| Among various sources of tropospheric CO, methane oxidation (MO) is commonly assumed
to be least uncertain term due to the fairly well studied kinetics of the reaction of CH4 with
OH. Many studies on CO tropospheric budget (including forward and inverse modelling with
CTMs and AC-GCMs) employ simplified treatment of MO source in their chemistry
schemes, i.e.Âparameterising the photochemical production of CO using a “net reaction” that
can be written as
CH4Â+ÂOHÂ/ λÂCOÂ+ (products),
where the yield λ approximates the effect of the chemistry regime and removal of
intermediates from the CH4 oxidation chain, and is duly used as one of the fitting parameters.
The estimates of λ, however, are hitherto inconsistent: Depending on the chemistry and
dry/wet deposition schemes used, reckoned average tropospheric λ values vary within 0.6-1,
whilst recent model parameterisations tend to favour almost complete conversion of CH4 to
CO. The issue of such large uncertainty in CO yield from CH4 is especially important for the
SH CO, where more than 50% of its inventory is typically attributed to the MO source in
austral summer.
In this study we scrutinise the MO source of CO using the ECHAM/MESSy Atmospheric
Chemistry (EMAC) model employing elaborate chemistry mechanisms and tools to
directly infer the value of λ, which is a diagnosed variable rather than an assumed
parameter. Three chemical mechanisms differing in complexity of the MO cycle
are used in simulations based on the EMAC evaluation study setup (detailed in
JöckelÂetÂal.,Â2010): (1)ÂThe reference (REF) mechanism which represents the
“standard” MO chemistry in EMAC including CH3O2, CH3OH, CH3OOH, HCHO and
HCOOH, (2)ÂThe extension of REF that resolves CH3 and CH3O intermediates and
reactions of CH3O2 with peroxy radicals, foremost HO2 (BASE mechanism), and
(3)ÂFurther extension of BASE with pathways of MO involving formation and
destruction of organic nitrates, plus reactions of CH3O2/CH3/HCHO with Ox and
HOx from known laboratory studies that were previously unaccounted for (FULL
mechanism).
We obtain yearly tropospheric value of λ in REF and BASE simulations at 0.94 and 0.96,
i.e.Âin line with the more recent estimates. The NH-SH difference in average λ reaches 0.07
in the boundary layer (BL) and 0.02 in the free troposphere (FT), respectively. The local λ
values minimise at the surface at ~0.4 (continents) and ~0.7 (oceans) and progressively
increase with altitude. This result, however, is only partly explicable in terms of the
removal processes’ location, as we diagnose low sensitivity of λ to the wet scavenging
and dry deposition efficiencies. In the FT, in contrast, we find local λ values often
exceeding unity (i.e., local CO production from MO is greater than CH4 sink via OH),
indicating efficient vertical transport of the intermediates from the BL. Despite the large
spatiotemporal variations in local λ values, the tropospheric integral yield of CO from
CH4 appears to be a very robust characteristic in EMAC. A substantially lower
average tropospheric λ value (below 0.9) is obtained in the FULL simulation, a clear
result of the changes to the MO chain chemical regime. The largest impact on λ has
the enhanced production of HCOOH in the FT (mostly due to HCHO+HO2 and
CH3O2+OH reactions), which augments irreversible removal of the intermediates from the
CH4/CO chain. We further discuss the details and implications of these preliminary
results.
References:
Jöckel,ÂP., Kerkweg,ÂA., Pozzer,ÂA., Sander,ÂR., Tost,ÂH., Riede,ÂH., Baumgaertner,ÂA.,
Gromov,ÂS.,Âand Kern,ÂB.: Development cycle 2 of the Modular Earth Submodel System
(MESSy2), Geosci.ÂModelÂDev., 3, 717-752, doi:Â10.5194/gmd-3-717-2010, 2010. |