Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
effects that are weakly wavelength-dependent. In any case, comparison of measured irradiances with modeled irradiances suffers from the limited availability and
quality of data necessary to describe the atmosphere (Schwander et al., 1997).
Several studies have compared measured irradiances with model simulations. Measurements made under clear sky conditions and no snow cover demonstrate quite
convincingly that lower ozone values result in higher UV irradiance levels at the surface of the Earth; these data sets have been used to quantify the dependence
statistically (McKenzie et al., 1991; Booth and Madronich, 1994; Kerr et al., 1994). When these measured dependencies are compared with those computed for clearsky
conditions, no aerosols, and low ground reflectivity (no snow cover), reasonable agreement has generally been found (McKenzie et al., 1991; Wang and Lenoble,
1994; Forster et al., 1995).
The availability of aerosol optical depth measurements in the UV has allowed studies of the effects of particulates on ground-level irradiance. Mayer et al. (1997)
compare clear-sky UV spectral data obtained at Garmish-Partenkirchen, Germany, between 1994-96 with model simulations. The model simulations use ozone and
aerosol optical depth measurements as inputs. Systematic differences between measured irradiance spectra and model results were between -11% and +2%. It was
necessary to introduce ground-level aerosols into the model to achieve agreement to within 5%. From total ozone, aerosol optical depth, and spectral UV irradiance
measurements made under clear-sky conditions at Toronto between 1989-91, Kerr (1997) demonstrates that most of the observed variability of UV irradiance between
300 and 325 nm can be explained by ozone and aerosols. The remaining unexplained variability is 4% at 300 nm and 2% at 325 nm. Comparison of the observed
dependence of UV irradiance on aerosol optical depth with model results suggests that typical aerosols over Toronto are slightly absorbing (Krotkov et al., 1998). The
model also shows that a single scattering albedo of about 0.95 for aerosols gives the best agreement with the Toronto data.
Surface UV irradiance is also reduced by atmospheric sulfur dioxide (SO2 ), which has strong absorption features at UV wavelengths and occurs both naturally from
volcanic emissions and anthropogenically from industrial sources (Zerefos, 1997; Kerr et al., 1998). The presence of SO2 can interfere with the measurement of ozone
and estimates of ozone and UV trends at sites affected by local air pollution (Bais et al., 1993; De Meur and De Backer, 1993). However, measurements made at
several sites in less-polluted situations suggest that the effects of SO2 on UV over wider areas are small (Fioletov et al., 1997).
A method developed recently to calculate surface spectral UV irradiance uses Total Ozone Mapping Spectrometer (TOMS) satellite measurements of ozone and UV
reflectivity with a radiative transfer model (Eck et al., 1995; Herman et al., 1996; Krotkov et al., 1998). Comparison of model results with ground-based measurements
made at Toronto under clear skies indicates agreement of absolute irradiance to about 2% after correction for the angular response of the ground-based instrument.
The effects of surface albedo have been considered in the UV-A (324 nm), where there is negligible ozone absorption, by observing the difference between
measurements made with and without snow cover at several sites (Wardle et al., 1997). The presence of snow was found to enhance irradiance differently from one
site to another. The minimum enhancement was 8% at Halifax, Canada; the maximum was 39% at Churchill, Canada. The difference between these two sites is likely
to be a result of differences in the surrounding terrain and snow texture. For example, the clean snow on the flat terrain around Churchill would result in a higher
average surface albedo than at Halifax, where snow would be dirtier in the suburban areas and not present on nearby open water. Model results show an
enhancement of about 50% for an albedo of about 1 (Deguenther et al., 1998; Krotkov et al., 1998). Although there are no direct measurements of albedo available
when snow is present, the model gives quite reasonable effective albedo values of about 20% at Halifax and 90% at Churchill. Model results of Deguenther et al.
(1998) show that irradiance values are affected by surface albedo (snow cover) at distances up to 40 km; most of the dependence is influenced by albedo within a
radius of 10 km.
Variability in cloud cover is the largest contributor to short-term changes in surface UV irradiance. It is possible to include the effects of clouds in radiative transfer
calculations to various levels of approximation. However, routinely available observational data do not allow a rigorous characterization of cloud optical properties.
Measurements show that UV spectral transmittance depends on cloud type, cloud thickness, and whether there are absorbers within the cloud. Although detailed
quantification of these dependencies requires further research, some general conclusions can be made. The effects of thin clouds are weakly (