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Ces travaux s'inscrivent dans le cadre du programme national de MEsure des RetombéesAtmosphériques (MERA). Ils portent sur la recherche de l'origine des précipitationscollectées entre 1997 et 1999 dans trois (Morvan, Iraty, Le Casset) des onze stations duréseau MERA localisées en différents points du territoire français. Deux méthodesstatistiques ont été utilisées dans cette étude. Les régions à l'origine des fortesconcentrations mesurées au site récepteur ont d'abord été déterminées à l'aide d'un modèle(méthode de Seibert) combinant les mesures réalisées sur site et les rétrotrajectoires demasses d'air puis, dans un second temps les différents profils de transport atmosphérique,leur fréquence et concentrations associées ont été évaluées à l'aide d'une classificationpar Nuées Dynamiques (méthode K-means/distance Euclidienne simple) des rétrotrajectoires demasses d'air. Le test de Kruskal-Wallis a été utilisé pour vérifier si les médianes desconcentrations associées à chaque classe sont statistiquement différentes. L'étude réaliséeà Iraty (Pyrénées) et au Casset (Alpes) a montré que ces deux stations sont influencéesdifféremment du Morvan. Plus exactement, ces deux sites ne sont pas, ou pratiquement pas,influencés par les zones d'Europe centrale ou du Nord-Ouest fortement émettrices de SO2, deNOx et de NH3. Seul le pH des précipitations collectées à Iraty sembledépendre des émissions de SO2 et de NOx d'une de ces zones. Iraty et le Casset sont trèsinfluencées par les émissions anthropiques et par les poussières d'origine terrestres enprovenance d'Afrique du Nord. Néanmoins, les niveaux de concentrations mesurés dans les fluxen provenance d'Afrique du Nord sont similaires pour Iraty, le Casset et le Morvan (sauf enions calcium, pour lequel le Casset et Iraty montrent de fortes concentrations). Une autrerégion européenne peut influencer les niveaux en composés acidifiants mesurés au Casset, ils'agit de l'Italie et de la zone localisée au niveau de l'ex-Yougoslavie. Mais, les niveauxde concentrations qui en résultent sont faibles par rapport à ceux mesurés dans certainsflux arrivant au Morvan.The chemistry of precipitation in France was examined using data from the Frenchatmospheric deposition network (MERA). In order to examine the source-receptor relationshipsresponsible for acid rain at three background sites in France, a receptor-oriented model wasapplied to the precipitation data collected from 1997 to 1999. This methodology combinedprecipitation and chemical data with air parcel backward trajectories to establishconcentration field maps of likely contributing sources. Then, a clustering technique usingpartitioning methods (K-means/Euclidian distance) was performed to backward trajectories andthe distributions of mixing samples associated with backward trajectories in each clusterwere compared. The Kruskal-Wallis test was used to verify that the concentration mediansassociated with each cluster were statistically significant. The results of this studydemonstrated that two stations (Iraty and le Casset) were not influenced by the same sourcesas Morvan. Specifically, these sites were less influenced by high emissions from Central orNorthwestern Europe when compared to Morvan. Only the pH seemed under the influence ofSO2 and NOx emissions from one of these areas. Iraty and Le casset are veryinfluenced by anthropogenic emissions and the crustal sources around the Mediterranean Basinand North Africa. Other European areas (e.g. Italy) can influence the concentrationsrecorded at Le Casset but the levels of concentration are lower than those measured atMorvan.This paper represents a complete statistical analysis of wet-only deposition chemistrydata for three stations (Iraty, Le Casset and Morvan). Two statistical methods were used inthis study. In order to examine the source-receptor relationships responsible for acid rainat these three background sites in France, a receptor-oriented model was applied to theprecipitation data collected from 1997 to 1999. This methodology combined chemical data withair parcel backward trajectories to establish concentration field maps of likelycontributing sources. This receptor-oriented model was developed by Seibert and it assumesthat if a trajectory endpoint falls in a grid cell (i,j), the air mass is assumed to collectcomponents emitted in this cell and once the components are incorporated, they aretransported along the trajectory to the receptor site. This model doesn't take into accountthe atmospheric diffusion and the removal mechanisms occurring during the trajectory fromthe sources to the receptor. Finally, a concentration field map for the selected species wascalculated taking into account all grid cells. For mapping, the grid cells counting fewerthan 10 endpoints were not taken in consideration because the confidence of their resultswas considered too low. The role of three-dimensional backward trajectories is fundamental,so we used three different information sources: the French Institute of Meteorology,Météo-France; the British Atmospheric Data Centre (BADC); and the Atmospheric EnvironmentService Long Range Transport model of Air Pollution (AES-LRTAP), Canada. These trajectorymodels were compared for different chemical species. All data were projected in the EMEPgrid (150 x 150 km) for establishment of the concentration field map. A clustering techniqueby partitioning methods (K-means/Euclidian distance) was performed on backward trajectoriesand the distributions of mixing samples associated with backward trajectories in eachcluster were compared. The Kruskal-Wallis test was used to verify that the medianconcentrations associated with each cluster were statistically significant.The results of this study for Morvan determined five classes of backward trajectoriesassociated with the precipitation collected at this station located in the centre of France.The fluxes from SW and WSW sectors contribute for 52% of events, while the fluxes of NW andE contribute for 31% of events but are mainly responsible for high concentrations ofsulphates, nitrates, ammonium and hydronium ion. Regions found to be responsible for rainevents coincide with European regions known for their high anthropogenic emissions ofSO2 and NOx (Great Britain, North of France, Belgium, The Netherlands andthe North of sea).The results for Iraty (South of France) yielded five classes of backward trajectoriesassociated with the precipitation collected in this station. The fluxes from W sectors (NNW,NW, W and WSW) were responsible for 71% of events, while the flux of S (low wind) wasresponsible for 29% of events but is mainly responsible for high concentrations ofsulphates, nitrates, ammonium and calcium. High concentrations of hydronium ion wereidentified in the NNW sector.The results for Le Casset (East region and mountainous) gave four classes of backwardtrajectories associated with the precipitation collected in this station. The fluxes from Wand WSW sectors were responsible for 35% of events, while the flux of SSW was responsiblefor 43% and the flux from the SE was responsible for 22% of events. This last sector wasmainly responsible for high concentrations of sulphates, nitrates, ammonium and calcium. Theconcentrations measured at this station were low. Regions found to be responsible for rainevents coincide with southern and eastern areas known for their high anthropogenic emissionsof SO2 and NOx (north Africa, northern Italy, Yugoslavia).All these results demonstrate that the Iraty and Le Casset stations were notinfluenced by the same sources as Morvan. Specifically, these sites were less influenced bythe high emissions from central or northwestern Europe than Morvan. Only the measurement ofpH seemed to be under the influence of SO2 and NOx emissions of one ofthese areas. Iraty and Le Casset were very influenced by the anthropogenic emissions and thecrustal sources around the Mediterranean Basin and North Africa. Other European areas (e.g.,Italy) can influence the concentrations recorded at Le Casset but the levels were lower thanthose measured at Morvan. A relation between sulphates, nitrates and ammonium was identifiedfor Morvan and Le Casset. This observation suggests that aerosol transport ofNH4 HSO4, (NH4)2SO4 and NH4 NO3 is occurring

revue des sciences de l'eau

Détermination à l'aide d'un modèle récepteur des zones sources à l'origine des concentrations mesurées dans les précipitations collectées en trois sites du réseau MERA (France)