Crystalline silicate dust around evolved stars I. The sample stars

This is the first paper in a series of three where we present the first comprehensive inventory of solid state emission bands observed in a sample of 17 oxygen-rich circumstellar dust shells surrounding evolved stars. The data were taken with the Short and Long Wavelength Spectrographs on board of the Infrared Space Observatory (ISO) and cover the 2.4 to 195 micron wavelength range. The spectra show the presence of broad 10 and 18 micron bands that can be attributed to amorphous silicates. In addition, at least 49 narrow bands are found whose position and width indicate they can be attributed to crystalline silicates. Almost all of these bands were not known before ISO. We have measured the peak positions, widths and strengths of the individual, continuum subtracted bands. Based on these measurements, we were able to order the spectra in sequence of decreasing crystalline silicate band strength. We found that the strength of the emission bands correlates with the geometry of the circumstellar shell, as derived from direct imaging or inferred from the shape of the spectral energy distribution. This naturally divides the sample into objects that show a disk-like geometry (strong crystalline silicate bands), and objects whose dust shell is characteristic of an outflow (weak crystalline silicate bands). All stars with the 33.6 micron forsterite band stronger than 20 percent over continuum are disk sources. We define spectral regions (called complexes) where a concentration of emission bands is evident, at 10, 18, 23, 28, 33, 40 and 60 micron. We derive average shapes for these complexes and compare these to the individual band shapes of the programme stars.Comment: 41 pages, 20 figures, accepted by A&A. Tables 4 to 20 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A

Global modeling of tropospheric iodine aerosol

Natural aerosols play a central role in the Earth system. The conversion of dimethyl sulfide to sulfuric acid is the dominant source of oceanic secondary aerosol. Ocean emitted iodine can also produce aerosol. Using a GEOS-Chem model, we present a simulation of iodine aerosol. The simulation compares well with the limited observational data set. Iodine aerosol concentrations are highest in the tropical marine boundary layer (MBL) averaging 5.2 ng (I) m −3 with monthly maximum concentrations of 90 ng (I) m −3. These masses are small compared to sulfate (0.75% of MBL burden, up to 11% regionally) but are more significant compared to dimethyl sulfide sourced sulfate (3% of the MBL burden, up to 101% regionally). In the preindustrial, iodine aerosol makes up 0.88% of the MBL burden sulfate mass and regionally up to 21%. Iodine aerosol may be an important regional mechanism for ocean-atmosphere interaction

The geochemical cycling of reactive chlorine through the marine troposphere

Heterogeneous reactions involving sea‐salt aerosol in the marine troposphere are the major global source for volatile inorganic chlorine. We measured reactant and product species hypothesized to be associated with these chemical transformations as a function of phase, particle size, and altitude over the North Atlantic Ocean during the summer of 1988. Concentrations of HCl were typically less than 1.0 ppbv near the sea surface and decreased with altitude and with distance from the U.S. east coast. Concentrations of Cl volatilized from aerosols were generally equivalent to the corresponding concentrations of HCl and ranged from less than detection limits to 125 nmol m−3 STP. Highest absolute and percentage losses of particulate Cl were typically associated with elevated concentrations of anthropogenic combustion products. Concentrations of product nss SO42− and N03− in coarse aerosol fractions indicate that on average only 38% of measured Cl− deficits could be accounted for by the combined effects of acid‐base desorption and reactions involving nonacidic N gases. We hypothesize a mechanism for the Cl loss initiated by reaction of O3 at sea‐salt aerosol surfaces, generating Cl2 followed by rapid photochemical conversion of Cl2 to HCl via Cl atoms (Cl˙) and eventual recapture of HCl by the aerosol. Simulations with a zero‐dimension (0‐D) photochemical model suggest that oxidation by Cl˙ may be an important tropospheric sink for dimethyl sulfide and hydrocarbons. Under low‐NOx conditions, the rapid cycling of reactive Cl would provide a catalytic loss mechanism for O3, which would possibly explain the low O3 concentrations often observed above the world\u27s oceans

A simple model of the tracer flux from the Mururoa lagoon to the Pacific

It is seen that a simple ordinary differential equation may be sufficient to predict the mass of tracers that may be present in the Mururoa lagoon, as well as the flux from the lagoon to the Pacific. This model depends on a key parameter, namely the lagoon turnover time, which is determined from the results of a complex, three-dimensional, hydrodynamic model

L'environnement océanique de l'archipel des Tuamotu (Polynésie Française)

Les atolls de l'archipel des Tuamotu (Pacifique tropical central sud) sont situés dans la partie centrale du grand tourbillon (gyre) anticyclonique du Pacifique sud où la subsidence de l'air entretient un climat aride et une évaporation prépondérante. le régime bi-modal d'alizés créé par le double système de hautes pressions subtropicales (Iles de Pâques - Iles Kermadec) entraîne en zone tropical une dérive générale des eaux vers l'ouest, le Courant Equatorial Sud (CES), dont la vitesse ne dépasse pas 10 cm.s-1. La Zone de Convergence des alizés du Pacifique Sud (ZCPS) à forte variabilité saisonnière en position et intensité, favorise une contre-circulation vers l'est, le Contre-Courant Equatorial Sud (CCES). (D'après résumé d'auteur

Wave-induced flow over Mururoa atoll reef

A unique long-term timeseries of current over Mururoa atoll reef is presented here. Comparison with TOPEX/POSEIDON satellite altimeter data reveals that the daily-averaged cross-reef current is wave-driven and varies at the synoptic time scale. This current also exhibits variations at or near the tidal frequency. In order to understand this evolution, a previously published analytical model of the wave-induced flow over a reef flat is used (SYMONDS et al., 1995). Forced by the altimeter data and the tide, it gives results which agree well with field data. It appears that the model is able to reproduce the observed long-term variations of the inward current and also its variations at tidal frequency. Furthermore, the model gives an explanation of the observed six-hour period signal. It reveals that the response of the cross-reef current to the tidal forcing highly depends on the reef geometry and particularly on the water depth over the reef flat. This confirms that the response of the current over a reef flat at or near the tidal frequency is site-specific