A numerical model was developed for simulating dispersion of automotive exhaust gas near complex urban roadways and buildings. The model is based on two or three dimensional numerical solution of advection-diffusion equation by Fnite Differemce Method. The first stage of the model solves wind velosity field with mixing length sub-model for estimating eddy viscosity, and the second stage calculates the advection and diffusion of pollutant in the calculated wind field. Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm and its variation (SIMPLER and SIMPLEST), as well as newly developed algorithm, which was simplified from the SIMPLER algorithm, were applied for calculation of wind velocity. New algorithm showed stable convergence for various configurations.
As the firststep of the validation of the model, two dimensional cross wind model was applied to flat area and urban street canyon, where data on field experiment of tracer gas dispersion is available. Results from numerical model well reproduced the concentration profile for both areas.

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The finite difference model to predict automotive exhaust gas dispersion near ueban roadways, formulated by the previous paper, was applied to three dimensional problems. In order to validate the model, tracer gas dispersion experiment using scale model in a wind tunnel was carried out. A number of imaginary urban districts with a variety of configurations for roadway and surrounding buildings were set up using wooden scale models. Roadway configuration includes flat, open cut, raised bank and elevated, and buildings configuration includes street canyon of differnet heights. Ethane, used as a tracer gas, was emitted from small holes punched on brazen pipe, of which concentrations were measured continuously at a number of points on the scale model. Both for two dimensional and three dimensional configurations, results from numerical simulation coincided fairely well with concentration distribution measured by wind tunnel experiment. Application of variable size grid in the numerical model improved the coincidence with experimental results. The value of diffusion parameter of the model, calibrated by the wind tunnel experiment, was smaller than the value reported by the previous paper based on the comparison with field experimental data.

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In order to clarify the diffusion of the automotive exhaust gas in the roadway surrounded by middle-sized and high buildings, two air tracer (SF₆) dispersion experiments were carried out. One case was under windy condition and the other was under calm condition. Following conclusions were obtained through the analysis of these data.
(1) When the wind at the roof-top height was perpendicular to the road, high concentration of the tracer gas was distributed toward the windward in the roadway.
(2) Through analyzing the wind data of this case, circulation wind was suggested to occur in the roadway.The distributions of the tracer gas concentration toward the leeward direction were not varied according to the wind speed at the roof-top height. In the calm condition, SF₆ was detected at the sampling points in the windward side.
(3) In the calm condition, residence time of SF₆ was more than 12hrs after SF₆ release was finished.

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For elucidation of the actual state of human exposure to carcinogen and its related compounds in the diesel exhaust, the deposition ratio of particles to respiratory tract was estimated. Sample exhaust, the air added a small amount of the exhaust from the diesel engines operating under 1000rpm and 90% roaded, or idling, was introduced into a tent in which two volunteers were sitting. The particles in exhalation were collected by Hand Rudolph mask, filter holder (teflon binding glass fiber filter) and respirometer which were connected in series. The particles in the air in tent corresponding to inhaled air were collected by the low volume air sampler of low-nois model. The concentrations of PAHs in the collected particles were determined by the PAH-microanalysis methods. From the results, the apparent deposition ratio was reached about 50%. It was suggested that the deposition amount of PAH to respiratory tract increased linearity with the concentration of those in the inhaled air.

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A field study on the diffusion around a cube was conducted in the urban canopy in Sep. and Nov., 1992 in Sapporo, Japan. It was found that in the urban canopy turbulence intensities were very strong, and the concentration patterns on the rooftop of the cube and on the ground level differed from those obtained from wind tunnel studies with low turbulence intensities. The relationship between the velocity fluctuations and the concentration variations was also investigated using two ultrasonic anemometers and fast-response concentration detectors. The results show that the reverse flow on the roof was caused by the ejection motions, which exist in the upwind velocity field, and also that the tracer gas released from the center of the cube roof was transported upwind by this reverse flow on the roof.

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In order to explain NO₂ pollution mechanism in the envions of main traffic roads, NO₂ concentration inthe period from year-end through beginning of next year was measured with TEA capsule.
The following three results were found out.
Firstly, the relations between NO₂ concentrations in the edges of R-43 (Route-No.43) and R-2 (Route-No.2) roads, and the number of passing cars through there could not be found to be linear.
Secondly, however the difference between the relation in the period from year-end through the six daysin the beginning of next year, and the relation in the periods except the above period could be found to bepositive.
Thirdly, NO₂ concentrations in edges of R-43 road were found to relate with the wind-direction andvelocity.

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