Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
The introduction of mixed supersonic and subsonic fleets in the future will have the potential to modify the impact of aviation on UVery. Present calculations predict
only marginal changes to the values of UVery in the tropics and increases in mid-latitudes. For example, the predicted changes relative to the corresponding
background at 45°N in July are +0.6% in 2015 and +0.3% in 2050. For 45°S in January, the predicted changes in UVery are +0.4% for 2015 and +0.2% in 2050,
relative to the corresponding background atmospheres. Although these estimated changes may be considered small, they do have significantly larger uncertainty
limits. The limits for a particular confidence range are difficult to determine. The %UVery changes are dominated by changes in ozone; the values for these changes
are supplied by Chapter 4, which has also considered three components when assessing the uncertainty for the impact of the hybrid fleet on ozone. These
components are the spread obtained by a number of models for a range of plausible scenarios, uncertainties in chemical rate coefficients, and uncertainties introduced
by inaccurate treatment of atmospheric circulation in the models. Chapter 4 concluded that the annually averaged impact on ozone for the Northern Hemisphere of a
hybrid fleet in 2050, including 1,000 High-Speed Civil Transports (HSCTs), would be in the range of -3.5 to +1% compared with the impact of the subsonic fleet, with a
best estimate of -1%. This result again represents the 67% likelihood range with a confidence in this uncertainty range of "fair." As with estimated subsonic impacts, we
believe that uncertainties in the changes in ozone caused by the hybrid fleet are much greater than any other uncertainties in the calculation of changes in UVery.
Chapter 4 has considered only the uncertainty estimate for an annually averaged Northern Hemisphere value, whereas reporting of the UV calculations requires
estimates of the uncertainties for a range of latitudes and seasons. To achieve this, Chapter 5 has taken the 67% likelihood range for percent change in UVery to be
from (-2% + the best estimate of the percent change) to (+3% + the best estimate of the percent change).
The magnitude of the changes calculated for the impact of aviation may be compared to those calculated for the background atmosphere for the period 1970 to 2050.
Expressed relative to 1970, the calculated changes in UVery at 45°N in July are +8% for 1992, +3% for 2015, and -3% for 2050. The changes calculated for the three
background atmospheres reflect the changing levels of halogens and NO x in the stratosphere and expected increases in NO x in the troposphere, particularly at
northern mid-latitudes, as a result of increased industrial activity. Observed changes in total ozone from 1970 to 1992 imply smaller percentage increases in UVery,
indicating the degree of uncertainty in the model predictions.
Increases in the abundance of atmospheric aerosols or the frequency of cirrus clouds would, in general, lead to a decrease in ground-level UV irradiance, where this
change is only weakly dependent on wavelength. The change in aerosol loading expected from increased aircraft operations between the present and 2050 is small
relative to the natural aerosol background and to anthropogenic influences other than those related to aviation. The effect of the aircraft-related increase in aerosols is
to reduce UV irradiance by less than 0.1%. Calculations indicate that aircraft-related increases in contrails lead to a decrease in UV irradiance of less than 0.2% in an
area-averaged sense in regions where 5% of the sky is covered.
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