Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Representation of heterogeneous processes on cold aerosols (PSCs) is not consistent among the participating models (Sections 4.3.1.3 and 4.3.3.5.). These
differences are manifest when increased abundances of NO Y and H 2 O from supersonic aircraft are considered. As mentioned previously, the transport fields of
participating assessment models are significantly different; therefore, the amount of supersonic H 2 O and NOy dispersion between hemispheres and vertically within the
same hemisphere is also different. Three models (AER, GSFC, and UNIVAQ) investigated the impact on total O 3 when cold aerosol representation was not included
(S4b). These models removed their representation of SAD, dehydration, and denitrification that would be associated with cold aerosol processes. The results were
mixed. When cold aerosol processes were included in the model chemistry formulation, the model-derived percentage change in Northern Hemisphere total column O3 increased by factors of 1.1 and 1.5 for the GSFC and AER models, respectively (S1c vs. S4b). However, in the Southern Hemisphere the GSFC model-derived
depletion in total O3 increased by 2.5 when cold chemistry processes were included. There was minimal change in Southern Hemisphere average O3 depletion in the
AER model. When the EI(NO x ) is set to zero (S4a), a similar conclusion is also drawn (compare to S1b) by the AER and GSFC models. These results are consistent
with the dispersion of supersonic H2O and NOy between the AER and GSFC models. The UNIVAQ model did show sensitivity to supersonic emissions in the total O3 hemispheric averages; because the absolute impact was small and centered around zero, however, it is difficult to draw any significant conclusions.
4.3.3.6. Ambient Chlorine Sensitivity
Total radical chlorine (Cl y ) abundance in the atmosphere is predicted to decrease in the future as a result of international agreements controlling production of
anthropogenic organic source gases such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) (WMO 1995, 1999). Therefore, several sensitivity
studies were performed to investigate the impact of ambient Cl y levels on a given supersonic aircraft perturbation. Here, atmospheric chlorine amounts were varied by
changing halocarbon boundary conditions. Four different ground halocarbon amounts were considered, which resulted in reactive Cl y levels of 1.0, 2.0, 3.0, and 4.0
ppbv in the upper stratosphere. For reference, the 3.0 and 2.0 ppbv Cl y levels are representative of predicted 2015 and 2050 atmospheric conditions, respectively. In
all Cl y sensitivity studies, only EI(NO x )=5 and a fleet of 500 supersonic aircraft were considered. Sensitivity studies were conducted under volcanically clean conditions
without aircraft sulfur emissions (SA0) and with sulfur emissions assuming 50% SO 2 gas-to-particle conversion in the plume (SA1). Without aircraft sulfur emissions,
there was little discernible sensitivity to background Cl y abundance (S1c, S5a, S6a, and S9a). With sulfur emissions included, however, total ozone depletion for all
three participating models was smallest when Cl y abundance was 1.0 ppbv. As Cl y increased in abundance, the model-derived ozone depletion increased, maximizing
and leveling off between 3.0 and 4.0 ppbv Cl y (S1f, S5b, S6b, and S9c). Among the participating models, AER showed the most sensitivity to Cl y abundance, followed
by GSFC and UNIVAQ. It should be noted that the Cl y =2 ppbv scenario (S9c) had N 2 O and CH 4 boundary conditions consistent with the year 2050. The other three
Cl y sensitivity scenarios (S1f, S5b, and S6b) had N 2 O and CH 4 abundances consistent with the year 2015. The AER model did investigate Cl y sensitivity to N 2 O and
CH 4 boundary conditions by repeating the Cl y =2.0 ppbv scenario with N 2 O and CH 4 values representative of 2015 conditions. The model-derived change in NH total
O 3 was -0.6% (instead of -0.7% under 2050 CH 4 and N 2 O conditions).
4.3.3.7. Fleet Design
4.3.3.7.1. Supersonic cruise altitude sensitivity
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