Rate Constants for the Gas-Phase Reactions of the OH Radical with Selected Carbamates and Lactates

Abstract

Using a relative rate method, rate constants have been measured for the gas-phase reactions of the volatile carbamates CH3NHC(O)OCH3, CH3NHC(O)OCH2CH3, CH3CH2NHC(O)OCH3 and CH3CH2NHC(O)OCH2CH3 and the lactates CH3CH(OH)C(O)OCH3, and CH3CH(OH)C(O)OCH2CH3 with the OH radical at 296 ± 2 K of (in units of 10-12 cm3 molecule-1 s-1) 4.3 ± 1.2, 8.3 ± 2.2, 10.5 ± 2.9, 14.4 ± 3.7, 2.8 ± 0.8, and 3.9 ± 1.1, respectively, where the error limits include the estimated overall uncertainties in the rate constants for the reference compounds. Rate constants were also measured for the reactions of the carbamates with NO3 radicals and O3, with respective NO3 radical and O3 reaction rate constants (in cm3 molecule-1 s-1 units) of CH3NHC(O)OCH3, (8.5 ± 3.2) × 10-16 and <4 × 10-20; CH3NHC(O)OCH2CH3, (9.4 ± 3.5) × 10-16 and <4 × 10-20; CH3CH2NHC(O)OCH3, (1.8 ± 0.8) × 10-15 and <6 × 10-20; and CH3CH2NHC(O)OCH2CH3, (2.6 ± 1.3) × 10-15 and <3 × 10-20. The OH radical reaction rate constants for the carbamates have been used to extend a previously developed estimation method to carbamates of the structure R1NHC(O)OR2 (where R1 and R2 are alkyl groups only), allowing OH radical reaction rate constants and hence tropospheric lifetimes to be calculated for carbamates used in agriculture for which experimental data are not available.