Atmospheric Chemistry of Dimethyl Carbonate: Reaction with OH Radicals, UV Spectra of CH 3 OC(O)OCH 2 and CH 3 OC(O)OCH 2 O 2 Radicals, Reactions of CH 3 OC(O)OCH 2 O 2 with NO and NO 2 , and Fate of CH 3 OC(O)OCH 2 O Radicals M. Bilde, T. E. Møgelberg, J. Sehested,* and O. J. Nielsen* Section for Chemical ReactiVity, EnVironmental Science and Technology Department, Risø National Laboratory, DK-4000 Roskilde, Denmark T. J. Wallington,* M. D. Hurley, S. M. Japar, and M. Dill Research Staff, SRL-E3083, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053 V. L. Orkin, T. J. Buckley, R. E. Huie, and M. J. Kurylo Center for Chemical Physics, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: June 5, 1996; In Final Form: March 4, 1997 X A flash photolysis-resonance fluorescence technique was used to study the rate constant for the reaction of OH radicals with dimethyl carbonate over the temperature range 252-370 K. The rate constant exhibited a weak temperature dependence, increasing at both low and high temperature from a minimum value of approximately 3.1 × 10 -13 cm 3 molecule -1 s -1 near room temperature. Pulse radiolysis/transient UV absorp- tion techniques were used to study the ultraviolet absorption spectra and kinetics of CH 3 OC(O)OCH 2 and CH 3 OC(O)CH 2 O 2 radicals at 296 K. Absorption cross sections of CH 3 OC(O)OCH 2 and CH 3 OC(O)OCH 2 O 2 at 250 nm were (3.16 ( 0.34) × 10 -18 and (3.04 ( 0.43) × 10 -18 cm 2 molecule -1 , respectively. Rate con- stants measured for the self-reactions of CH 3 OC(O)OCH 2 and CH 3 OC(O)OCH 2 O 2 radicals and reactions of CH 3 OC(O)OCH 2 O 2 radicals with NO and NO 2 were (5.6 ( 1.1) × 10 -11 , (1.27 ( 0.21) × 10 -11 , (1.2 ( 0.2) × 10 -11 , and (1.2 ( 0.2) × 10 -11 cm 3 molecule -1 s -1 , respectively. The rate constant for reaction of F atoms with dimethyl carbonate was determined by a pulse radiolysis absolute rate technique to be (6.1 ( 0.9) × 10 -11 cm 3 molecule -1 s -1 . A FTIR smog chamber system was used to show that, in 760 Torr of air at 296 K, CH 3 OC(O)OCH 2 O radicals are lost via three competing processes: 42 ( 15% via reaction with O 2 , 14 ( 2% via H atom elimination, and 44 ( 10% via decomposition and/or isomerization. Relative rate techniques were used to measure rate constants for the reactions of F atoms with CH 3 OC(O)OCH 3 , (6.4 ( 1.4) × 10 -11 cm 3 molecule -1 s -1 , and Cl atoms with CH 3 OC(O)OCH 3 , CH 3 OC(O)OCH 2 Cl, CH 3 OC(O)- OCHO, and HC(O)OC(O)OCHO, (2.3 ( 0.8) × 10 -12 , (4.6 ( 2.8) × 10 -13 , (1.7 ( 0.1) × 10 -13 , and (1.7 ( 0.1) × 10 -14 cm 3 molecule -1 s -1 , respectively. Results are discussed in the context of the atmospheric chemistry of CH 3 OC(O)OCH 3 . 1. Introduction The use of oxygenated compounds in motor vehicle fuels is accelerating rapidly. In the United States this change in fuel composition has been pushed by the 1990 Clean Air Act Amendments. These amendments mandate the use of oxygen- ated fuels in areas of the United States which exceed the National Ambient Air Quality Standard for carbon monoxide during the winter and in the nine worst summer smog areas. Under ideal conditions, oxygenated fuel components perform three simultaneous functions: to increase the fuel oxygen content (and thereby reduce CO emissions from carburated vehicles), to enhance the fuel octane value, and to lower the fuel’s Reid vapor pressure (RVP). The currently preferred oxygenated fuel additives, ethanol and methyl tert-butyl ether (MTBE), have limitations. In particular, the use of ethanol effectively raises fuel RVP and, therefore, fuel-related evapora- tive emissions. For MTBE and other ethers such as ethyl tert- butyl ether (ETBE) and tert-amyl methyl ether (TAME), the relatively low oxygen content requires high blending volumes to meet fuel oxygen requirements. For example, to meet a 2.7 wt % O standard requires 15.1% (by volume) MTBE in a standard fuel. Other oxygenated compounds, including organic carbonates, 1 have been suggested as potential oxygenated fuel additives because their use can minimize these problems. For example, dimethyl carbonate and diethyl carbonate have very high oxygen contents (53.3 and 40.6 wt %, respectively) compared to MTBE (18.2% wt oxygen), and their high boiling points can lead to a reduction in the RVP of the blended fuel. 1 The potential use of organic carbonates in motor vehicle fuels introduces a general need to map out the oxidation mechanism of oxygenated organic molecules and necessitates an under- standing of the environmental impact of such compounds if they are released into the atmosphere. This work addresses the oxidation mechanisms and atmospheric degradation of dimethyl carbonate (DMC). Upon release to the atmosphere, DMC reacts with OH radicals: As will be discussed, our measurements of k 1 are suggestive of both direct hydrogen abstraction as well as the formation of an addition complex that serves to facilitate such H atom abstrac- tion. Thus, for atmospheric considerations, the products from reaction 1 can be considered as written. The oxygenated alkyl X Abstract published in AdVance ACS Abstracts, April 15, 1997. CH 3 OC(O)OCH 3 + OH f CH 3 OC(O)OCH 2 + H 2 O (1) 3514 J. Phys. Chem. A 1997, 101, 3514-3525 S1089-5639(96)01664-7 CCC: $14.00 © 1997 American Chemical Society