Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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EXECUTIVE SUMMARY
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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 (NO x )
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 twodimensional
(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 midlatitudes,
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%).
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