875 research outputs found
Stomatal control of leaf fluxes of carbonyl sulfide and CO<sub>2</sub> in a <i>Typha</i> freshwater marsh
Carbonyl sulfide (COS) is an emerging tracer to constrain land photosynthesis at canopy to global scales, because leaf COS and CO2 uptake processes are linked through stomatal diffusion. The COS tracer approach requires knowledge of the concentration normalized ratio of COS uptake to photosynthesis, commonly known as the leaf relative uptake (LRU). LRU is known to increase under low light, but the environmental controls over LRU variability in the field are poorly understood due to scant leaf scale observations.Here we present the first direct observations of LRU responses to environmental variables in the field. We measured leaf COS and CO2 fluxes at a freshwater marsh in summer 2013. Daytime leaf COS and CO2 uptake showed similar peaks in the mid-morning and late afternoon separated by a prolonged midday depression, highlighting the common stomatal control on diffusion. At night, in contrast to CO2, COS uptake continued, indicating partially open stomata. LRU ratios showed a clear relationship with photosynthetically active radiation (PAR), converging to 1.0 at high PAR, while increasing sharply at low PAR. Daytime integrated LRU (calculated from daytime mean COS and CO2 uptake) ranged from 1 to 1.5, with a mean of 1.2 across the campaign, significantly lower than the previously reported laboratory mean value (∼ 1.6). Our results indicate two major determinants of LRU – light and vapor deficit. Light is the primary driver of LRU because CO2 assimilation capacity increases with light, while COS consumption capacity does not. Superimposed upon the light response is a secondary effect that high vapor deficit further reduces LRU, causing LRU minima to occur in the afternoon, not at noon. The partial stomatal closure induced by high vapor deficit suppresses COS uptake more strongly than CO2 uptake because stomatal resistance is a more dominant component in the total resistance of COS. Using stomatal conductance estimates, we show that LRU variability can be explained in terms of different patterns of stomatal vs. internal limitations on COS and CO2 uptake. Our findings illustrate the stomata-driven coupling of COS and CO2 uptake during the most photosynthetically active period in the field and provide an in situ characterization of LRU – a key parameter required for the use of COS as a photosynthetic tracer
Strong low-frequency quantum correlations from a four-wave mixing amplifier
We show that a simple scheme based on nondegenerate four-wave mixing in a hot
atomic vapor behaves like a near-perfect phase-insensitive optical amplifier,
which can generate bright twin beams with a measured quantum noise reduction in
the intensity difference of more than 8 dB, close to the best optical
parametric amplifiers and oscillators. The absence of a cavity makes the system
immune to external perturbations, and the strong quantum noise reduction is
observed over a large frequency range.Comment: 4 pages, 4 figures. Major rewrite of the previous version. New
experimental results and further analysi
Squeezed Light and Entangled Images from Four-Wave-Mixing in Hot Rubidium Vapor
Entangled multi-spatial-mode fields have interesting applications in quantum
information, such as parallel quantum information protocols, quantum computing,
and quantum imaging. We study the use of a nondegenerate four-wave mixing
process in rubidium vapor at 795 nm to demonstrate generation of
quantum-entangled images. Owing to the lack of an optical resonator cavity, the
four-wave mixing scheme generates inherently multi-spatial-mode output fields.
We have verified the presence of entanglement between the multi-mode beams by
analyzing the amplitude difference and the phase sum noise using a dual
homodyne detection scheme, measuring more than 4 dB of squeezing in both cases.
This paper will discuss the quantum properties of amplifiers based on
four-wave-mixing, along with the multi mode properties of such devices.Comment: 11 pages, 8 figures. SPIE Optics and Photonics 2008 proceeding (San
Diego, CA
Use of Lagrangian simulations to hindcast the geographical position of propagule release zones in a Mediterranean coastal fish
The study of organism dispersal is fundamental for elucidating patterns of connectivity between populations, thus crucial for the design of effective protection and management strategies. This is especially challenging in the case of coastal fish, for which information on egg release zones (i.e. spawning grounds) is often lacking. Here we assessed the putative location of egg release zones of the saddled sea bream (Oblada melanura) along the south-eastern coast of Spain in 2013. To this aim, we hindcasted propagule (egg and larva) dispersal using Lagrangian simulations, fed with species-specific information on early life history traits (ELTs), with two approaches: 1) back-tracking and 2) comparing settler distribution obtained from simulations to the analogous distribution resulting from otolith chemical analysis. Simulations were also used to assess which factors contributed the most to dispersal distances. Back-tracking simulations indicated that both the northern sector of the Murcia region and some traits of the North-African coast were hydrodynamically suitable to generate and drive the supply of larvae recorded along the coast of Murcia in 2013. With the second approach, based on the correlation between simulation outputs and field results (otolith chemical analysis), we found that the oceanographic characteristics of the study area could have determined the pattern of settler distribution recorded with otolith analysis in 2013 and inferred the geographical position of main O. melanura spawning grounds along the coast. Dispersal distance was found to be significantly affected by the geographical position of propagule release zones. The combination of methods used was the first attempt to assess the geographical position of propagule release zones in the Mediterranean Sea for O. melanura, and can represent a valuable approach for elucidating dispersal and connectivity patterns in other coastal species
Light forces in ultracold photoassociation
We study the time-resolved photoassociation of ultracold sodium in an optical
dipole trap. The photoassociation laser excites pairs of atoms to molecular
states of large total angular momentum at high intensities (above 20
kW/cm). Such transitions are generally suppressed at ultracold
temperatures by the centrifugal barriers for high partial waves. Time-resolved
ionization measurements reveal that the atoms are accelerated by the dipole
potential of the photoassociation beam. We change the collision energy by
varying the potential depth, and observe a strong variation of the
photoassociation rate. These results demonstrate the important role of light
forces in cw photoassociation at high intensities.Comment: 7 pages, 3 figure
A soil diffusion–reaction model for surface COS flux: COSSM v1
Soil exchange of carbonyl sulfide (COS) is the second largest COS flux in terrestrial ecosystems. A novel application of COS is the separation of gross primary productivity (GPP) from concomitant respiration. This method requires that soil COS exchange is relatively small and can be well quantified. Existing models for soil COS flux have incorporated empirical temperature and moisture functions derived from laboratory experiments but not explicitly resolved diffusion in the soil column. We developed a mechanistic diffusion–reaction model for soil COS exchange that accounts for COS uptake and production, relates source–sink terms to environmental variables, and has an option to enable surface litter layers. We evaluated the model with field data from a wheat field (Southern Great Plains (SGP), OK, USA) and an oak woodland (Stunt Ranch Reserve, CA, USA). The model was able to reproduce all observed features of soil COS exchange such as diurnal variations and sink–source transitions. We found that soil COS uptake is strongly diffusion controlled and limited by low COS concentrations in the soil if there is COS uptake in the litter layer. The model provides novel insights into the balance between soil COS uptake and production: a higher COS production capacity was required despite lower COS emissions during the growing season compared to the post-senescence period at SGP, and unchanged COS uptake capacity despite the dominant role of COS emissions after senescence. Once there is a database of soil COS parameters for key biomes, we expect the model will also be useful to simulate soil COS exchange at regional to global scales
Strong relative intensity squeezing by 4-wave mixing in Rb vapor
We have measured -3.5 dB (-8.1 dB corrected for losses) relative intensity
squeezing between the probe and conjugate beams generated by stimulated,
nondegenerate four-wave mixing in hot rubidium vapor. Unlike early observations
of squeezing in atomic vapors based on saturation of a two-level system, our
scheme uses a resonant nonlinearity based on ground-state coherences in a
three-level system. Since this scheme produces narrowband, squeezed light near
an atomic resonance it is of interest for experiments involving cold atoms or
atomic ensembles.Comment: Submitted to Optics Letter
Effects of a physiotherapy and occupational therapy intervention on mobility and activity in care home residents: a cluster randomised controlled trial
Objective To compare the clinical effectiveness of a programme of physiotherapy and occupational therapy with standard care in care home residents who have mobility limitations and are dependent in performing activities of daily living
Ultraslow propagation of matched pulses by four-wave mixing in an atomic vapor
We have observed the ultraslow propagation of matched pulses in nondegenerate
four-wave mixing in a hot atomic vapor. Probe pulses as short as 70 ns can be
delayed by a tunable time of up to 40 ns with little broadening or distortion.
During the propagation, a probe pulse is amplified and generates a conjugate
pulse which is faster and separates from the probe pulse before getting locked
to it at a fixed delay. The precise timing of this process allows us to
determine the key coefficients of the susceptibility tensor. The presence of
gain in this system makes this system very interesting in the context of
all-optical information processing.Comment: 5 pages, 4 figure
Kampala Trauma Score (KTS): Is it a new triage tool?
In North America, injury surveillance has generated an accumulation of data regarding trauma events and outcomes through the implementation of trauma registries. Trauma registries have been predominantly instituted in response to the desire to be accredited by the American College of Surgeons (ACS). Committee on Trauma as an ACS Trauma Center1. These registries play a key role in research that determines epidemiological patterns of injury, advances injury prevention actions and finally, measures outcomes2. For example, they have been used extensively in the US to monitor reductions in mortality which has been attributed to the institution of “trauma systems”, coordinated hospital-based systems for the management of injured patients3. As interventions to reduce and prevent injuries are sought, registries allow the calculation of injury rates, identification of causes and measurement of outcomes produced by implementation of interventions4. Trauma registry data is made universally comparable across different strata through the routine use of trauma scores, such as the revised trauma score (RTS) and injury severity score (ISS)
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