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Mei-Kao Liu, David C. Whitney, and Philip M. Roth

Abstract

This paper presents the results of a study in which a numerical model for photochemical air pollution was used to assess the effect of atmospheric parameters on concentration levels of both primary and secondary air pollutants in an urban area. Quantitative estimates were obtained of relative changes in CO, NO, O3 and NO2 concentrations in response to relative changes in wind speed, vertical diffusivity, mixing depth, radiation intensity and emission rate. The results show that although concentration levels of all four pollutants are most sensitive to variations in wind speed, they are also sensitive to variations in emission rate and, to a lesser degree, to variations in mixing depth and vertical diffusivity. For nitric oxide and ozone, radiation intensity is another important parameter affecting the concentration distributions.

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Mei-Kao Liu, Douglas A. Stewart, and Donald Henderson

Abstract

This paper describes the use of a regional-scale air quality model as a diagnostic tool for analyzing problems associated with acid rain. The model, which is hybrid in nature, consists of a puff module and a grid module. The puff module computes the evolution of individual puffs, such as the horizontal and vertical standard deviations of the puff spreads and the location of the center of mass, emitted continuously from each major point source. It also determines the location at which the puff will be released to the grid module and the amount of oxidation and deposition along the trajectory. The grid module then follows the transport, diffusion, and chemical reactions of these aged puffs, as well as emissions from a variety of diffuse sources. Elaborate schemes for both dry and wet deposition have also been incorporated into the model. This model has been exercised for two real-time meteorological scenarios—a dry case and a two-day rainstorm episode in the Northern Great Plains. On the basis of model calculations, atmospheric budgets for SO2 and sulfate over the modeling region have been estimated.

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Gary E. Moore, Christopher Daly, Mei-Kao Liu, and Shi-Jian Huang

Abstract

A dry three-dimensional mesoscale model was used to study the diurnal cycle of mountain-valley winds in the southern San Joaquin Valley during a summer day. A scheme for interpolating potential temperature was developed to provide hourly temperature fields to initialize and force the dynamically predicted wind fields. A simplified modeling approach was used to produce steady state solutions that are dynamically consistent with the momentum equation and supplied temperature fields. Model performance was evaluated by comparing observed and predicted surface winds. Some features of the wind field flow aloft were qualitatively examined with regard to their importance in air quality studies.

The morning drainage-upslope transition and the evening reversal of upslope flow were realistically simulated throughout most of the valley. The variation of wind speeds throughout the valley and over the course of the day were simulated with an average bias of 9% of the average wind. Wind directions were simulated with an overall average bias of 5° and midday hourly correlation coefficients of typically r = 0.8. Model performance was below average during the morning and evening transition periods, when thermal forcing is at a minimum and valley winds are light and variable. At midday, the model produces strong upward vertical motions near the ridge crests and divergence-driven subsidences at the foot of the mountains typical of observations made in mountain-valley systems. During the morning, modeled drainage flow down the mountains results in a convergence zone in the southern and narrowest part of the valley, resulting in rising motions; down-valley flow, sometimes observed in mountain-valley systems, also occurs. The model is best suited for applications in mountain-valley regions for which wind observations are sparse and do not adequately reflect thermally driven circulation.

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