The Pressure Dependence of the OH Radical Yield from Ozone−Alkene Reactions

Abstract

The OH radical is the key oxidizing agent in the troposphere, and ozone-alkene reactions appear to be a significant and sometimes dominant source of new HO x radicals in urban and rural air. In this work, we report the first study of the pressure dependence of the OH radical yield for the ozonolysis of ethene, propene, 1-butene, trans-2-butene, and 2,3-dimethyl-2-butene over the range 20-760 Torr and of trans-3-hexene and cyclopentene over the range 200-760 Torr. Low-pressure experiments were performed in a long-path evacuable FTIR cell or a steady-state flow-tube reactor in series with a gas chromatograph/flame ionization detector and FTIR cell. We have also investigated the effect of adding SF 6 at atmospheric pressure for ethene, 1-butene, and trans-2-butene, in a collapsible Teflon chamber. OH formation increased almost 3-fold for ethene at low pressures, from 0.22 ( 0.06 at 760 Torr to 0.61 ( 0.18 at 20 Torr, and increased somewhat for propene from 0.33 ( 0.07 at 760 Torr to 0.46 ( 0.11 at 20 Torr. A pressure dependence of the OH formation yield was not observed for 1-butene, trans-2-butene, 2,3-dimethyl-2-butene, trans-3-hexene, or cyclopentene over the ranges studied. Density functional theory calculations at the B3LYP/6-31G(d,p) level are presented to aid in understanding the trends observed. They lead to the proposal that the formation of a hydroperoxide via a diradical pathway can compete with the formation of the carbonyl oxide for the ethene primary ozonide.

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

“…Table 1 shows the assignment of different oligomer series observed in the ESI mass spectra. In the absence of a scavenger, OH radicals formed by ozonolysis with a yield 61 of B47%, will react in air with trans-3-hexene, producing primarily CH 3 CH 2 CH(OH)CH(OO )CH 2 CH 3 (hereafter referred to as HE-RO 2 , MW 133) as the main RO 2 radicals. As observed in Fig.…”

Technical note: Measuring tropospheric OH and HO<sub>2</sub> by laser-induced fluorescence at low pressure. A comparison of calibration techniques

“…The OH yield reported from scavenger studies does not exhibit a pressure dependence for T2B in the range 27-1013 hPa (Fenske et al, 2000;Hard et al, 2002). It has been argued that TCIs can undergo reactions with water-vapor (Neeb et al, 1997;Tobias and Ziemann, 2001), resulting in an OH yield that would depend on the water-vapor concentration.…”

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II – gas phase reactions of organic species

“…This assertion was supported by further LIF studies in which HO production in the ozonolysis of 2,3-dimethyl-2-butene, at one atmosphere of pressure, was seen to increase over significantly longer time-scales (Kroll et al, 2001b). A study by Johnson et al (2001) looked at the HO production efficiency of the reaction of ozone with 2-methyl-2-butene in the presence of relatively high concentrations of scavenger species believed to react with Criegee intermediates. Under the conditions of these latter experiments (with concentrations of scavenger species in excess of those that would be encountered in the troposphere), no effect upon the yield of HO was observed.…”

“…Studies have been made of the reactions of RCHOO with aldehydes (Su et al, 1980;Fenske et al, 2000;Cox and Penkett, 1972;Hanst et al, 1958;Kan et al, 1981;Niki et al, 1975Niki et al, , 1981Neeb et al, 1995), SO 2 Su et al, 1980, Cox andPenkett, 1972;Kerr and Calvert, 1984;, H 2 O (Cox and Penkett, 1972;Akimoto et al, 1979;Kan et al, 1981), carboxylic acids (Neeb et al, 1995; and methanol , but detailed kinetic data are often lacking. Relative rate coefficients have been derived by , Su et al (1980) and Suto et al (1985) based on experimental measurements of the rates of consumption of molecular reactants relative to consumption of SO 2 in systems involving RCHOO Criegee intermediates.…”

“…In the absence of direct kinetic measurements of the absolute rate coefficients for any of the RCHOO bimolecular reactions (excepting the recent measurement of Fenske et al (2000) for CH 3 CHOO + CH 3 CHO which cannot be used with any of the relative-rate measurements made to date), both Atkinson and Lloyd (1984) and Kerr and Calvert (1984) have suggested that k 6 should be equated to the rate coefficient for the structurally analogous reactions, RO 2 +NO→RO+NO 2 , with k 6 =7×10 −12 cm 3 molecule −1 s −1 at 298 K. While this seems a reasonable proposition, it is desirable to obtain experimental verification. At present it is difficult to see how any direct measurements could be made with RCHOO systems involving O 3 -alkene reactions owing to the complex chemistry involved.…”