Nitrogen and oxygen isotope constraints on the origin of atmospheric nitrate in coastal Antarctica

Throughout the year 2001, aerosol samples were collected continuously for 10 to 15 days at the French Antarctic Station Dumont d'Urville (DDU) (66°40' S, l40°0' E, 40 m above mean sea level). The nitrogen and oxygen isotopic ratios of particulate nitrate at DDU exhibit seasonal variations that are among the most extreme observed for nitrate on Earth. In association with concentration measurements, the isotope ratios delineate four distinct periods, broadly consistent with previous studies on Antarctic coastal areas. During austral autumn and early winter (March to mid-July), nitrate concentrations attain a minimum between 10 and 30 ng m^−3 (referred to as Period 2). Two local maxima in August (55 ng m^−3) and November/December (165 ng m^−3) are used to assign Period 3 (mid-July to September) and Period 4 (October to December). Period 1 (January to March) is a transition period between the maximum concentration of Period 4 and the background concentration of Period 2. These seasonal changes are reflected in changes of the nitrogen and oxygen isotope ratios. During Period 2, which is characterized by background concentrations, the isotope ratios are in the range of previous measurements at mid-latitudes: δ¹⁸O_vsmow=(77.2±8.6)‰; Δ¹⁷O=(29.8±4.4)‰; δ¹⁵N_air=(−4.4±5.4)‰ (mean ± one standard deviation). Period 3 is accompanied by a significant increase of the oxygen isotope ratios and a small increase of the nitrogen isotope ratio to δ¹⁸O_vsmow=(98.8±13.9)‰; Δ¹⁷O=(38.8±4.7)‰ and δ¹⁵N_air=(4.3±8.20‰). Period 4 is characterized by a minimum ¹⁵N/¹⁴N ratio, only matched by one prior study of Antarctic aerosols, and oxygen isotope ratios similar to Period 2: δ¹⁸O_vsmow=(77.2±7.7)‰; Δ¹⁷O=(31.1±3.2)‰; δ¹⁵N_air=(−32.7±8.4)‰. Finally, during Period 1, isotope ratios reach minimum values for oxygen and intermediate values for nitrogen: δ¹⁸O_vsmow=63.2±2.5‰; Δ¹⁷O=24.0±1.1‰; δ¹⁵N_air=−17.9±4.0‰). Based on the measured isotopic composition, known atmospheric transport patterns and the current understanding of kinetics and isotope effects of relevant atmospheric chemical processes, we suggest that elevated tropospheric nitrate levels during Period 3 are most likely the result of nitrate sedimentation from polar stratospheric clouds (PSCs), whereas elevated nitrate levels during Period 4 are likely to result from snow re-emission of nitrogen oxide species. We are unable to attribute the source of the nitrate during periods 1 and 2 to local production or long-range transport, but note that the oxygen isotopic composition is in agreement with day and night time nitrate chemistry driven by the diurnal solar cycle. A precise quantification is difficult, due to our insufficient knowledge of isotope fractionation during the reactions leading to nitrate formation, among other reasons

Acoustic properties of turbofan inlets

The finite element codes were improved using Hermitian elements and numerical integration of element relations. The question of real variable versus complex variable formulation was resolved and an integrated civil engineering system was implemented on the Georgia Tech Cyber 70/74. Efforts are underway to restructure the program to obtain the most efficient use of array storage

Origin of Enantioselection in Chiral Alcohol Oxidation Catalyzed by Pd[(-)-sparteine]Cl2

A kinetic investigation into the origin of enantioselectivity for the Pd[(-)-sparteine]Cl2-catalyzed aerobic oxidative kinetic resolution (OKR) is reported. A mechanism to account for a newly discovered chloride dissociation from Pd[(-)-sparteine]Cl2 prior to alcohol binding is proposed. The mechanism includes (1) chloride dissociation from Pd[(-)-sparteine]Cl2 to form cationic Pd(-)-sparteine]Cl, (2) alcohol binding, (3) deprotonation of Pd-bound alcohol to form a Pd-alkoxide, and (4) â-hydride elimination of Pd-alkoxide to form ketone product and a Pd-hydride. Utilizing the addition of (-)-sparteine HCl to control the [Cl-] and [H+] and the resulting derived rate law, the key microscopic kinetic and thermodynamic constants were extracted for each enantiomer of sec-phenethyl alcohol. These constants allow for the successful simulation of the oxidation rate in the presence of exogenous (-)-sparteine HCl. A rate law for oxidation of the racemic alcohol was derived that allows for the successful prediction of the experimentally measured krel values when using the extracted constants. Besides a factor of 10 difference between the relative rates of â-hydride elimination for the enantiomers, the main enhancement in enantiodetermination results from a concentration effect of (-)-sparteine HCl and the relative rates of reprotonation of the diastereomeric Pd-alkoxides

Avoiding catastrophic failure in correlated networks of networks

Networks in nature do not act in isolation but instead exchange information, and depend on each other to function properly. An incipient theory of Networks of Networks have shown that connected random networks may very easily result in abrupt failures. This theoretical finding bares an intrinsic paradox: If natural systems organize in interconnected networks, how can they be so stable? Here we provide a solution to this conundrum, showing that the stability of a system of networks relies on the relation between the internal structure of a network and its pattern of connections to other networks. Specifically, we demonstrate that if network inter-connections are provided by hubs of the network and if there is a moderate degree of convergence of inter-network connection the systems of network are stable and robust to failure. We test this theoretical prediction in two independent experiments of functional brain networks (in task- and resting states) which show that brain networks are connected with a topology that maximizes stability according to the theory.Comment: 40 pages, 7 figure

Helping and Cooperation in Children with Autism

Helping and cooperation are central to human social life. Here, we report two studies investigating these social behaviors in children with autism and children with developmental delay. In the first study, both groups of children helped the experimenter attain her goals. In the second study, both groups of children cooperated with an adult, but fewer children with autism performed the tasks successfully. When the adult stopped interacting at a certain moment, children with autism produced fewer attempts to re-engage her, possibly indicating that they had not formed a shared goal/shared intentions with her. These results are discussed in terms of the prerequisite cognitive and motivational skills and propensities underlying social behavior

Relative reactivity of alkenyl alcohols in the palladium-catalyzed redox-relay Heck reaction

The relative rates of alkenyl alcohols in the Pd-catalyzed redox-relay Heck reaction were measured in order to examine the effect of their steric and electronic properties on the rate-determining step. Competition experiments between an allylic alkenyl alcohol and two substrates with differing chain lengths revealed that the allylic alcohol reacts 3–4 times faster in either case. Competition between di- and trisubstituted alkenyl alcohols provided an interesting scenario, in which the disubstituted alkene was consumed first followed by reaction of the trisubstituted alkene. Consistent with this observation, the transition structures for the migratory insertion of the aryl group into the di- and trisubstituted alkenes were calculated with a lower barrier for the former. An internal competition between a substrate containing two alcohols with differing chain lengths demonstrated the catalyst's preference for migrating toward the closest alcohol. Additionally, it was observed that increasing the electron-density in the arene boronic acid promotes a faster reaction, which correlates with Hammett [sigma-rho] values to give a [rho] of −0.87

The Early Sociocognitive Battery: a clinical tool for early identification of children at risk for social communication difficulties and ASD?

BACKGROUND: A substantial proportion of preschool children referred to speech and language therapy (SLT) services have social communication difficulties and/or autistic spectrum disorders (SC&/ASD) that are not identified until late childhood. These 'late' diagnosed children miss opportunities to benefit from earlier targeted interventions. Prior evidence from a follow-up clinical sample showed that preschool performance on the Early Sociocognitive Battery (ESB) was a good predictor of children with social communication difficulties 7-8 years later. AIMS: The aims were three-fold: (1) to determine the impact of child/demographic factors on ESB performance in a community sample of young children; (2) to assess the ESB's concurrent validity and test-retest reliability; and (3) to use cut-offs for 'low' ESB performance derived from the community sample data to evaluate in a clinical sample the predictiveness of the ESB at 2-4 years for outcomes at 9-11 years, including parent-reported SC&/ASD diagnosis. METHODS & PROCEDURES: A community sample of 205 children aged 2-4 years was assessed on the ESB and a receptive vocabulary test. A subsample (n = 20) was retested on the ESB within 2 weeks. Parents completed a questionnaire providing background child/demographic information. The clinical sample from our previous study comprised 93 children assessed on the ESB at 2;6 to < 4;0 whose parents completed the Social Responsiveness Scale (SRS), our measure of social communication, when the children were 9-11 years. Cut-offs for 'low' ESB performance derived from the community sample were used to determine the predictive validity of 'low' ESB scores for social communication outcomes and parent-reported SC&/ASD diagnosis according to age of ESB assessment. OUTCOME & RESULTS: Findings from the community sample confirmed the ESB as psychometrically robust, sensitive to age and language delay, and, in contrast to the receptive vocabulary measure, unaffected by bilingualism. While overall associations between ESB performance and later social communication difficulties in the clinical sample were particularly strong for the youngest age group (2;6 to < 3;0; r = .71, p < .001), 'low' ESB performance was equally predictive across age groups and overall identified 89% of children with 'late' SC&/ASD diagnoses (sensitivity), and 75% of those without (specificity). CONCLUSIONS & IMPLICATIONS: Results indicate that the ESB is a valid preschool assessment suitable for use with children from diverse language backgrounds. It identifies deficits in key sociocognitive skills and is predictive of social communication difficulties in school-age children that had not been identified in preschool clinical assessment, supporting earlier targeted interventions for these children

The Hugging Team: The Role of Technology in Business Networking Practices

Abstract. Technological devices for social networking are produced in droves and networking through media seems to be the way of getting ahead in business. We examine what role technology plays in the creation, development and maintenance of business relationships among entrepreneurs in Copenhagen. We find that mediated communication is useful in all stages of relational maintenance but only in a supportive role in relational development where co-presence and shared personal experiences take center-stage, generating trust necessary for business relationships to work. These trust-developing experiences take effort and hard work and although they can be successfully supported and even facilitated through the use of communication technologies, they need not be replaced or made simpler. The difficulties of creating these experiences make working business relationships viable in the uncertain and risky world of entrepreneurship