The impact of the chemical production of methyl nitrate from the NO + CH₃O₂ reaction on the global distributions of alkyl nitrates, nitrogen oxides and tropospheric ozone: a global modelling study

Abstract. The formation, abundance and distribution of organic nitrates are relevant for determining the production efficiency and resident mixing ratios of tropospheric ozone (O₃) on both regional and global scales. Here we investigate the effect of applying the recently measured direct chemical production of methyl nitrate (CH₃ONO₂) during NO_x recycling involving the methyl-peroxy radical on the global tropospheric distribution of CH₃ONO₂ and the perturbations introduced towards tropospheric NO_x and O₃ using the TM5 global chemistry transport model. By comparisons against numerous observations, we show that the global surface distribution of CH₃ONO₂ can be largely explained by introducing the chemical production mechanism using a branching ratio of 0.3%, when assuming a direct oceanic emission source of ~0.15 Tg N yr⁻¹. On a global scale, the chemical production of CH₃ONO₂ converts 1 Tg N yr⁻¹ from nitrogen oxide for this branching ratio. The resident mixing ratios of CH₃ONO₂ are found to be highly sensitive to the dry deposition velocity that is prescribed, where more than 50% of the direct oceanic emission is lost near the source regions, thereby mitigating the subsequent effects due to long-range and convective transport out of the source region. For the higher alkyl nitrates (RONO₂) we find improvements in the simulated distribution near the surface in the tropics (10° S–10° N) when introducing direct oceanic emissions equal to ~0.17 Tg N yr⁻¹ . In terms of the vertical profile of CH₃ONO₂, there are persistent overestimations in the free troposphere and underestimations in the upper troposphere across a wide range of latitudes and longitudes when compared against data from measurement campaigns. This suggests either a missing transport pathway or source/sink term, although measurements show significant variability in resident mixing ratios at high altitudes at global scale. For the vertical profile of RONO₂, TM5 performs better at tropical latitudes than at mid-latitudes, with similar features in the comparisons to those for CH₃ONO₂. Comparisons of CH₃ONO₂ with a wide range of surface measurements shows that further constraints are necessary regarding the variability in the deposition terms for different land surfaces in order to improve on the comparisons presented here. For total reactive nitrogen (NO_y) ~20% originates from alkyl nitrates in the tropics and subtropics, where the introduction of both direct oceanic emissions and the chemical formation mechanism of CH₃ONO₂ only makes a ~5% contribution to the total alkyl nitrate content in the upper troposphere when compared with aircraft observations. We find that the increases in tropospheric O₃ that occur due oxidation of CH₃ONO₂ originating from direct oceanic emission is negated when accounting for the chemical formation of CH₃ONO₂, meaning that the impact of such oceanic emissions on atmospheric lifetimes becomes marginal when a branching ratio of 0.3% is adopted.