Aviation and the Global Atmosphere

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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.

Calculations for Subsonic Aircraft Scenarios

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.

Calculations for Supersonic Aircraft Scenarios

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.

  • For low emission indices of NOx [5-10 g nitrogen dioxide (NO2) kg-1 fuel], the predicted decrease in O3 is dominated by emitted H2O.

  • The amount of emitted sulfur dioxide (SO2) is important in determining the magnitude of the calculated ozone decrease. The response to sulfur depends on how much SO2 is converted to sulfate particles in the aircraft exhaust plume, with larger depletions accompanying larger conversions.
  • The calculated change in O3 also depends on background sulfate surface area density (SAD), which is variable because of volcanic input. The computed change in O3 is not very sensitive to projected changes in chlorine loading and trace gases for an assumed fleet size of 500 aircraft, using a background sulfate SAD condition in the model simulation.
  • Changes in cruise altitude produce different results: Higher (lower) cruise altitudes result in larger (smaller) O3 depletions.

    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.

  • The calculated range for Northern Hemisphere annual average total O3 change is-1.3 to 0.0% for a fleet of 500 aircraft with an EI(NOx)=5 in 2015 flying in a low background sulfate SAD stratosphere. The O3 change is computed to be more positive in a higher background sulfate SAD stratosphere.
  • The calculated range for Northern Hemisphere annual average total O3 change is-1.4 to -0.1% for a fleet of 1,000 aircraft with an EI(NOx)=5 in 2050.
  • Model simulations show that O3 depletion occurs throughout most of the stratosphere, except in the tropical LS. Some models calculate an O3 increase in the lowest part of the mid-latitude stratosphere.

    Carry-Through Computations to Chapters 5 and 6

    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:

  • Deficiencies in model representation of transport processes
  • Deficiencies in model representation of chemical processes
  • Unknown or missing chemical and physical processes
  • Limited knowledge about the chemical composition of the future atmosphere and subsequent changes in atmospheric temperatures and winds resulting from climate change effects
  • Limitations in model resolution and dimensionality.

    Uncertainties in the computations were estimated as follows:

  • For the subsonic case, the uncertainty was taken to be a factor of 2, and was based on the differences among the various tropospheric models. The factor of 2 uncertainty was chosen for both the 2015 and 2050 atmospheres. This factor does not reflect additional uncertainties in future emissions, chemistry, and climate. Therefore, the confidence of the calculations is "fair" for 2015 and "poor" for 2050.
  • For the supersonic case, the range of all model calculations was passed on as a qualitative indication of the uncertainty. The estimate of the likely uncertainty range is significantly larger than this model range. For example, the central value for the Northern Hemisphere annual average total O3 change is -0.8% for a fleet of 1,000 aircraft in 2050, with a model range of -1.4 to -0.1%. A more likely uncertainty range for the possible Northern Hemisphere annual average total O3 change from this supersonic fleet of aircraft is +1 to -3.5%, and a "fair" confidence is associated with this range.

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