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Abstract:
We present a comparison of chemistry-transport
models (TransCom-N2O) to examine the importance of atmospheric
transport and surface fluxes on the variability of
N2O mixing ratios in the troposphere. Six different models
and two model variants participated in the inter-comparison
and simulations were made for the period 2006 to 2009.
In addition to N2O, simulations of CFC-12 and SF6 were
made by a subset of four of the models to provide information
on the models’ proficiency in stratosphere–troposphere
exchange (STE) and meridional transport, respectively. The
same prior emissions were used by all models to restrict differences
among models to transport and chemistry alone.
Four different N2O flux scenarios totalling between 14 and
17 TgN yr−1 (for 2005) globally were also compared. The
modelled N2O mixing ratios were assessed against observations
from in situ stations, discrete air sampling networks and
aircraft. All models adequately captured the large-scale patterns
of N2O and the vertical gradient from the troposphere
to the stratosphere and most models also adequately captured
the N2O tropospheric growth rate. However, all models underestimated
the inter-hemispheric N2O gradient by at least
0.33 parts per billion (ppb), equivalent to 1.5 TgN, which,
even after accounting for an overestimate of emissions in the
Southern Ocean of circa 1.0 TgN, points to a likely underestimate
of the Northern Hemisphere source by up to 0.5 TgN
and/or an overestimate of STE in the Northern Hemisphere.
Comparison with aircraft data reveal that the models overestimate
the amplitude of the N2O seasonal cycle at Hawaii
(21 N, 158 W) below circa 6000 m, suggesting an overestimate
of the importance of stratosphere to troposphere transport
in the lower troposphere at this latitude. In the Northern
Hemisphere, most of the models that provided CFC-12
simulations captured the phase of the CFC-12, seasonal cycle,
indicating a reasonable representation of the timing of STE. However, for N2O all models simulated a too early
minimum by 2 to 3 months owing to errors in the seasonal
cycle in the prior soil emissions, which was not adequately
represented by the terrestrial biosphere model. In the Southern
Hemisphere, most models failed to capture the N2O and
CFC-12 seasonality at Cape Grim, Tasmania, and all failed at
the South Pole, whereas for SF6, all models could capture the
seasonality at all sites, suggesting that there are large errors
in modelled vertical transport in high southern latitudes.
