Atmospheric Fate of Methyl Isocyanate, CH3NCO: OH and Cl Reaction Kinetics and Identification of Formyl Isocyanate, HC(O)NCO

CH3NCO, methyl isocyanate, is a toxic compound that is formed in the atmosphere following the use of agricultural and industrial precursors as well as emission from biomass burning. The atmospheric abundance and chemistry of CH3NCO, however, is presently not sufficiently well characterized to evaluate its overall environmental and health impacts. In this study, rate coefficients for the gas-phase reaction of OH radicals (k(1)) and Cl-atoms (k(2)) with CH3NCO were measured using pulsed laser photolysis-laser-induced fluorescence and relative rate methods at 295-375 and 296 K, respectively. We obtained k(1) (295 = 1.36 X 10(-13) cm(3) molecule(-1) s(-1 )with the temperature dependence given by the Arrhenius expression k(1) (295-375 K) = 6.11 X 10(-12) exp(-(1119 +/- 62)/T) cm(3) molecule(-1) s(-1). The Cl atom rate coefficient was determined to be k(2) (296 5.58 X 10(-)(12) cm(3) molecule(-1) s(-1). k(1) (295 K) from this study is compared with a previous room temperature measurement and an explanation for the significantly greater literature value, a factor of 26.6, is discussed. As part of this work, the stable end products following the OH radical- and Cl atom-initiated oxidation of CH3NCO were obtained in an environmental chamber reactor with infrared absorption detection. Formyl isocyanate, HC(O)NCO, was identified by comparison with its theoretically calculated infrared absorption spectrum as a major stable end product. A CH3NCO atmospheric degradation mechanism is proposed. In addition, quantitative UV and infrared absorption spectra of CH3NCO are reported.