	-	35	65 -	99
		950	-	46	54 -	99
	Mn 54 Al 44 C 2 AQ	100 -	-	-	-
		550	-	100 -	-	-
		650	-	84	6 -	88
		850	-	79	21 -	100
		950	-	96	4 -	100
	Mn 55 Al 45	AQ	100 -	-	-	-
		550	-	75 -	25 -
		650	-	-	33	67 100
	3.3. Magnetic properties

Figure 4. XRD patterns of sintered at 650 °C bulk samples. The Mn 54 Al 44 C 2 alloy contains more ferromagnetic τ -phase than other compositions.

Magnetic measurements were performed on melt-spun ribbons (annealed in the furnace or during in-situ heating/cooling in VSM) and sintered bulk samples (table 2). The

Table 2 .

 2 Magnetic properties of annealed at 550 °C melt-spun ribbons and sintered at higher temperatures (650-950 °C) bulk samples in applied field of 1.9 T. curves of two compositions having ferromagnetic behaviour are presented in figure5and figure6. These results agree well with XRD data (table 1) as τ -phase responsible for hard magnetism is observed after sintering only in Mn 49 Al 49 C 2 and Mn 54 Al 44 C 2 .

	Composition Temperature Magnetization Remanence Coercivity
	(at.%)	T ht (°C)	µ 0 M s (T)	µ 0 M r (T)	µ 0 H a (T)
	Mn 49 Al 49 C 2 550	0.38	0.22	0.23
		650	0.15	0.08	0.31
		850	0.22	0.09	0.13
		950	0.24	0.09	0.10
	Mn 54 Al 44 C 2 550	0.55	0.31	0.18
		650	0.50	0.26	0.19
		850	0.47	0.16	0.09
		950	0.59	0.14	0.05
	Mn 55 Al 45	550	0.43	0.24	0.14
	room temperature magnetization


J Ringuet

E Leoz-Garziandia

H Budzinski

CARACTERISATION DES DERIVES DU BENZO[A]PYRENE (B[A]P), EN ATMOSPHERE REELLE ET EN LABORATOIRE

Polycyclic Aromatic Hydrocarbons (PAH) are emitted by all the combustion sources and are present in both the particulate and the gas phases. Since they are potentially carcinogenic and/or mutagenic they have been widely studied. Benzo[a]pyrene (B[a]P) have been chosen as a tracer for PAH because of his toxicity. It is mostly present in the particular phase (90%). Very reactive, it can be oxidized to form nitrated and oxygenated by-products which can be more toxic than B[a]P itself. These reactions can occur in the air and/or on the sampling support. The purpose of this work is to determine the major place of reaction, to identify the oxidizing species responsible for these degradations and by-products formation, for a better B[a]P quantification. Field campaign and reactor tests will allow to identify the reaction products and oxidants, to determine the major place of degradation, to improve B[a]P monitoring. Analytical and sampling methods will be described. The analysis of filters sampled with an impactor will be presented.

Résumé :

Les Hydrocarbures Aromatiques Polycycliques (HAP) sont des composés émis sous forme gazeuse et particulaire, lors de combustions incomplètes. Ces composés potentiellement cancérigènes et/ou mutagènes sont réglementés. C'est le benzo[a]pyrène (B[a]P) qui a été choisi comme traceur pour cette famille de composé par la IV ème directive fille européenne (2004/107/CE), en raison de sa forte toxicité. Ce composé qui est présent à plus de 90% en phase particulaire est très réactif. Il peut réagir avec la plupart des oxydants gazeux pour former des composés nitrés et oxygénés qui peuvent potentiellement être plus toxiques que le B[a]P lui-même. Ces réactions peuvent avoir lieu dans l'air (en amont du prélèvement) et/ou sur le support de prélèvement. Le but de l'étude est de déterminer le lieu de réaction majoritaire, puis d'identifier les espèces oxydantes responsables de ces dégradations, ainsi que les produits formés, afin de proposer des solutions permettant de mieux quantifier le B[a]P. Pour cela, une étude de terrain couplée à des tests en réacteur permettront dans un premier temps, d'identifier les produits de réactions et les oxydants, puis de déterminer le lieu majoritaire de dégradation. La méthode analytique, ainsi que les méthodes de prélèvements seront décrits. Les résultats présentés sont issus de filtres prélevés avec un impacteur en cascade.

Introduction

Les hydrocarbures aromatiques polycycliques (HAP) sont des composés émis sous forme gazeuse et particulaire, principalement lors de combustions incomplètes. Leur surveillance est importante puisqu'ils sont potentiellement cancérigènes et mutagènes (classés 1 par l'IARC pour certains). C'est le cas du benzo[a]pyrène (B[a]P) qui a été choisi comme traceur pour cette famille de composés par la IV ème directive fille européenne (2004/107/CE) (valeur cible de 1 ng/m 3 en moyenne annuelle sur la fraction PM10). Ce choix a été dicté par le fait qu'il représente à lui seul près de 40% de la cancérogenèse totale des HAP. Cependant, le B[a]P pose des problèmes de quantification dans l'atmosphère. Il est en effet très réactif et peut réagir avec de nombreux composés gazeux tels que l'ozone, les radicaux OH ou les oxydes d'azotes, mais également avec la lumière. Les principaux produits de dégradation listés dans la bibliographie sont des dérivés nitrés et oxygénés (Finlayson-Pitts et Pitts Jr, 2000) qui s'avèrent pour certains d'entre eux plus dangereux d'un point de vue toxicologique que le B[a]P lui-même.

Etant donné que la réaction du B[a]P avec les espèces oxydantes de l'atmosphère et la lumière peut avoir lieu dans l'air (avant prélèvement) ou sur le support de prélèvement (durant le prélèvement), les principaux enjeux de ce travail sont :

• L'identification du lieu de réaction majoritaire ;

• L'identification des espèces oxydantes responsables de cette dégradation ;

• La caractérisation des espèces formées (origine primaire / secondaire, lien taille / toxicité) ; Et ceci afin de proposer des solutions permettant de bien quantifier le B[a]P.

Cependant, en fonction du lieu majoritaire de réaction, les axes de progrès proposés seront différents. En effet, si la réaction a lieu majoritairement dans l'atmosphère, l'étude de caractérisation des espèces formées permettra de rendre compte du réel impact du B[a]P et des autres HAP sur la pollution atmosphérique ; et d'orienter des travaux de toxicité sur des nouvelles espèces. S'il s'avérait que la dégradation pendant la phase de prélèvement est la plus importante, l'identification des espèces oxydantes responsables de cette dégradation permettra le développement des solutions métrologiques à mettre en place lors du prélèvement. A ce jour, des « dénudeurs » à ozone (Goriaux et al., 2006) permettent d'éliminer l'ozone en amont du support de prélèvement.

Ce travail présente les premiers résultats d'analyse de filtres issus d'un impacteur en cascade qui prélève les particules en fonction de leur taille et peut donner des informations sur les origines primaires ou secondaires des HAP nitrés et oxygénés identifiés.

Méthodologie de l'étude

Cette étude combine des travaux en laboratoire et sur le terrain : Etudes en laboratoire : ces travaux ont pour but de mettre au point la méthode d'analyse des dérivés nitrés et oxygénés des HAP et de la valider grâce à des tests effectués en réacteur chimique. Les tests permettront également de déterminer les produits majoritaires de dégradation du B[a]P ainsi que les composés oxydants les plus réactifs vis-à-vis de ce dernier. Des tests réalisés en présence de multi-polluants ainsi que des essais de photo oxydation seront également réalisés pour refléter au mieux les conditions atmosphériques. Les particules utilisées seront issues de prélèvements de terrain ou générées avec un brûleur et seront exposés à des concentrations contrôlées d'oxydants.

Etudes de terrain : ces travaux permettront d'identifier le lieu de réaction majoritaire du B[a]P. Pour ce faire des prélèvements avec différents appareils munis ou non de pièges à ozone seront réalisés sur différents sites (urbains, périurbains et ruraux). La caractérisation des dérivés des HAP par tranche granulométrique sera également effectuée à l'aide d'un impacteur en cascade. Les HAP oxygénés et nitrés pouvant être émis directement dans l'atmosphère ou formés par réaction des espèces oxydantes avec les HAP, la taille des particules auxquelles ils seront associés, pourra nous donner des informations sur leur origine primaire et/ou secondaire (formés par réaction).

Matériels et méthodes

Méthode analytique

La méthode analytique utilisée est basée sur l'extraction par ASE (accelerated solvant extraction), la purification sur cartouche SPE (solid phase extraction), puis l'analyse en GC/MS en ionisation chimique négative. Les HAP et plus particulièrement les HAP oxygénés et nitrés étant présents à l'état de traces, cette technique présente l'avantage d'être très sensible (limite de quantification de l'ordre du dixième de pg injecté pour les HAP nitrés). Cette méthode a été décrite au cours de précédents travaux (Albinet et al., 2006).

Les campagnes de terrain

Dans le cadre de ce travail, des campagnes de prélèvements sur le terrain sont prévus. Deux d'entre elles ont déjà été réalisées. Chacune d'entre elles était intégrée dans des projets français ou européens sur des sites spécifiques. La première s'est déroulée en janvier 2009 à Grenoble dans le cadre du projet FORMES (Fraction Organique de l'aérosol urbain : Méthodologie d'Estimation des Sources). Il s'agissait d'un site de fond urbain, caractérisé par un fort apport particulaire dû au chauffage au bois, à des couches d'inversion basses et à une topographie particulière (vallée). La deuxième campagne s'est déroulée à Saclay en juillet 2009 dans le cadre du projet Européen MEGAPOLI (Megacities : Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation). Ce site péri-urbain présentait la principale caractéristique d'être situé potentiellement dans le panache de la ville de Paris. La concentration en oxydants gazeux élevée, cumulée à la forte photochimie et au temps de transport entraîne des concentrations importantes en polluants secondaires dans le panache.

Sur ces deux campagnes, les préleveurs utilisés pour notre étude étaient des préleveurs haut débit (30 m 3 /h) de type digitel équipés ou non de dénudeurs à ozone et un impacteur en cascade à douze étages de type MOUDI.

Les essais en laboratoire

Comme expliqué plus haut, afin d'étudier l'effet de différents oxydants, ainsi que les produits formés, des essais en réacteurs seront réalisées. Deux réacteurs différents seront utilisés, un réacteur à écoulement (Figure 1) pour les réactions en présence de dioxyde d'azote et/ou de radicaux OH°. Pour les réactions réalisées en présence d'ozone, un réacteur semi-statique (Figure 2) sera utilisé.

Avancement de l'étude, premiers résultats

Les travaux effectués jusqu'à l'heure ne permettent malheureusement pas de présenter des résultats définitifs. Néanmoins, les essais préliminaires réalisés à l'INERIS permettent de proposer une distribution plus détaillée de certains HAP oxygénés (Figure 3 et Figure 4).

Il apparaît sur cette figure que grâce à l'utilisation de l'impacteur en cascade à 12 étages MOUDI, une meilleure distribution granulométrique peut être réalisée. Ainsi en les comparant aux figures issues d'un impacteur Andersen à 6 étages, il apparaît que la 9,10-Anthraquinone présente une distribution bimodale centrée autour de 0,1 et 10 µm. Ces résultats semblent indiquer que la 9,10-Anthraquinone a une double origine primaire et secondaire. En ce qui concerne les autres produits, il apparaît ici aussi que le MOUDI permet d'établir une répartition granulométrique plus détaillée. Ainsi, il semble que la 9-Phenathrènecarboxaldéhyde et la benzanthrone sont présents en majeure partie sur des particules de 0,1 µm de diamètre, dites alvéolaires.

Ces résultats ne sont que préliminaires et nécessitent d'être complétés par les analyses de filtres prélevés lors des deux campagnes de terrain décrites précédemment.

La mise en place de méthodes de prélèvement tel que le MOUDI ont déjà permis l'acquisition des données prometteuses et ont été utilisées lors des deux campagnes de mesures et seront déployées lors de campagnes à venir. De plus, l'optimisation et la validation de la méthode analytique permettra l'analyse des filtres issus de ces campagnes et l'exploitation des résultats.

Références bibliographiques

Albinet, A., Leoz-Garziandia, E., Budzinski, H., ViIlenave, E., (2006). Simultaneous analysis of oxygenated and nitrated polycyclic aromatic hydrocarbons on standard reference material 1649a (urban dust) and on natural ambient air samples by gas chromatography-mass spectrometry with negative ion chemical ionisation. Journal of Chromatography A 1121, 106-113. Albinet, A., Leoz-Garziandia, E., Budzinski, H., Villenave, E., Jaffrezo, J.L., (2008). Nitrated and oxygenated derivatives of polycyclic aromatic hydrocarbons in the ambient air of two French alpine valleys Part 2: Particle size distribution. Atmospheric Environment 42, 55-64.

Finlayson-Pitts, B.J., Pitts Jr, J.N., (2000). Chemistry of the upper and lower atmosphere, New York.969 pp Goriaux, M., Jourdain, B., Temime, B., Besombes, J.L., Marchand, N., Albinet, A., Leoz-Garziandia, E., Wortham, H., (2006)

Figure 1 :Figure 2 :

 12 Figure 1 : Réacteur à écoulement de Bordeaux 1 (Réactions OH° et NO 2 )

Figure 3 :Figure 4 :

 34 Figure 3 : Répartition de certains HAP oxygénés en fonction de la taille des particules (données obtenues avec un impacteur 12 étages de type MOUDI)

  . Field Comparison of Particulate PAH Measurements Using a Low-Flow Denuder Device and Conventional Sampling Systems. pp. 6398-6404.