The reaction of HO 2 with CH 3 C(O)O 2 is examined using flash photolysis and FTIR smog chamber techniques. Time-resolved UV spectroscopy is used to follow the transient peroxy species. It yields reasonable concentration versus time profiles for CH 3 C(O)O 2 and HO 2 , but indicates anomalously high levels of secondary CH 3 O 2 radicals. Transient IR diode laser absorption confirms the HO 2 decay rates; however, the anticipated reaction model substantially underestimates the observed decay. The model is augmented by assuming that, in analogy with formaldehyde, there exists a reaction between HO 2 and acetaldehyde (the precursor for CH 3 C(O)O 2 ). Consistent with this, the fitted rate for the hypothesized reaction increases with increasing initial acetaldehyde level. Relative rate measurements reveal that chlorine atoms remove more CH 3 CHO relative to CH 3 OH in air as compared to nitrogen diluent. This supports the hypothesis since, in the presence of oxygen, HO 2 is formed and presents an additional acetaldehyde removal pathway. Employing the augmented model, analyses of HO 2 decay traces yield a CH 3 C(O)O 2 + HO 2 rate constant of k 1 ) (3.9 -2.3 +5.0) × 10 -13 e (1350(250)/T cm 3 s -1 . Reasons are discussed for why the present rate constants are 2-3 times larger than previously reported. FTIR-smog chamber studies reveal the reaction to proceed via two channels to (a) peracetic acid and O 2 and to (b) acetic acid and O 3 , with a branching fraction at 295 K that is less than half of the literature value. Time-resolved UV absorption measurements support this smaller fraction; averaged together the two methods give k 1b /k 1 ) 0.12 ( 0.04. As part of this work, relative rate techniques are used to measure k(Cl+CH 3 C-(O)OH) ) (2.5 ( 0.3) × 10 -14 cm 3 s -1 and k(Cl+CH 3 C(O)OOH) ) (4.5 ( 1.0) × 10 -15 cm 3 s -1 at 295 K.

This study explores the influence of ethanol on particulate matter (PM) emissions from gasoline direct injection (GDI) vehicles, a technology introduced to improve fuel economy and lower CO 2 emissions, but facing challenges to meet next-generation emissions standards. Because PM formation in GDI engines is sensitive to a number of operating parameters, two engine calibrations are examined to gauge the robustness of the results. As the ethanol level in gasoline increases from 0% to 20%, there is possibly a small (<20%) benefit in PM mass and particle number emissions, but this is within test variability. When the ethanol content increases to >30%, there is a statistically significant 30%-45% reduction in PM mass and number emissions observed for both engine calibrations. Particle size is unaffected by ethanol level. PM composition is primarily elemental carbon; the organic fraction increases from ∼5% for E0 to 15% for E45 fuel. Engine-out hydrocarbon and NO x emissions exhibit 10-20% decreases, consistent with oxygenated fuel additives. These results are discussed in the context of the changing commercial fuel and engine technology landscapes.

Flash photolysis of CldCF3CHOhI gas mixtures (M = N2, SF6) combined with transient diode laser detection of CO is used to investigate the kinetics of the CF3CO + M -CF3 + CO + M dissociation reaction and the competition with the CF3CO self-reaction. The studies are carried out over a temperature range 223-353 K and a pressure range 25-650 Torr. The temperature dependent low-and high-pressure-limiting rate constants in NZ are given by k2.0 = (3.9 x 10-")e-(255M650fl cm3 s-I and &2,.. = (1.5 x 10")e-(424M660Y7 s-I, respectively, with F, = 0.6. Essentially the same high-pressure rate constant is obtained for a bath gas of SF6. These results clarify a previous discrepancy of about 10 k c d m o l in the activation energy. As part of this study, we report a rate constant of k3 = (2.5 f 1.5) x lo-'' cm3 s-' for the reaction CF3CO + CF3CO -(CF3CO)2 and a rate constant of k4 = (6.1 f 2.5) x cm3 s-I for CF3CO + C12 -CF3C(O)C1+ C1. Both are essentially independent of temperature over the range 223-353 K.

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