Pressure and temperature dependence of methyl nitrate formation in the CH _3O _2 + NO reaction

摘要

The branching ratio β = k _(1b)/k _(1a) for the formation of methyl nitrate, CH _3ONO _2, in the gas-phase CH _3O _2 + NO reaction, CH _3O _2 + NO → CH _3O + NO _2 (1a), CH _3O _2 + NO → CH _3ONO _2 (1b), has been determined over the pressure and temperature ranges 50-500 Torr and 223-300 K, respectively, using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. At 298 K, the CH _3ONO _2 yield has been found to increase linearly with pressure from 0.33 ± 0.16 at 50 Torr to 0.80 ± 0.54 at 500 Torr (errors are 2σ). Decrease of temperature from 300 to 220 K leads to an increase of β by a factor of about 3 in the 100-200 Torr range. These data correspond to a value of β ≈ 1.0 ± 0.7 over the pressure and temperature ranges of the whole troposphere. Atmospheric concentrations of CH _3ONO _2 roughly estimated using results of this work are in reasonable agreement with those observed in polluted environments and significantly higher compared with measurements in upper troposphere and lower stratosphere.