Air Pollution, the Automobile, and Public Health (1988) / Chapter Skim
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Atmospheric Transport and Dispersion of Air Pollutants Associated with Vehicular Emissions
Atmospheric Transport and Dispersion of Air Pollutants Associated with Vehicular Emissions
Pages 77-98
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From page 77... ...
. SAMSON University of Michigan Definitions of Transport and Dispersion / 78 Meteorological Parameters / 78 Scales of Motion / 81 Transport and Dispersion: Theory and Applications / 82 Near Field / 82 Urban-Scale Transport and Dispersion / 88 Regional-Scale Transport / 89 Summary 1 92 Summary of Research Recommendations / 93 Air Pollution, the Automobile, and Public Health.
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From page 78... ...
. Concentrations associated with nonmoving vehicles as might be encountered in traffic queues, parking structures, and street canyons are determined by emission rates and the wind flows and turbulence produced by the interaction of the local wind with complex structures such as buildings and roadside sound barriers.
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From page 79... ...
Apart from natural transport and dispersion processes, moving vehicles themselves cause considerable mixing that influences pollutant concentrations within about 100 m of a highway. The aerodynamic drag of a moving vehicle causes a turbulent wake in which pollutants initially mix.
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From page 80... ...
These days occur during the coldest winter months when air stagnates in the valleys (Zimmerman and McKenzie 1974; Bowling 1984; Femau and Samson 1985~. Also, there are potential problems in urban street canyons, where the complexities of wind flow can lead to high pollutant concentrations under certain conditions.
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From page 81... ...
Generally, vehicular emissions are produced by moving vehicles, but it is also possible for idling vehicles to contribute to high pollutant concentrations, for example, in a parking structure or lot, a street canyon, or a highway queue at rush hour. Consequently, the dispersions associated both with moving vehicles (line sources)
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From page 82... ...
Wind speed and direction can change substantially near the surface, especially near the highway where they are influenced by the motion of the vehicles.) Atmospheric Transport and Dispersion of Air Pollutants The theoretical description of transport and dispersion processes has been developed for a continuously emitting line source by Calder (1973)
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From page 83... ...
values despite the changing ambient stability illustrates the dominance of traff~cinduced turbulence over ambient turbulence in the near field of a line source. To compensate for the poor predictions of simple Gaussian line-source models under certain conditions, a number of modifications to the basic Gaussian approach have been developed.
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From page 84... ...
The exhaust heat convection could turbulence led to overpredictions in con- play an important role in the dispersion of centration when the wind was nearly par allel to the axis of the highway. An increase in mechanical mixing led to a decrease in pollutants near highways under light wind conditions with heavy traffic by lifting the exhaust plume (plume rise)
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From page 85... ...
The airflow in street canyons is neither steady nor homogeneous, and street segments do not approximate infinite line sources. Likewise, exhaust from parking structures is rarely vented through an isolated, elevated point source.
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From page 86... ...
Very few computer simulation models have been developed for street canyons because of the complexity of building geometry and wind flow. A vortex of airflow is expected when the flow above building height is perpendicular to the axis of the canyon, as shown schematically in figure 4.
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From page 87... ...
In summary, studies of the transport and dispersion of pollutants from complex urban situations suffer from a lack of reliable data and the uniqueness of each situation. Street canyons in particular pose special problems because of the complicated nature of the wind flows relative to specific geometries of buildings.
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From page 88... ...
The sites for this program should include urban street canyons, parking structures (including truck stops) , and other complex environments, for example, where sound-reducing roadway barriers and covered highway cuts influence air movement.
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From page 89... ...
1 he concentrations ot (my were not consistently related to the morn ing mixing height, possibly because there was insufficient time for the pollutant to mix to the mixing height, and possiblywhich are emitted principally from elevated because of the unreliability of the method for calculating the morning mixing height. The concentrations of O3 were not related to the maximum mixing height, possibly because of the dominance of other factors such as solar intensity and upwind pollutant concentrations.
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From page 90... ...
. slgn1tlcant atlases In atmospheric transport calculations.
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From page 91... ...
This assumes a 10 m/see wind speed at about 40° latitude. (Adapted with permission from Draxler and Taylor 1982, and from the American Meteorological Society.)
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From page 92... ...
It is known that the concentrations of pollutants associated with moving vehicles are determined by several factors, including their emission rate from the vehicle, mixing induced by vehicle motion, wind speed and direction relative to axis of the highway, intensity of ambient atmospheric turbulence, reactions to or from other chemical species, and the rate of removal to the surface (deposition)
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From page 93... ...
Simulating the transport of pollutants within street canyons and parking structures is very difficult and virtually impossible to extrapolate to other sites or times. The flow of air in street canyons is neither steady nor homogeneous, and representative wind flow measurements are difficult to obtain.
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From page 94... ...
LOW PRIORITY Recommendation 5 Urban-Scale Transport Develop an automated, remote-sensing wind-profiling system for use in defining wind flow within an urban atmosphere. Recommendlation 7 Regional-Scale Transport Define the most efficient method for augmenting the existing NWS rawinsonde measurement network to satisfy the data needs of regional-scale air pollution transport models.
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From page 95... ...
1979. Dispersion of carbon monoxide from roadways at low wind speeds,J.
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From page 96... ...
1974. A Scale Model Study of the Dispersion of Pollution in Street Canyons, Paper presented at the 67th Annual Meeting of the Air Pollution Control Assn., Denver, Colo., June 9-13 1974.
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From page 97... ...
1980. Pollutant dispersion of vehicle exhaust gases in street canyons, Proceedings ofthe 5th International Clean Air Congress, Buenos Aires.
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