Determination of Equilibrium Constants for the Reaction between Acetone and HO_2 Using Infrared Kinetic Spectroscopy

The reaction between the hydroperoxy radical, HO_2, and acetone may play an important role in acetone removal and the budget of HO_x radicals in the upper troposphere. We measured the equilibrium constants of this reaction over the temperature range of 215–272 K at an overall pressure of 100 Torr using a flow tube apparatus and laser flash photolysis to produce HO_2. The HO_2 concentration was monitored as a function of time by near-IR diode laser wavelength modulation spectroscopy. The resulting [HO_2] decay curves in the presence of acetone are characterized by an immediate decrease in initial [HO_2] followed by subsequent decay. These curves are interpreted as a rapid (<100 μs) equilibrium reaction between acetone and the HO_2 radical that occurs on time scales faster than the time resolution of the apparatus, followed by subsequent reactions. This separation of time scales between the initial equilibrium and ensuing reactions enabled the determination of the equilibrium constant with values ranging from 4.0 × 10^(–16) to 7.7 × 10^(–1)8 cm^3 molecule^(–1) for T = 215–272 K. Thermodynamic parameters for the reaction determined from a second-law fit of our van’t Hoff plot were Δ_(r)H°_(245) = −35.4 ± 2.0 kJ mol^(–1) and Δ_(r)S°_(245) = −88.2 ± 8.5 J mol^(–1) K^(–1). Recent ab initio calculations predict that the reaction proceeds through a prereactive hydrogen-bonded molecular complex (HO_2–acetone) with subsequent isomerization to a hydroxy–peroxy radical, 2-hydroxyisopropylperoxy (2-HIPP). The calculations differ greatly in the energetics of the complex and the peroxy radical, as well as the transition state for isomerization, leading to significant differences in their predictions of the extent of this reaction at tropospheric temperatures. The current results are consistent with equilibrium formation of the hydrogen-bonded molecular complex on a short time scale (100 μs). Formation of the hydrogen-bonded complex will have a negligible impact on the atmosphere. However, the complex could subsequently isomerize to form the 2-HIPP radical on longer time scales. Further experimental studies are needed to assess the ultimate impact of the reaction of HO_2 and acetone on the atmosphere

Dissipative and Dispersive Optomechanics in a Nanocavity Torque Sensor

Dissipative and dispersive optomechanical couplings are experimentally observed in a photonic crystal split-beam nanocavity optimized for detecting nanoscale sources of torque. Dissipative coupling of up to approximately $500$ MHz/nm and dispersive coupling of $2$ GHz/nm enable measurements of sub-pg torsional and cantilever-like mechanical resonances with a thermally-limited torque detection sensitivity of 1.2$\times 10^{-20} \text{N} \, \text{m}/\sqrt{\text{Hz}}$ in ambient conditions and 1.3$\times 10^{-21} \text{N} \, \text{m}/\sqrt{\text{Hz}}$ in low vacuum. Interference between optomechanical coupling mechanisms is observed to enhance detection sensitivity and generate a mechanical-mode-dependent optomechanical wavelength response.Comment: 11 pages, 6 figure

Confronting the US's Highly Uneven State-by-State Distribution of Diagnostic Radiology Residency Positions

The state-by-state distribution of diagnostic radiology residency positions is important for several reasons. The location in which residents train is positively correlated with their eventual practice location, and states with relatively few residents may have greater difficulty meeting needs for radiology services ( 1 ). The presence of residents can also enrich the educational experiences of other learners such as medical students, but this is less likely to happen when residents are in short supply ( 2 ). Finally, because residency training programs can promote a culture of inquiry, a dearth of residents may undermine intellectual engagement within a professional community ( 3 )

Herbivory promotes dental disparification and macroevolutionary dynamics in grunters (teleostei: Terapontidae), a freshwater adaptive radiation

Trophic shifts into new adaptive zones have played major (although often conflicting) roles in reshaping the evolutionary trajectories of many lineages. We analyze data on diet, tooth, and oral morphology and relate these traits to phenotypic disparification and lineage diversification rates across the ecologically diverse Terapontidae, a family of Australasian fishes. In contrast to carnivores and most omnivores, which have retained relatively simple, ancestral caniniform tooth shapes, herbivorous terapontids appear to have evolved a variety of novel tooth shapes at significantly faster rates to meet the demands of plant-based diets. The evolution of herbivory prompted major disparification, significantly expanding the terapontid adaptive phenotypic continuum into an entirely novel functional morphospace. There was minimal support for our hypothesis of faster overall rates of integrated tooth shape, spacing, and jaw biomechanical evolution in herbivorous terapontids in their entirety, compared with other trophic strategies. There was, however, considerable support for accelerated disparification within a diverse freshwater clade containing a range of specialized freshwater herbivores. While the evolutionary transition to herbivorous diets has played a central role in terapontid phenotypic diversification by pushing herbivores toward novel fitness peaks, there was little support for herbivory driving significantly higher lineage diversification compared with background rates across the family

Quantifying cosmic variance

We determine an expression for the cosmic variance of any "normal" galaxy survey based on examination of M* +/- 1 mag galaxies in the SDSS DR7 data cube. We find that cosmic variance will depend on a number of factors principally: total survey volume, survey aspect ratio, and whether the area surveyed is contiguous or comprised of independent sight-lines. As a rule of thumb cosmic variance falls below 10% once a volume of 10^7h_0.7^-3Mpc^3 is surveyed for a single contiguous region with a 1:1 aspect ratio. Cosmic variance will be lower for higher aspect ratios and/or non-contiguous surveys. Extrapolating outside our test region we infer that cosmic variance in the entire SDSS DR7 main survey region is ~7% to z < 0.1. The equation obtained from the SDSS DR7 region can be generalised to estimate the cosmic variance for any density measurement determined from normal galaxies (e.g., luminosity densities, stellar mass densities and cosmic star-formation rates) within the volume range 10^3 to 10^7 h^-3_0.7Mpc^3. We apply our equation to show that 2 sightlines are required to ensure cosmic variance is <10% in any ASKAP galaxy survey (divided into dz ~0.1 intervals, i.e., ~1 Gyr intervals for z <0.5). Likewise 10 MeerKAT sightlines will be required to meet the same conditions. GAMA, VVDS, and zCOSMOS all suffer less than 10% cosmic variance (~3%-8%) in dz intervals of 0.1, 0.25, and 0.5 respectively. Finally we show that cosmic variance is potentially at the 50-70% level, or greater, in the HST Ultra Deep Field depending on assumptions as to the evolution of clustering. 100 or 10 independent sightlines will be required to reduce cosmic variance to a manageable level (<10%) for HST ACS or HST WFC3 surveys respectively (in dz ~ 1 intervals). Cosmic variance is therefore a significant factor in the z>6 HST studies currently underway.Comment: Accepted for publication in MNRA

A Composite Seyfert 2 X-ray Spectrum: Implications for the Origin of the Cosmic X-ray Background

We present a composite 1-10 keV Seyfert 2 X-ray spectrum, derived from ASCA observations of a distance-limited sample of nearby galaxies. All 29 observed objects were detected. Above ~3 keV, the composite spectrum is inverted, confirming that Seyfert 2 galaxies as a class have the spectral properties necessary to explain the flat shape of the cosmic X-ray background spectrum. Integrating the composite spectrum over redshift, we find that the total emission from Seyfert 2 galaxies, combined with the expected contribution from unabsorbed type 1 objects, provides an excellent match to the spectrum and intensity of the hard X-ray background. The principal uncertainty in this procedure is the cosmic evolution of the Seyfert 2 X-ray luminosity function. Separate composite spectra for objects in our sample with and without polarized broad optical emission lines are also presented.Comment: 11 pages (AASTeX), including 3 figures. Accepted for publication in ApJ Letter

The Power of Community: The Role of Community-Based Organizations in Mitigating COVID-19-Related Impacts on Well-Being Among South Florida's Minoritized Populations

The rapid closure of schools and businesses during the COVID-19 pandemic has had a significant impact in South Florida, highlighting diverse challenges to community well-being. Community-based organizations (CBOs) have served as a source of support for community members, particularly in stressful times. Using a combination of quantitative and qualitative methods, and in collaboration with Breakthrough Miami, a CBO committed to addressing the educational opportunity gap in Miami-Dade County, we conducted a study to assess challenges to the well-being of systematically minoritized families during the heart of the COVID-19 pandemic. The study included longitudinal survey data collected via ecological momentary assessment and in-depth interviews with Breakthrough Miami families to gather richer and nuanced data regarding indicators of well-being and CBO engagement. Quantitative results indicated that as perceived stress increased over time, so too did emotional and informational support. Follow-up qualitative interviews indicated that 100% of families identified Breakthrough Miami as a source of academic support during the pandemic. Additionally, families cited the important role Breakthrough Miami played beyond their standard academic programs and services in coping with the most significant sources of stress reported through the pandemic: meeting basic needs and threats to socioemotional well-being. Implications for CBOs are addressed

The Power of Community: The Role of Community-Based Organizations in Mitigating COVID-19-Related Impacts on Well-Being Among South Florida's Minoritized Populations

The rapid closure of schools and businesses during the COVID-19 pandemic has had a significant impact in South Florida, highlighting diverse challenges to community well-being. Community-based organizations (CBOs) have served as a source of support for community members, particularly in stressful times. Using a combination of quantitative and qualitative methods, and in collaboration with Breakthrough Miami, a CBO committed to addressing the educational opportunity gap in Miami-Dade County, we conducted a study to assess challenges to the well-being of systematically minoritized families during the heart of the COVID-19 pandemic. The study included longitudinal survey data collected via ecological momentary assessment and in-depth interviews with Breakthrough Miami families to gather richer and nuanced data regarding indicators of well-being and CBO engagement. Quantitative results indicated that as perceived stress increased over time, so too did emotional and informational support. Follow-up qualitative interviews indicated that 100% of families identified Breakthrough Miami as a source of academic support during the pandemic. Additionally, families cited the important role Breakthrough Miami played beyond their standard academic programs and services in coping with the most significant sources of stress reported through the pandemic: meeting basic needs and threats to socioemotional well-being. Implications for CBOs are addressed

Arabica-like flavour in a heat tolerant wild coffee species

There are numerous factors to consider when developing climate resilient coffee crops, including the ability to tolerate altered climatic conditions, meet agronomic and value chain criteria, and satisfy consumer preferences for flavour (aroma and taste). We evaluated the sensory characteristics and key environmental requirements for the enigmatic narrow-leaved coffee (Coffea stenophylla), a wild species from Upper West Africa1. We confirm historical reports of a superior flavour1-3, and uniquely and remarkably, reveal a sensory profile analogous to high quality Arabica coffee. We demonstrate that this species grows and crops under the same range of key climatic conditions as (sensorially inferior) robusta and Liberica coffee4-9, and has a mean annual temperature 6.2–6.8⁰C higher than Arabica coffee, even under equivalent rainfall conditions. This species substantially broadens the climate envelope for high quality coffee, and could provide an important resource for the development of climate resilient coffee crop plants