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James P. Killus and Gary E. Moore

Abstract

The composition of canister hydrocarbon data collected at four surface sites and from aircraft during a 1985 field experiment in California's south-central coast air basin was analyzed to determine the source. Statistical routines from a commonly used software package (SAS) were used to perform a principal component extraction on a 15-variable vector space containing the most important hydrocarbons (as well as CO) in the air quality samples. The ratios of each of the 15 hydrocarbon species to total nonmethane hydrocarbon (NMHC) were plotted as a function of the ethane-to-acetylene (E/A) ratio. The plots serve as an estimator of the source profile and were consistent with the source profiles obtained by the principal component analysis.

The source reconciliation analysis identified two principal sources. One appears to be rich in acetylene, carbon monoxide (CO), and some unique hydrocarbons. This source corresponds closely to accepted hydrocarbon profiles for automobile emissions. The second major source is high in methane, ethane, and propane, but also contains reasonable amounts of butane and pentane. This type of composition suggests a geogenic source. In addition to these two major sources, clean oceanic air was identified as a third component. The biogenic compound isoprene also occurred in some samples.

Canister data from three of four sites were dominated by a profile of trace gases associated with a geogenic source. The reactivity of the geogenic air mass is about 50% that of urban air. The dominance of such a low-reactive emission source affects the amount of ozone predicted by photochemical modeling of the region.

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David G. Strimaitis, Gary E. Moore, and Sharon G. Douglas

Abstract

This paper contains an analysis of data obtained from measurements of the concentration of tracer gases released during the four intensive measurement periods of the 1985 South-Central Coast Cooperative Air Monitoring Program (SCCCAMP). These tracer experiments were designed to document mesoscale circulation patterns in the coastal region of Santa Barbara and Ventura counties in southern California. Analyses of these concentration data are aimed at evaluating the design of the experiments, describing the movement of the tracers and comparing transport patterns to measured winds, and evaluating the ability of trajectory calculations, which are based on winds from a diagnostic wind model to reproduce those patterns of transport.

The study concludes that patterns of recirculation over the 2-day period of the tests are successfully documented, revealing transport in the Santa Barbara Channel that is consistent with the circulation about the midchannel eddy, and diurnal patterns of onshore–offshore flow. Trajectories based on an interpolation of observed winds disagreed with the observed transport patterns of tracer material, especially for releases that initially traveled into the channel during the early morning hours. Apparently, the circulation in the channel and the complex flow near the coast was not sufficiently resolved by the measurement network.

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Steven R. Hanna, David G. Strimaitis, Joseph S. Scire, Gary E. Moore, and Robert C. Kessler

Abstract

The subject of this paper is an overview of the data analysis collected during the comprehensive five-week South-Central Coast Cooperative Air Monitoring Program (SCCCAMP 1985) in California during September and October 1985. The various data analyses have been designed to develop a better understanding of the atmospheric processes that cause elevated ozone concentrations in the region, which includes Ventura and Santa Barbara counties.

The data analyses lead to the conclusion that the ozone episodes observed during the field study are typical of those occurring during the past several years and are most strongly correlated with clear skies, high temperatures (i.e., warm high pressure), and with pressure gradients that would imply an easterly component to the wind flow. These episodes are also marked by mixing depths of 100 m or less over the water and by low mixing depths (i.e., a few hundred meters) over the coastal plains. While it is clear that much of the local ozone is caused by local sources, it is also evident that during many ozone episodes, ozone and its precursors are advected into the region from sources to the east, in the vicinity of Los Angeles. Observed ozone and wind patterns suggest that this advection sometimes takes place at elevations of a few hundred meters through inland valleys and along the coast, and sometimes it takes place in a shallow layer near the surface over the coastal ocean. Because of the presence of time-varying sea breezes and mesoscale eddies, it is possible for recirculation of pollutants to occur, as verified by tracer experiments conducted during the field study.

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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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