Rain water-aerosol trace metal relationships at cap ferrat: A coastal site in the Western Mediterranean

The factors controlling the distributions of the trace metals Al, Co, Ni, Cu and Pb in a series of 25 individual rain water samples collected at Cap Ferrat, a site on the Western Mediterranean coast, are interpreted in relation to aerosols taken simultaneously at the same site. The trace metal chemistry and pH of the rain waters are constrained by the scavenged aerosols, which are composed of a mixture of urban-dominated (European) and crust-dominated (Saharan) components. Thus, the pH values of the rain waters, which range between 3.95 to 6.77, reflect the type of aerosol scavenged from the air; urban-dominated aerosol components giving rise to acidic rains, and crust-dominated aerosol resulting in neutral to basic rains. The average solubilities of the trace metals in the rain waters increase in the order Al (17%), Co (36%), Ni (53%), Pb (65%) and Cu (76%). The paniculate ↔ dissolved speciation of the non-crust-dominated metals Cu and Pb varies with pH, being more soluble at lower pH values, and exhibits the classical pH ‘adsorption edge’. However, the pH of rain can vary during an individual rain event in response to the sequential scavenging of crust-dominated and urban-dominated aerosol components. As a result, the solubility of non-crust-dominated trace metals, such as Pb, can also vary sequentially during an individual rain event; the maximum solubility being related to a ‘dip’ in pH associated with the scavenging of urban-rich aerosol components, followed by a return to the initial pH as the pH-influencing components are exhausted. Data from the present study therefore indicate that the pH-controlled trace metal solubility relationship reported for individual rain events can also occur sequentially in the same event. The particulate material in the rain waters does not contain the relatively high concentrations of Ni, Cu and Pb found in the parent aerosols, and its composition approaches that of crust-dominated aerosols transported to the Mediterranean. Data from the present study, together with those for other Western European coastal locations, indicate that there is a Pb-Cu fractionation between aerosols and rainwaters which results in a significantly greater fraction of the aerosol Pb, relative to Cu, escaping precipitation scavenging in the coastal zone and so becoming available for long-range atmospheric transport.     

Mechanisms of compaction of quartz sand at diagenetic conditions

The relative contribution of cracking, grain rearrangement, and pressure solution during experimental compaction of quartz sand at diagenetic conditions was determined through electron and optical microscopy and image analysis. Aggregates of St. Peter sand (255±60 μm diameter grain size and porosity of approximately 34%) were subjected to creep compaction at effective pressures of 15, 34.5, 70, and 105 MPa, temperatures of 22 and 150°C, nominally dry or water-saturated (pore fluid pressure of 12.5 MPa) conditions, and for times up to one year. All aggregates displayed transient, decelerating creep, and volume strain rates as low as 2×10⁻¹⁰ s⁻¹ were achieved. The intensity of fracturing and degree of fragmentation increase with volume strain and have the same dependence on volume strain at all conditions tested, indicating that impingement fracturing and grain rearrangement were the main mechanisms of compaction throughout the creep phase. The increase in fracture density and decrease in acoustic emission rate at long times under wet conditions reflect an increase in the contribution of subcritical cracking. No quantitative evidence of significant pressure solution was found, even for long-term creep at 150°C and water-saturated conditions. Comparison of our findings to previous work suggests that pressure solution could become significant at temperatures or times somewhat greater than investigated here.     

Movement and propagation of multicellular convective storms

Migration velocities of convective storms are summarized for six situations, with different environmental wind fields. Small-and medium-sized storms generally moved to left of the direction of, and at speeds somewhat less than, the vector mean wind in the troposphere. Large-diameter (ca. 20–30 km) storms generally deviated to the right, in proportion to their sizes and to the veering of wind with height. This behavior, and the tendency for large storms to move appreciably slower than the mean wind, are even more pronounced when giant clusters of thunderstorms are considered. An example is analyzed in which a multicellular storm, 80 km wide, moved 55° to right of the mean wind and with half its speed. This behavior results from a characteristic pattern of propagation, in which new cells tend to form on the general upwind side of the cluster, with the larger and more intense cells developing on its right flank. The individual cells move through the cluster, dissipating on approach to its advancing and left flanks. Preferential formation of cells toward the rear side of the cluster is shown to be compatible with the probable origin and trajectories (relative to the moving storm) of air ascending from the lower part of the subcloud layer. The sometimes-observed rapid movement of large multicellular storms to left of the mean wind is partly accounted for by an opposite (left forward flank) pattern of propagation.