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Search Results: 1 - 10 of 673358 matches for " S. A. Hayward "
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Isoprene emission from wetland sedges
A. Ekberg, A. Arneth, H. Hakola, S. Hayward,T. Holst
Biogeosciences (BG) & Discussions (BGD) , 2009,
Abstract: High latitude wetlands play an important role for the surface-atmosphere exchange of carbon dioxide (CO2) and methane (CH4), but fluxes of biogenic volatile organic compounds (BVOC) in these ecosystems have to date not been extensively studied. This is despite BVOC representing a measurable proportion of the total gaseous C fluxes at northern locations and in the face of the high temperature sensitivity of these systems that requires a much improved process understanding to interpret and project possible changes in response to climate warming. We measured emission of isoprene and photosynthetic gas exchange over two growing seasons (2005–2006) in a subarctic wetland in northern Sweden with the objective to identify the physiological and environmental controls of these fluxes on the leaf scale. The sedge species Eriophorum angustifolium and Carex rostrata were both emitters of isoprene. Springtime emissions were first detected after an accumulated diurnal mean temperature above 0°C of about 100 degree days. Maximum measured growing season standardized (basal) emission rates (20°C, 1000 μmol m 2 s 1) were 1075 (2005) and 1118 (2006) μg C m 2 (leaf area) h 1 in E. angustifolium, and 489 (2005) and 396 (2006) μg C m 2 h 1 in C. rostrata. Over the growing season, basal isoprene emission varied in response to the temperature history of the last 48 h. Seasonal basal isoprene emission rates decreased with leaf nitrogen (N), which may be explained by the typical growth and resource allocation pattern of clonal sedges as the leaves age. The observations were used to model emissions over the growing season, accounting for effects of temperature history, links to leaf assimilation rate and the light and temperature dependencies of the cold-adapted sedges.
Leaf isoprene emission in a subarctic wetland sedge community
A. Ekberg,A. Arneth,H. Hakola,S. Hayward
Biogeosciences Discussions , 2008,
Abstract: High latitude wetlands play an important role for the surface-atmosphere exchange of carbon dioxide (CO2) and methane (CH4), but fluxes of biogenic volatile organic compounds (BVOC) in these ecosystems have to date not been extensively studied. This is despite BVOC representing a measurable proportion of the total gaseous C fluxes at northern locations and in the face of the high temperature sensitivity of these systems that requires a much improved process understanding to interpret and project possible changes in response to climate warming. We measured emission of isoprene and photosynthetic gas exchange over two growing seasons (2005–2006) in a subarctic wetland in northern Sweden with the objective to identify the physiological and environmental controls of these fluxes on the leaf scale. The sedge species Eriophorum angustifolium and Carex rostrata were both emitters of isoprene, and springtime emissions were first detected after an accumulated diurnal mean temperature above 0°C of about 100 degree days. Maximum measured growing season standardized (basal) emission rates (20°C, 1000 μmol m 2 s 1) were 1075 (2005) and 1118 (2006) μg C m 2 (leaf area) h 1 in E. angustifolium, and 489 (2005) and 396 (2006) μg C m 2 h 1 in C. rostrata. Over the growing season, basal isoprene emission varied in response to the temperature history of the last 48 h. Seasonal basal isoprene emission rates decreased also with leaf nitrogen (N), which may be explained by the typical growth and resource allocation pattern of clonal sedges as the leaves age. The observations were used to model emissions over the growing season, accounting for effects of temperature history, links to leaf assimilation rate and the light and temperature dependencies of the cold-adapted sedges.
BVOC ecosystem flux measurements at a high latitude wetland site
T. Holst,A. Arneth,S. Hayward,A. Ekberg
Atmospheric Chemistry and Physics Discussions , 2008,
Abstract: In this study, we present summertime concentrations and fluxes of biogenic volatile organic compounds (BVOCs) measured at a sub-arctic wetland in northern Sweden using a disjunct eddy-covariance (DEC) technique based on a proton transfer reaction mass spectrometer (PTR-MS). The vegetation at the site was dominated by Sphagnum, Carex and Eriophorum spp. The performance of the DEC system was assessed by comparing H3O+-ion cluster formed with water molecules (H3O+(H2O) at m37) with water vapour concentration measurements made using an adjacent humidity sensor, and from a comparison of sensible heat fluxes for high frequency and DEC data obtained from the sonic anemometer. These analyses showed no significant PTR-MS sensor drift over a period of several weeks and only a small flux-loss due to high-frequency spectrum omissions. This loss was within the range expected from other studies and the theoretical considerations. Standardised (20°C and 1000 μmol m 2 s 1 PAR) summer isoprene emission rates of 323 μg C m 2 (ground area) h 1 were comparable with findings from more southern boreal forests, and fen-like ecosystems. On a diel scale, measured fluxes indicated a stronger temperature dependence when compared with emissions from temperate or (sub)tropical ecosystems. For the first time, to our knowledge, we report ecosystem methanol fluxes from a sub-arctic ecosystem. Maximum daytime emission fluxes were around 270 μg m 2 h 1 (ca. 100 μg C m 2 h-1) and measurements indicated some nocturnal deposition. The measurements reported here covered a period of 50 days (1 August to 19 September 2006), approximately one half of the growing season at the site, and allowed to investigate the effect of vegetation senescence on daily BVOC fluxes and on their temperature and light responses. Long-term measurements of BVOC are still lacking for nearly all ecosystems and only a very few studies about seasonal or even interannual variation of BVOC emissions have been published so far, particularly for northern ecosystems. The results presented here will be useful for testing process understanding obtained in laboratory studies and for model evaluation, improving our understanding of biogeochemical cycles in a region which is likely to be sensitive to climate change and currently undergoes rapid changes due to global warming.
Can Winter-Active Bumblebees Survive the Cold? Assessing the Cold Tolerance of Bombus terrestris audax and the Effects of Pollen Feeding
Emily L. Owen, Jeffrey S. Bale, Scott A. L. Hayward
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0080061
Abstract: There is now considerable evidence that climate change is disrupting the phenology of key pollinator species. The recently reported UK winter activity of the bumblebee Bombus terrestris brings a novel set of thermal challenges to bumblebee workers that would typically only be exposed to summer conditions. Here we assess the ability of workers to survive acute and chronic cold stress (via lower lethal temperatures and lower lethal times at 0°C), the capacity for rapid cold hardening (RCH) and the influence of diet (pollen versus nectar consumption) on supercooling points (SCP). Comparisons are made with chronic cold stress indices and SCPs in queen bumblebees. Results showed worker bees were able to survive acute temperatures likely to be experienced in a mild winter, with queens significantly more tolerant to chronic cold temperature stress. The first evidence of RCH in any Hymenoptera is shown. In addition, dietary manipulation indicated the consumption of pollen significantly increased SCP temperature. These results are discussed in the light of winter active bumblebees and climate change.
Mid-Infrared Imaging of Alpha Orionis
S. A. Rinehart,T. L. Hayward,J. R. Houck
Physics , 1998, DOI: 10.1086/305961
Abstract: We have imaged the M2Iab supergiant star Alpha Ori at 11.7 micron and 17.9 micron using the Hale 5-m telescope and an array camera with sub-arcsecond angular resolution. Our images reveal the circumstellar dust shell extending out to approximately 5" from the star. By fitting two multi-parameter models to the data, we find values for the dust shell inner radius, the temperature at this radius, the percentage flux produced by the circumstellar material at each of our wavelengths, and the radial dependence of the dust mass density. We present profiles of the mean surface brightness as a function of radius and the power detected from within annular rings as a function of radius.
Comment on "On the tunneling through the black hole horizon" [arXiv:0910.3934]
S. A. Hayward,R. Di Criscienzo,M. Nadalini,L. Vanzo,S. Zerbini
Physics , 2009,
Abstract: The arguments of the above article [arXiv:0910.3934] do not apply to the papers which it criticizes, and contain several key errors, including a fundamental misunderstanding about the equivalence principle.
Local Hawking temperature for dynamical black holes
S. A. Hayward,R. Di Criscienzo,M. Nadalini,L. Vanzo,S. Zerbini
Physics , 2008,
Abstract: A local Hawking temperature is derived for any future outer trapping horizon in spherical symmetry, using a Hamilton-Jacobi variant of the Parikh-Wilczek tunneling method. It is given by a dynamical surface gravity as defined geometrically. The operational meaning of the temperature is that Kodama observers just outside the horizon measure an invariantly redshifted temperature, diverging at the horizon itself. In static, asymptotically flat cases, the Hawking temperature as usually defined by the Killing vector agrees in standard cases, but generally differs by a relative redshift factor between the horizon and infinity, being the temperature measured by static observers at infinity. Likewise, the geometrical surface gravity reduces to the Newtonian surface gravity in the Newtonian limit, while the Killing definition instead reflects measurements at infinity. This may resolve a longstanding puzzle concerning the Hawking temperature for the extremal limit of the charged stringy black hole, namely that it is the local temperature which vanishes. In general, this confirms the quasi-stationary picture of black-hole evaporation in early stages. However, the geometrical surface gravity is generally not the surface gravity of a static black hole with the same parameters.
Reply to Comments on "Invariance of the tunneling method for dynamical black holes" arXiv:0907.2020
S. A. Hayward,R. Di Criscienzo,M. Nadalini,L. Vanzo,S. Zerbini
Physics , 2009,
Abstract: We point out basic misunderstandings about quantum field theory, general relativity and partial derivatives in the above Comments. In reply to a second comment on our first reply by the same author, we also identify precisely where the author's original calculation goes wrong and correct it, yielding the same local Hawking temperature as obtained by the Hamilton-Jacobi method.
Local temperature for dynamical black holes
Sean A. Hayward,R. Di Criscienzo,M. Nadalini,L. Vanzo,S. Zerbini
Physics , 2008, DOI: 10.1063/1.3141237
Abstract: A local Hawking temperature was recently derived for any future outer trapping horizon in spherical symmetry, using a Hamilton-Jacobi tunneling method, and is given by a dynamical surface gravity as defined geometrically. Descriptions are given of the operational meaning of the temperature, in terms of what observers measure, and its relation to the usual Hawking temperature for static black holes. Implications for the final fate of an evaporating black hole are discussed.
The Health System and Population Health Implications of Large-Scale Diabetes Screening in India: A Microsimulation Model of Alternative Approaches
Sanjay Basu?,Christopher Millett?,Sandeep Vijan?,Rodney A. Hayward,Sanjay Kinra?,Rahoul Ahuja?,John S. Yudkin
PLOS Medicine , 2015, DOI: 10.1371/journal.pmed.1001827
Abstract: Background Like a growing number of rapidly developing countries, India has begun to develop a system for large-scale community-based screening for diabetes. We sought to identify the implications of using alternative screening instruments to detect people with undiagnosed type 2 diabetes among diverse populations across India. Methods and Findings We developed and validated a microsimulation model that incorporated data from 58 studies from across the country into a nationally representative sample of Indians aged 25–65 y old. We estimated the diagnostic and health system implications of three major survey-based screening instruments and random glucometer-based screening. Of the 567 million Indians eligible for screening, depending on which of four screening approaches is utilized, between 158 and 306 million would be expected to screen as “high risk” for type 2 diabetes, and be referred for confirmatory testing. Between 26 million and 37 million of these people would be expected to meet international diagnostic criteria for diabetes, but between 126 million and 273 million would be “false positives.” The ratio of false positives to true positives varied from 3.9 (when using random glucose screening) to 8.2 (when using a survey-based screening instrument) in our model. The cost per case found would be expected to be from US$5.28 (when using random glucose screening) to US$17.06 (when using a survey-based screening instrument), presenting a total cost of between US$169 and US$567 million. The major limitation of our analysis is its dependence on published cohort studies that are unlikely fully to capture the poorest and most rural areas of the country. Because these areas are thought to have the lowest diabetes prevalence, this may result in overestimation of the efficacy and health benefits of screening. Conclusions Large-scale community-based screening is anticipated to produce a large number of false-positive results, particularly if using currently available survey-based screening instruments. Resource allocators should consider the health system burden of screening and confirmatory testing when instituting large-scale community-based screening for diabetes.
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