skip to main content


Search for: All records

Creators/Authors contains: "Dennis, L."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. A molecular catalyst attached to an electrode sur-face can in principle offer the advantages of both homogeneous and heterogeneous catalysis. Unfortunately, some molecular catalysts constrained to a surface lose much or all of their solution performance. In contrast, we have found that when a small molecule [2Fe–2S] catalyst is incorporated into metallopolymers of the form PDMAEMA–g–[2Fe–2S] (PDMAEMA = poly(2-dimethylamino)ethyl methacrylate) and adsorbed to the sur-face, the observed rate of hydrogen production increases to kobs > 105 s-1 per active site with lower overpotential, increased life-time, and tolerance to oxygen. Herein, the electrocatalytic performances of these metallopolymers with different length polymer chains are compared to reveal the factors that lead to this high performance. It was anticipated that smaller metallopolymers would have faster rates due to faster electron and proton transfers to more accessible active sites, but the experiments show that the rates of catalysis per active site are largely independent of the polymer size. Molecular dynamics modelling reveals that the high performance is a consequence of adsorption of these metallopolymers on the surface with natural assembly that brings the [2Fe–2S] catalytic sites into close contact with the electrode surface while maintaining exposure of the sites to protons in solution. The assembly is conducive to fast electron transfer, fast proton transfer, and a high rate of catalysis regardless of polymer size. These results offer a guide to enhancing the performance of other electrocatalysts with incorporation into a polymer that provides optimal interaction of the catalyst with the electrode and with solution. 
    more » « less
  2. Abstract

    Tectonic processes control hot spring temperature and geochemistry, yet how this in turn shapes microbial community composition is poorly understood. Here, we present geochemical and 16 S rRNA gene sequencing data from 14 hot springs from contrasting styles of subduction along a convergent margin in the Peruvian Andes. We find that tectonic influence on hot spring temperature and geochemistry shapes microbial community composition. Hot springs in the flat-slab and back-arc regions of the subduction system had similar pH but differed in geochemistry and microbiology, with significant relationships between microbial community composition, geochemistry, and geologic setting. Flat-slab hot springs were chemically heterogeneous, had modest surface temperatures (up to 45 °C), and were dominated by members of the metabolically diverse phylum Proteobacteria. Whereas, back-arc hot springs were geochemically more homogenous, exhibited high concentrations of dissolved metals and gases, had higher surface temperatures (up to 81 °C), and host thermophilic archaeal and bacterial lineages.

     
    more » « less
  3. As one part of an NSF-sponsored Data Science Fellowship at Cal Poly, San Luis Obispo, a group of faculty offered a unique one-unit quarter-long seminar on the history of ideas behind the core principles of Data Science. We present an overview of this seminar, its learning objectives, and outcomes and lessons learned. 
    more » « less
  4. Since about 1980, the tropical Pacific has been anomalously cold, while the broader tropics have warmed. This has caused anomalous weather in midlatitudes as well as a reduction in the apparent sensitivity of the climate associated with enhanced low-cloud abundance over the cooler waters of the eastern tropical Pacific. Recent modeling work has shown that cooler temperatures over the Southern Ocean around Antarctica can lead to cooler temperatures over the eastern tropical Pacific. Here we suggest that surface wind anomalies associated with the Antarctic ozone hole can cause cooler temperatures over the Southern Ocean that extend into the tropics. We use the short-term variability of the Southern Annular Mode of zonal wind variability to show an association between surface zonal wind variations over the Southern Ocean, cooling over the Southern Ocean, and cooling in the eastern tropical Pacific. This suggests that the cooling of the eastern tropical Pacific may be associated with the onset of the Antarctic ozone hole. 
    more » « less
  5. Abstract The vertical profile of clear-sky radiative cooling places important constraints on the vertical structure of convection and associated clouds. Simple theory using the cooling-to-space approximation is presented to indicate that the cooling rate in the upper troposphere should increase with surface temperature. The theory predicts how the cooling rate depends on lapse rate in an atmosphere where relative humidity remains approximately a fixed function of temperature. Radiative cooling rate is insensitive to relative humidity because of cancellation between the emission and transmission of radiation by water vapor. This theory is tested with one-dimensional radiative transfer calculations and radiative-convective equilibrium simulations. For climate simulations that produce an approximately moist adiabatic lapse rate, the radiative cooling profile becomes increasingly top-heavy with increasing surface temperature. If the temperature profile warms more slowly than a moist adiabatic profile in mid-troposphere, then the cooling rate in the upper troposphere is reduced and that in the lower troposphere is increased. This has important implications for convection, clouds and associated deep and shallow circulations. 
    more » « less
  6. Abstract Satellite observations of tropical maritime convection indicate an afternoon maximum in anvil cloud fraction that cannot be explained by the diurnal cycle of deep convection peaking at night. We use idealized cloud-resolving model simulations of single anvil cloud evolution pathways, initialized at different times of the day, to show that tropical anvil clouds formed during the day are more widespread and longer lasting than those formed at night. This diurnal difference is caused by shortwave radiative heating, which lofts and spreads anvil clouds via a mesoscale circulation that is largely absent at night, when a different, longwave-driven circulation dominates. The nighttime circulation entrains dry environmental air that erodes cloud top and shortens anvil lifetime. Increased ice nucleation in more turbulent nighttime conditions supported by the longwave cloud-top cooling and cloud-base heating dipole cannot compensate for the effect of diurnal shortwave radiative heating. Radiative–convective equilibrium simulations with a realistic diurnal cycle of insolation confirm the crucial role of shortwave heating in lofting and sustaining anvil clouds. The shortwave-driven mesoscale ascent leads to daytime anvils with larger ice crystal size, number concentration, and water content at cloud top than their nighttime counterparts. Significance Statement Deep convective activity and rainfall peak at night over the tropical oceans. However, anvil clouds that originate from the tops of deep convective clouds reach their largest extent in the afternoon hours. We study the underlying physical mechanisms that lead to this discrepancy by simulating the evolution of anvil clouds with a high-resolution model. We find that the absorption of sunlight by ice crystals lofts and spreads the daytime anvil clouds over a larger area, increasing their lifetime, changing their properties, and thus influencing their impact on climate. Our findings show that it is important not only to simulate the correct onset of deep convection but also to correctly represent anvil cloud evolution for skillful simulations of the tropical energy balance. 
    more » « less
  7. Las perspectivas paleoetnobotánicas son esenciales para comprender las formas de vida pasadas, pero siguen estando infrarrepresentadas en investigaciónes Paleoindios. Presentamos nuevos datos arqueobotánicos y de radiocarbono de características de combustión dentro de componentes culturales estratificados en las Cuevas Connley ( Connley Caves ), Oregon, que reafirman la inclusión de plantas en la dieta de grupos Paleoindios. Restos botánicos de tres rascos en Cueva 5 de las Cuevas Connley muestran que las personas buscaban diversos taxones de tierras secas y una gama limitada de plantas de humedales durante los meses de verano y otoño. Estos datos añaden nuevos taxones a los conocidos de la economía alimentaria del Pleistoceno y apoyan la idea de que los grupos equipados con útiles de tradiciones Western Stemmed tenían dietas amplias y flexibles. Cuando se ve a escala continental, este trabajo contribuye a un creciente cuerpo de investigación lo que indica que las estrategias de subsistencia adaptadas regionalmente estaban en su lugar al menos durante el período Younger Dryas y que algunos recolectores en el Lejano Oeste pueden haber incorporado una gama más amplia de plantas, incluidas semillas pequeñas, verduras de hoja verde, frutas, cactus, y geófitos en sus dietas antes que otros grupos Paleoindios en otras partes de Norte América. Apariciónes más frecuentes de varios recursos diversos y percibidos de ser menos importantes en la economía vegetal emergente de Paleoindio sugiere que es posible que necesitemos explorar una variedad de variables nutricionales para explicar ciertos aspectos del comportamiento de recolectores de épocas tempranos. 
    more » « less