Observations Of The Morning Transition Of The Convective Boundary Layer

The morning transition between the stable nocturnal situation and the daytime convective boundary layer (CBL) is of interest both for basic understanding and for initializing prognostic models. While the morning growth phase of the CBL has been studied in detail, relatively little has been published on the transition itself. In this paper, conventional observations of surface temperature, humidity, and turbulent fluxes,and data from a meteorological tower, are combined with measurements of the onset of convection by boundary-layer wind profilers to explore the timing and behaviour of the transition period. The transition is defined here as the period between sunrise and the time at which the depth ofconvection reaches about 200 m AGL. Diagnostic relationships based on surface heat flux, the temperature difference between 2 m and 200 m, and bulk Richardson number are explored. The transition is foundto be enabled by surface heating relaxing the surface stability, while the warming of the layerbetween 2 m and 200 m is in large part due to shear-driven entrainment.     

Carbon isotope analyses of n-alkanes in dust from the lower atmosphere over the central eastern Atlantic

Atmospheric dust samples collected along a transect off the West African coast have been investigated for their lipid content and compound-specific stable carbon isotope compositions. The saturated hydrocarbon fractions of the organic solvent extracts consist mainly of long-chain n-alkanes derived from epicuticular wax coatings of terrestrial plants. Backward trajectories for each sampling day and location were calculated using a global atmospheric circulation model. The main atmospheric transport took place in the low-level trade-wind layer, except in the southern region, where long-range transport in the mid-troposphere occurred. Changes in the chain length distributions of the n-alkane homologous series are probably related to aridity, rather than temperature or vegetation type. The carbon preference of the leaf-wax n-alkanes shows significant variation, attributed to a variable contribution of fossil fuel- or marine-derived lipids. The effect of this nonwax contribution on the δ¹³C values of the two dominant n-alkanes in the aerosols, n-C₂₉ and n-C₃₁ alkane, is, however, insignificant. Their δ¹³C values were translated into a percentage of C₄ vs. C₃ plant type contribution, using a two-component mixing equation with isotopic end-member values from the literature. The data indicate that only regions with a predominant C₄ type vegetation, i.e. the Sahara, the Sahel, and Gabon, supply C₄ plant-derived lipids to dust organic matter. The stable carbon isotopic compositions of leaf-wax lipids in aerosols mainly reflect the modern vegetation type along their transport pathway. Wind abrasion of wax particles from leaf surfaces, enhanced by a sandblasting effect, is most probably the dominant process of terrigenous lipid contribution to aerosols.     

The nitrogen distribution in soils formed under widely differing climatic conditions

Amino acids, amino sugars and ammonia were determined in 6N HCl hydrolyzates of soils formed under arctic, cool temperate, subtropical and tropical climates. Soils from warmer climates yielded relatively more amino acids and amino sugars but less NH₃ than did those from colder regions. The amino acid composition of all soils, however, was remarkably similar, resembled that of lake sediments and was somewhat similar to that of bacteria, suggesting the importance of microorganisms in their formation. Climate appears to have little effect on the amino acid composition of soils.     

Potassium-argon ages from the Ceneri Zone,Southern Swiss Alps

The Ceneri Zone is a unit of the crystalline basement of the Southern Alps. Its northern boundary is the Tonale Line segment of the Periadriatic Line, an important tectonic lineament separating the Oligocene and younger features of the Central Alps from the older metamorphic and structural trends of the Southern Alps. Unmetamorphosed Permian and younger sedimentary units lap onto the Southern Alpine basement from the south.Potassium-argon results from the Ceneri Zone define a Hercynian age pattern typical for the basement of continental Europe. This pattern extends to within at least 100 meters of the Tonale Line. Thus, amphibolite facies metamorphism in this region occurred around 325 m.y. ago. The geochronologic similarity of the Southern Alps to many other European regions must be taken into account in megatectonic theories.In detail, the Hercynian age pattern of the Ceneri Zone is complicated. Some hornblendes have apparent ages between the Hercynian and a Caledonian value (430 m.y.). They probably retained some radiogenic argon during the Hercynian upper amphibolite facies metamorphism. In addition, mica results between 200 and 300 m.y. have a strong geographic correlation. Apparently, the northwestern portion of the Ceneri Zone was reheated or mildly metamorphosed during the Upper Triassic to Lower Jurassic. A relationship between these ages and 170–180 m.y. ages from the neighboring Ivrea-Verbano Zone seems likely. No geologic evidence for any post-Hercynian event has been noted as yet in the Ceneri Zone.     

Fractionation of sulfur isotopes during laboratory synthesis of pyrite at low temperatures

The reaction products and the accompanying sulfur isotope fractionations during the reaction of H₂S with goethite in aqueous media at 22–24°C for periods from 0.5 hr to 65 days were studied. Fine-grained pyrite formed within two days and was isotopically 0.8‰ lighter than the H₂S source. After 65 days reaction time the pyrite had nearly the same isotopic value as the H₂S. Aqueous precipitation of pyrite from H₂S and goethite at room temperature involved three major steps, namely: (1) the rapid oxidation of H₂S and reduction of Fe³⁺ during which elemental S is formed; (2) the formation of acid-volatile sulfides and the disappearance of elemental S; and (3) the formation of pyrite at the expense of acid-volatile sulfides.     

Atmospheric trace metals: global cycles and assessment of man's impact

Global data are presented for sources of atmospheric input for 20 trace metals, and the relative importance of natural and anthropogenic sources is assessed. Interference factors are calculated as (total anthropogenic emissions/total natural emissions) × 100. For lithophile metals such as Fe and Mn, interference factors are small. In contrast, the atmophile metals, such as As, Se and Hg, exhibit large interference factors. A significant degree of correlation exists between interference factors and enrichment factors, where enrichment factor is defined as the metal/Al ratio in atmospheric particulates divided by the metal/Al ratio in soils. For many of the trace metals, enrichment factors are of the same order of magnitude at high latitudes in both the Northern and Southern Hemispheres, and are larger at high latitude than at mid latitude.A simple mathematical model is used to calculate present-day enrichment factors in both hemispheres based on natural and anthropogenic influxes, effluxes, and transfer between hemispheres. The calculated enrichment factors are in good agreement with the observed enrichment factors for lithophile metals at both mid and high latitude, and for atmophile metals at mid latitude. However, calculated enrichment factors for atmophile metals are lower than observed enrichment factors at high latitude. To explain these results, we propose that for Hg, As and Se, and perhaps for other atmophile metals, there are significant fluxes from the sea surface to the atmosphere. If the estimated low-temperature fluxes of As, Se and Hg from the land and sea surfaces are included in the interference factor calculations for these metals, the factors are reduced to less than 100%.