Search

International audience; Assessments of carbon monoxide emissions through inverse modeling are dependent on the modeled abundance of the hydroxyl radical (OH) which controls both the primary sink of CO and its photochemical source through hydrocarbon oxidation. However, most chemistry-transport models (CTMs) fall short of reproducing constraints on hemispherically-averaged OH levels derived from methylchloroform (MCF) observations. Here we construct five different OH fields compatible with MCF-based analyses, and we prescribe those fields in a global CTM to infer CO fluxes based on IASI CO columns. Each OH field leads to a different set of optimized emissions. Comparisons with independent data (surface, ground-based remotely-sensed, aircraft) indicate that the inversion adopting the lowest average OH level in the Northern Hemisphere (7.8 × 105molec. cm−3, ca. 18% lower than the best estimate based on MCF measurements) provides the best overall agreement with all tested observation datasets.

Dust has been implicated as a pathway for endosulfan transport to the riverine environment in New South Wales. Anthropogenic dust, expressed as total suspended sediment (TSP), and particle associated endosulfan emissions were estimated on a cotton (Gossypium hirsutum L.) farm in northern New South Wales, Australia, using a combination of direct measurements and modelling. A vehicle travelling at 80 km h−1 on an unsealed road was a greater source of TSP emission (3.7 g m−1) than an 8-m wide inter-row cultivator travelling at 8 km h−1 (1.7 g m−1). However, the particle size distribution of the TSP from inter-row cultivation was finer (mode of 19–22 μm) than that from vehicular traffic on unsealed roads (mode of 32 μm) and hence may be transported further. Endosulfan source strength from inter-row cultivation was 3.6 μg m−1 of travel (or 0.45 μg m−2) which was only 6.0E-4% of that applied, 4 days after endosulfan application. This was slightly higher than the endosulfan source strength from vehicular traffic on an unsealed road (3.1 μg m−1 of travel), only 2 days after spraying. On unsealed roads, particle-associated endosulfan mass fractions declined rapidly with time due to volatilisation and photodegradation and a decrease in endosulfan-enriched source sediment due to the removal by repeated vehicle passes. For unsealed roads, TSP at 0.63 m height showed endosulfan-enrichment ratios of approximately four, compared to the surface soil 0–1 cm depth, for inter-row cultivation and vehicular traffic.

In a field experiment the transfer of PCDD/PCDF from contaminated soils to apples and pears was investigated. The soil PCDD/PCDF contamination differed in total PCDD/PCDF content and in the homologue pattern. PCDD/PCDF content in whole fruits ranged from 1 - 4 ng/kg FW. Soil PCDD/PCDF contamination was not correlated with the PCDD/PCDF concentrations of the fruits. The higher PCDD/PCDF concentrations of the peel in comparison to the pulp indicate that air-borne PCDD/PCDF is the major pathway for fruit contamination. Therefore peeling of fruits can possibly reduce PCDD/PCDF intake.

The optical properties of a one-dimensional (1D) electron gas with only one or two occupied subbands have been studied in semiconductor quantum wires, obtained by electron beam lithography and subsequent low-energy ion bombardment of modulation-doped GaAs quantum wells. Large optical singularities have been observed at the Fermi level, both in optical absorption and emission. They disappear at temperatures comparable to the Fermi energy. The 1D singularities are much larger and sharper than in 2D systems due to the lack of certain hole recoil effects in 1D. The experimental data are in qualitative agreement with theoretical results based on the exact diagonalization of finite chains.

Modulation-doped quantum wire structures have been fabricated in the extreme quantum limit in which only the lowest one-dimensional (1D) subband is occupied by electrons. A pronounced Fermi edge singularity is observed for the first time in the absorption and emission spectra of the 1D electron gas. It has a strong temperature dependence determined by the Fermi energy and is much sharper than in two dimensions. The experimental results agree qualitatively with exact diagonalization studies of finite Hubbard chains.

In a field experiment, the PCDD/PCDF transfer pathways from soil into carrots, lettuce and peas has been investigated. PCDD/PCDF contamination levels in soil varied between 5 ng I-TEq/kg on the control plot and 56 ng I-TEq/kg on the contaminated plot. PCDD/PCDF levels in carrots were threefold higher in the contaminated plot than in the control plot, which was a result of a tenfold increase in the PCDD/PCDF levels of the peel. PCDD/PCDF levels in lettuce and peas were not higher when grown on the contaminated plot and were much lower than in carrots, which indicates that the PCDD/PCDF in lettuce and peas from both plots are of atmospheric origin.

The Keldysh-Schwinger nonequilibrium Green's function theory is used to derive on the level of a screened Hartree-Fock approximation the relevant equations which determine the optical properties of laser-excited semiconductors. The equation for the optical interband polarization is derived for four different cases: (1) For situations in which the resonantly excited carriers are in quasi-equilibrium and in an (a) spatially homogeneous, or (b) spatially inhomogeneous state. (2) For ultrashort pulse excitation in which the excitations are in a coherent (unrelaxed) state; (a) for nonresonant excitation of the exciton, and (b) for resonant band-to-band excitation. In case (1a) a Bethe-Salpeter equation in the screened ladder approximation results, in case (2b) we obtain a generalization of Stahl's coherent band edge equation. In case (2a) our result agrees with the polarization equation of Schmitt-Rink and Chemla and explains the optical Stark shift of the exciton, while in case (2b) our result describes the spectral hole-burning (light induced gap) with an unrelaxed electron-hole population.