Possible dissolution pathways participating in the Mg corrosion reaction

The Mg corrosion reaction sequence was considered in terms of possible elementary reaction steps, and their plausibility in relation to their thermodynamic status. The uni-positive Mg+ reaction sequence is thermodynamically favoured, and can occur by a number of elementary steps. There are a number of possible dissolution reactions (which are defined are reactions which contribute no electrons to the corroding Mg electrode). It is impossible at this stage to conclusively determine the reaction mechanism without knowledge of the species present at the magnesium water interface. (C) 2014 Elsevier Ltd. All rights reserved

A snapshot of biodiversity protection in Antarctica.

Threats to Antarctic biodiversity are escalating, despite its remoteness and protection under the Antarctic Treaty. Increasing human activity, pollution, biological invasions and the omnipresent impacts of climate change all contribute, and often combine, to exert pressure on Antarctic ecosystems and environments. Here we present a continent-wide assessment of terrestrial biodiversity protection in Antarctica. Despite Antarctic Specially Protected Areas covering less than 2% of Antarctica, 44% of species (including seabirds, plants, lichens and invertebrates) are found in one or more protected areas. However, protection is regionally uneven and biased towards easily detectable and charismatic species like seabirds. Systematic processes to prioritize area protection using the best available data will maximize the likelihood of ensuring long-term protection and conservation of Antarctic biodiversity

Towards constraints on fossil fuel emissions from total column carbon dioxide

We assess the large-scale, top-down constraints on regional fossil fuel emissions provided by observations of atmospheric total column CO_2, X_CO_2. Using an atmospheric general circulation model (GCM) with underlying fossil emissions, we determine the influence of regional fossil fuel emissions on global X_CO_2 fields. We quantify the regional contrasts between source and upwind regions and probe the sensitivity of atmospheric X_CO_2 to changes in fossil fuel emissions. Regional fossil fuel X_CO_2 contrasts can exceed 0.7 ppm based on 2007 emission estimates, but have large seasonal variations due to biospheric fluxes. Contamination by clouds reduces the discernible fossil signatures. Nevertheless, our simulations show that atmospheric fossil X_CO_2 can be tied to its source region and that changes in the regional X_CO_2 contrasts scale linearly with emissions. We test the GCM results against X_CO_2 data from the GOSAT satellite. Regional X_CO_2 contrasts in GOSAT data generally scale with the predictions from the GCM, but the comparison is limited by the moderate precision of and relatively few observations from the satellite. We discuss how this approach may be useful as a policy tool to verify national fossil emissions, as it provides an independent, observational constraint

Free energy landscapes for homogeneous nucleation of ice for a monatomic water model

We simulate the homogeneous nucleation of ice from supercooled liquid water at 220 K in the isobaric-isothermal ensemble using the MW monatomic water potential. Monte Carlo simulations using umbrella sampling are performed in order to determine the nucleation free energy barrier. We find the Gibbs energy profile to be relatively consistent with that predicted by classical nucleation theory; the free energy barrier to nucleation was determined to be ~18 kT and the critical nucleus comprised ~85 ice particles. Growth from the supercooled liquid gives clusters that are predominantly cubic, whilst starting with a pre-formed subcritical nucleus of cubic or hexagonal ice results in the growth of predominantly that phase of ice only.Comment: 11 pages, 6 figures; updated with nucleation rates and additional comparisons with some newly published paper

Beyond reasons and obligations: A dual-role approach to reasons and supererogation

peer reviewedDual-role approaches to reasons say, roughly, that reasons can relate to actions in two fundamentally different ways: they can either require conformity, or justify an action without requiring that it be taken. This paper develops a formal dual-role approach, combining ideas from defeasible logic and practical philosophy. It then uses the approach to shed light on the phenomenon of supererogation and resolve a well-known puzzle about supererogation, namely, Horton’s All or Nothing Problem

Emissions of greenhouse gases from a North American megacity

Atmospheric column abundances of carbon dioxide (CO_2), carbon monoxide (CO), methane (CH_4) and nitrous oxide (N_2O) have been measured above the South Coast air basin (SCB), a densely populated urban region of Southern California, USA, which includes Los Angeles and the surrounding suburbs. Large diurnal variations in CO and CH_4 are observed which correlate well with those in CO_2. Weaker correlations are seen between N_2O and CO_2, with large uncertainties. We compute yearly SCB emissions of CO and CH_4 to be 1.4 ± 0.3 Tg CO and 0.6 ± 0.1 Tg CH_4. We compare our calculated emissions to the California Air Resources Board (CARB) and the Emission Database for Global Atmospheric Research (EDGAR) estimates. Our measurements confirm that urban emissions are a significant source of CH_4 and in fact may be substantially higher than currently estimated. If our emissions are typical of other urban centers, these findings suggest that urban emissions could contribute 7–15% to the global anthropogenic budget of methane

Toward accurate CO_2 and CH_4 observations from GOSAT

The column-average dry air mole fractions of atmospheric carbon dioxide and methane (X_(CO_2) and X_(CH_4)) are inferred from observations of backscattered sunlight conducted by the Greenhouse gases Observing SATellite (GOSAT). Comparing the first year of GOSAT retrievals over land with colocated ground-based observations of the Total Carbon Column Observing Network (TCCON), we find an average difference (bias) of −0.05% and −0.30% for X_(CO_2) and X_(CH_4) with a station-to-station variability (standard deviation of the bias) of 0.37% and 0.26% among the 6 considered TCCON sites. The root-mean square deviation of the bias-corrected satellite retrievals from colocated TCCON observations amounts to 2.8 ppm for X_(CO_2) and 0.015 ppm for X_(CH_4). Without any data averaging, the GOSAT records reproduce general source/sink patterns such as the seasonal cycle of X_(CO_2) suggesting the use of the satellite retrievals for constraining surface fluxes

A method for evaluating bias in global measurements of CO_2 total columns from space

We describe a method of evaluating systematic errors in measurements of total column dry-air mole fractions of CO_2 (X_(CO_2)) from space, and we illustrate the method by applying it to the v2.8 Atmospheric CO_2 Observations from Space retrievals of the Greenhouse Gases Observing Satellite (ACOS-GOSAT) measurements over land. The approach exploits the lack of large gradients in X_(CO_2) south of 25° S to identify large-scale offsets and other biases in the ACOS-GOSAT data with several retrieval parameters and errors in instrument calibration. We demonstrate the effectiveness of the method by comparing the ACOS-GOSAT data in the Northern Hemisphere with ground truth provided by the Total Carbon Column Observing Network (TCCON). We use the observed correlation between free-tropospheric potential temperature and X_(CO_2) in the Northern Hemisphere to define a dynamically informed coincidence criterion between the ground-based TCCON measurements and the ACOS-GOSAT measurements. We illustrate that this approach provides larger sample sizes, hence giving a more robust comparison than one that simply uses time, latitude and longitude criteria. Our results show that the agreement with the TCCON data improves after accounting for the systematic errors, but that extrapolation to conditions found outside the region south of 25° S may be problematic (e.g., high airmasses, large surface pressure biases, M-gain, measurements made over ocean). A preliminary evaluation of the improved v2.9 ACOS-GOSAT data is also discussed

Total column CO_2 measurements at Darwin, Australia – site description and calibration against in situ aircraft profiles

An automated Fourier Transform Spectroscopic (FTS) solar observatory was established in Darwin, Australia in August 2005. The laboratory is part of the Total Carbon Column Observing Network, and measures atmospheric column abundances of CO_2 and O_2 and other gases. Measured CO_2 columns were calibrated against integrated aircraft profiles obtained during the TWP-ICE campaign in January–February 2006, and show good agreement with calibrations for a similar instrument in Park Falls, Wisconsin. A clear-sky low airmass relative precision of 0.1% is demonstrated in the CO2 and O2 retrieved column-averaged volume mixing ratios. The 1% negative bias in the FTS X_(CO_2) relative to the World Meteorological Organization (WMO) calibrated in situ scale is within the uncertainties of the NIR spectroscopy and analysis

"Liquid, gel and soft glass" phase transitions and rheology of nanocrystalline cellulose suspensions as a function of concentration and salinity

The colloidal size and rod morphology of nanocrystalline cellulose (NCC) lead to suspensions with useful phase and gelation behaviours as well as complex rheologies. However, these have not been comprehensively evaluated previously. Here we report the detailed phase behaviour of sulphonated NCC aqueous suspensions as a function of concentration and salinity. Four phases - liquid, viscoelastic, repulsive glass and attractive glass/gel - are identified in terms of their distinct rheological behaviours. The liquid-solid transitions (LSTs) are determined rheologically, and these are supported by a simplified model based on the DLVO theory that indicates the importance of charge in determining the phase behaviour. Rheology is also used to investigate the solid-solid transition from a repulsive glass to an attractive gel with increasing salt at high NCC concentrations. A time-dependent aging phenomenon is observed in suspensions with a composition just below the LSTs, and the implications of this on the dynamics occurring during gelation processes are discussed. This work can be directly applied to the development of structure-function relationships and the expanding utilisation of NCC suspensions, whilst also providing a basis for the study of charged colloidal rods more generally and evaluation of theoretical models