Investigations of Transport in Complex Atmospheric Flow Systems. I. Small Scale Studies of Diffusion through Porous Media, Impact of Fumehood Exhaust Reentry on Indoor Air Quality, and Pollutant Transport Near an Isolated Island. II. Pollutant Transport in Mountain-Valley and Coastal Regions of California

Citation

Reible, Danny David (1982) Investigations of Transport in Complex Atmospheric Flow Systems. I. Small Scale Studies of Diffusion through Porous Media, Impact of Fumehood Exhaust Reentry on Indoor Air Quality, and Pollutant Transport Near an Isolated Island. II. Pollutant Transport in Mountain-Valley and Coastal Regions of California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/X6WE-JD66. https://resolver.caltech.edu/CaltechETD:etd-12212006-103605

Abstract

This thesis details some applications of tracer techniques from laboratory scale studies of diffusion in porous media to the analysis of the transport and dispersion of pollutants in the mountain-valley and coastal environments that form the majority of the state of California.

Chapter 1 describes a technique for estimating gaseous diffusivities in porous media that is based on the general solution to Fick's second law for diffusion in a tube between two well-mixed volumes. In beds of essentially non-porous particles, the ratio of the measured effective diffusivity to the air diffusivity of a gas was found to be proportional to the bed porosity raised to the 1.43 power, a result in agreement with previous studies on similar materials. High moisture content (>15-20% moisture in sand) was found to significantly reduce the gas diffusivity with respect to that found in dry materials.

Chapter 2 indicates the importance of ventilation system imbalance upon the reentrainment of pollutants exhausted from a building. Tracer was released from a fumehood in a "clean" room at the Jet Propulsion Laboratory. Indoor concentrations as high as 235 PPB/gr-mole tracer released/hr were observed due to infiltration of the exhausted tracer. This concentration is about an order of magnitude higher than has been observed in buildings with more balanced ventilation systems. Predictions of single and multi-compartment stirred-tank models were compared to the dynamics of the tracer infiltration. A simple one-compartment model provided a better description of the infiltration dynamics than a three-compartment model suggested by the design of the ventilation system.

Chapter 3 describes a series of atmospheric tracer studies of the transport and dispersion of pollutants over the ocean and near an isolated island cape. The experiments were designed to determine the impact of local sources on a background air quality sampling program. The horizontal dispersion of the tracer over the ocean surface could be approximated by the Gaussian plume model assuming a neutrally stable atmosphere, in general agreement with the expected atmospheric stability. Tracer releases from the surface of the isolated cape indicated that an essentially well-mixed separated zone existed above and downwind of the cape. The height of this zone extended to 35-40% above the height of the cape, about the same height as the wake downwind of an isolated building. Limited mixing between the separated zone and the freestream resulted in a sharp concentration gradient above this height.

Chapter 5 indicates the difficulties of describing the behavior of pollutants in complex terrain. A series of tracer experiments conducted in the northern and central California Coastal Mountains are described. The Gaussian plume model could be used to describe the dispersion of the tracer during strong, unidirectional winds. During an elevated tracer release, however, wind directional shear with altitude led to plume bifurcation, with the majority being transported through a stable nighttime drainage layer to ground level. The transport through the stable layer occurred at a vertical velocity of about 2 cm/s, surprisingly rapid transport between stably stratified layers of the atmosphere.

Chapter 6 describes the uncertainties associated with mass balance and Gaussian parameter estimates from tracer data. The uncertainty in the calculated final result can be less than the errors (assumed random) associated with any individual experimental measurement, indicating that such calculations can be made with greater accuracy than would initially be expected.

Chapter 7 details the transport of pollutants in the San Joaquin Valley during stable wintertime conditions. The relatively limited net ventilation of the valley indicates that pollutants can remain within the valley for several days subsequent to their release. During one tracer experiment, about 50% of the released tracer was observed to be well-mixed within the southern valley about 72 hours after the beginning of the release. The most significant ventilation mechanism for the valley during the winter was the occasional passage of low pressure frontal systems. Long periods without frontal system passage can lead to significant pollutant buildup.

Chapter 8 describes the transport of pollutants in the San Joaquin Valley during summertime conditions. While much more effectively ventilated than during the winter, the increased solar insolation leads to significant ozone levels within the valley. A strong influx of air at the northern mouth of the valley is balanced during the day by a corresponding efflux at its southern end and by daytime upslope flow on the Sierra Nevada Mountains. At night, an eddy forms in the southern valley due to low level stabilization and terrain blockage of the afternoon efflux over the southern boundary of the valley. This eddy grows as more air is entrained from the influx at the northern mouth of the valley. An accelerated layer of air aloft also develops during the night due to surface layer stabilization and decoupling. These dynamic flow structures are significant factors in the transport and dispersion of pollutants in the valley during the summer.

Chapter 9 details the impact of the San Joaquin Valley on the northern Mojave Desert. The transport of pollutants from the southern valley was linked through both tracer and aerosol data to the rapid nighttime reduction in visibility in the northern Mojave Desert. Unlike winter conditions, most of the pollutants in the southern valley were transported out of the valley within a day after their release.

Chapter 10 describes the impact on the Sierra Nevada Mountains of pollutant sources within the San Joaquin Valley. Tracer released within the valley was efficiently transported upslope, impacting National Park and Forest areas. The maximum concentrations observed upslope could be approximated with the Gaussian plume model, assuming very unstable atmospheric conditions. Nighttime stabilization arrested the upslope movement of the tracer and led to slope and valley impacts throughout the night. The limited nighttime ventilation of the slopes may result in the significant ozone concentrations typically observed at slope sites throughout the night.

Chapter 11 describes the transport characteristics of the Sacramento Valley, the northern half of the California Central Valley. Tracer experiments indicated that San Francisco Bay area pollutants have only a small effect on the air quality in the Sacramento Valley. A midday flow divergence over Sacramento resulted in tracer impacts in both the northern part of the valley and the slopes northeast of the city. A counterclockwise eddy that forms in the southern valley during the morning was a potential mechanism for recirculating aged pollutants within the valley. During one tracer experiment, most of the released tracer was trapped within an elevated layer of air, a potentially important mechanism for multi-day impacts of pollutants.

Chapter 12 evaluates the transport of pollutants in the Santa Barbara Channel off the coast of southern California. Limited vertical mixing combined with diurnal wind reversals resulted in multi-day onshore impacts of the tracer released offshore. Efficient lateral mixing of the tracer during wind reversals led to a widespread coastal impact from a single point source. The existence of many point sources could result in a diluted background concentration (i.e. after wind reversals) that equals or exceeds the concentration directly downwind of a single source.

Chapter 13 develops a two layer model of the atmosphere that semi-quantitatively incorporates much of the basic transport structure observed in the above studies. The method of characteristics and the method of moments were used to examine the implications of the model. The model indicates that the air aloft must be considered in order to accurately predict the impact of a pollutant source, especially when considering the multi-day or long range impact of the source.

Thesis Files