Atmospheric Chemistry of N-Methylmethanimine (CH₃N═CH₂): A Theoretical and Experimental Study

The OH-initiated photo-oxidation of N-methylmethanimine, CH₃N═CH₂, was investigated in the 200 m³ EUPHORE atmospheric simulation chamber and in a 240 L stainless steel photochemical reactor employing time-resolved online FTIR and high-resolution PTR-ToF-MS instrumentation and in theoretical calculations based on quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations forecast the OH reaction to primarily proceed via H-abstraction from the ═CH₂ group and π-system C-addition, whereas H-abstraction from the -CH₃ group is a minor route and forecast that N-addition can be disregarded under atmospheric conditions. Theoretical studies of CH₃N═CH₂ photolysis and the CH₃N═CH₂ + O₃ reaction show that these removal processes are too slow to be important in the troposphere. A detailed mechanism for OH-initiated atmospheric degradation of CH₃N═CH₂ was obtained as part of the theoretical study. The photo-oxidation experiments, obstructed in part by the CH₃N═CH₂ monomer-trimer equilibrium, surface reactions, and particle formation, find CH₂═NCHO and CH₃N═CHOH/CH₂═NCH₂OH as the major primary products in a ratio 18:82 ± 3 (3σ-limit). Alignment of the theoretical results to the experimental product distribution results in a rate coefficient, showing a minor pressure dependency under tropospheric conditions and that can be parametrized k(T) = 5.70 × 10^-14 × (T/298 K)^3.18 × exp(1245 K/T) cm³ molecule^-1 s^-1 with k₂₉₈ = 3.7 × 10^-12 cm³ molecule^-1 s^-1. The atmospheric fate of CH₃N═CH₂ is discussed, and it is concluded that, on a global scale, hydrolysis in the atmospheric aqueous phase to give CH₃NH₂ + CH₂O will constitute a dominant loss process. N₂O will not be formed in the atmospheric gas phase degradation, and there are no indications of nitrosamines and nitramines formed as primary products.