Showing papers by "University of the Aegean published in 1998"

ISORROPIA: A New Thermodynamic Equilibrium Model for Multiphase Multicomponent Inorganic Aerosols

Abstract: A computationally efficient and rigorous thermodynamic model that predicts the physical state and composition of inorganic atmospheric aerosol is presented. One of the main features of the model is the implementation of mutual deliquescence of multicomponent salt particles, which lowers the deliquescence point of the aerosol phase. The model is used to examine the behavior of four types of tropospheric aerosol (marine, urban, remote continental and non-urban continental), and the results are compared with the predictions of two other models currently in use. The results of all three models were generally in good agreement. Differences were found primarily in the mutual deliquescence humidity regions, where the new model predicted the existence of water, and the other two did not. Differences in the behavior (speciation and water absorbing properties) between the aerosol types are pointed out. The new model also needed considerably less CPU time, and always shows stability and robust convergence.

From scalar to vector equilibrium problems in the quasimonotone case

Abstract: In a unified approach, existence results for quasimonotone vector equilibrium problems and quasimonotone (multivalued) vector variational inequality problems are derived from an existence result for a scalar equilibrium problem involving two (rather than one) quasimonotone bifunctions. The results in the vector case are not only obtained in a new way, but they are also stronger versions of earlier existence results.

Quantitative mapping of air pollution density using Earth observations: a new processing method and application to an urban area

Abstract: Urban air pollution can be quantified in terms of atmospheric turbidity if satellite images of similar geometry acquired under clear atmosphere and pollution conditions are radiometrically compared. This comparison can be conveniently carried out by evaluating 'the apparent contrast reduction' due to aerosols over land. However, this method is subject to misclassification due to ground-reflectance temporal variations. This paper proposes the complementary application of 'the temperature attenuation' procedure, which allows one to decouple the results of the previous method from such variations and, thus, increase confidence in classification results. The respective image processing code developed has been tested successfully on Landsat-5/TM data of the urban area of Athens.

Multivariate Mixed Models for Open-Ended Contingent Valuation Data: Willingness To Pay For Conservation of Monk Seals

Abstract: Although dichotomous choice (DC) contingent valuation (CV) has been recommended by the US NOAA 'blue-ribbon' panel for large-scale contingent valuation studies, useful information can still be obtained from smaller, open-ended (OE) studies, often undertaken as a precursor to a DC survey. The CV study considered here was carried out in Greece and looked at willingness-to-pay (WTP) for protecting the Mediterranean monk seal (Monachus-monachus) in the Aegean area. This is the most endangered seal in the world, and the application of the CV methodology was the first such application in Greece. The OE data consist of two responses: first, a binary response detailing whether or not respondents were in principle prepared to pay for the protection of this seal; secondly, those respondents who answered 'yes' to the first question were then asked to state their maximum WTP for such protection. A multivariate binomial – log-normal mixture model is used to develop a bid function including explanatory variables such as income, sex, age and education. Such a modelling approach provides an alternative to more commonplace tobit estimation. However, the model is extended to include further information which was collected on: (a) an increased WTP amount given in response to information that the initial WTP amount may not be enough to prevent the extinction of the seal; (b) respondents were asked to divide their final WTP amount between use, option and existence values, the latter requiring a multivariate model with four binary and four continuous responses per individual in the same model. The discussion focuses on the methodological issues raised with some comment on the substantive interpretation of results.

Quasimonotonicity and Pseudomonotonicity in Variational Inequalities and Equilibrium Problems

Abstract: In this paper we survey results concerning major properties of variational inequality problems and equilibrium problems under generalized monotonicity assumptions rather than monotonicity. Scalar and vectorial versions of these models are considered. The analysis is done for both pseudomonotone and quasimonotone maps and their variants.

Effects of Grazing Pressure on Succession Process and Productivity of Old Fields on Mediterranean Islands

Abstract: competitive under conditions of overgrazing and fire, accounted for 70%–85% of the total plant cover in all sites, except Santorini, where participation of spiny species was very low. In the recently abandoned sites, annuals accounted for 50%–60% of the standing biomass, but constituted only a tiny fraction in those abandoned for 30 years or more. This was not paralleled by any remarkable increase of the total plant cover. In fact, the yearly produced green biomass decreased with the years after abandonment. The productivity of old fields abandoned for a long time is insufficient to support the grazing animals present. This shortfall, given the animals' requirements, is most dramatic in Symi. The nonrecovering vegetation in the old fields of these mountainous islands constitutes a severe environmental threat; remedial measures appear imperative.

Evaluation of Phytoplankton Community Indices for Detecting Eutrophic Trends in the Marine Environment

Abstract: Twenty two diversity, evenness and dominance indices based on phytoplankton community structure were examined for their sensitivity to assess eutrophic conditions. Data from two sampling sites, one eutrophic and one oligotrophic, were used as reference data-sets for evaluating the indices. McNaughton's dominance index, cell number, species number (Hill's N0), Kothe's species deficit, Odum's species per thousand individuals and Evenness E2, E3 seemed to be the most sensitive indices for discriminating between eutrophic and oligotrophic conditions. A case study was also performed using a data-set from ten stations spaced along the coastal area of the city of Rhodes, Aegean Sea, Greece to find out which of the indices were appropriate to assess different levels of eutrophication. Kothe's species deficit, Hill's N0 (species number), Margalef's and Gleason's indices have shown the highest sensitivity to resolve oligotrophic, mesotrophic and eutrophic conditions.

The influence of product type on the purchasing structure

Abstract: This article reports on the influence of product type on the purchasing structure within selected phases of the purchasing process by using regression analysis on data from Greek manufacturing and utility enterprises. Our study examined the influence of two different categories of items, namely product incorporated items and MRO (maintenance, repair and operating) items, on various aspects of the purchasing cycle. The results suggested that parameters of purchasing structure varied considerably between the two product types and that their configuration depended on attributes such as product complexity and environmental uncertainty.

Modeling of aerosol properties related to direct climate forcing

Abstract: A long-term local experiment was designed with the purpose to accurately quantify aerosol parameters needed in order to estimate aerosol climate forcing at an anthropogenically perturbed continental site. Total light-scattering σλ,sp and backscattering σλ,bsp coefficients at wavelength λ, the hygroscopic growth factors with respect to scattering, ƒ(RH)λ,s, and the backscatter ratio bλ are the parameters considered in the paper. Reference and controlled relative humidity nephelometry measurements were taken at a ground level field sampling station, located near Bondville Illinois (40°03′12″N, W 88°22′19″W). Aerosol particle chemical composition and mass particle size distributions were also measured. The target parameters were also estimated from models. The modeling approach involved a two-step process. In the first step, aerosol properties were parameterized with an approach that made use of a modified thermodynamic equilibrium model, published laboratory measurements of single hygroscopic particle properties, and empirical mixing rules. In the second step, the parameterized aerosol properties were used as inputs into a code that calculate σλ,sp and σλ,bsp as functions of λ, RH, particle size, and composition. Comparison between the measured and the modeled results showed that depending on the assumptions, the differences between the modeled and observed results were within 5 to 28% for ƒ(RH)λ,s and within 22–35% for bλ at low RH and 0–20% for bλ at high RH. The temporal variation of the particle size distribution, the equilibrium state of the particles, and the hygroscopicity of the material characterized as residual were the major factors limiting the predictive ability of the models.

Development of categorical mapping for quantitative assessment of eutrophication

Abstract: Spatial distribution of nutrient and phytoplankton variables is often illustrated using categorical mapping for each variable. However, the assessment of eutrophication cannot be derived from a single parameter since a synthesis of the environmental variables related to eutrophication is required. These shortcomings are further complicated since it is difficult to discriminate between distinct trophic states along natural environmental gradients. In the present work, a methodological procedure for quantitative assessment of eutrophication at a spatial scale was examined in the Gulf of Saronicos, Greece, based on a thematic map generated from the synthesis of four variables characterising eutrophication. The categorical map of each variable was developed using the Kriging interpolation method and four trophic levels were indicated (eutrophic, upper-mesotrophic, lower-mesotrophic and oligotrophic) based on nutrient and phytoplankton concentration scaling. Multi-criteria choice methods were applied to generate a final categorical map showing the four trophic levels in the area. This synthesis of categorical maps for assessing eutrophication at a spatial scale is proposed as a methodological procedure appropriate for coastal management studies.

Forecasting paratransit ridership using discrete choice models with explicit consideration of availability

Abstract: In most developed countries, the population of the elderly and disabled is growing rapidly. These individuals require transportation service suited to their needs. Such service may be provided by applying emerging technologies to dial-a-ride transit. This research develops a methodology to quantitatively evaluate the impact of paratransit services on a traveler's mode choice behavior. The mode choice model explicitly considers availability of alternative modes and includes latent factors to account for taste heterogeneity. Stated preferences are also used to elicit preferences for new paratransit services. The methodology is empirically tested with data collected in Winston-Salem, North Carolina. The model system developed is applied to evaluate the effect of improving service attributes and the impact of the introduction of new cost-effective modes on modal shares. Results of the policy analysis indicate that (a) transit policy changes, such as fare reduction, would have little effect on automobile driver and automobile passenger shares; (b) an improved reservation system for dial-a-ride services would produce shifts in mode share; (c) the proposed new bus deviation service was favored; (d) free bus service reduces dial-a-ride share; and (e) an increase in awareness of a dial-a-ride system would significantly increase its share.

Generic security policies for healthcare information systems

Abstract: Healthcare Establishments (HCEs) have developed a major dependency on Information and Communications Technologies (ICT) in the last decade. The increasing reliance upon ICT has stressed the need to foster security in Healthcare Information Systems (HIS). Security policies may have a significant contribution to make to this effort, but they could cause portability and inter-operability problems. Moreover, policies that fail to take into account all the aspects of HIS security, the legal and regulatory requirements, and the effect of several stakeholders, may lead to ineffective and inefficient security measures. We argue that policies of a special category, named Generic Security Policies (GSPs), should be developed to provide policy-level harmonization and guidance to policy-makers within HCEs. We have reviewed five policies that appear as candidates and have used the results of this review to compile a set of guidelines for potential developers of GSPs.

Versions of normality and some weak forms of the axiom of choice

Abstract: We investigate the set theoretical strength of some properties of normality, in- cluding Urysohn's Lemma, Tietze-Urysohn Extension Theorem, normality of disjoint unions of normal spaces, and normality of F subsets of normal spaces.

On The Distribution Of Digits in Dyadic Expansions

Abstract: We estimate the Hausdorff dimension of certain Borel sets determined in dyadic expansion by products of r successive digits.

Management science semantics for object-oriented business modelling in BPR

Abstract: BPR methodologies should justify their capability for understanding an organization's strategy, business structure and processes. For this reason techniques that provide understanding of the current and future desired positions of the organization and the discrepancy between them as well as consistent business modelling techniques are required. One of the main drawbacks of existing business modelling methodologies is the lack of theoretical underpining associated with the concepts used in business modelling. This paper presents a BPR methodology called agent relationship morphism analysis (ARMA) that goes beyond the limitations of the existing BPR methodologies taking a holistic view of the organization. In ARMA the modelling of the business environment is achieved with the use of three perspectives: the structural, behavioural and process. A technique called agent relationship modelling (ARM) has been developed for modelling the structural perspective. The more dynamic organizational concepts are described in the behavioural and process perspectives. These perspectives are modelled in a technique called agent/object lifecycles (ALCs/OLCs).

Optimal seismic deconvolution: distributed algorithms

Abstract: Deconvolution is one of the most important aspects of seismic signal processing. The objective of the deconvolution procedure is to remove the obscuring effect of the wavelet's replica making up the seismic trace and therefore obtain an estimate of the reflection coefficient sequence. This paper introduces a new deconvolution algorithm. Optimal distributed estimators and smoothers are utilized in the proposed solution. The new distributed methodology, perfectly suitable for a multisensor environment, such as the seismic signal processing, is compared to the centralized approach, with respect to computational complexity and architectural efficiency. It is shown that the distributed approach greatly outperforms the currently used centralized methodology offering flexibility in the design of the data fusion network.

Adiabatic invariants and mixmaster catastrophes

Abstract: We present a rigorous analysis of the role and uses of the adiabatic invariant in the mixmaster dynamical system. We propose a new invariant for the global dynamics which in some respects has an improved behavior over the commonly used one. We illustrate its behavior in a number of numerical results. We also present a new formulation of the dynamics via catastrophe theory. We find that the change from one era to the next corresponds to a fold catastrophe, during the Kasner shifts the potential is an implicit function form whereas, as the anisotropy dissipates, the mixmaster potential must become a Morse 0-saddle. We compare and contrast our results to many known works on the mixmaster problem and indicate how extensions could be achieved. Further exploitation of this formulation may lead to a clearer understanding of the global mixmaster dynamics.

Disjoint Unions of Topological Spaces and Choice

Abstract: We nd properties of topological spaces which are not shared by disjoint unions in the absence of some form of the Axiom of Choice. Introduction and Terminology This is a continuation of the study of the roll the Axiom of Choice plays in general topology. See also (vd), (gt), (wgt), and (hkrr). Our primary concern will be the use of the axiom of choice in proving properties of disjoint unions of topological spaces (See Denition 1, part 11.) For example, in set theory with choice the disjoint union of metrizable topological spaces is a metrizable topological space. The usual proof of this fact begins with the choice of metrics for the component spaces. We will show that the use of some form of choice cannot be avoided in this proof and in fact without choice the disjoint union of metrizable spaces may not even be metacompact. In section 1 we show that many assertions about disjoint unions of topological spaces are equivalent to the axiom of multiple choice. Models of set theory and corresponding independence results are described in section 2. In section 3, we study the roll the Axiom of Choice plays in the properties of disjoint unions of collectionwise Hausdor and collectionwise normal spaces. We begin with the denitions of the symbols and terms we will be using. Denition 1. 1. A familyK of subsets of a topological space (X;T )i sl.f .( locally nite) i each point of X has a neighborhood meeting a nite number of elements ofK. 2. X is paracompact i X is T2 and every open coverU of X has a l.f.o.r. (locally nite open renement)V.T hat is,V is a locally nite open cover ofX and every member ofV is included in a member ofU. 3. A familyK of subsets of X is p.f. (point nite) i each element of X belongs to only nitely many members ofK. 4. X is metacompact i each open coverU of X has an o.p.f.r. (open point nite renement). 5. An open coverU =fUi : i2 kg of X is shrinkable ithere exists an open cover V =fVi : i2 kg of non-empty sets such that V i Ui for all i2 k. V is also called a shrinking ofU. 6. X is a PFCS space i every p.f. open cover of X is shrinkable.