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5.1. Factors that Determine Ground-Level Ultraviolet Irradiance
This chapter considers the potential impact of aviation on ground-level UV
irradiance by using changes in ozone and cloudiness reported in preceding chapters.
A series of radiative transfer calculations based on this information yields
estimated changes in UV irradiance associated with various aviation scenarios
in the years 2015 and 2050 and places these scenarios in context relative to
changes expected from other causes during this time period.
Because of strong attenuation by atmospheric ozone, practically no solar radiation
reaches the ground at wavelengths shorter than 290 nm. The wavelength range
of interest extends from this short wavelength ozone cutoff to 400 nm because
biological sensitivities, including the reference action spectrum for erythema
(defined as a reddening of human skin in response to irradiation) (McKinlay
and Diffey, 1987), extend through both the UV-B (wavelengths 280-315 nm) and
the UV-A (315-400 nm). To characterize UV radiation, this chapter adopts the
UVery defined here as the erythemally weighted irradiance, A(l)E(l), integrated
over wavelength and expressed in W m-2. The quantity A(l) is the biological
weighting function of McKinlay and Diffey (1987); E(l) is the spectral irradiance
received on a horizontal surface. This chapter presents calculations of UVery
at local noon. Although action spectra exist for a variety of biological effects
(UNEP, 1994), the erythemal weighting has international recognition and is the
basis for the widely used UV index. In general, action spectra that are more
sharply peaked toward short wavelengths in the UV-B lead to weighted irradiances
that are more sensitive to changes in ozone. The discussion in UNEP (1994) addresses
this issue; no further detail is required here.
Absorption by ozone is the most important single process that influences the
transmission of UV-B radiation through the atmosphere. This absorption leads
to a sharp reduction in ground-level spectral irradiance as wavelength decreases
from 315 nm. Although the bulk of the absorption occurs at stratospheric altitudes,
tropospheric ozone is also important. The dependence of transmission on the
geometrical path taken by sunlight, hence on solar zenith angle, leads to a
strong dependence of ground-level UV irradiance on latitude, season, and local
time. In addition, molecular scattering is significant in the UV and leads to
a diffuse irradiance at the surface of the Earth under clear, aerosol-free skies
that is comparable to or larger than the direct solar beam, where the relative
magnitudes are functions of solar zenith angle. Furthermore, ground-level irradiance
increases as surface albedo increases, as a result of backscattering of radiation
reflected from the ground. Additional discussion of the factors involved in
the transfer of UV radiation appears in Kerr (1997).
Recent studies of radiative transfer in the UV emphasize the roles of clouds
and aerosols, primarily sulfates and soot, in altering ground-level irradiance
(Seckmeyer et al., 1996; Kerr, 1997). Attenuation of UV sunlight by clouds and
aerosols arises primarily from backscattering of radiation to space, although
absorption by soot-both freely suspended in the atmosphere and incorporated
into cloud drops-can be non-negligible under certain circumstances, particularly
in polluted urban areas. The temporal variability inherent in clouds and their
geometrical complexity hinder realistic radiative transfer modeling, although
simple parameterizations using satellite-based measurements of cloud reflectivities
are valuable in assessing the attenuation provided by cloudy skies (Eck et al.,
1995; Frederick and Erlick, 1995). The effects of backscattering by aerosols
are implicit in such measurements.
Emissions from aircraft could influence the UV irradiance incident on the biosphere
by altering the ozone abundance in the stratosphere or troposphere, the abundance
and optical properties of atmospheric aerosols, and the geographic extent or
optical characteristics of clouds. Our ability to model these three influences
decreases in accuracy as one moves from ozone to aerosols to cloudiness. The
following sections consider the natural variability in ground-level UV irradiance,
comparisons between measurements and calculations, our capability to detect
changes, and estimations of the changes that could occur as a consequence of
future aviation.
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