13624
J. Phys. Chem. 1996, 100, 13624-13628
Direct Kinetics Study of the CH3C(O)O2 + NO Reaction Using Chemical Ionization Mass Spectrometry Peter W. Villalta†,‡ and Carleton J. Howard* Aeronomy Laboratory, NOAA, EnVironmental Research Laboratories, Boulder, Colorado, 80303 ReceiVed: May 16, 1996X
A direct measurement of the CH3C(O)O2 + NO gas-phase reaction rate coefficient over the temperature range 200-402 K was made using chemical ionization mass spectrometric detection of the CH3C(O)O2 reactant. A significant temperature dependence was observed, and a temperature dependent expression of k(T) ) (8.1 ( 1.3) × 10-12 exp{(270 ( 60)/T} cm3 molecule-1 s-1 was determined. The 298 K rate coefficient, k ) (2.0 ( 0.3) × 10-11 cm3 molecule-1 s-1, agrees well with results from previous indirect measurements. NO2, CH3, and CO2 were positively identified as products originating from the reaction. The question of whether CH3 and CO2 are direct products of the reaction or result from the thermal decomposition of CH3C(O)O could not be answered. The 298 K rate coefficients for the reactions of SF6-, I-, and O3- with CH3C(O)O2 +4 +7 were measured to be (7-2 ) × 10-10, (9-5 ) × 10-10, and g2 × 10-10 cm3 molecule-1 s-1, respectively.
Introduction The peroxyacetyl radical, CH3C(O)O2, is formed in the troposphere in the course of oxidation of many organic species with two or more carbons.1 In polluted air, the peroxyacetyl radical reacts primarily with NO and NO2.
CH3C(O)O2 + NO f CH3C(O)O + NO2 M
CH3C(O)O2 + NO2 98 CH3C(O)O2NO2
(1) (2)
While the products of reaction 1 have not been conclusively identified, those shown above have been assumed to be correct.2 The degree to which these competing reactions occur has a large impact on regional air pollution. Reaction 1 initiates the production of ozone via the conversion of NO to NO2.1 Further generation of O3 is expected if the CH3C(O)O radical falls apart owing to thermal instability to form CO2 and a methyl radical.
CH3C(O)O f CH3 + CO2
(3)
The oxidation of the methyl radical leads to the conversion of additional NO to NO2. Reaction 2 produces CH3C(O)O2NO2 (PAN), which acts as a reservoir for NOX (NO and NO2) and peroxyacetyl radicals and therefore can lead to the transport of these species to relatively unpolluted regions of the troposphere.1 In spite of the importance of reactions 1 and 2, reaction 1 has never been directly examined. The ratio of the rate coefficients for reactions 1 and 2 (k1/k2) has been determined in several studies.3-7 The ratio was measured at various temperatures (283-328 K) in several of the studies4-7 and found to be independent of temperature in each case, although the temperature ranges covered were modest. The recommended value for the 298 K rate coefficient of reaction 1 is derived by using the measured k1/k2 ratios and the recommended2,8 rate coefficient for reaction 2 and is reported +2.4 ) × 10-11 cm3 molecule-1 s-1 in the JPL NASA as (2.4-0.8 +1.2 2 ) × 10-11 cm3 molecule-1 s-1 in the evaluation and (2.0-0.7 IUPAC evaluation.8 The recommended E/R value is reported † Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO. ‡ Present Address: Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts, 01821-3976. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)01415-3 CCC: $12.00
as 0 ( 200 K2 and 0 ( 600 K,8 respectively. In the first instance, the E/R value is based upon analogy with other RO2 + NO reactions. In the second instance, the E/R value is based upon the lack of temperature dependence found in studies by Kirchner et al. (304-321 K)6 and Tuazon et al. (283-313 K).7 The products of reaction 1 are thought to be CH3C(O)O and NO2 but have never been directly observed. In the present study, our primary objective is to make direct measurements of the rate coefficient for reaction 1 over a wide temperature range (200-402 K) using a flow tube reactor with chemical ionization mass spectrometric detection. An investigation of product formation will also be undertaken by searching for the production of CH3C(O)O, NO2, CO2, and CH3. Experimental Section Apparatus. The experimental apparatus consists of a neutral flow tube reactor coupled to an ion flow tube/quadrupole mass spectrometer. It has been described in detail previously9,10 and therefore will be only briefly outlined here. The gas from the neutral flow reactor enters the ion flow tube through a Pyrex valve used to control the neutral flow tube pressure (1.2-6.0 Torr) relative to the ion flow tube pressure (∼0.5 Torr). He (>99.9995%) flows of 16.7-22.5 STP cm3 s-1 are used in the neutral flow tube with flow speeds of 10002740 cm s-1. NO flows ranging from 8.2 × 10-5 to 3.6 × 10-3 STP cm3 s-1 are delivered to the flow tube through a 120 cm long × 0.64 cm o.d. Pyrex injector and result in NO concentrations of 3.5 × 1011 to 1.0 × 1013 molecules cm-3 in the neutral flow tube. The NO is delivered to the flow tube in two ways either as pure NO or out of a Pyrex bulb containing a mixture of 0.66% NO in He. In both cases, the NO (g99.0%) used is passed at ∼2 atm through a dry-ice-cooled silica gel trap to reduce the amount of nitrogen oxide impurities present. After they are trapped, the NO2 impurity in the NO is estimated to be
