Algal blooms (increased abundance of phytoplankton) are an increasingly common phenomenon which has been causally linked to increased fluxes of nutrient (particularly nitrogenous) compounds to ...aquatic ecosystems. These blooms have implications for water quality and human health in addition to ecosystem productivity, health and ecological diversity. Anthropogenic emissions of nitrogen to the atmosphere are estimated to be comparable to, or greater than, biogenic emissions but are considerably more concentrated in space. Although fluvial pathways typically dominate the annually averaged nitrogen flux to coastal waters, atmosphere–surface exchange represents a significant component of the total flux and may be particularly critical during the summertime when both the riverine input and ambient nutrient concentrations are often at a minimum. In this chapter, we present an overview of the physical and chemical processes which dictate the quantity (and direction) of atmosphere–surface fluxes of trace chemicals to (and above) water surfaces with particular emphasis on the role of particles.
Dry deposition (transfer to the surface in the absence of precipitation) of particles is determined by meteorological conditions, atmospheric concentrations, surface type/condition and the specific chemical and physical properties of the particle. Dry deposition can be conceptualized as a three-step process: (1) the gas or particle is moved toward the surface by thermally or mechanically driven eddies; (2) it is transferred by diffusion across a thin layer close to the surface where turbulence is absent; and (3) the gas or particle is captured by the surface. In the case of larger particles a second parallel pathway exists; particles are drawn towards the surface by gravity. Atmospheric particles determine dry deposition fluxes not only by serving as a conduit for transfer but also because of their action as sources or sinks of trace gases. The example given here is the transfer of nitric acid to sea salt particles as a result of heterogeneous chemistry acting as a competing sink to surface removal. To illustrate the importance of current uncertainties in our understanding of dry deposition processes and to highlight the role of some of the key parameters in determining the transfer rate (the deposition velocity) a simple model of particle dry deposition is presented. The model describes the calculation of the rate at which a particle of a given size and chemical composition will be moved towards the surface under given environmental conditions. Observational and experimental techniques for measuring dry deposition fluxes are also reviewed. The techniques used for gases are largely reliant on use of highly temporally resolved sampling (e.g., concentrations sampled 10 times per second) or highly accurate and precise measurements of concentrations, either in the vertical to resolve the gradient to or from the surface or conditionally sampled by the direction of transfer (to or from the surface). These stringent measurement requirements represent significant barriers to application to measurement of particle dry deposition fluxes although, as discussed, innovative solutions are now becoming available.
In the final section, we examine meteorological controls on deposition to the coastal zone. This region of the world’s oceans and seas is most significantly impacted by human activities. More than half of the world’s population lives within 100 km of a coast and hence the overwhelming majority of anthropogenic fluxes to aquatic systems occur in the coastal zone. We discuss the particular challenges that arise from efforts to simulate and measure fluxes close to the coastline. These arise in part from the complexity of atmospheric flow in this region where energy and chemical fluxes are highly inhomogeneous in space and time and thermally generated atmospheric circulations are commonplace.
Many highly productive marine ecosystems exhibit nitrogen limitation or co-limitation. This article is a status review of research into the exchange of nitrogen between the atmosphere and these ...ecosystems with a particular focus on reactive nitrogen compounds. A summary of research conducted over the past ten years is presented and a perspective given as to remaining uncertainities and research needs. Looking toward development of coastal management modeling tools, we illustrate the processes that need to be resolved in order to accurately simulate the flux from the atmosphere and provide guidance on the required resolution of such models.
The project described here seeks to answer questions regarding the role increased nitrogen (N) deposition is playing in enhanced carbon (C) sequestration in temperate mid-latitude forests, using ...detailed measurements from an AmeriFlux tower in southern Indiana (Morgan-Monroe State Forest, or MMSF). The measurements indicate an average atmosphere-surface N flux of approximately 6 mg-N m(-2) day(-1) during the 2000 growing season, with approximately 40% coming from dry deposition of ammonia (NH3), nitric acid (HNO3), and particle-bound N. Wet deposition and throughfall measurements indicate significant canopy uptake of N (particularly NH4+) at the site, leading to a net canopy exchange (NCE) of -6 kg-N ha(-1) for the growing season. These data are used in combination with data on the aboveground C:N ratio, litterfall flux, and soil net N mineralization rates to indicate the level of potential perturbation of C sequestration at this site.
Analysis of data collected between 1950 and 1995 in the state of Indiana indicates that the state experiences an average of nearly 19 tornadoes each year, which occur on approximately 8 or 9 days ...during an average year. Analysis of storm dimensions indicates that the annual probability of a point location in Indiana being impacted by a tornado is 1.4 × 10
-4
per year. The largest number of the more intense tornadoes (F2 and higher) typically occur from March to June, during the afternoon and evening, although tornadoes have been observed in each month of the year and each hour of the day. Inter-annual variability of tornadic events in Indiana is large and may reflect both physical causes (e.g., the variability of cyclone passages or jet stream location) and recording bias (e.g., increased public awareness during the 1950s). Using the data set from Indiana, statistical analyses of the temporal and spatial variability of tornadoes are undertaken. Analyses of tornado reports by county indicate that, in addition to area and population, surface roughness is a significant determinant of event frequency. Analyses presented herein also indicate that in contrast to previous studies, (1) no statistically significant hebdomadal cycle of tornado reporting is evident in the data and (2) the diurnal cycle of tornado records is evident for all F-scale classes and is not confined largely to the weak tornado categories. Analyses of the physical dimensions and F-scale ratings of tornadoes in Indiana indicate that there has been a decline in reported tornado intensity, width, and length between 1950 and 1995. These changes are in accord with evidence of a decreasing number of reports of strong and violent tornadoes across the United States over this period and appears to be principally a result of reporting biases at the beginning of the record. Key words: tornado climatology, temporal and spatial variability, reporting bias.
Wind speed trends over the contiguous USA Pryor, Sara C; Barthelmie, R J; Takle, G S
IOP conference series. Earth and environmental science,
02/2009, Volume:
6, Issue:
9
Journal Article
Peer reviewed
Open access
This article was submitted without an abstract, please refer to the full-text PDF file.
A National Science Foundation-sponsored container runtimes investigation was
conducted by the Aristotle Cloud Federation to better understand the challenges
of selecting and using Docker, ...Singularity, and X-Containers. The main goal of
this investigation was to identify the "pain points" experienced by users when
selecting and using containers for scientific research and to share lessons
learned. Application performance characteristics are included in this report as
well as user experiences with Kubernetes and container orchestration on cloud
and HPC platforms. Scientists, research computing practitioners, and educators
may find value in this report when considering the use and/or deployment of
containers or when preparing students to meet the unique challenges of using
containers in scientific research.
Posttranslational lipidation provides critical modulation of the functions of some proteins. Isoprenoids (i.e., farnesyl or geranylgeranyl groups) are attached to cysteine residues in proteins ...containing C-terminal CAAX sequence motifs (where A is an aliphatic residue and X is any residue). Isoprenylation is followed by cleavage of the AAX amino acid residues and, in some cases, by additional proteolytic cuts. We determined the crystal structure of the CAAX protease Ste24p, a zinc metalloprotease catalyzing two proteolytic steps in the maturation of yeast mating pheromone a-factor. The Ste24p core structure is a ring of seven transmembrane helices enclosing a voluminous cavity containing the active site and substrate-binding groove. The cavity is accessible to the external milieu by means of gaps between splayed transmembrane helices. We hypothesize that cleavage proceeds by means of a processive mechanism of substrate insertion, translocation, and ejection.
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