Although aircraft generally fly in corridors, their atmospheric effects are expected to propagate far beyond those regions.
For subsonic aircraft flying in the troposphere and the lowermost part of the stratosphere, emissions of oxides of nitrogen (NOx) are the primary cause for the model-calculated increase in ozone (O3) in the upper troposphere (UT). Emissions of NOx, water vapor (H2O), and sulfate from supersonic aircraft cruising in the stratosphere are expected to decrease the column abundance of O3.
The effects of aircraft emissions are strongly dependent on flight altitudes: Emissions in the troposphere and the stratosphere have distinctly different effects on O3. Although the global chemical transport models used in this assessment attempt to simulate both the troposphere and the stratosphere, these models have been developed for simulating either the troposphere or the stratosphere, but not both. Thus, the assessment of O3 impact focuses on subsonic aircraft effects using tropospheric models and supersonic effects using stratospheric models. This approach is probably valid for supersonic aircraft, which cruise exclusively in the stratosphere (19 km), but it may be problematic for subsonic aircraft that have a sizable amount of emissions in the lowest levels of the stratosphere (around 12 km at mid-latitudes). The impact of subsonic emissions on the stratosphere has not been fully evaluated using models and requires further investigation.
Model calculations were designed to assess the effects of added NOx. They concluded that there would be an increase in ozone concentration and a decrease in methane (CH4) concentration.
NOx-All models compute increases of NOx in the upper troposphere (UT)/lower stratosphere (LS) of 50-150 pptv at 12 km at mid-latitudes for 2015; these increases are significant compared to the average background levels of 50-200 pptv.
O3-The O3 increase is restricted to northern mid- and high-latitudes, with maximum increases in the UT and LS. Subsonic aircraft are predicted to cause a maximum annual average O3 increase of 7-11 ppbv in 2015 in the latitude band 30-60�N at 10-13 km altitude. This result corresponds to an increase of approximately 5-10%.
The calculated increase in global average O3 with increasing NOx emission from aircraft is in broad agreement among different models. Although the models show general linearity for O3 increase from NOx emission, O3 production is less efficient at high NOx emission. The global average O3 increase from aircraft NOx emissions is not very sensitive to projected changes in atmospheric composition for the model scenarios investigated here.
The O3 increase may be mitigated somewhat by emitted sulfate, leading to production of chlorine moNOxide (ClO), bromine moNOxide (BrO), and hydrogen dioxide (HO2) in the LS. This process, however, has not been included in the model calculations presented in this report.
CH4-As a result of increases in tropospheric hydroxyl (OH) caused by aircraft NOx emissions, CH4 removal rates are computed to increase by 1.6-2.9% in 2015 and 2.3-4.3% in 2050. The possible influence of changes in CH4 on climate are discussed in Chapter 6.
Because the supersonic fleet is still in its design stage, the range of emissions, fleet size, and cruise altitude covered by supersonic scenarios is larger than for subsonic aircraft. For a nominal cruise altitude of 19 km, the largest impacts of proposed supersonic aircraft occur in the stratosphere.
The predicted decrease of O3 in the stratosphere is most sensitive to emissions of H2O, oxides of sulfur (SOxO), and NOx.
Results reported here correspond to changes in O3 when a supersonic fleet of aircraft replaces a portion of subsonic flights. The baseline computations assume that supersonic aircraft have a cruise altitude of 19 km, an emission index for water EI(H2O)=1230 (1230 g H2O kg-1 fuel), EI(NOx)=5 (5 g NO2 kg-1 fuel), and a range of values for EI(SO2).
H2O-The increase in H2O is calculated to be a maximum of 0.4-0.7 ppmv in the Northern Hemisphere mid-to high-latitude LS for a fleet of 500 aircraft, compared to a background of 3-4 ppmv.
Total Active Nitrogen (NOy)-The increase in NOy is calculated to be a maximum of 0.6-1.0 ppbv in the Northern Hemisphere mid-to high-latitude LS for a fleet of 500 aircraft, compared to a background of 3-10 ppbv.
SO2-Emission and conversion of SO2 to sulfate particles in the plume are still very uncertain for supersonic aircraft; thus, we summarize a range of scenarios. The SAD would increase 20-100% between 15 and 20 km in the 30-90�N latitude band for EI(SO2)=0.4 and a range of assumptions about gas to particle conversion in the plume.
O3-Each model shows different distributions of O3 depletion and enhancement that probably reflect different methodologies of transport and chemistry incorporated in individual models. The results summarized below are from different models over a range of scenarios.
Most Likely Values
For each scenario, a single model was used to propagate changes in constituents
in Chapters 5 and 6.
The various model computations exhibit a range of results that to some degree reflects uncertainties. However, this range does not define the uncertainties and, indeed, it is very difficult at this time to quantify them. Factors that contribute include the following:
Uncertainties in the computations were estimated as follows:
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