Photochemistry in the Arctic Free Troposphere: Ozone Budget and Its Dependence on Nitrogen Oxides and the Production Rate of Free Radicals

Local ozone production and loss rates for the arctic free troposphere (58–85° N, 1–6 km, February–May) during the TroposphericOzone Production about the Spring Equinox (TOPSE) campaign were calculated using a constrained photochemical box model. Estimates were made to assess the importance of local photochemical ozone production relative to transport in accounting for the springtime maximum in arctic free tropospheric ozone. Ozone production and loss rates from our diel steady-state box model constrained by median observations were first compared to two point box models, one run to instantaneous steady-state and the other run to diel steady-state. A consistent picture of local ozone photochemistry was derived by all three box models suggesting that differences between the approaches were not critical. Our model-derived ozone production rates increased by a factor of 28 in the 1–3 km layer and a factor of 7 in the 3–6 kmlayer between February and May. The arctic ozone budget required net import of ozone into the arctic free troposphere throughout the campaign; however, the transport term exceeded the photochemical production only in the lower free troposphere (1–3 km) between February and March. Gross ozone production rates were calculated to increase linearly with NO_x mixing ratiosup to 300 pptv in February and for NO_x mixing ratios up to 500 pptv in May. These NO_x limits are an order of magnitude higher thanmedian NO_x levels observed, illustrating the strong dependence ofgross ozone production rates on NO_x mixing ratios for the majority of theobservations. The threshold NO_x mixing ratio needed for netpositive ozone production was also calculated to increase from NO_x 10pptv in February to 25 pptv in May, suggesting that the NO_x levels needed to sustain net ozone production are lower in winter than spring. This lower NO_x threshold explains how wintertime photochemical ozone production can impact the build-up of ozone over winter and early spring. There is also an altitude dependence as the threshold NO_x neededto produce net ozone shifts to higher values at lower altitudes. This partly explains the calculation of net ozone destruction for the 1–3 km layerand net ozone production for the 3–6 km layer throughout the campaign.     

Induced Magnetization of Magnetite-titanomagnetite in Alternating Fields Ranging from 400 A/m to 80,000 A/m; Low-field Susceptibility (100 – 400 A/m) and Beyond

For remanence-bearing minerals (RBM) such as magnetite-titanomagnetite, susceptibility to induced magnetism (M) measured in alternating fields (H _AC ) is field-dependent. However, for fields ≤ 400 A/m, measured in an AC induction coil instrument (at 19,100 Hz), susceptibility k ₀ = M/H _AC is sufficiently linear to provide a reproducible rock (or mineral) magnetic characteristic and its anisotropy may be related to arrangements of minerals in rock, or for single mineral grains to their crystalline or shape anisotropy. For any remanence-bearing mineral at higher fields k _HF (= M/H _AC ) is not constant and the term susceptibility is not normally used. This study bridges the responses between traditional low-field susceptibility measurements and those due to high applied fields, for example when studying hysteresis or saturation magnetization of RBM. Where |k _HF | is measured in alternating fields that peak significantly above 400 A/m the M(H _AC ) relation is forced to follow a hysteresis loop in which |k _HF | > k ₀ for small |H _AC | and where |k _HF | decreases to zero for very large fields that achieve saturation magnetization. Hysteresis nonlinearity is due to remanence acquired with one field direction requiring a reverse field for its cancellation. We investigate the transition from initial, traditional “low-field” susceptibility (k ₀ ) measurements at 60 A/m, through 24 different fields from 400 A/m to 40,000 A/m (for very high k ₀ to 80,000 A/m). This reveals M(H _AC ) dependence beyond from conventional k ₀ through the range of hysteresis behavior in fields equal to and exceeding that required to achieve saturation magnetization (M _S ). We show k _HF increases with peak H _AC until the peak field is slightly less than saturation magnetization in natural rock samples rich in magnetite (TM₀ = Fe₃O₄) and TM₆₀ (Fe_2.4Ti_0.6O₄). All sample suites predominantly contain multidomain grains with subordinate pseudo-single domain and single-domain grains. k/k ₀ increases by ≤ 5% for fields up to 2 kA/m. Above 4 kA/m k/ k ₀ increases steeply and peaks, usually between 24 kA/m and 30 kA/m where all grains magnetic moments are activated by H _AC since this exceeds the coercive force of most grains. For higher peak H _AC , k/k ₀ declines sharply as increased H _AC values more effectively flip M with each field-direction switch, leading to the low gradient at distal portions of the hysteresis loop. For M₀-TM₆₀ bearing rocks, susceptibility peaks for fields ~12 kA/m and for magnetite rich rocks up to 24 kA/m. These values are approximately 10% of saturation magnetizations (M _S ) reported for the pure minerals from hysteresis DC field measurements. Both the field at peak k/k ₀ and the peak k/k ₀ value appear to be controlled by the dominant domain structure; multidomain behavior has larger k/k ₀ peaks at lower H _AC . Stacked k/k ₀ versus H _AC curves for each sample suite (n = 12 to n = 39) were successfully characterized at the 95% level by a polynomial fit that requires the cubic form k/k ₀ = a + bH + cH ² + dH ³. Thus, for most M-TM bearing rocks, susceptibility and anisotropy of susceptibility (AMS) measurements made on different instruments would be sufficiently precise for most geological applications, if peak alternating fields are ≤700 A/m.     

Brine pool deposition for the Zn–Pb–Cu massive sulphide deposits of the Bathurst mining camp, New Brunswick, Canada. I. Comparisons with the Iberian pyrite belt

The Ordovician Zn–Pb–Cu massive sulphide ore deposits of the Bathurst mining camp share many features with those of the Devonian/Carboniferous Iberian pyrite belt, particularly the tendency to large size (tonnage and metal content); shape, as far as can be determined after allowing for deformation; metal content, particularly Fe/Cu, Pb/Zn and Sn; mineral assemblages (pyrite + arsenopyrite ± pyrrhotite and lack or rarity of sulphates); sulphide textures (particularly framboidal pyrite); lack of chimney structures and rubble mounds; irregular metal or mineral zoning; and the low degree of zone refining compared to Hokuroku ores. The major differences between the provinces are the lack of vent complexes and the presence of Sn–Cu ores in the Iberian pyrite belt. There are also similarities in the geological setting of the two camps: both lie within continental terranes undergoing arc-continent and continent–continent collision, and in each case massive sulphide mineralisation followed ophiolite obduction; the ore deposits are associated with bimodal volcanic rocks derived from MORB and continental crust and marine shales; and mineralisation was locally accompanied or followed by deposition of iron formations.Fluid inclusion data from veins in stockworks from at least six of the Iberian massive sulphide deposits point to sulphide deposition having taken place in basins containing mostly spent saline, ore-forming fluids (brine pools), and it is suggested that most of the major features of the Bathurst deposits can be explained by similar processes. The proposed model is largely independent of ocean sulphate and O₂ content, whereas low values of each are requisites for the current, spreading-plume model of sulphide deposition in the Bathurst camp.     

Soil–structure interaction in the time domain using halfspace Green's functions

A time-domain formulation is proposed for the transient response analysis of general, three-dimensional structures resting on a homogeneous, elastic halfspace subjected to either external loads or seismic motions. The formulation consists of two parts: (a) the time domain formulation of the soil behaviour and (b) the coupling of the corresponding soil algorithms to the Finite Element Code ANSYS. As far as the structure is concerned, this coupling opens the way for the analysis of non-linear soil–structure interaction. The approach is based on halfspace Green's functions for displacements elicited by Heaviside time-dependent surface point loads. Hence, the spatial discretisation can be confined to the contact area between the foundation and the soil, i.e. no auxiliary grid beyond the foundation as for conventional boundary element formulations is required. The method is applied to analyse the dynamic response of a railway track due to a moving wheel set by demonstrating the influence of ‘through-the-soil coupling’.     

Fractal dimensions of individual flocs and floc populations in streams

The fractal dimension of an individual floc is a measure of the complexity of its external shape. Fractal dimensions can also be used to characterize floc populations, in which case the fractal dimension indicates how the shape of the smaller flocs relates to that of the larger flocs. The objective of this study is to compare the fractal dimensions of floc populations with those of individual flocs, and to evaluate how well both indicate contributions of sediment sources and reflect the nature and extent of flocculation in streams. Suspended solids were collected prior to and during snowmelt at upstream and downstream sites in two southern Ontario streams with contrasting riparian zones. An image analysis system was used to determine area, longest axis and perimeter of flocs. The area–perimeter relationship was used to calculate the fractal dimension, D, that characterizes the floc population. For each sample, the fractal dimension, D_i , of the 28 to 30 largest individual flocs was determined from the perimeter–step‐length relationship. Prior to snowmelt, the mean value of D_i ranged from 1·19 (Cedar Creek, downstream) to 1·22 (Strawberry Creek, upstream and downstream). A comparison of the means using t‐tests indicates that most samples on this day had comparable mean values of D_i . During snowmelt, there was no significant change in the mean value of D_i at the Cedar Creek sites. In contrast, for Strawberry Creek the mean value of D_i at both sites increased significantly, from 1·22 prior to snowmelt to 1·34 during snowmelt. This increase reflects the contribution of sediment‐laden overland flow to the sediment load. At three of the sampling sites, the increase in fractal dimensions was accompanied by a decreases in effective particle size, which can be explained by an increase in bed shear stress. A comparison of fractal dimensions of individual flocs in a sample with the fractal dimensions of the floc populations indicates that both fractal dimensions provide similar information about the temporal changes in sediment source contributions, about the contrasting effectiveness of the riparian buffer zones in the two basins, and about the hydraulic conditions in the streams. Nevertheless, determining the individual fractal dimensions of a set of large flocs in a sample is very time consuming. Using fractal dimensions of floc populations is therefore the preferred method to characterize suspended matter. Copyright © 2000 John Wiley & Sons, Ltd.     

Recapturing quality field experiences and strengthening teaching–research links

Because of unwieldy numbers, and in an attempt to strengthen teaching–research links, the teaching team of 'Field Research Methods' discarded a class fieldtrip in favour of more flexible fieldwork. The new approach continues to use inquiry-based learning but has research problems more closely linked to staff interests. Students are responsible for most aspects of the research process including some logistical planning of fieldwork. Although often anxious at first, students welcomed this approach and appreciated the host of transferable and research skills they developed. Tutors benefited from the enhanced learning outcomes for students and a strengthening of teaching–research links.