Sesquiterpene emissions from vegetation: a review

International audienceThis literature review summarizes the environmental controls governing biogenic sesquiterpene (SQT) emissions and presents a compendium of numerous SQT-emitting plant species as well as the quantities and ratios of SQT species they have been observed to emit. The results of many enclosure-based studies indicate that temporal SQT emission variations appear to be dominated mainly by ambient temperatures although other factors contribute (e.g., seasonal variations). This implies that SQT emissions have increased significance at certain times of the year, especially in late spring to mid-summer. The strong temperature dependency of SQT emissions also creates the distinct possibility of increasing SQT emissions in a warmer climate. Disturbances to vegetation (from herbivores and possibly violent weather events) are clearly also important in controlling short-term SQT emissions bursts, though the relative contribution of disturbance-induced emissions is not known. Based on the biogenic SQT emissions studies reviewed here, SQT emission rates among numerous species have been observed to cover a wide range of values, and exhibit substantial variability between individuals and across species, as well as at different environmental and phenological states. These emission rates span several orders of magnitude (10s?1000s of ng gDW-1 h?1). Many of the higher rates were reported by early SQT studies, which may have included artificially-elevated SQT emission rates due to higher-than-ambient enclosure temperatures and disturbances to enclosed vegetation prior to and during sample collection. When predicting landscape-level SQT fluxes, modelers must consider the numerous sources of variability driving observed SQT emissions. Characterizations of landscape and global SQT fluxes are highly uncertain given differences and uncertainties in experimental protocols and measurements, the high variability in observed emission rates from different species, the selection of species that have been studied so far, and ambiguities regarding controls over emissions. This underscores the need for standardized experimental protocols, better characterization of disturbance-induced emissions, screening of dominant plant species, and the collection of multiple replicates from several individuals within a given species or genus as well as a better understanding of seasonal dependencies of SQT emissions in order to improve the representation of SQT emission rates

Equidimensional modelling of flow and transport processes in fractured porous systems I

Flow and transport in fractured porous media play an important role for many environmental applications, e.g. the design of disposal systems for hazardous waste. The different hydraulic properties of the fractures and the surrounding rock matrix have a strong influence on the behaviour of the physical processes existing on site. In the two papers of this conference, we will present a new numerical concept to describe saturated flow and transport processes in arbitrarily fractured porous media. We will use an equidimensional approach where fracture and matrix are discretized with elements of the same dimension. To solve the problem, we developed a two-level multigrid method based on a hierarchical decomposition into a fracture problem and a matrix problem. This decoupled treatment of fracture and matrix allows us to handle the locally governing physical processes appropriately. In this paper we will also present convergence comparisons with classical multigrid and algebraic multigrid methods (AMG). In Neunhäuserer et al. (this issue, part II) we will discuss the effect of equidimensionality on the modelling results and the influence of the chosen transport discretisation technique

Equidimensional modelling of flow and transport processes in fractured porous systems II

In fractured formations, the vastly different hydraulic properties of fractures and porous matrix lead to a considerable mass exchange between fracture and matrix, strongly affecting the flow and transport conditions in the domain of interest. This plays an important role for many environmental applications, e.g. the design of disposal systems for hazardous waste. In two papers, we display a new numerical concept describing saturated flow and transport processes in arbitrarily fractured porous media. An equidimensional approach is developed using elements of the same dimension for fracture and matrix discretisation. In Gebauer et al. (this issue, part I) we introduced a two-level multigrid method based on a hierarchical decomposition designed to solve equidimensional fracture-matrix-problems. In this paper we will discuss the effect of equidimensionality on the modelling results. Furthermore, the influence of the chosen transport discretisation technique will be shown

Selective antimicrobial modulation of the intestinal microbial flora for infection prevention in patients with hematological malignancies

Contains fulltext : 4455.pdf (publisher's version ) (Open Access

A reassessment of Antarctic plateau reactive nitrogen based on ANTCI 2003 airborne and ground based measurements

The first airborne measurements of nitric oxide (NO) on the Antarctic plateau have demonstrated that the previously reported elevated levels of this species extend well beyond the immediate vicinity of South Pole. Although the current database is still relatively weak and critical laboratory experiments are still needed, the findings here suggest that the chemical uniqueness of the plateau may be substantially greater than first reported. For example, South Pole ground-based findings have provided new evidence showing that the dominant process driving the release of nitrogen from the snowpack during the spring/summer season (post-depositional loss) is photochemical in nature with evaporative processes playing a lesser role. There is also new evidence suggesting that nitrogen, in the form of nitrate, may undergo multiple recycling within a given photochemical season. Speculation here is that this may be a unique property of the plateau and much related to its having persistent cold temperatures even during summer. These conditions promote the efficient adsorption of molecules like HNO3 (and very likely HO2NO2) onto snow-pack surface ice where we have hypothesized enhanced photochemical processing can occur, leading to the efficient release of NOx to the atmosphere. In addition, to these process-oriented tentative conclusions, the findings from the airborne studies, in conjunction with modeling exercises suggest a new paradigm for the plateau atmosphere. The near-surface atmosphere over this massive region can be viewed as serving as much more than a temporary reservoir or holding tank for imported chemical species. It defines an immense atmospheric chemical reactor which is capable of modifying the chemical characteristics of select atmospheric constituents. This reactor has most likely been in place over geological time, and may have led to the chemical modulation of some trace species now found in ice cores. Reactive nitrogen has played a critical role in both establishing and in maintaining this reactor. © 2007 Elsevier Ltd. All rights reserved

Gas Component Transport Across the Soil-Atmosphere Interface for Gases of Different Density: Experiments and Modeling

We investigate the influence of near-surface wind conditions on subsurface gas transport and on soil-atmosphere gas exchange for gases of different density. Results of a sand tank experiment are supported by a numerical investigation with a fully coupled porous medium-free flow model, which accounts for wind turbulence. The experiment consists of a two-dimensional bench-scale soil tank containing homogeneous sand and an overlying wind tunnel. A point source was installed at the bottom of the tank. Gas concentrations were measured at multiple horizontal and vertical locations. Tested conditions include four wind velocities (0.2/1.0/2.0/2.7 m/s), three different gases (helium: light, nitrogen: neutral, and carbon dioxide: heavy), and two transport cases (1: steady-state gas supply from the point source; 2: transport under decreasing concentration gradient, subsequent to termination of gas supply). The model was used to assess flow patterns and gas fluxes across the soil surface. Results demonstrate that flow and transport in the vicinity of the surface are strongly coupled to the overlying wind field. An increase in wind velocity accelerates soil-atmosphere gas exchange. This is due to the effect of the wind profile on soil surface concentrations and due to wind-induced advection, which causes subsurface horizontal transport. The presence of gases with pronounced density difference to air adds additional complexity to the transport through the wind-affected soil layers. Wind impact differs between tested gases. Observed transport is multidimensional and shows that heavy as well as light gases cannot be treated as inert tracers, which applies to many gases in environmental studies. © 2020. The Authors

Mehrgittermethoden und adaptive Diskretisierungsverfahren zur Simulation von Strömungs- und Transportprozessen in Kluftaquiferen

Im Zusammenhang mit der Deponierung von Schadstoffen und der Trinkwassergewinnung kommt der Simulation von Strömungs- und Transportprozessen in geklüfteten Bodenzonen eine große Bedeutung zu. Die oft sehr unterschiedlichen hydraulischen Eigenschaften von Kluftsystem und umgebender Gesteinsmatrix prägen das Strömungs- und Transportverhalten stark. Die daraus resultierende Heterogenität der physikalischen Prozesse ebenso wie die Komplexität der zugrundeliegenden Geometrie stellt hohe Anforderungen an die numerische Modellierung. In der vorliegenden Arbeit werden Modellansätze verwendet, die Klüfte und Matrix diskret beschreiben. Um die Nachteile der bisher verwendeten Kopplung von Elementen unterschiedlicher Dimension (keine Flußerhaltung am Kluft-Matrix-Übergang) zu vermeiden, werden Kluft und Matrix mit Elementen gleicher Dimension vernetzt, wobei in den Klüften degenerierte Elemente zugelassen werden. Alle weiteren Bausteine des Lösungsprozesses müssen daher robust gegenüber verschwindender Kluftweite (reduziertes Problem) sein. Die Diskretisierung der Strömungsgleichung wird mit einem Standard-Galerkin-Finite-Elemente-Verfahren durchgeführt, für die Transportgleichung wird ein modifiziertes Boxverfahren eingesetzt. Zur Lösung der Strömungsgleichung wird ein neuartiges Mehrgitterverfahren verwendet, das eine hierarchische Zerlegung des Lösungsraums in einen Kluft- und einen Matrixraum beinhaltet. Dies ermöglicht zu einem späteren Zeitpunkt die Anwendung unterschiedlicher, an den jeweils dominanten physikalischen Prozeß angepaßter Diskretisierungsverfahren in Kluft und Matrix

Numerical Methods for Flow in Fractured Porous Media

In this work we present the mathematical models for single-phase flow in fractured porous media. An overview of the most common approaches is considered, which includes continuous fracture models and discrete fracture models. For the latter, we discuss strategies that are developed in literature for its numerical solution mainly related to the geometrical relation between the fractures and porous media grids