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

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 NOx
recycling involving the methyl-peroxy radical on the global tropospheric distribution of CH3ONO2 and the perturbations introduced towards tropospheric NOx 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 TgN yr−1. On a global scale, the chemical production of CH3ONO2 converts 1 TgNyr−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 TgN 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 (NOy) 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.