Air quality model evaluation data for organics. 5. C₆-C₂₂ 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.