J. C. Ruiz-Suárez

Short-term ozone forecasting by artificial neural networks

Abstract In this work we report preliminary results of a study aiming to develop an intelligent tool for performing ozone forecasting in the polluted atmosphere of Mexico City. This tool is based in the paradigm of neural networks. Two neural models are used in this work, namely, the Bidirectional Associative Memory (BAM) and the Holographic Associative Memory (HAM). We analyse and preprocess daily patterns of meteorological variables and concentrations of pollutants as measured by five monitoring stations in Mexico City. These patterns are used to train both neural networks and then we use them to predict ozone at one point in the city. Preliminary results are reported and some conclusions are drawn.

Vibration-induced granular segregation: a phenomenon driven by three mechanisms.

The segregation of large spheres in a granular bed under vertical vibrations is studied. In our experiments, we systematically measure rise times as a function of density, diameter, and depth, for two different sinusoidal excitations. The measurements reveal that, at low frequencies, inertia and convection are the only mechanisms behind segregation. Inertia (convection) dominates when the relative density is greater (less) than one. At high frequencies, where convection is suppressed, fluidization of the granular bed causes either buoyancy or sinkage and segregation occurs.

Two‐ and three‐body forces in the interaction of He atoms with Xe overlayers adsorbed on (0001) graphite

In order to address the problem of three‐body interactions in gas–surface scattering, we considered the collision of a He atom with the (0001) surface of graphite coated by a monolayer of Xe. To eliminate the uncertainties connected with errors in the two‐body He–Xe interaction, we determined the latter by crossed‐beam differential collision cross‐section measurements performed at two energies (67.2 and 22.35 meV). These scattering data together with room‐temperature bulk diffusion data are then fitted with a Hartree–Fock–dispersion–type function to yield an interaction potential that explains most of the properties of this system within the experimental errors and represents an improvement on previously published He–Xe potentials. Helium diffraction measurements are then carried out from the Xe overlayer and the dependence of the specular intensity from the angle of incidence is carefully determined. Further, a He–surface potential is constructed by adding together the following terms: (1) the He–Xe pair...

Inertia in the Brazil nut problem

The rise dynamics of a large particle, in a granular bed under vertical vibrations, is experimentally studied with an inductive device designed to track the particle while it climbs through the granulate under different conditions. A model based on energy considerations is presented to explain our experimental data, drawing the important conclusion that it is the inertia of the particle, assisted by Reynolds dilatancy, the driven force behind its ascension mechanism. The ascension reveals a friction profile within the column which remains unchanged for different accelerations.

Sensitivity analysis of a UV radiation transfer model and experimental photolysis rates of NO2 in the atmosphere of Mexico City

Abstract Photolysis of key species such as nitrogen dioxide, ozone and aldehydes, are the elementary reactions leading to the formation of photochemical smog. Calculations and experimental measurements (in particular, nitrogen dioxide) of these key reactions are reported, as well as, the sensitivity of them to the ozone column, local albedo and properties of the urban aerosol layer in Mexico City. A radiation transfer model (RTM ), based on the delta-Eddington approximation, was used in the calculations. The results show the importance of providing local resolution for photolysis rates by considering different local conditions within the modeling domain of air quality models.

Solar absorptance and thermal emittance of cermets with large particles

We evaluate, theoretically, the solar absorptance and thermal emittance of cermets with large particles. In particular, we consider Co and Ni particles with radii from 0.05 to 0.13 µm, embedded in binders of alumina and silica, respectively. For these particle sizes, it is necessary to consider the effect of the multiple scattering of light in the material for the visible and near infrared parts of the spectrum. The response of the particles is calculated using the Lorenz-Mie theory, and the multiple scattering effects are taken into account through a four-flux radiative transfer model. In the medium infrared, the Lorenz-Mie theory cannot be used because of the high absorption of the binder. The reflectance of the cermets in this regime is calculated using the Maxwell-Garnett effective medium approximation. In general, moderate values of the absorptance-to-emittance ratio are found.

Average path-length parameter of diffuse light in scattering media

We use Monte Carlo simulations to study in detail the propagation of light in a plane-parallel medium containing scattering particles. In particular, we compute the forward and backward average path-length parameters (FAPP and BAPP, respectively) of four-flux radiative transfer models as functions of the optical depth. Strong dependence on the single scattering albedo and phase function asymmetry is found for both quantities. In general the values of the FAPP decrease with increasing absorption, whereas the opposite occurs for the BAPP. A similar effect is produced when changing from isotropic phase functions to phase functions with a large asymmetry in the forward direction. We present analytical results for the asymptotic values of the FAPP and BAPP as functions of albedo for the particular case of isotropic scattering. Our results differ markedly from the predictions obtained recently with two multiple-scattering models by Vargas and Niklasson [J. Opt. Soc. Am. A 14, 2243 (1997); Appl. Opt. 36, 3735 (1997)]. The differences found point out the intrinsic limitations of these models.

Phase transition in liquid drop fragmentation

A liquid droplet is fragmented by a sudden pressurized-gas blow, and the resulting droplets, adhered to the window of a flatbed scanner, are counted and sized by computerized means. The use of a scanner plus image recognition software enables us to automatically count and size up to tens of thousands of tiny droplets with a smallest detectable volume of approximately 0.02 nl . Upon varying the gas pressure, a critical value is found where the size distribution becomes a pure power law, a fact that is indicative of a phase transition. Away from this transition, the resulting size distributions are well described by Fishers model at coexistence. It is found that the sign of the surface correction term changes sign, and the apparent power-law exponent tau has a steep minimum, at criticality, as previously reported in nuclear multifragmentation studies. We argue that the observed transition is not percolative, and introduce the concept of dominance in order to characterize it. The dominance probability is found to go to zero sharply at the transition. Simple arguments suggest that the correlation length exponent is nu=1/2 . The sizes of the largest and average fragments, on the other hand, do not go to zero abruptly but behave in a way that appears to be consistent with recent predictions of Ashurst and Holian.

Spectral selectivity of cermets with large metallic inclusions

In order to address the problem of the optical reflectance of cermets beyond the quasistatic dipolar limit, we consider Monte Carlo multiple scattering simulations as well as a radiative transfer study. Slabs of alumina with inclusions of Cu, Au, Co and Cr are studied; reflectance coefficients are calculated as a function of wavelength, considering metal inclusion diameters up to 2.0 μm. We show that despite the incoherent diffuse propagation of light inside the slab, strong selectivity is achieved by particle diameters around 0.2 μm, even for very low metal concentrations.

The interaction of local anesthetics with lipid membranes

Molecular Dynamic Simulations are performed to evaluate the interaction of lidocaine, procaine and tetracaine with a lipid membrane. The main interest is to evaluate the structural changes produced by these local anesthetics in the bilayers. Penetration trajectories, interaction energies, entropy changes and an order parameter are calculated to quantify the destabilization of the lipid configurations. We show that such structural parameters give important information to understand how anesthetic agents influence the structure of plasma membranes. Graphic processing units (GPUs) are used in our simulations.