Pressure and Temperature Dependence of Methyl Nitrate Formation in the CH₃O₂+ NO Reaction

Abstract

The branching ratio β = k(1b)/k(1a) for the formation of methyl nitrate, CH(3)ONO(2), in the gas-phase CH(3)O(2) + NO reaction, CH(3)O(2) + NO → CH(3)O + NO(2) (1a), CH(3)O(2) + NO → CH(3)ONO(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(3)ONO(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(3)ONO(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.

The impact of the chemical production of methyl nitrate from the NO + CH<sub>3</sub>O<sub>2</sub> reaction on the global distributions of alkyl nitrates, nitrogen oxides and tropospheric ozone: a global modelling study

“…The long-range transport of CH 3 ONO 2 out of the tropics below 800 hPa at higher latitudes is shown to be negligible due to loss by deposition at the surface. The global tropospheric lifetime of CH 3 ONO 2 in EMISS is ∼ 20 days, well within the estimates of between 7 days to a month as given in the literature (Butkovskaya et al, 2012). Chemical destruction predominantly occurs in the tropics, where dry deposition accounts for ∼ 50 % of the total sink term.…”

“…The comparison clearly shows that HIGHBR results in an excess of CH 3 ONO 2 , with mixing ratios approximately an order of magnitude too high for the NH sites shown and a factor of two too high for Antarctica. Assuming that there is not an important sink process missing from the simulations, such as heterogeneous uptake into particles, these comparisons suggest that the lower limit of the branching ratio measured by Butkovskaya et al (2012) seems the most plausible value.…”

“…The CH 4 distribution in the boundary layer is constrained by nudging to a seasonally dependant latitudinal gradient up to 500 hPa using the method given in Williams et al (2013). In total, six sensitivity studies are defined and listed in Neu et al (2008) and applied between 10 • S-10 • N; (2) DEMISS, which applies a direct a priori oceanic emission flux for CH 3 ONO 2 twice that of EMISS to represent an upper limit; (3) LOWBR, with which we adopt a low branching ratio of 0.3 % for the chemical formation of CH 3 ONO 2 (the lower limit of the experimental data in Butkovskaya et al, 2012); (4) HIGHBR with a branching ratio of 1.0 %; (5) FULL, with which we reduce the direct emission by 50 % compared to EMISS and apply the low branching ratio of 0.3 %; (6) DDEP, applied in the same manner as FULL, except we double the dry deposition flux of CH 3 ONO 2 ; and (6) FLIGHT, applied in the same manner as DEMISS, except that we use a branching ratio of 0.025 % which is the mean value suggested by Flocke et al (1998a) derived from measurements made in the upper troposphere (UT; above 10 km until the tropopause). In the EMISS, FULL, DDEP and FLIGHT simulations we also include the direct emissions of RONO 2 from the ocean, adopting an estimate of 15.62 mg m −2 s −1 (∼ 0.17 Tg N yr −1 ), which is twice the a priori estimate calculated for C 2 H 5 ONO 2 in Neu et al (2008) so as to represent the release of other alkyl nitrates such as e.g.…”

“…Sander et al, 2011) due to the lack of an independent measurement for this branching ratio. Using the same technique, Butkovskaya et al (2012) recently detected the direct formation of methyl nitrate (CH 3 ONO 2 ) from Reaction (2). The branching ratio (Reaction 10) inferred for tropospheric conditions is 1.0 ± 0.7 %, and it has a weak temperature and pressure dependence:…”

Theoretical study on the atmospheric reaction of CH3O2with OH

“…Methyl peroxy radical (CH 3 O 2 ) has drawn wide attention owing to its importance in the photochemical cycling of ozone in atmospheric chemistry . Several experimental data for CH 3 O 2 reactions with molecules or radicals, such as O 3 , NO, NO 2 , ClO, IO, Cl, and Br, have been reported. There are also some theoretical researches on the kinetics or mechanisms of CH 3 O 2 reactions with F, Cl, Br, NO, ClO, IO, and HO 2 .…”