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| Titel |
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 |
| VerfasserIn |
J. E. Williams, G. Le Bras, A. Kukui, H. Ziereis, C. A. M. Brenninkmeijer |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 5 ; Nr. 14, no. 5 (2014-03-07), S.2363-2382 |
| Datensatznummer |
250118465
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| Publikation (Nr.) |
 copernicus.org/acp-14-2363-2014.pdf |
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| Zusammenfassung |
| 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 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 (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. |
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