Peroxyacyl nitrates RC(O)OONO{sub}2 play an important role in urban air quality and tropospheric chemistry. They also receive attention as mutagens, phytotoxins, and possible air quality indicators of changes in vehicle fuel composition. Ambient concentrations of PAN (R = CH{sub}3) and PPN (R = C{sub}2H{sub}5) have been measured during summer 1997 at two southern California locations, Azusa (July 14-October 16) and Simi Valley (June 18-October 16). The highest concentrations were 4.8 ppb for PAN and 0.72 ppb for PPN in Azusa and 3.0 ppb for PAN and 0.28 ppb for PPN in Simi Valley. Ambient levels of PAN and PPN during summer 1997 were lower than those measured in the last three studies carried out in southern California in the summers of 1990, 1991, and 1993. Average PPN/PAN concentration ratioswereaboutthe same in Azusa (0.142±0.025, n= 132) and in Simi Valley (0.135 ± 0.028, n = 138). The PPN/PAN ratio measured in Azusa was the same as that measured at that location in 1993 prior to the introduction in 1996 of California Phase 2 reformulated gasoline. Diurnal variations of PAN and PPN generally followed those of ozone with respect to time of day but not with respect to amplitude. The PAN/ozone ratio was lower in Simi Valley than in Azusa, and daytime minima were recorded at both locations. The amount of PAN lost by thermal decomposition accounted for large fractions of the amount of PAN formed (measured + decomposed) during daytime hours at both locations. The amount of PAN lost by thermal decomposition was higher in Azusa and was up to ca. 8.5 ppb, i.e., 4-5 times more than that measured, when afternoon temperatures were ca. 40 ℃.
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