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The formation, abundance and distribution of organic nitrates are relevantfor determining the production efficiency and resident mixing ratios oftropospheric ozone (O3) on both regional and global scales. Here weinvestigate the effect of applying the recently measured direct chemicalproduction of methyl nitrate (CH3ONO2) during NOx recycling involving themethyl-peroxy radical on the global tropospheric distribution of CH3ONO2 andthe perturbations introduced towards tropospheric NOx and O3 using the TM5global chemistry transport model. By comparisons against numerous observations,we show that the global surface distribution of CH3ONO2 can be largelyexplained by introducing the chemical production mechanism using a branchingratio of 0.3%, when assuming a direct oceanic emission source of~0.15 Tg N yr−1. On a global scale, the chemical productionof 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 sensitiveto the dry deposition velocity that is prescribed, where more than50% of the direct oceanic emission is lost near the source regions,thereby mitigating the subsequent effects due to long-range and convectivetransport out of the source region. For the higher alkyl nitrates (RONO2) wefind improvements in the simulated distribution near the surface in thetropics (10° S–10° N) when introducing direct oceanicemissions equal to ~0.17 Tg N yr−1 . In terms of thevertical profile of CH3ONO2, there are persistentoverestimations in the free troposphere and underestimations in the uppertroposphere across a wide range of latitudes and longitudes when comparedagainst data from measurement campaigns. This suggests either a missing transport pathway orsource/sink term, although measurements show significant variability inresident mixing ratios at high altitudes at global scale. For the vertical profile ofRONO2, TM5 performs better at tropical latitudes than atmid-latitudes, with similar features in the comparisons to those forCH3ONO2. Comparisons of CH3ONO2 with a wide range of surfacemeasurements shows that further constraints are necessary regarding thevariability in the deposition terms for different land surfaces in order toimprove on the comparisons presented here. For totalreactive nitrogen (NOy) ~20% originates from alkylnitrates in the tropics and subtropics, where the introduction of bothdirect oceanic emissions and the chemical formation mechanism of CH3ONO2only makes a ~5% contribution to the total alkyl nitratecontent in the upper troposphere when compared with aircraft observations.We find that the increases in tropospheric O3 that occur due oxidation ofCH3ONO2 originating from direct oceanic emission is negated whenaccounting for the chemical formation of CH3ONO2, meaning that the impact ofsuch oceanic emissions on atmospheric lifetimes becomes marginal whena branching ratio of 0.3% is adopted

atmospheric chemistry and physics

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