Atmospheric Chemistry of Dimethyl Carbonate:  Reaction with OH Radicals, UV Spectra of CH₃OC(O)OCH₂ and CH₃OC(O)OCH₂O₂ Radicals, Reactions of CH₃OC(O)OCH₂O₂ with NO and NO₂, and Fate of CH₃OC(O)OCH₂O Radicals

Abstract

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 absorption 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 constants 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 .