Increase in hospital admissions for acute childhood asthma in Cape Town, 1978 - 1990

To determine whether hospital admissions for acute childhood asthma were rising in Cape Town in line with the experience of other countries, Red Cross War Memorial Children's Hospital's records for the period 1978 - 1990 were analysed. These were compared with total admissions for non-surgical causes and lower respiratory tract illness as well as those for bronchiolitis and pneumonia. Asthma admissions showed a sharp upward trend from 1978 to 1984, a slower rise through 1987 and a levelling off since. The profile of hospital admissions for respiratory illness was also analysed. Black children were under-represented among asthma admissions compared with those for pneumonia. Asthma admissions occurred throughout the year but showed seasonal peaks in May and November. Reasons for these trends and patterns are discussed, as well as hypotheses for further research into the epidemiology of asthma in South Africa

Numerical test of the damping time of layer-by-layer growth on stochastic models

We perform Monte Carlo simulations on stochastic models such as the Wolf-Villain (WV) model and the Family model in a modified version to measure mean separation $\ell$ between islands in submonolayer regime and damping time $\tilde t$ of layer-by-layer growth oscillations on one dimension. The stochastic models are modified, allowing diffusion within interval $r$ upon deposited. It is found numerically that the mean separation and the damping time depend on the diffusion interval $r$, leading to that the damping time is related to the mean separation as ${\tilde t} \sim \ell^{4/3}$ for the WV model and ${\tilde t} \sim \ell^2$ for the Family model. The numerical results are in excellent agreement with recent theoretical predictions.Comment: 4 pages, source LaTeX file and 5 PS figure

Growth of Patterned Surfaces

During epitaxial crystal growth a pattern that has initially been imprinted on a surface approximately reproduces itself after the deposition of an integer number of monolayers. Computer simulations of the one-dimensional case show that the quality of reproduction decays exponentially with a characteristic time which is linear in the activation energy of surface diffusion. We argue that this life time of a pattern is optimized, if the characteristic feature size of the pattern is larger than $(D/F)^{1/(d+2)}$, where $D$ is the surface diffusion constant, $F$ the deposition rate and $d$ the surface dimension.Comment: 4 pages, 4 figures, uses psfig; to appear in Phys. Rev. Let

Parameterizing anisotropic reflectance of snow surfaces from airborne digital camera observations in Antarctica

The surface reflection of solar radiation comprises an important boundary condition for solar radiative transfer simulations. In polar regions above snow surfaces, the surface reflection is particularly anisotropic due to low Sun elevations and the highly anisotropic scattering phase function of the snow crystals. The characterization of this surface reflection anisotropy is essential for satellite remote sensing over both the Arctic and Antarctica. To quantify the angular snow reflection properties, the hemispherical-directional reflectance factor (HDRF) of snow surfaces was derived from airborne measurements in Antarctica during austral summer in 2013/14. For this purpose, a digital 180∘ fish-eye camera (green channel, 490–585 nm wavelength band) was used. The HDRF was measured for different surface roughness conditions, optical-equivalent snow grain sizes, and solar zenith angles. The airborne observations covered an area of around 1000 km × 1000 km in the vicinity of Kohnen Station (75∘0′ S, 0∘4′ E) at the outer part of the East Antarctic Plateau. The observations include regions with higher (coastal areas) and lower (inner Antarctica) precipitation amounts and frequencies. The digital camera provided upward, angular-dependent radiance measurements from the lower hemisphere. The comparison of the measured HDRF derived for smooth and rough snow surfaces (sastrugi) showed significant differences, which are superimposed on the diurnal cycle. By inverting a semi-empirical kernel-driven bidirectional reflectance distribution function (BRDF) model, the measured HDRF of snow surfaces was parameterized as a function of solar zenith angle, surface roughness, and optical-equivalent snow grain size. This allows a direct comparison of the HDRF measurements with the BRDF derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite product MCD43. For the analyzed cases, MODIS observations (545–565 nm wavelength band) generally underestimated the anisotropy of the surface reflection. The largest deviations were found for the volumetric model weight fvol (average underestimation by a factor of 10). These deviations are likely linked to short-term changes in snow properties

Modulation of Neuronal Signal Transduction Systems by Extracellular ATP

The secretion of ATP by stimulated nerves is well documented. Following repetitive stimulation, extracellular ATP at the synapse can accumulate to levels estimated to be well over 100 Μ M. The present study examined the effects of extracellular ATP in the concentration range of 0.1–1.0 m M on second-messenger-generating systems in cultured neural cells of the clones NG108-15 and NIE-115. Cells in a medium mimicking the physiological extracellular environment were used to measure 45 Ca 2+ uptake, changes in free intracellular Ca 2+ levels by the probes aequorin and Quin-2, de novo generation of cyclic GMP and cyclic AMP from intracellular GTP and ATP pools prelabeled with [ 3 H]guanosine and [ 3 H]adenine, respectively, and phosphoinositide metabolism in cells preloaded with [ 3 H]inositol and assayed in the presence of LiCI. Extracelluar ATP induced a concentration-dependent increase of 45 Ca 2+ uptake by intact cells, which was additive with the uptake induced by K + depolarization. The increased uptake involved elevation of intracellular free Ca 2+ ions, evidenced by measuring aequorin and Quin-2 signals. At the same concentration range (0.1–1.0 m M ), extracellular ATP induced an increase in [ 3 H]cyclic GMP formation, and a decrease in prostaglandin E 1 -stimulated [ 3 H]cyclic AMP generation. In addition, extracellular ATP (1 m M ) caused a large (15-fold) increase in [ 3 H]inositol phosphates accumulation, and this effect was blocked by including La 3+ ions in the assay medium. In parallel experiments, we found in NG 108–15 cells surface protein phosphorylation activity that had an apparent K m for extracellular ATP at the same concentration required to produce half-maximal effects on Ca 2+ uptake. Extracellular ATP at concentrations that can be produced in the synaptic cleft by repetitive stimulation but not during routine transmission can thus initiate a unique chain of events, which may play a role in the induction of long-term adaptive changes in neuronal function.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65952/1/j.1471-4159.1988.tb13263.x.pd

Cloud phase identification of Arctic boundary-layer clouds from airborne spectral reflection measurements: Test of three approaches

Arctic boundary-layer clouds were investigated with remote sensing and in situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. The clouds formed in a cold air outbreak over the open Greenland Sea. Beside the predominant mixed-phase clouds pure liquid water and ice clouds were observed. Utilizing measurements of solar radiation reflected by the clouds three methods to retrieve the thermodynamic phase of the cloud are introduced and compared. Two ice indices IS and IP were obtained by analyzing the spectral pattern of the cloud top reflectance in the near infrared (1500–1800 nm wavelength) spectral range which is characterized by ice and water absorption. While IS analyzes the spectral slope of the reflectance in this wavelength range, IS utilizes a principle component analysis (PCA) of the spectral reflectance. A third ice index IA is based on the different side scattering of spherical liquid water particles and nonspherical ice crystals which was recorded in simultaneous measurements of spectral cloud albedo and reflectance. Radiative transfer simulations show that IS, IP and IA range between 5 to 80, 0 to 8 and 1 to 1.25 respectively with lowest values indicating pure liquid water clouds and highest values pure ice clouds. The spectral slope ice index IS and the PCA ice index IP are found to be strongly sensitive to the effective diameter of the ice crystals present in the cloud. Therefore, the identification of mixed-phase clouds requires a priori knowledge of the ice crystal dimension. The reflectance-albedo ice index IA is mainly dominated by the uppermost cloud layer (τ<1.5). Therefore, typical boundary-layer mixed-phase clouds with a liquid cloud top layer will be identified as pure liquid water clouds. All three methods were applied to measurements above a cloud field observed during ASTAR 2007. The comparison with independent in situ microphysical measurements shows the ability of the three approaches to identify the ice phase in Arctic boundary-layer clouds

Rigid Singularity Theorem in Globally Hyperbolic Spacetimes

We show the rigid singularity theorem, that is, a globally hyperbolic spacetime satisfying the strong energy condition and containing past trapped sets, either is timelike geodesically incomplete or splits isometrically as space $\times$ time. This result is related to Yau's Lorentzian splitting conjecture.Comment: 3 pages, uses revtex.sty, to appear in Physical Review

Solitonic-exchange mechanism of surface~diffusion

We study surface diffusion in the framework of a generalized Frenkel-Kontorova model with a nonconvex transverse degree of freedom. The model describes a lattice of atoms with a given concentration interacting by Morse-type forces, the lattice being subjected to a two-dimensional substrate potential which is periodic in one direction and nonconvex (Morse) in the transverse direction. The results are used to describe the complicated exchange-mediated diffusion mechanism recently observed in MD simulations [J.E. Black and Zeng-Ju Tian, Phys. Rev. Lett. {\bf 71}, 2445-2448(1993)].Comment: 22 Revtex pages, 9 figures to appear in Phys. Rev.