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

Abstract

Abstract. The formation, abundance and distribution of organic nitrates are relevant for determining the production efficiency and resident mixing ratios of tropospheric ozone (O 3 ) on both regional and global scales. Here we investigate the effect of applying the recently measured direct chemical production of methyl nitrate (CH 3 ONO 2 ) during NO x recycling involving the methyl-peroxy radical on the global tropospheric distribution of CH 3 ONO 2 and the perturbations introduced towards tropospheric NO x and O 3 using the TM5 global chemistry transport model. By comparisons against numerous observations, we show that the global surface distribution of CH 3 ONO 2 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 CH 3 ONO 2 converts 1 Tg N yr −1 from nitrogen oxide for this branching ratio. The resident mixing ratios of CH 3 ONO 2 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 (RONO 2 ) 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 CH 3 ONO 2 , 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 RONO 2 , TM5 performs better at tropical latitudes than at mid-latitudes, with similar features in the comparisons to those for CH 3 ONO 2 . Comparisons of CH 3 ONO 2 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 CH 3 ONO 2 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 O 3 that occur due oxidation of CH 3 ONO 2 originating from direct oceanic emission is negated when accounting for the chemical formation of CH 3 ONO 2 , meaning that the impact of such oceanic emissions on atmospheric lifetimes becomes marginal when a branching ratio of 0.3 % is adopted.

Methyl, Ethyl, and Propyl Nitrates: Global Distribution and Impacts on Reactive Nitrogen in Remote Marine Environments

“…The branching ratio between reactions and (combined where relevant with the branching ratio for formation of the specific isomer) represents the yield of RONO 2 , referred to as α . For the species considered here, α increases from <1% for methyl nitrate (CH 3 ONO 2 ; Flocke et al, ) to ≈2% for ethyl nitrate (C 2 H 5 ONO 2 ; Ranschaert et al, ) to >3% for propyl nitrate (C 3 H 7 ONO 2 ; Atkinson et al, )—although precise measurement of α remains an important source of uncertainty in atmospheric RONO 2 budgets (Butkovskaya et al, ; ; Khan et al, ; Nault et al, ; Williams et al, ). While reaction has no net impact on available atmospheric NO x , reaction can recycle NO x , temporarily sequester NO x , or permanently remove NO x from the atmosphere, depending on the lifetime and fate of the RONO 2 produced (Fisher et al, ; Perring et al, ).…”

“…Using aircraft observations from the PEM‐Tropics campaigns, Neu et al () inferred a constant ocean flux of 0.35 Tg N yr −1 , mainly from the tropical Pacific with a small contribution from the Southern Ocean. However, the model used in their study did not include atmospheric production of RONO 2 via reactions , which likely exaggerated the size of the ocean source (Williams et al, ). The Neu et al () estimate was also hampered by the limitations of the aircraft data available at the time, including a lack of seasonal information and particularly large uncertainties for the Southern Ocean, where only one flight leg crossed south of 45°S.…”

Interferences in photolytic NO&lt;sub&gt;2&lt;/sub&gt; measurements: explanation for an apparent missing oxidant?

“…The proposed origin of the methyl nitrate (Williams et al, 2014) is shown in Reaction (R14) and formaldehyde, which is formed as a major by-product is shown in Reaction (R15) (Singh et al, 1995: Warneck andZerbach, 1992).…”

Carbon monoxide column retrieval for clear-sky and cloudy atmospheres: a full-mission data set from SCIAMACHY 2.3 µm reflectance measurements

“…In that case, the difference between the retrieved CH 4 column and the a priori CH 4 column from the chemical transport model TM5 (Williams et al, 2013(Williams et al, , 2014 reflects the light-path effect of aerosols and clouds on the observations due to shielding and photon path enhancement by multi-scattering (Vidot et al, 2012). For the subsequent retrieval of CO, we use the retrieved nonscattering CH 4 column to provide a first estimate for the cloud height.…”

“…Thereby, it supplies CO vertical column densities that we transformed to column averaged mixing ratios by calculating the dry-air column from the surface pressure at the station sites. The TCCON instruments measure in the same spectral range as SCIAMACHY and provide the vertical column densities of trace gases including CO with high precision (Wunch et al, 2010(Wunch et al, , 2011. Because of the lack of profile information, it is not possible to apply averaging kernels, and NDACC/TCCON data must be compared directly with the SCIAMACHY cloudysky retrievals.…”