Kinetics of the reaction of CH3O2+NO:: A temperature and pressure dependence study with chemical ionization mass spectrometry

The overall rate coefficient (k(4)) for the reaction CH3O2 + NO --> products (4) has been measured by using the turbulent flow technique with chemical ionization mass spectrometry (CIMS) for the detection of reactants and products. The temperature dependence of the rate coefficient was investigated between 193 and 300 K. Across the temperature range the experimentally determined rate coefficients were fitted by using an Arrhenius type analysis to yield the expression k(4) = (1.75(-0.24)(+0.28)) x 10(-12) exp[(435 +/- 35)/T] cm(3) molecule(-1) s(-1). Experiments were carried out at 100 and 200 Torr total pressure within the stated temperature range, where the rate coefficients were shown to be invariant with pressure. The branching ratio of the reaction was also assessed as a function of temperature and was found to proceed 100 +/- 10% via the channel forming CH3O + NO2, there being no discernible increase in the yield of CH3ONO2 at low temperatures. This work represents the first study of the branching ratio as a function of temperature and pressure. Previous studies have shown that the rate coefficient displays a negative temperature dependence, with the suggestion that the reaction rate increases with increasing pressure as well as increasing with decreasing temperature. This study lends weight to the assertion that there is no pressure effect (in agreement with a recent theoretical study) and that differences between previous studies at low temperature are most likely to be experimental errors. A model of the troposphere has been used to assess the impact of the experimental error of the rate coefficients determined in this study on predicted concentrations of a number of key species, including O-3, OH, HO2, NO, and NO,. In all cases it is found that the propagated error is very small and will not in itself be a major cause of uncertainty in modelled concentrations. However, at low temperatures where there is a wide discrepancy between existing kinetic studies, modelling using the range of kinetic data in the literature shows a relatively large variation for CH3O2, HCHO, and minor reservoir species such as HO2NO2 and CH3O2NO2. Such a discrepancy may have implications for observationally driven models operating in regions of the troposphere below 240 K.