Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
indicate that the differences were less than 1% for CSIRO and LLNL; a few percent for AER, LARC, and SLIMCAT; and a few percent up to 10-15% for GSFC. Similar
results are not available for the other models, and it is not possible to conclude that the chemical solvers in all models are equivalent throughout the atmosphere;
however, the choice of chemical solver does not appear to be a major uncertainty in our ability to perform assessment calculations.
Uncertainty in photochemical rate constants has been propagated through a 2-D assessment model using a Monte Carlo approach for an EI(NOx )=15 HSCT fleet with
background aerosols (SA0) and no sulfur emissions (Stolarski et al., 1995). The effect of uncertainties in gas-phase reaction rate coefficients on perturbutions of O3 are estimated to yield an uncertainty on the order of 1% (1-sigma) in calculated Northern Hemisphere O3 column perturbations. However, these experiments did not
include uncertainties in photolytic cross-sections and heterogeneous processes, which could enhance the uncertainty significantly. Recently, box model sensitivityuncertainty
calculations for O3 depletion from supersonic aircraft emissions, again with EI(NOx )=15 and background aerosols, were performed at the most perturbed
locale using localized outputs from a 2-D model (Dubey et al., 1997). Guided by these sensitivities, 2-D model integrations were completed with nine targeted input
parameters altered to 1/3 of their 1-sigma uncertainties to put error bounds on the predicted O 3 change. Results indicated local O 3 loss of 1.5 ± 3% in regions of large
NO x injections. We note, however, that these sensitivity studies were all carried out with relatively high EI(NO x ) values [three times the EI(NO x ) taken as standard for
the current assessment], which is likely to further modify the intrinsic sensitivities of the models to perturbations.
4.4.2.4. Polar Stratospheric Cloud, High Cirrus Cloud, and Aerosol Processing
Uncertainty in our knowledge of the present atmosphere affects our ability to calculate
formation of and the chemical processing associated with PSCs. This uncertainty is
particularly acute in the Arctic winter because early springtime temperatures are higher in
the Arctic than in the Antarctic, and are quite close to the threshold temperature for the
formation of PSCs. Thus, a temperature error of a few degrees can make a substantial
difference in calculating the extent of processing. Objectively analyzed temperatures-those
based on measurements and often processed for use in weather forecast models-are
frequently used in 3-D CTMs to estimate chemical processing. However, it appears that
UKMO and ECMWF objectively analyzed temperatures may be systematically too high by 1
to 2 K in winter (Knudsen, 1996; Pullen and Jones, 1997); this uncertainty will impact our
understanding of the microphysics associated with heterogeneous chemistry.
The question of processing of reservoir species such as HNO3 and HCl on PSCs remains
uncertain, and hysteresis effects may be important. That is, the formation of PSCs by
condensation and removal by evaporation may not be describable in terms of reversible
processes, and the temperature history of the air parcel may be important. Currently, most
models avoid this complication. Conversion of reservoir species may occur as a result of
dissolution of HCl in sulfate, forming a ternary solution, or may occur on frozen surfaces of
water or NAT ice surfaces. The sensitivity of O3 depletion to the details of these processes
appears to be less important for Antarctic conditions; many models yield relatively complete
conversion of reservoir species induced by cold vortex temperatures. However, as noted
above, this effect could be more important for the relatively warmer Arctic vortex. This
uncertainty associated with chemistry, combined with the dynamic uncertainty, suggests
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