Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
2050 calculated a nearly linear increase in O 3 perturbation as a result of increases in aircraft NO x emissions despite increases in background NO x levels. As noted
above, there are large differences among models in their calculation of O 3 transport and chemistry between the UT and LS, and it is in this region where O 3 has a
significant contribution to the infrared heating budget (Wang and Sze, 1980; Lacis et al., 1990; Fortuin et al., 1995).
Given the limitations of the tropospheric models to estimate O3 perturbations in the tropopause region-in particular perturbations in the LS-a comparison of fractions of
O3 change that occur in the troposphere is made. Considering different characteristics of each model, meteorological and numerical, the estimated tropopause height
may differ considerably among them. In addition, the "real" tropopause height may vary locally by at least 2 km, depending on the season and the local meteorology.
Thus, we have applied the following crude estimate of tropopause height to all models: 8 km poleward of 65°, 10 km between 65° and 50°, 12 km between 50° and
35°, 14 km between 35° and 25°, and 16 km in the tropics. These figures are consistent with average tropopause heights used by the Total Ozone Mapping
Spectrometer (Fishman et al., 1990).
The fraction of O3 change from 1992 aircraft emissions that occurs in the troposphere varies between 0.65 and 0.75, with an uncertainty of about 15%. The fraction
calculated for the stratospheric 2-D AER model was 0.81. The ECHAm3 /CHEM model predicts the lowest tropospheric fraction (0.65). As noted above, the assumption
is that this fraction of 0.65 represents minimum O3 generation in the troposphere from subsonic emissions by the current (1992) fleet.
However, the model simulations just discussed did not include the generation of sulfate by aircraft, and consideration of this factor may alter the picture somewhat.
Because no 3-D tropospheric models were available to investigate sensitivity to sulfate emissions, an attempt to assess their significance was made using the AER
and UNIVAQ 2-D microphysical models. Although these models are limited in their applicability to the UT, they do yield results that suggest further work is required to
evaluate the significance of calculated O 3 generation by subsonics.
Aircraft sulfur experiments have been carried out for 2015 using an EI(SO 2 )=0.4 for a fleet flying nominally at 10 km. It was further assumed that the SO 2 emitted was
converted in the plume to 10 nm (radius) particles with 50 and 100% efficiency. Table 4-14a summarizes the model predictions of aerosol increases at middle northern
latitudes for the 10-14 km height range.
The importance of transport is clear: Although new particles are formed at about 11-km altitude, the models yield significant changes of SAD up to about 14 km, thus
creating a potentially important link between subsonic emissions and stratospheric O 3 . This conclusion is consistent with the results of the 1992 tracer experiment (see
Section 3.3.4), in which 2-D and 3-D models calculate the largest fuel tracer mixing ratios in an altitude layer from about 10 to 14 km.
Table 4-14b gives concomitant O 3 column changes. The AER and UNIVAQ models show that the O 3 column increase from atmospheric NO x aircraft perturbation
tends to be mitigated when sulfate SAD is changed by the aircraft. This result is mainly a consequence of more heterogeneous reactions on sulfate particles, leading to
the release of ClO, BrO, and HO 2 in this layer. However, the reduction in magnitude of O 3 generation is much stronger in the UNIVAQ model as compared to the AER
model. Clearly, this issue requires more study.
4.4.2. Uncertainties in Calculated Effects of Supersonic Aircraft Emissions
4.4.2.1. Transport
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