70,946 research outputs found
Development of a Fast and Detailed Model of Urban-Scale Chemical and Physical Processing
Abstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/).A reduced form metamodel has been produced to simulate the effects of physical, chemical, and meteorological processing of highly reactive trace species in hypothetical urban areas, which is capable of efficiently simulating the urban concentration, surface deposition, and net mass flux of these species. A polynomial chaos expansion and the probabilistic collocation method have been used for the metamodel, and its coefficients were fit so as to be applicable under a broad range of present-day and future conditions. The inputs upon which this metamodel have been formed are based on a combination of physical properties (average temperature, diurnal temperature range, date, and latitude), anthropogenic properties (patterns and amounts of emissions), and the surrounding environment (background concentrations of certain species).
Probability Distribution Functions (PDFs) of the inputs were used to run a detailed parent chemical and physical model, the Comprehensive Air Quality Model with Extensions (CAMx), thousands of times. Outputs from these runs were used in turn to both determine the coefficients of and test the precision of the metamodel, as compared with the detailed parent model. The deviations between the metamodel and the parent mode for many important species (O3, CO, NOx, and BC) were found to have a weighted RMS error less than 10% in all cases, with many of the specific cases having a weighted RMS error less than 1%. Some of the other important species (VOCs, PAN, OC, and sulfate aerosol) usually have their weighted RMS error less than 10% as well, except for a small number of cases. These cases, in which the highly non-linear nature of the processing is too large for the third order metamodel to give an accurate fit, are explained in terms of the complexity and non-linearity of the physical, chemical, and meteorological processing. In addition, for those species in which good fits have not been obtained, the program has been designed in such a way that values which are not physically realistic are flagged.
Sensitivity tests have been performed, to observe the response of the 16 metamodels (4 different meteorologies and 4 different urban types) to a broad set of potential inputs. These results were compared with observations of ozone, CO, formaldehyde, BC, and PM10 from a few well observed urban areas, and in most of the cases, the output distributions were found to be within ranges of the observations.
Overall, a set of efficient and robust metamodels have been generated which are capable of simulating the effects of various physical, chemical, and meteorological processing, and capable of determining the urban concentrations, mole fractions, and fluxes of species, important to human health and the climate.Federal Agencies and industries that sponsor the MIT Joint Program on the Science and Policy of Global Change
Feature Selection via Coalitional Game Theory
We present and study the contribution-selection algorithm (CSA), a novel algorithm for feature selection. The algorithm is based on the multiperturbation shapley analysis (MSA), a framework that relies on game theory to estimate usefulness. The algorithm iteratively estimates the usefulness of features and selects them accordingly, using either forward selection or backward elimination. It can optimize various performance measures over unseen data such as accuracy, balanced error rate, and area under receiver-operator-characteristic curve. Empirical comparison with several other existing feature selection methods shows that the backward elimination variant of CSA leads to the most accurate classification results on an array of data sets
Optimal Iris Fuzzy Sketches
Fuzzy sketches, introduced as a link between biometry and cryptography, are a
way of handling biometric data matching as an error correction issue. We focus
here on iris biometrics and look for the best error-correcting code in that
respect. We show that two-dimensional iterative min-sum decoding leads to
results near the theoretical limits. In particular, we experiment our
techniques on the Iris Challenge Evaluation (ICE) database and validate our
findings.Comment: 9 pages. Submitted to the IEEE Conference on Biometrics: Theory,
Applications and Systems, 2007 Washington D
Statistical Matrix for Electroweak Baryogenesis
In electroweak baryogenesis, a domain wall between the spontaneously broken
and unbroken phases acts as a separator of baryon (or lepton) number,
generating a baryon asymmetry in the universe. If the wall is thin relative to
plasma mean free paths, one computes baryon current into the broken phase by
determining the quantum mechanical transmission of plasma components in the
potential of the spatially changing Higgs VEV. We show that baryon current can
also be obtained using a statistical density operator. This new formulation of
the problem provides a consistent framework for studying the influence of
quasiparticle lifetimes on baryon current. We show that when the plasma has no
self-interactions, familiar results are reproduced. When plasma
self-interactions are included, the baryon current into the broken phase is
related to an imaginary time temperature Green's function.Comment: 20 pages, no figures, Late
Progress in Electroweak Baryogenesis
Recent work on generating the excess of matter over antimatter in the early
universe during the electroweak phase transition is reviewed.Comment: 50 pages (figures on request), uses harvmac (table of contents
correct for "l" format), UCSD-93-2,BU-HEP-93-
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