Stratospheric ozone chemistry in the Antarctic: what determines the lowest ozone values reached and their recovery?

Balloon-borne observations of ozone from the South Pole Station have been reported to reach ozone mixing ratios below the detection limit of about 10 ppbv at the 70 hPa level by late September. After reaching a minimum, ozone mixing ratios increase to above 1 ppmv on the 70 hPa level by late December. While the basic mechanisms causing the ozone hole have been known for more than 20 yr, the detailed chemical processes determining how low the local concentration can fall, and how it recovers from the minimum have not been explored so far. Both of these aspects are investigated here by analysing results from the Chemical Lagrangian Model of the Stratosphere (CLaMS). As ozone falls below about 0.5 ppmv, a balance is maintained by gas phase production of both HCl and HOCl followed by heterogeneous reaction between these two compounds in these simulations. Thereafter, a very rapid, irreversible chlorine deactivation into HCl can occur, either when ozone drops to values low enough for gas phase HCl production to exceed chlorine activation processes or when temperatures increase above the polar stratospheric cloud (PSC) threshold. As a consequence, the timing and mixing ratio of the minimum ozone depends sensitively on model parameters, including the ozone initialisation. The subsequent ozone increase between October and December is linked mainly to photochemical ozone production, caused by oxygen photolysis and by the oxidation of carbon monoxide and methane

What causes the irregular cycle of the atmospheric tape recorder signal in HCN?

Variations in the mixing ratio of long-lived trace gases entering the stratosphere in the tropics are carried upward with the rising air with the signal being observable throughout the tropical lower stratosphere. This phenomenon, referred to as "atmospheric tape recorder" has previously been observed for water vapor, CO2, and CO which exhibit an annual cycle. Recently, based on Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) satellite measurements, the tape recorder signal has been observed for hydrogen cyanide (HCN) but with an approximately two-year period. Here we report on a model simulation of the HCN tape recorder for the time period 2002-2008 using the Chemical Lagrangian Model of the Stratosphere (CLaMS). The model can reproduce the observed pattern of the HCN tape recorder signal if time-resolved emissions from fires in Indonesia are used as lower boundary condition. This finding indicates that inter-annual variations in biomass burning in Indonesia, which are strongly influenced by El Nino events, control the HCN tape recorder signal. A longer time series of tropical HCN data will probably exhibit an irregular cycle rather than a regular biannual cycle. Citation: Pommrich, R., R. Muller, J.-U. Grooss, G. Gunther, P. Konopka, M. Riese, A. Heil, M. Schultz, H.-C. Pumphrey, and K. A. Walker (2010), What causes the irregular cycle of the atmospheric tape recorder signal in HCN?, Geophys. Res. Lett., 37, L16805, doi:10.1029/2010GL044056

Horizontal transport affecting trace gas seasonality in the Tropical Tropopause Layer (TTL)

We analyze horizontal transport from midlatitudes into the tropics (in-mixing) and its impact on seasonal variations of ozone, carbon monoxide and water vapor in the Tropical Tropopause Layer (TTL). For this purpose, we use three-dimensional backward trajectories, driven by ECMWF ERA-Interim winds, and a conceptual one-dimensional model of the chemical composition of the TTL. We find that the fraction of in-mixed midlatitude air shows an annual cycle with maximum during NH summer, resulting from the superposition of two inversely phased annual cycles for in-mixing from the NH and SH, respectively. In-mixing is driven by the monsoonal upper-level anticyclonic circulations. This circulation pattern is dominated by the Southeast Asian summer monsoon and, correspondingly, in-mixing shows an annual cycle. The impact of in-mixing on TTL mixing ratios depends on the in-mixed fraction of midlatitude air and on the meridional gradient of the particular species. For CO the meridional gradient and consequently the effect of in-mixing is weak. For water vapor, in-mixing effects are negligible. For ozone, the meridional gradient is large and the contribution of in-mixing to the ozone maximum during NH summer is about 50%. This in-mixing contribution is not sensitive to the tropical ascent velocity, which is about 40% too fast in ERA-Interim. As photochemically produced ozone in the TTL shows no distinct summer maximum, the ozone annual anomaly in the upper TTL turns out to be mainly forced by in-mixing of ozone-rich extratropical air during NH summer

Königin Elisabeth und die zeitgenössische englische Literatur

von Oberlehrer Dr. PommrichProgr.-Nr. 78

Selbstdiffusion in siliziumreichen Legierungsschmelzen

In der vorliegenden Arbeit wurde die Selbstdiffusion von Nickel, Titan und Cobalt in flüssigem Silizium (Si) experimentell ermittelt. Die chemisch hochreaktiven Schmelzen wurden tiegelfrei mit der elektromagnetischen Levitation prozessiert. Die Selbstdiffusion wurde mit quasi-elastischer Neutronenstreuung am TOFTOF (FRM-II, Garching) bestimmt. Für einen weiten Temperaturbereich ober- und unterhalb des Liquiduspunktes wurden Selbstdiffusionskoeffizienten im Bereich von $10^{-8}m^{2/s}$ gemessen. Es wird vermutet, dass die Diffusion in flüssigem Si von der Majoritätskomponente also Si dominiert wird. Die erhaltenen Ergebnisse können als Eingangsdaten für Erstarrungssimulationen dienen. Weiterhin wurde die Viskosität von flüssigem Zr64Ni36 mit der Methode des oszillierenden Tropfens in einer elektrostatischen Levitationsanlage bestimmt. Ein Vergleich der gewonnenen Viskositätsdaten mit Diffusionsdaten dieser Legierung aus der Literatur zeigte eine ähnliche Temperaturabhängigkeit

Self-diffusion of 3d transition metals in liquid silicon alloys

Self-diffusion coefficients of titanium and cobalt in a silicon melt with 10 at.% metal content were measured in a temperature range between 180 K above and below the liquidus temperature, respectively, with quasielastic neutron scattering. The chemical reactive melts were processed container-less by electromagnetic levitation. Whereas, in crystalline silicon self diffusion coefficients of Ti and Co impurities differ by more than 3 orders of magnitude close to the melting point, in the liquid state values of self-diffusion coefficients of Ti and Co, are equal within error bars and are of the order of 10-8 m2/s. This is in line with a liquid-like, collective transport mechanism that is dominated by the mobility of the surrounding Si atoms

Relation between self-diffusion and viscosity in dense liquids: New experimental results from electrostatic levitation

By using the technique of electrostatic levitation, the Ni self-diffusion, density, and viscosity of liquid Zr64Ni36 have been measured in situ with high precision and accuracy. The inverse of the viscosity, η, measured via the oscillating drop technique, and the self-diffusion coefficient D, obtained from quasielastic neutron scattering experiments, exhibit the same temperature dependence over 1.5 orders of magnitude and in a broad temperature range spanning more than 800 K. It was found that Dη=const for the entire temperature range, contradicting the Stokes-Einstein relation