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Published June 15, 1998 | public
Journal Article

Air Quality Model Evaluation Data for Organics. 5. C_6−C_(22) Nonpolar and Semipolar Aromatic Compounds


The concentrations of 86 vapor-phase, semivolatile, and particle-phase aromatic compounds are measured during a severe Los Angeles photochemical smog episode. The measurements are part of a larger experiment designed to acquire a nearly complete description of organic air pollutants for use in verifying the predictions of photochemical airshed models for organics. Compound classes formed by grouping all aromatic hydrocarbons having the same number of aromatic rings show progressively declining concentrations as the number of aromatic rings is increased. Examination of the partitioning of the polycyclic aromatic hydrocarbons (PAH) between the gas and particle phases shows the transition from purely gaseous PAH at low molecular weight to purely particle-phase PAH at high molecular weight, with compounds such as the mutagen cyclopenta[cd]pyrene present about equally in both gas and particle phases. Primary aromatics, both the vapor-phase mono-aromatics and the PAH, show evidence of depletion by atmospheric chemical reaction over downwind transport with apparent depletion rates generally increasing as the degree of substitution of the aromatic rings increases. In contrast, many nitro-PAH and some oxy-PAH accumulate during downwind transport, consistent with their likely formation as products of atmospheric chemical reactions. Historical data generally show that aromatics concentrations declined substantially from the 1950s to the 1980s, but that concentrations measured during the present 1993 experiment are very similar to those measured during the mid-1980s including the August episode of the 1987 SCAQS experiment. The present experiment provides baseline data prior to the introduction of California Phase II reformulated gasoline that can be used in future years to examine the effect of the reduced aromatic content of that Phase II gasoline.

Additional Information

© 1998 American Chemical Society. Received for review April 17, 1997. Revised manuscript received September 22, 1997. Accepted January 26, 1998. We thank Mr. Ed Ruth of UCLA for assistance with acquisition of the GC−MS data. Michael Hannigan and Claudine Butcher assisted in preparing for the field experiments. We appreciate the help of the 13 California Institute of Technology graduate students and staff who participated in air monitoring site operations for their assistance. We thank William Bope (South Coast Air Quality Management District) for logistics support in Azusa, Long Beach, and Los Angeles; Lowell Ashbaugh (California Air Resources Board), Eric Grosjean and Daniel Grosjean (DGA, Inc.), and Gervaise McKay (Unisearch Associates) for logistics support in Claremont. This research was supported by the Electric Power Research Institute under Agreement RP3189-03 and by the Caltech Center for Air Quality Analysis.

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August 19, 2023
October 19, 2023