Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
The change in SAD of PSCs from HSCT aircraft emissions has been calculated by the AER, GSFC and UNIVAQ 2-D models. The surface area increase is a
consequence of the stratospheric injection of H 2 O and NOy from aircraft. This process has the effect of increasing the saturation ratios of both water vapor and nitric
acid, producing an enhancement of PSC particle mass and SAD. Changes in H 2 O and NOy from HSCT aircraft at 18 km and 60°N and 60°S are listed in Tables 4-13a
and 4-13b. The average additional SAD of PSC1+PSC2 aerosols in the 12-24 km altitude layer and poleward of 60°N during December-January-February (DJF) or
poleward of 60°S during June-July-August (JJA) are shown in Tables 4-13c and 4-13d. These results should be interpreted with caution because PSC distributions are
highly localized.
All three models assume PSC1 to be solid NAT particles, although the treatment for PSC1
and PSC2 formation varies from model to model. AER and GSFC use a thermodynamic
equilibrium between gas and condensed phase for HNO 3 and H 2 O (Hanson and
Mauersberger, 1988), with an imposed size distribution. Supersaturation ratios of 10 and 1.4
are required before PSC1 and PSC2 form in the GSFC model, thus lowering the
temperature threshold for particle nucleation by about 3 and 2 K, respectively. Denitrification
and dehydration are included in the models by calculating HNO 3 and H 2 O loss terms from
sedimentation. The UNIVAQ model makes the calculation by integrating on the NAT/ice
particle size distribution; the AER and GSFC models assume that all condensed HNO 3 and
H 2 O form NAT and ice particles of a prescribed size distribution. No supersaturation is
assumed in the AER model. In these two models, perturbations of sulfate aerosols from
aircraft emission of SO2 leave unaffected the PSC. The models do allow H2O and HNO3 changes from aircraft to affect PSC. The UNIVAQ model allows interaction between sulfate
aerosols and PSC by adopting a microphysical code for both types of particles (Pitari et al.,
1993). As the atmosphere cools, PSC1 particles are first formed by nucleating on
preexisting sulfate aerosols; below the frost point, PSC1 become coated with ice, thus
forming PSC2 particles. With this scheme, a simultaneous presence of sulfate, PSC1, and
PSC2 particles is possible, and size distributions are calculated for all particle types.
The spread in absolute change of PSC SADs shown in Tables 4-13c and 4-13d is not
surprising because of the different schemes for PSC formation adopted in the models. In
particular, the absence of supersaturation assumptions in the AER model consistently
produces larger PSC SAD. The spread in relative changes is closely related to different
relative increases of H 2 O and NOy. About 50-60% (70-90%) of the calculated PSC
perturbation in the Northern Hemisphere (Southern Hemisphere) is caused by HSCT H 2 O
emissions when EI(NO x )=5. The more important role of H 2 O emissions in the Antarctic
Figure 4-9: Northern Hemisphere total O3 column change as a
function of cruise altitude of the supersonic fleet in 2015 with EI(NOx)
=5, SA0 sulfate distribution, and no sulfur aircraft emissions.
winter is a consequence of persistent PSC2 particles there; in the Arctic winter, PSC1 particles contribute to almost the entire available PSC SAD.
4.3.3.5.3. Column ozone sensitivity to PSCs
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