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Abstract
A numerical, advection-diffusion air pollution transport model is used to describe dispersion of emissions from urban area-type sources for several atmospheric stability situations, The estimation of appropriate velocity and diffusivity fields for use as meteorological input data is discussed in detail. Profiles representing the constant-flux surface layer are used in a nondimensional transport equation to obtain concentration fields over a limited horizontal and vertical scale. A wind spiral model and diffusivity profile after Blackadar is used to simulate dispersion under neutral stability conditions on an urban scale and in three dimensions for both a steady source and a single puff. Analysis of the trajectory and spread statistics for the puff suggests that, on an urban scale, the combined effects of vertical diffusion and transverse wind components result in effective cross-wind diffusion coefficients an order of magnitude greater than typical values of vertical diffusion coefficients. Superposition of the concentration fields of a two-dimensional calculation demonstrates the effect of different spatial distributions of sources on ground-level concentrations.
In the last sections, diffusivity profiles are postulated for investigation of the effects of varying stability and mixing height on concentration distributions on an urban scale. Results indicate that ground-level concentrations vary by a factor of three or so over a range of stability conditions from neutral to slightly unstable for infinite mixing heights. The influence of time variations of mixing height on ground-level concentrations is examined in terms of the relative meteorological time scales and corresponding departures from quasi-steady-state concentration values.
Abstract
A numerical, advection-diffusion air pollution transport model is used to describe dispersion of emissions from urban area-type sources for several atmospheric stability situations, The estimation of appropriate velocity and diffusivity fields for use as meteorological input data is discussed in detail. Profiles representing the constant-flux surface layer are used in a nondimensional transport equation to obtain concentration fields over a limited horizontal and vertical scale. A wind spiral model and diffusivity profile after Blackadar is used to simulate dispersion under neutral stability conditions on an urban scale and in three dimensions for both a steady source and a single puff. Analysis of the trajectory and spread statistics for the puff suggests that, on an urban scale, the combined effects of vertical diffusion and transverse wind components result in effective cross-wind diffusion coefficients an order of magnitude greater than typical values of vertical diffusion coefficients. Superposition of the concentration fields of a two-dimensional calculation demonstrates the effect of different spatial distributions of sources on ground-level concentrations.
In the last sections, diffusivity profiles are postulated for investigation of the effects of varying stability and mixing height on concentration distributions on an urban scale. Results indicate that ground-level concentrations vary by a factor of three or so over a range of stability conditions from neutral to slightly unstable for infinite mixing heights. The influence of time variations of mixing height on ground-level concentrations is examined in terms of the relative meteorological time scales and corresponding departures from quasi-steady-state concentration values.
Abstract
A numerical, grid-element model has been developed for the study of air pollution transport from urban area-type sources. This advection-diffusion model is especially useful for the estimation of air pollution concentrations under conditions of spatial and time varying emissions, velocities and diffusion rates. The “pseudo-diffusive” errors associated with conventional finite-difference approximations to advective transport are eliminated by a material-conserving computation procedure involving the zeroth, first and second moments of the concentration distribution within each grid element. Extensions of the procedure are suggested for retention of sub-grid-scale resolution of concentration values necessary in the study of transport of chemically reactive materials, or for the incorporation of emissions from point and line sources. A novel procedure is presented for the numerical simulation of horizontal diffusion from area sources which can be used to model empirically observed dispersive growth rates.
Abstract
A numerical, grid-element model has been developed for the study of air pollution transport from urban area-type sources. This advection-diffusion model is especially useful for the estimation of air pollution concentrations under conditions of spatial and time varying emissions, velocities and diffusion rates. The “pseudo-diffusive” errors associated with conventional finite-difference approximations to advective transport are eliminated by a material-conserving computation procedure involving the zeroth, first and second moments of the concentration distribution within each grid element. Extensions of the procedure are suggested for retention of sub-grid-scale resolution of concentration values necessary in the study of transport of chemically reactive materials, or for the incorporation of emissions from point and line sources. A novel procedure is presented for the numerical simulation of horizontal diffusion from area sources which can be used to model empirically observed dispersive growth rates.
This article summarizes a workshop convened under the direction of the AMS Steering Committee for the EPA (Environmental Protection Agency) Cooperative Agreement on Air Quality Modeling. The purpose of the workshop was to address the status of our understanding of dispersion in complex or mountainous terrain settings, with a specific focus on the ability of current technologies to predict air pollution concentrations in different terrain settings. The meteorological phenomena of importance for estimating the effects of elevated plumes interacting with high terrain are described in detail. Current understanding of the problems associated with lee-side flows and valley meteorology are also addressed. The present operational status of physical and mathematical modeling capabilities is summarized in relation to various terrain configurations.
The participants provided a number of recommendations, both on the general use of the present science and also on seven specific areas where further research is needed in order to substantially improve our ability to reliably assess the effects of dispersion in complex terrain. These specific recommendations are accompanied by more-general perspectives regarding the fundamental complexity of stratified-fluid dynamics when coupled with topographic environments.
This article summarizes a workshop convened under the direction of the AMS Steering Committee for the EPA (Environmental Protection Agency) Cooperative Agreement on Air Quality Modeling. The purpose of the workshop was to address the status of our understanding of dispersion in complex or mountainous terrain settings, with a specific focus on the ability of current technologies to predict air pollution concentrations in different terrain settings. The meteorological phenomena of importance for estimating the effects of elevated plumes interacting with high terrain are described in detail. Current understanding of the problems associated with lee-side flows and valley meteorology are also addressed. The present operational status of physical and mathematical modeling capabilities is summarized in relation to various terrain configurations.
The participants provided a number of recommendations, both on the general use of the present science and also on seven specific areas where further research is needed in order to substantially improve our ability to reliably assess the effects of dispersion in complex terrain. These specific recommendations are accompanied by more-general perspectives regarding the fundamental complexity of stratified-fluid dynamics when coupled with topographic environments.
