Applications of a Numerical Air Pollution Transport Model to Dispersion in the Atmospheric Boundary Layer

Bruce A. Egan Harvard School of Public Health, Dept. of Environmental Health Sciences, Boston, Mass. 02115

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James R. Mahoney Harvard School of Public Health, Dept. of Environmental Health Sciences, Boston, Mass. 02115

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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.

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