Factor Analysis of Hydrocarbon Species in the South-Central Coast Air Basin

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.