Abstract
The use of global three-dimensional (3-D) models
with satellite observations of CO2 in inverse modeling
studies is an area of growing importance for understanding
Earth’s carbon cycle. Here we use the GEOS-Chem model
(version 8-02-01) CO2 mode with multiple modifications in
order to assess their impact on CO2 forward simulations.
Modifications include CO2 surface emissions from shipping
(0.19 PgC yr−1), 3-D spatially-distributed emissions from
aviation (0.16 PgC yr−1), and 3-D chemical production of
CO2 (1.05 PgC yr−1). Although CO2 chemical production
from the oxidation of CO, CH4 and other carbon gases is
recognized as an important contribution to global CO2, it is
typically accounted for by conversion from its precursors at
the surface rather than in the free troposphere. We base our
model 3-D spatial distribution of CO2 chemical production
on monthly-averaged loss rates of CO (a key precursor
and intermediate in the oxidation of organic carbon) and
apply an associated surface correction for inventories that
have counted emissions of CO2 precursors as CO2. We
also explore the benefit of assimilating satellite observations
of CO into GEOS-Chem to obtain an observation-based
estimate of the CO2 chemical source. The CO assimilation corrects for an underestimate of atmospheric CO abundances
in the model, resulting in increases of as much as 24% in
the chemical source during May–June 2006, and increasing
the global annual estimate of CO2 chemical production
from 1.05 to 1.18 Pg C. Comparisons of model CO2 with
measurements are carried out in order to investigate the
spatial and temporal distributions that result when these
new sources are added. Inclusion of CO2 emissions from
shipping and aviation are shown to increase the global CO2
latitudinal gradient by just over 0.10 ppm (3%), while
the inclusion of CO2 chemical production (and the surface
correction) is shown to decrease the latitudinal gradient by
about 0.40 ppm (10%) with a complex spatial structure
generally resulting in decreased CO2 over land and increased
CO2 over the oceans. Since these CO2 emissions are omitted
or misrepresented in most inverse modeling work to date,
their implementation in forward simulations should lead to
improved inverse modeling estimates of terrestrial biospheric
fluxes.