Nonsequential Two-Photon Double Ionization of Atoms: Identifying the Mechanism

We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of \textit{ab initio} calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process is fully described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime.Comment: 4 pages, 4 figure

Mind the gap – Relevant design for laboratory oil exposure of fish as informed by a numerical impact assessment model

Laboratory experiments provide knowledge of species-specific effects thresholds that are used to parameterize impact assessment models of oil contamination on marine ecosystems. Such experiments typically place individuals of species and life stages in tanks with different contaminant concentrations. Exposure concentrations are usually fixed, and the individuals experience a shock treatment being moved from clean water directly into contaminated water and then back to clean water. In this study, we use a coupled numerical model that simulates ocean currents and state, oil dispersal and fate, and early life stages of fish to quantify oil exposure histories, specifically addressing oil spill scenarios of high rates and long durations. By including uptake modelling we also investigate the potential of buffering transient high peaks in exposure. Our simulation results are the basis for a recommendation on the design of laboratory experiments to improve impact assessment model development and parameterization. We recommend an exposure profile with three main phases: i) a gradual increase in concentration, ii) a transient peak that is well above the subsequent level, and iii) a plateau of fixed concentration lasting ~3 days. In addition, a fourth phase with a slow decrease may be added

Embryonic exposure to produced water can cause cardiac toxicity and deformations in Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae

Regular discharges of produced water from the oil and gas industry represents the largest direct discharge of effluent into the marine environment worldwide. Organic compound classes typically reported in produced water include saturated hydrocarbons, monoaromatic and polyaromatic hydrocarbons (MAHs, PAHs) as well as oxygenated compounds, such as phenols, acids and ketones. This forms a cocktail of known and suspect toxicants, but limited knowledge is yet available on the sub-lethal toxicity of produced water to cold-water marine fish species. In the present work, we conducted a 4-day exposure of embryos of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) to produced water extracts equivalent to 1:50, 1:500 and 1:5000 times dilutions of raw effluent. No significant reduction in survival or hatching success was observed, however, for cod, hatching was initiated earlier for exposed embryos in a concentration-dependent manner. During recovery, significantly reduced embryonic heart rate was observed for both species. After hatch, larvae subjected to embryonic exposure to produced water extracts were smaller, and displayed signs of cardiotoxicity, jaw and craniofacial deformations. In order to improve risk assessment and regulation of produced water discharges, it is important to identify which produced water components contribute to these effects.acceptedVersio

A comparison of Eulerian and Lagrangian methods for vertical particle transport in the water column

A common task in oceanography is to model the vertical movement of particles such as microplastics, nanoparticles, mineral particles, gas bubbles, oil droplets, fish eggs, plankton, or algae. In some cases, the distribution of the vertical rise or settling velocities of the particles in question can span a wide range, covering several orders of magnitude, often due to a broad particle size distribution or differences in density. This requires numerical methods that are able to adequately resolve a wide and possibly multi-modal velocity distribution. Lagrangian particle methods are commonly used for these applications. A strength of such methods is that each particle can have its own rise or settling speed, which makes it easy to achieve a good representation of a continuous distribution of speeds. An alternative approach is to use Eulerian methods, where the partial differential equations describing the transport problem are solved directly with numerical methods. In Eulerian methods, different rise or settling speeds must be represented as discrete classes, and in practice, only a limited number of classes can be included. Here, we consider three different examples of applications for a water column model: positively buoyant fish eggs, a mixture of positively and negatively buoyant microplastics, and positively buoyant oil droplets being entrained by waves. For each of the three cases, we formulate a model for the vertical transport based on the advection–diffusion equation with suitable boundary conditions and, in one case, a reaction term. We give a detailed description of an Eulerian and a Lagrangian implementation of these models, and we demonstrate that they give equivalent results for selected example cases. We also pay special attention to the convergence of the model results with an increasing number of classes in the Eulerian scheme and with the number of particles in the Lagrangian scheme. For the Lagrangian scheme, we see the convergence, as expected for a Monte Carlo method, while for the Eulerian implementation, we see a second-order () convergence with the number of classes

A comparison of Eulerian and Lagrangian methods for vertical particle transport in the water column

A common task in oceanography is to model the vertical movement of particles such as microplastics, nanoparticles, mineral particles, gas bubbles, oil droplets, fish eggs, plankton, or algae. In some cases, the distribution of vertical rise or settling velocities of the particles in question can span a wide range, covering several orders of magnitude, often due to a broad particle size distribution or differences in density. This requires numerical methods that are able to adequately resolve a wide and possibly multi-modal velocity distribution. Lagrangian particle methods are commonly used for these applications. strength of such methods is that each particle can have its own rise or settling speed, which makes it easy to achieve a good representation of a continuous distribution of speeds. An alternative approach is to use Eulerian methods, where the partial differential equations describing the transport problem are solved directly with numerical methods. In Eulerian methods, different rise or settling speeds must be represented as discrete classes, and in practice only a limited number of classes can be included. Here, we consider three different examples of applications for a water-column model: positively buoyant fish eggs, a mixture of positively and negatively buoyant microplastics, and positively buoyant oil droplets being entrained by waves. For each of the three cases we formulate a model for the vertical transport, based on the advection-diffusion equation with suitable boundary conditions and in one case a reaction term. We give a detailed description of an Eulerian and a Lagrangian implementation of these models, and we demonstrate that they give equivalent results for selected example cases. We also pay special attention to the convergence of the model results with increasing number of classes in the Eulerian scheme, and the number of particles in the Lagrangian scheme. For the Lagrangian scheme, we see the 1/√Np convergence as expected for a Monte Carlo method, while for the Eulerian implementation, we see a second order (1/N2k) convergence with the number of classes

High overlap between traditional ecological knowledge and forest conservation found in the Bolivian Amazon

Unidad de excelencia María de Maeztu MdM-2015-0552Altres ajuts: FBBVA research grant (BIOCON_06_106-07)It has been suggested that traditional ecological knowledge (TEK) may play a key role in forest conservation. However, empirical studies assessing to what extent TEK is associated with forest conservation compared with other variables are rare. Furthermore, to our knowledge, the spatial overlap of TEK and forest conservation has not been evaluated at fine scales. In this paper, we address both issues through a case study with Tsimane' Amerindians in the Bolivian Amazon. We sampled 624 households across 59 villages to estimate TEK and used remote sensing data to assess forest conservation. We ran statistical and spatial analyses to evaluate whether TEK was associated and spatially overlapped with forest conservation at the village level. We find that Tsimane' TEK is significantly and positively associated with forest conservation although acculturation variables bear stronger and negative associations with forest conservation. We also find a very significant spatial overlap between levels of Tsimane' TEK and forest conservation. We discuss the potential reasons underpinning our results, which provide insights that may be useful for informing policies in the realms of development, conservation, and climate. We posit that the protection of indigenous cultural systems is vital and urgent to create more effective policies in such realms

Crop pests and predators exhibit inconsistent responses to surrounding landscape composition

The idea that noncrop habitat enhances pest control and represents a win–win opportunity to conserve biodiversity and bolster yields has emerged as an agroecological paradigm. However, while noncrop habitat in landscapes surrounding farms sometimes benefits pest predators, natural enemy responses remain heterogeneous across studies and effects on pests are inconclusive. The observed heterogeneity in species responses to noncrop habitat may be biological in origin or could result from variation in how habitat and biocontrol are measured. Here, we use a pest-control database encompassing 132 studies and 6,759 sites worldwide to model natural enemy and pest abundances, predation rates, and crop damage as a function of landscape composition. Our results showed that although landscape composition explained significant variation within studies, pest and enemy abundances, predation rates, crop damage, and yields each exhibited different responses across studies, sometimes increasing and sometimes decreasing in landscapes with more noncrop habitat but overall showing no consistent trend. Thus, models that used landscape-composition variables to predict pest-control dynamics demonstrated little potential to explain variation across studies, though prediction did improve when comparing studies with similar crop and landscape features. Overall, our work shows that surrounding noncrop habitat does not consistently improve pest management, meaning habitat conservation may bolster production in some systems and depress yields in others. Future efforts to develop tools that inform farmers when habitat conservation truly represents a win–win would benefit from increased understanding of how landscape effects are modulated by local farm management and the biology of pests and their enemies

Two-photon double ionization of helium by attosecond laser pulses: Evidence of highly correlated electron motion

We apply a recently developed ab initio numerical framework to investigate the angular distributions of the emitted electrons in the immediate proximity of the threshold for the two-photon double ionization of helium. Provided one of the electrons is emitted perpendicular to the laser polarization direction, it is found that the angular distribution of the other electron is characterized by three lobes. The results are similar to those recently reported for the corresponding process in the hydrogen negative ion [R. Nepstad and M. Førre, Phys. Rev. A 84, 021402(R) (2011)], and provide further evidence of highly correlated electron dynamics in the vicinity of the double ionization threshold

Numerical analysis of boundary conditions in a Lagrangian particle model for vertical mixing, transport and surfacing of buoyant particles in the water column

Lagrangian particle models are used for many applications in the ocean sciences, including transport modelling of oil spills, fish eggs and larvae, plastics, and sediment particles. In the context of oil spill modelling, there are numerous papers discussing entrainment of surface oil by breaking waves. However, for the opposite process, i.e., droplets reaching the surface and joining the surface slick, we have found no discussion in the literature of the exact steps involved. Given the wide use of particle-based models in oil spill modelling, it is important to establish a consistent recipe for treating the surface boundary. We investigate a Lagrangian particle model for the vertical transport, surfacing and resuspension of buoyant material in the water column. By modifying the behaviour at the surface boundary, the model can be applied to materials such as oil droplets, that form a surface slick, and hence see the surface as an absorbing boundary, and to particles that do not form a surface slick, such as fish eggs and microplastics. For slick-forming materials, we also consider resuspension from the surface slick, (e.g., entrainment of surface oil by breaking waves). While we restrict our attention to positively buoyant materials, the model is equally applicable to the settling of negatively buoyant particles, such as sediment grains and marine snow. We consider three case studies, each designed to allow a detailed and direct comparison of the Lagrangian model to an Eulerian model based on numerical solution of the advection-diffusion-reaction equation. We demonstrate that the two models give the same results when the boundary at the surface is treated correctly.Numerical analysis of boundary conditions in a Lagrangian particle model for vertical mixing, transport and surfacing of buoyant particles in the water columnpublishedVersio