Research Projects

Project at a Glance

Project Status: complete

Report Published May 1982:

Title: Impact of transport from the South Coast Air Basin on ozone levels in the southeast desert air basin

Principal Investigator / Author(s): Lehrman, D.

Contractor: California Institute of Technology and Meteorology Research, Inc.

Contract Number: A0-145-32


Research Program Area: Atmospheric Processes

Topic Areas: Transport


Abstract:

An observational program was carried out during July-August 1981 to investigate the impact of transport from the South Coast Air Basin on ozone levels in the Southeast Desert Air Basin. The study was conducted jointly by the California Institute of Technology, Division of Chemistry and Chemical Engineering and Meteorology Research, Inc. The present volume (Volume I) is an Executive Summary of the program. Volume II gives the detailed results of the study. Volumes III and IV are data volumes prepared by the California Institute of Technology and Meteorology Research, Inc., respectively.

A total of eight tracer releases were carried out by the California Institute of Technology using SF6 as the tracer material. A total of 34-hourly sampler sites were established for the study. Not all sites were utilized for any given test. Numerous syringe samples were taken during automobile traverses and aircraft flights.

A total of five ozone monitoring stations were established for the duration of the program to supplement the existing ozone network. Aircraft flights were made on each test day by MRI to sample air quality and meteorological parameters. Pibal wind observations were usually made at four locations in support of each test. One of the locations also released an Airsonde at intervals during each test to obtain additional information on vertical temperature structure.

Three tracer releases were made from the western part of the Los Angeles basin (Culver City, Carson and Garden Grove) to document transport into the desert from this area. Two releases were made from the eastern part of the basin (Ontario and South Fontana). One release each was made along the Burbank-Newhall-Palmdale exit route and one in Cajon Pass to investigate transport along these paths. A final release was made at Brawley to examine transport from the Imperial Valley northward into the Coachella Valley.

Tracer trajectories from the western part of the Los Angeles basin followed routes through Soledad Canyon, Cajon Pass, San Gorgonio Pass, up the slopes of the San Gabriel and San Bernardino Mts. and into the Elsinore convergence zone, depending on release location and time of day. Trajectories from the eastern part of the basin and from the immediate pass areas followed expected routes into the desert through the pass and up the mountain slopes. Significant tracer concentrations were observed in the desert from all seven releases made in the Los Angeles basin.

Ozone monitoring data were available during the program from Mt. Wilson, Mt. Baldy, Lake Gregory and Fawnskin (Big Bear). Peak hourly ozone values at Mt. Baldy and Lake Gregory during the field program were the same as Fontana (35 pphm). Maximum observed concentration at Mt. Wilson was 29 pphm. The Fawnskin maximum was only 15 pphm. These data and supporting evidence indicate that a major exit route for pollutants from the Los Angeles basin lies along the slopes of the San Gabriel Mts., through Cajon Pass and extending to the western slopes of the San Bernardino Mts. To the east of Lake Gregory, the transport of pollutants into the San Bernardino Mts. Decreases sharply. Other major routes also exist through San Gorgonio Pass and Soledad Canyon. Low-level pollutant transport occurs through the passes and affects the desert areas immediately downwind. In all passes, however, there was evidence of a separate, higher ozone layer moving through the pass which was associated with slope flow along the shoulders of the pass. This upper layer was brought to the surface, on occasion, by lee wave action in the Coachella Valley. On the two observational days in the Mojave Desert, the upper layer remained aloft in spite of strong surface heating.

During the summer months a strong flow of air occurs from the San Joaquin Valley into the western portion of the Mojave Desert, continuing eastward toward Barstow. There is a zone of confluence between this flow and the flow through Soledad Canyon which appears to lie near or slightly north of Edwards AFB on a mean basis. Most frequent wind direction streamlines through Edwards and Barstow suggest an influence from the San Joaquin Valley while Palmdale and Lancaster are most frequently under the influence of the Soledad Canyon flow. Highest ozone concentrations at Edwards AFB, however, appear to be associated with flow from Soledad Canyon which brings air into the desert from areas of large emission sources in Los Angeles County. There was no evidence from ozone concentration, tracer data or wind flow patterns of a Soledad Canyon influence as far north as China Lake.

Carry-over of pollutants in the desert was observed in the form of deep layers of ozone in early morning sampling flights. These layers resulted from transport from the Los Angeles basin during the previous afternoon and evening. On two mornings, peak observed concentrations aloft were 10 pphm or more (Lucerne Valley and Palm Springs). A comparison was made at Palm Springs and Indio of those cases which showed clear evidence of late evening transport from the Los Angeles basin and those where the significance of the transport was much less or non-existent. During the following forenoon the data suggested that the principal effect of the transport was to raise the overall ozone background in the area. The magnitude of the diurnal increase in ozone concentration (from morning minimum to midday maximum) did not appear to be related to the transport of pollutants during the previous evening.

Recommendations include further studies on the effect of slope transport from the Los Angeles basin and the influence of the San Joaquin Valley flow on the Mojave Desert. The importance of the eastern San Gabriel Valley as an early morning reservoir of basin pollutant should also be explored.


 

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