Uncertainty and sensitivity analyses of OH-initiated dimethyl sulphide (DMS) oxidation kinetics

A numerical experiment has been conducted on the OH-initiated tropospheric oxidation of DMS. This involved the selection of a set of reactions describing the OH-initiated oxidation kinetics and the conversion of the present level of uncertainty of the system into uncertainty ranges and distributions for the relevant system parameters (kinetic constants and initial concentrations). Uncertainties have been propagated through the model onto the output variables of interest. This has allowed (a) the uncertainty in model prediction to be quantified and compared with observations (uncertainty analysis) and (b) the relative importance of each input parameter in determining the output uncertainty to be quantified (sensitivity analysis). Output considered were the ratio of MSA/(SO₂ + H₂SO₄) concentration at a given time, the ratio SO₂/H₂SO₄, the total peroxynitrate species concentrations and the relative fraction of SO₂ and H₂SO₄ formed through the various pathways. Conditional upon the model and data assumptions underlying the experiment, the following main conclusions were drawn:

1. The possibility of direct formation of SO₃ without SO₂ as intermediate as suggested by Bandyet al. (1992) and Yinet al. (1990), involving direct thermal decomposition of CH₃SO₃ · does not seem to play a major role in the overall generation of sulphate. This is relevant to the issue of gas to particle conversion over remote areas.
2. Reaction of CH₃SOO · intermediate may be the most important pathway to the formation of SO₂.
3. The dominating peroxynitrate is CH₃S(O)₂O₂NO₂.

Through sensitivity analysis the kinetic constants have been identified which — because of their uncertainty and of their impact on the output — mostly contribute to the output uncertainty.     

Natural evidence for rapid abiogenic hydrothermal generation of CH4

Although recent hydrothermal experiments imply that abiogenic methane (CH₄) generation from hydrothermal reduction of CO₂ can occur, evidence from natural systems was still lacking. Based on the chemical and isotopic equilibrium signatures of low-temperature fumarolic gas discharges, we are able to provide hard evidence for its natural occurrence, namely in three subduction-related bi-phase hydrothermal systems of the Mediterranean, whose temperatures range from 260 to 470 °C. The attainment of equilibrium and the time spans of recent volcanic dormancy allowed us to calculate minimum rates for chemical and isotopic equilibration. These are significantly higher than those previously reported and might be due to the presence of a saturated water vapor phase in the investigated systems. The fact that nature provides conditions enabling relatively fast production of hydrocarbons from CO₂ strongly supports the concerns that were recently raised from laboratory experiments. These address the use of the carbon isotope composition of reduced carbon in Archean sediments as a tracer of early life and the occurrence of CH₄ on extraterrestrial planets as a bioindicator. In view of the potential role of abiogenic CH₄ as a precursor of life, we also present an estimate of abiogenic hydrothermal CH₄ fluxes throughout the Archean. It is not expected that these fluxes exceeded 80 Mt/yr during the past 4.0 Ga. This, however, would have been enough to facilitate HCN production on the prebiotic Earth.     

Long cores from small lakes—recovering up to 100 m-long lake sediment sequences with a high-precision rod-operated piston corer (Usinger-corer)

We present a unique, versatile piston corer for recovering continuous lake and bog sediment sequences with superior accuracy and quality. The main components of the system and their function are described, with special focus on measures for obtaining long, continuous lake sediment sections up to a current maximum length of 95 m. Examples of lake sediment profiles obtained with this system from different climatic zones are presented.     

Geophysical and hydrogeologic investigation of groundwater in the Karoo stratigraphic sequence at Sawmills in northern Matabeleland,Zimbabwe: a case history

Geophysical and hydrogeological investigations have been carried out around Sawmills in Zimbabwe, Africa. The investigations are components of a larger investigation to assess the groundwater potential of the Karoo sedimentary basin with regards to supplying water to Bulawayo City. The Sawmills area was selected due to the availability of borehole logs indicating favourable stratigraphy for groundwater availability and due to the high yields from the aquifers measured from these boreholes. Data collected using two geophysical methods are presented here: transient electromagnetic (TEM) and continuous vertical electrical sounding (CVES) data. The data have also been processed using laterally constrained inversion (LCI). Because the CVES provides greater detail in the shallow subsurface, whereas TEM is more effective at depth, a more accurate image of the entire subsurface profile is provided based on using both methods. The results suggest that LCI of CVES and TEM data, in the subsurface at the required depths at Sawmills, is able to provide a substantially more accurate image of the subsurface than either method alone. The hydrogeological interpretation of the geophysical data is valuable for determining the depth to and thickness of the potential aquifer horizon(s) and for identifying the position of potential recharge zones.     

Effective Roughness Calculated from Satellite-Derived Land Cover Maps and Hedge-Information used in a Weather Forecasting Model

In numerical weather prediction, climate and hydrologicalmodelling, the grid cell size is typically larger than the horizontal length scales of variations in aerodynamicroughness, surface temperature and surface humidity. These local land cover variations give rise to sub-gridscale surface flux differences. Especially the roughness variations can give a significantly differentvalue between the equilibrium roughness in each of the patches as compared to the aggregated roughness value,the so-called effective roughness, for the grid cell. The effective roughness is a quantity that secures thephysics to be well-described in any large-scale model. A method of aggregating the roughness step changesin arbitrary real terrain has been applied in flat terrain (Denmark) where sub-grid scale vegetation-drivenroughness variations are a dominant characteristic of the landscape. The aggregation model is a physicaltwo-dimensional atmospheric flow model in the horizontal domain based on a linearized version of theNavier Stoke equation. The equations are solved by the Fast Fourier Transformation technique, hence the codeis very fast. The new effective roughness maps have been used in the HIgh Resolution Limited Area Model(HIRLAM) weather forecasting model and the weather prediction results are compared for a number of casesto synoptic and other observations with improved agreement above the predictions based on currentstandard input. Typical seasonal springtime bias on forecasted winds over land of +0.5 m s^-1 and-0.2 m s^-1 in coastal areas is reduced by use of the effective roughness maps.     

Critical shear stress for mechanical twinning of jadeite—an experimental study

Coarse-grained natural jadeitite samples from Myanmar were experimentally deformed in a Griggs-type solid-medium apparatus at strain rates of 2·10⁻⁵ and 5·10⁻⁶ s⁻¹ and temperatures of 900 and 1000 °C. The microfabrics of the deformed samples are investigated by scanning electron microscopy (SEM) using the electron backscatter diffraction (EBSD) technique. The critical shear stress for twinning in the (100) [001] system is derived from the orientation distribution of jadeite crystals with and without mechanical twins. The results indicate a homogeneous stress field within the sample and a critical shear stress of 150±25 MPa, which compares well to that determined by Kollé and Blacic [J. Geophys. Res. 87 (1982) 4019] for mechanical twinning of other clinopyroxenes. With the critical shear stress known, mechanical twinning of jadeite can be used as a paleopiezometer for high stress tectonic environments.