Aviation and the Global Atmosphere

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The solar ultraviolet (UV) irradiance incident on the surface of the Earth is responsible for a variety of biological effects (UNEP, 1994). This radiation varies greatly with local time, altitude, latitude, season, and meteorological conditions. For a given solar elevation, the transmission of UV sunlight through the atmosphere depends on absorption, predominantly by ozone; scattering and absorption by aerosols; and scattering by clouds. Aircraft emissions have the potential to alter each of these processes, and hence to influence the solar radiation field at biologically relevant UV wavelengths. To characterize UV radiation, this chapter adopts the erythemal dose rate (UVery), which is defined as the irradiance on a horizontal surface, at local solar noon, integrated over wavelength, with a wavelength-dependent weighting factor to account for the sunburning effect of the radiation as a function of wavelength and expressed in W m-2.

The calculated impacts of present and future fleets of aircraft on atmospheric ozone-hence on the UVery-are compared with those calculated from other expected changes in the composition of the atmosphere, including changes in bromine and chlorine content and expected increases in emissions of oxides of nitrogen (NOx) resulting from combustion at the surface. For these calculations, 1970 is taken as the reference year; a combination of results from three-dimensional (3-D) and two-dimensional (2-D) chemical transport models is used to predict changes in ozone for the period 1970 to 2050.

The calculated changes in UVery show strong dependencies on latitude, season, composition of the background atmosphere, and, in the case of aircraft impacts, whether the aviation fleet is assumed to have a component of supersonic aircraft. For present and projected fleets of subsonic aircraft, the calculations predict a decrease in UVery relative to the corresponding background atmosphere, which contains no aircraft emissions. The biggest changes are calculated for northern mid-latitudes, where present and expected emissions are greatest. For example, at 45�N in July the change in UVery relative to the corresponding background atmosphere is predicted to range from -0.5% in 1992 to -1.3% in 2050; the decrease in UV is brought about by the increase in ozone in the upper troposphere resulting from aircraft emissions. The corresponding range at 45�S, where present and predicted air traffic is substantially less, is -0.1% in 1992 and -0.3% in 2050 for January.

Ozone changes for the range of scenarios considered for these calculations have been obtained from Chapter 4. For calculations of the impact of subsonic aircraft on UVery, Chapter 4 has given a factor of 2 as the uncertainty for calculated ozone changes. These uncertainties are taken as the 67% likelihood range. We believe that uncertainty in the calculation of ozone changes is by far the greatest uncertainty in the determination of changes in UVery; accordingly, we have not added additional uncertainties to the range supplied by Chapter 4. In addition, our assessment of the confidence in these calculations is as given by Chapter 4: that is, "fair" for 2015 and "poor" for 2050. Accordingly, the calculated change for 2050 at 45�N in July, for example, can be expressed as-1.3% (-2.5 to -0.7%).

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 NOx in the stratosphere and expected increases in NOx 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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