Nitramine and nitrosamine formation is a minor pathway in the atmospheric oxidation of methylamine: A theoretical kinetic study of the CH3NH + O2 reaction

The atmospheric oxidation of amines proceeds via initial radical attack at C-H or N-H bonds to form carbon- and nitrogen-centered radicals, respectively. It is conventionally assumed that nitrogen-centered aminyl radicals react slowly with oxygen in the troposphere and associate predominantly with the radicals (NO)-N-center dot and NO2 center dot to form toxic nitrosamines and nitramines. We have used theoretical kinetic modeling techniques to study the prototypical (CH3NH)-H-center dot + O-2 reaction and have shown that it proceeds to CH2NH + HO2 center dot under tropospheric conditions with a rate coefficient of 3.6 x 10(-17) cm(3) molecule(-1) s(-1). Although this value is low compared to the competing NOx reactions (similar to 10(-11) cm(3) molecule(-1) s(-1)), the much higher concentration of O-2 versus NOx in air makes it the dominant process in the atmospheric oxidation of methylamine for NOx concentrations below 100 ppb. The mechanism identified here is available to amines with primary, secondary, and tertiary alpha carbons and suggests that they may be less likely to form nitramines and nitrosamines than is currently thought.