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 modeling study

Abstract

Abstract. The formation, abundance and distribution of organic nitrates are relevant for determining the production efficiency and resident mixing ratios of tropospheric ozone (O3) at 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 comparing 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 ~0.29 Tg N yr−1. The resident mixing ratios are found to be highly sensitive towards the dry deposition velocity of CH3ONO2 that is prescribed, where more than 50% of the direct oceanic emission of CH3ONO2 is lost near the source regions thereby mitigating subsequent effects on tropospheric composition due to long range and convective transport. For the higher alkyl nitrates (C2 and above) we find improvements in their simulated distribution in the tropics in TM5 improves when introducing direct oceanic emissions of ~0.17 Tg N yr−1. For the tropical upper troposphere (UT) a significant low model bias for all alkly nitrates occurs due to either missing transport pathways or chemical precursors, although measurements show significant variability in resident mixing ratios at high altitudes with respect to both latitude and longitude. For total reactive nitrogen (NOy) ~20% originates from alkyl nitrates in the tropical and extra-tropical UT, where the introduction of both direct oceanic emission sources and the chemical production of CH3ONO2 only increases NOy by ~5% when compared with aircraft observations. We find that the increases in tropospheric O3 due to direct oceanic emissions are mitigated by introducing the direct chemical production of CH3ONO2 resulting in rather moderate effects on nitrogen oxides and tropospheric O3.