Abstract. The interaction of mineral dust with N 2 O 5 was investigated using both airborne mineral aerosol (using an aerosol flow reactor with variable relative humidity) and bulk samples (using a Knudsen reactor at zero humidity). Both authentic (Saharan, SDCV) and synthetic dust samples (Arizona test dust, ATD and calcite, CaCO 3 ) were used to derive reactive uptake coefficients (γ ). The aerosol experiments (Saharan dust only) indicated efficient uptake, with e.g. a value of γ (SDCV)=(1.3±0.2)×10 −2 obtained at zero relative humidity. The values of γ obtained for bulk substrates in the Knudsen reactor studies are upper limits due to assumptions of available surface area, but were in reasonable agreement with the AFT measurements, with: γ (SDCV)=(3.7±1.2)×10 −2 , γ (ATD)=(2.2±0.8)×10 −2 and γ (CaCO 3 )=(5±2)×10 −2 . The errors quoted are statistical only. The results are compared to literature values and assessed in terms of their impact on atmospheric N 2 O 5 .

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Abstract. The interaction of mineral dust with N2O5 was investigated using both airborne mineral aerosol (using an aerosol flow reactor with variable relative humidity) and bulk samples (using a Knudsen reactor at zero humidity). Both authentic (Saharan, SDCV) and synthetic dust samples (Arizona test dust, ATD and calcite, CaCO3) were used to derive reactive uptake coefficients (γ). The aerosol experiments (Saharan dust only) indicated efficient uptake, with e.g. a value of γ (SDCV)=(1.3±0.2)×10−2 obtained at zero relative humidity. The values of γ obtained for bulk substrates in the Knudsen reactor studies are upper limits due to assumptions of available surface area, but were in reasonable agreement with the AFT measurements, with: γ(SDCV)=(3.7±1.2)×10−2, γ(ATD)=(2.2±0.8)×10−2 and γ(CaCO3)=(5±2)×10−2. The errors quoted are statistical only. The results are compared to literature values and assessed in terms of their impact on atmospheric N2O5.

Although modern gravity dam spillways include often steep chutes operating in skimming flow regime, succession of free-falling nappes (i.e. nappe flow regime) are more common on low gradient chutes and cascades, and this flow situation received little attention to date. New experiments were conducted in nappe flows without hydraulic jump in two large-size facilities with flat slopes. The flow on the stepped cascade displayed complex, three-dimensional patterns. Detailed air-water flow measurements were performed in the jet, at nappe impact and in the downstream flow region. Key results demonstrated that the flow on each step was rapidly varied, highly three-dimensional and strongly aerated. RÉSUMÉBien que les déversoirs modernes de barrage poids incluent souvent des pentes raides fonctionnant en régime écumant, les successions de nappes en chute libre (i.e. régime d'écoulement en nappe) sont plus courantes sur les déversoirs et les cascades à pentes faibles, or cette forme d'écoulement n'a reçu que peu d'attention jusqu'ici. De nouvelles expériences ont été entreprises sur des écoulements en nappe sans ressaut hydraulique dans deux équipements de grande taille avec des pentes faibles. L'écoulement, sur une cascade en gradin présentait des configurations tridimensionnelles complexes. Des mesures détaillées de l'écoulement air-eau ont été effectuées dans le jet, à l'impact de la nappe et dans la zone d'écoulement aval. Les résultats principaux ont démontré que l'écoulement sur chaque marche était rapidement variable, largement tridimensionnel et fortement aéré.