A study of the formation of N2O in the reaction of NO3(A2E′) with N2

The rate of formation of N₂O via the thermochemically favourable reaction of NO₃(A²E) with N₂, which would represent an additional source of stratospheric N₂O and therefore NO_x, has been investigated. Mixtures of NO₂+O₃ in synthetic air were photolysed at 662 nm. No evidence was found for the production of N₂O via this pathway, the upper limit for the quantum yield of nitrous oxide formation being % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiiYdd9qrFfea0dXdf9vqai-hEir8Ve% ea0de9qq-hbrpepeea0db9q8as0-LqLs-Jirpepeea0-as0Fb9pgea% 0lrP0xe9Fve9Fve9qapdbaqaaeGacaGaaiaabeqaamaabaabcaGcba% GaeqOXdy2aaSbaaSqaamaaBaaameaadaWgaaqaamaaBaaabaGaamOt% amaaBaaabaGaaGOmaiaad+eaaeqaaaqabaaabeaaaeqaaaWcbeaatu% uDJXwAK1uy0HMmaeHbfv3ySLgzG0uy0HgiuD3BaGqbaOGae8hzIqOa% aGimaiaac6cacaaI2aGaaiyjaaaa!4E60!\[\phi _{_{_{_{N_{2O} } } } } \le 0.6\% \]. However, a dark conversion of NO_x to N₂O was observed and is attributed tentatively to a heterogeneous reaction on the wall of the reaction vessel. This process, although most likely to be insignificant in the atmosphere, needs to be taken into consideration in laboratory investigations or field studies of N₂O emission or deposition.     

Dynamical diagnosis of the breakup of the stratospheric polar vortex in the Northern Hemisphere

The research on climate change in polar regions, especially on the role of polar in the global climate system, has gain unprecedented level of interest. It has been the key scientific issue of the International Polar Year program (IPY, 2007―2008). In this paper, we dealt with the debate upon the breakup time of the stratospheric polar vortex in boreal spring. An observational study of the relation between strato- spheric polar vortex breakup and the extra-tropical circulation was performed. The mean breakup date―when the winter westerly at the core of polar jet turns to summer easterly―is about April 10. The breakup time has large interannual variation with a time span of about 2 months. It also has a long-term trend with the 1990s and 2000s witnessing more and more late breakups of polar vortex. Composite of wind speed at the core of polar jet for the extremely early and late breakup years shows that late years have two periods of westerly weakening while early breakup years have only one. The first weakening in the late years happens in middle January with wind speed dropping sharply from more than 40 m s?1 to about 15 m s?1. This is accompanied with anomalous activities of planetary waves in both strato- sphere and troposphere; while the second weakening in the late breaking years is mainly the results of diabatic heating with very weak wave activities. In early breakup years, the transition from westerly to easterly is rapid with wind speed dropping from more than 30 m s?1 to less than ?10 m s?1 within a month. This evolution is associated with a strong bidirectional dynamical coupling of the stratosphere and troposphere. The circulation anomalies at low troposphere are also analyzed in the extremely early and late breakup years. It shows that there are significant differences between the two kinds of extreme years in the geopotential height and temperature composite analysis, indicating the dynamical cou- pling of stratosphere and troposphere with the evolution of stratospheric polar vortex.     

Impact of Accurate Photolysis Calculations on the Simulation of Stratospheric Chemistry

The interpretation of atmospheric measurements and the forecasting of the atmospheric composition require a hierarchy of accurate chemical transport and global circulation models. Here, the results of studies using Bremens Atmospheric Photochemical Model (BRAPHO) are presented. The focus of this study is given to the calculation of the atmospheric photolysis frequencies It is shown that the spectral high resolved simulation of the O₂ Schumann–Runge bands leads to differences in the order of 10% in the calculated O₂ photolysis frequency when compared with parameterizations used in other atmospheric models. Detailed treatment of the NO absorption leads to even larger differences (in the order of 50%) compared to standard parameterizations. Refraction leads to a significant increase in the photolysis frequencies at large solar zenith angles and, under polar spring conditions, to a significant change in the nighttime mixing ratio of some trace gases, e.g., NO₃. It appears that recent changes in some important rate constants significantly alter the simulated BrO_x- and HO_x-budgets in the mid-latitude stratosphere.     

High resolution provenancing of long travelled dust deposited on the Southern Alps, New Zealand

On 7 February 2000 an atypical orange discolouration of snowfields in the central Southern Alps, New Zealand occurred following the passage of a cold front. Analysis of snow samples identified fine orangey-brown dust mixed with much coarser grey dust. Air parcel forward trajectories from dust sources in southern and central Australia, where dust storms were reported on 4 February 2000, were computed to identify the deposits source. Geochemical analyses of the dust deposit using 26 trace elements, unaffected by regional air pollution and gravitational sorting, indicate that 20% of the dust was sourced from western New South Wales, with 45% from the eastern Eyre Peninsula of South Australia and the remaining 35% was local New Zealand dust. This provenancing approach provides a spatial resolution of long travelled dust sourcing not previously achieved.     

Photoelastic and numerical investigation of stress distributions around fault models under biaxial compressive loading conditions

This paper presents the results of a photoelastic and numerical study of stress distributions (contours and trajectories) around fault models of various geometries, submitted to a biaxial compressive load. It aims to describe typical biaxial stress behaviours and emphasize the existing differences with the well-known uniaxial compressive load case. Stress trajectories are sometimes shown by joint sets acting as markers of a paleostress field, and they can be interpreted by particular shallow tension–compression situations. At depth fractures can be reactivated, or can dilate under conditions of triaxial compression, and behaviour is essentially controlled by a high stress ratio (high σ₃/σ₁ ratio). In spite of the potential importance of such stress states on fracture permeability and fluid flow, and although they are frequently found at depth in a reservoir context, such stress conditions are poorly investigated, particularly in terms of stress perturbations.The presented analogue experiments consisted of compression tests done on polymethylmethacrylate (PMMA) plates; this material has mechanical properties comparable to those of brittle rocks in the upper crust, and presents birefringence. The samples contained open defects acting as faults, and the stress trajectories around these faults were investigated using a photoelastic device. Comparable numerical experiments were realised with a finite-element code (Franc 2D), using frictionless fault models.First, the effect of an increasing biaxial compressive load ratio σ₃/σ₁ on stress trajectories around an isolated open defect was explored. It was shown that the stress trajectories were drastically modified when σ₃/σ₁ increased from 0.2 to 0.4, this result being consistent with previous studies. In particular, when σ₃/σ₁ was superior or equal to 0.4, external isotropic points around where trajectories diverged (called repulsive isotropic points) appeared near the tips, but away from the defects. They tended to move away from the defects towards the main load direction when σ₃/σ₁ increased. The described isotropic points were points of stress trajectory divergence, i.e. points where stress decreased, implying that zones around them were strongly unfavourable to shear reactivation.Second, stress trajectories around fault models of various geometries (oblique isolated defect, dilational jog, compressive jog, and complex patterns) were studied, the applied biaxial compressive load ratio being 0.7. These biaxial stress trajectories were compared with similar uniaxial stress trajectories in order to explore the existing differences between the two regimes. It enabled new stress trajectory geometries to be described and interpreted. In particular, numerous external repulsive isotropic points were observed, and defect tips were shown to be zones of high convergence of stress trajectories. Furthermore, in contrast to the uniaxial compressive load case, stress trajectories and stress contours were geometrically similar within dilational and compressive jogs under biaxial compressive load. For both jog types, the centre of the overlapping zone, and the areas along the wall of a jog-defining fault and facing the overlapping tip of the other fault, proved to be zones of low mean stress, implying that fluids may migrate towards them from the tips of the faults, in response to mean stress gradients. Furthermore, the centre of the jogs exhibiting a high differential stress was a favourable area in terms of fracture reactivation, which may facilitate fluids transfer and storage.It was also observed that for both the uniaxial and biaxial compressive loads, isotropic zones were localised at the acute angle between branching defects.     

Column Observations of Stratospheric Gases at Søndre Strømfjord, Greenland During Winter 1994-1995

As part of the Second European Stratospheric Arctic and Mid-latitude Experiment (SESAME) field campaign, observations were made during the period from October 1994 to April 1995 at Søndre Strømfjord, Greenland (67°N, 53°W). Using a Fourier transform spectrometer, high resolution (0.06 cm-1) infrared spectra were recorded with the sun as a radiation source. Column amounts of trace gases including HCl, HF, HNO3, N2O and O3 are shown for five time periods during the course of the 1994-95 Arctic winter. Results are compared with lower stratospheric potential vorticity fields to describe variations in trace gas column amounts.     

Vertical profiles of Volcanic Aerosol and Polar Stratospheric Clouds Above Kiruna, Sweden: Winters 1993 and 1995

Vertical profiles of aerosol were measured in February 1993, and January - March 1995 using balloon-borne particle counters released from Kiruna, Sweden. Condensation nuclei (CN) and aerosol with radii 0.15 - 10.0 µm were measured in 8-12 size classes. The three flights in 1993 were within the polar vortex. Temperatures were below polar stratospheric cloud (PSC) threshold temperatures on one flight and a thin PSC was observed. The volcanic aerosol in the 1993 vortex was similar to that in 1992. In 1993, surface areas were 10 - 20 µm2 cm-3 and volumes 1 - 3 µm3 cm-3. In 1995 three of five flights were within the polar vortex. The volcanic aerosol had decreased to 3 - 7 µm2 cm-3 and 0.1 - 0.4 µm3 cm-3. The top of the volcanic aerosol layer in both years was near 500 K potential temperature (~20 km). A thick nitric acid and water PSC was observed in January 1995. In the thickest region of this PSC nearly all CN were observed to be activated, and surface areas of 5 - 10 µm2 cm-3 were calculated. The volumes observed in this PSC were closer to what would be expected for particles composed of nitric acid trihydrate than for ternary solution droplets. In 1993 the opposite was observed, the volumes in the thin PSC were closer to what would be expected for ternary solution droplets.     

Fast-J2: Accurate Simulation of Stratospheric Photolysis in Global Chemical Models

Modeling photochemistry in the stratosphere requires solution of the equationof radiative transfer over an extreme range of wavelengths and atmosphericconditions, from transmission through the Schumann–Runge bands ofO₂ in the mesosphere, to multiple scattering from troposphericclouds and aerosols. The complexity and range of conditions makes photolysiscalculations in 3-D chemical transport models computationally expensive. Thisstudy pesents a fast and accurate numerical method, Fast-J2, for calculatingphotolysis rates (J-values) and the deposition of solar flux in stratosphere.Fast-J2 develops an optimized, super-wide 11-bin quadrature for wavelengthsfrom 177 to 291 nm that concatenates with the 7-bin quadrature (291–850nm) already developed for the troposphere as Fast-J. Below 291 nm the effectsof Rayleigh scattering are implemented as a pseudo-absorption, and above 291nm the full multiple-scattering code of Fast-J is used. Fast-J2 calculates themean ultraviolet-visible radiation field for these 18 wavelength binsthroughout the stratosphere, and thus new species and new cross sections canbe readily implemented. In comparison with a standard, high-resolution,multiple-scattering photolysis model, worst-case errors in Fast-J2 do notexceed 5% over a wide range of solar zenith angles, altitudes(0–60 km), latitudes, and seasons where the rates are important inphotochemistry.     

Prediction in a socio-hydrological world

ABSTRACT

Water resource management involves public investments with long-ranging impacts that traditional prediction approaches cannot address. These are increasingly being critiqued because (1) there is an absence of feedbacks between water and society; (2) the models are created by domain experts who hand them to decision makers to implement; and (3) they fail to account for global forces on local water resources. Socio-hydrological models that explicitly account for feedbacks between water and society at multiple scales and facilitate stakeholder participation can address these concerns. However, they require a fundamental change in how we think about prediction. We suggest that, in the context of long-range predictions, the goal is not scenarios that present a snapshot of the world at some future date, but rather projection of alternative, plausible and co-evolving trajectories of the socio-hydrological system. This will both yield insights into cause–effect relationships and help stakeholders identify safe or desirable operating space.     

Influence of 850 mb trajectories on acid precipitation in southwestern British Columbia

Precipitation chemistry in southwestern British Columbia has been examined to determine if the shutdown of the ASARCO smelter in Tacoma, Washington in March of 1985 had a significant effect.The smelter emitted about 300 tonnes/day of sulphur dioxide and about one tonne/day of arsenic. The smelter had a supplementary control system (SCS) which was implemented during adverse meteorological conditions to meet ambient air standards. The SCS was in operation in October 1984.Five storms from October 1984, before the shutdown, that produced precipitation over the eight-station network in southwestern British Columbia, were considered. The 850 mb back trajectories and ionic loadings in the collected precipitation samples were compared. A similar comparison was made for three storms in October 1985, after the shutdown of the smelter. An examination of cases whose trajectories passed over the smelter before and after the shutdown indicated an over 50% decrease in volume weighted average sulphate concentration in precipitation over the Gulf Islands (islands in the southern Strait of Georgia). However, when the trajectory from the northwest passes over British Columbia coastal sources, higher excess SO₄ concentrations were observed in the precipitation over the Gulf Islands, indicating a linkage between these concentrations and the British Columbia coastal sources. There was no apparent change in values at stations farthest from the Strait of Georgia with one exception. It was also noted that there was reduction in arsenic in precipitation, which could be considered as a tracer, after the shutdown of the ASARCO smelter. This paper enlarges on the above observation with comparisons of the major meteorological and chemical characteristics related to eight storms.