5.4. Calculated Impact of Aviation on UV at the Surface of the Earth

Present and future fleets of aircraft have the capacity to modify the amount of UV arriving at the surface of the Earth as a result of changes brought about by:

The amount and distribution of ozone in the upper troposphere and lower stratosphere
The amount of cloud cover
The aerosol type, content, and distribution.

This section discusses the expected magnitudes of the effects of each of these changes and compares the calculated impact of aviation on UV at the ground with that resulting from other changes in the composition of the atmosphere.

Many of the results discussed in this section are expressed in terms of percent change in UVery between two scenarios being compared. The relationships between UVery, the erythemal weighting factor, the ground-level irradiance, and the erythemally weighted irradiance are illustrated in Figure 5-1 for 30°N for July and January.

So that percent change in UVery may be put in context in terms of absolute irradiance, Figure 5-2 shows the calculated spatial and seasonal variation of the absolute value for UVery with latitude for the 1992 background atmosphere.

5.4.1. Methodology for Treating Changes in Atmospheric Ozone

The underlying principle for the methodology described below is that calculations of ozone changes derived from chemical transport models should be related to measurements. For this chapter, calculations designed to determine ozone changes for the present and the future have been related to present-day measurements of ozone columns and profiles. Model calculations are then used to estimate the impact of aviation in 1970, 1992, 2015, and 2050. The modeled impacts are compared with other changes that are calculated to have occurred to ozone columns since 1970 and predicted to occur between 1992 and 2050. The radiative transfer calculations reported in this chapter were performed using a stand-alone version of the solar radiation module from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) 2-D chemical transport model (CTM), which has been described in Stolarski et al. (1995) and Randeniya et al. (1997). The method of Meier et al. (1982) is used to account for the effects of multiple scattering, and ray-tracing techniques are used to account for the curvature of the Earth. The CSIRO radiative transfer model participated in the photolysis benchmark intercomparison conducted as part of the 1995 Atmospheric Effects of Stratospheric Aircraft (AESA) assessment (Stolarski et al., 1995), and excellent agreement was obtained with the benchmark calculations. Intercomparisons have also been performed with the Tropospheric Ultraviolet and Visible (TUV) model developed by Madronich using calculations made at the Laboratory of Atmospheric Physics at the University of Thessaloniki. The radiative transfer equation in TUV was solved by using the discrete ordinate radiative transfer (DISORT) code in 16-stream mode (Stamnes et al., 1988). Erythemally weighted fluxes were calculated at the ground under clear-sky conditions for zenith angles from 0 to 80°. Using the same input parameters, agreement was obtained with the CSIRO model to within 2-3% for zenith angles up to 60° and within 5% for a zenith angle of 80°.

5.4.1.1. Ozone Columns and Profiles

Figure 5-3: Illustration of the derivation of columns used for UV calculations.

The calculations address the effect of aviation on UV at the surface in 1970, 1992, 2015, and 2050. For the years 2015 and 2050, there will be a range of scenarios that will be influenced primarily by the expected size and composition of the fleets and the amount and composition of emissions. In principle, however, the UV calculations require a representation of a background atmosphere and an atmosphere that includes the effects of aircraft emissions for each of the years 1970, 1992, 2015, and 2050. The term "background" in this context refers to an atmosphere derived from a calculation that includes all expected inputs other than those attributable to aircraft emissions. The calculated impacts of aviation on ozone for 1992, 2015, and 2050 are provided by Chapter 4. The small effect calculated for 1970 is deduced from the 1992 calculations, as discussed in Section 5.4.1.2.

The starting point for subsequent calculations is the representation of ozone columns and profiles for 1992. For this chapter, the columns have been derived from archived TOMS version 7 measurements (Herman et al., 1996). UV calculations have been performed at local noon for January, April, July, and October at the following latitudes: 65°S, 45°S, 30°S, 0, 30°N, 45°N, and 65°N. The monthly averaged and zonally averaged ozone columns derived from the TOMS data for these months and latitudes for the 10-year period 1983 to 1992 are shown in Table 5-1.

Because the values shown in Table 5-1 are averages of measured values, they already include the effects of present-day aircraft emissions on ozone. In other words, to obtain the 1992 background atmosphere, the effects of emissions from aircraft have to be removed from the values shown in Table 5-1. The manner in which this has been done for these calculations and the extension of the methods applied to obtain the required information for the years 1970, 2015, and 2050 are illustrated in Figure 5-3.

These absolute differences are applied to 1992ss to produce 1992bg, the 1992 background atmosphere, which is free of inputs from aircraft emissions. A model calculation is then carried out using the prescribed background scenario for 2015. The results of these calculations are compared with the similar calculation for 1992, and the absolute difference obtained from the model calculations is applied to 1992bg to obtain 2015bg, the 2015 background atmosphere. Comparison of a model calculation for a 2015 atmosphere that contains inputs attributable to aviation, assuming only subsonic aircraft will be operating, with the model calculation for the 2015 background atmosphere provides an estimate of absolute changes in ozone concentration between these two calculations. These differences are applied to 2015bg to produce 2015ss, the 2015 atmosphere that includes the effects of emissions from a purely subsonic fleet of aircraft. Clearly, an analogous approach can be adopted to obtain the required information for 2050 and for 1970. Chapter 4 has provided mixing ratio differences calculated for the impact of aviation on ozone profiles for the latitudes and seasons shown in Table 5-1. The authors of Chapter 4 chose results from the Oslo 3-D model and the Atmospheric and Environmental Research, Inc. (AER) 2-D model as representative of the subsonic and supersonic scenario calculations, respectively. Table 5-2 shows the scenarios considered here, with references to the relevant Chapter 4 tables. The method adopted in Chapter 4 for expressing the range in uncertainties for the ozone mixing ratio differences is discussed in Section 5.4.2.3. Figure 5-3 attempts to highlight the fact that, although 1992bg, 2015bg, and 2015ss are derived from model calculations, they have the measurements of 1992ss as a reference. The implication of this approach is that, although ozone concentrations derived from models clearly have uncertainties, the differences obtained between any two model calculations may be more accurate than the absolute concentrations in either model calculation. This assumption is questionable and must be regarded more as an assumption that allows UV calculations to proceed than one that can be defended strongly. The approach does have the advantage that UV calculations derived from 1992bg, 2015bg, and 2015ss are not based entirely on model calculations. It should be noted that, in assigning differences between model calculations to measured values in 1992ss, absolute differences in model calculations have been chosen rather than percentage differences. Again, the approach taken cannot be defended rigorously. Examination of the 3-D CTM results in Chapter 4 shows a very wide variation in the range of models for estimated ozone differences in the upper troposphere. Arguments can be advanced for adopting either absolute or percentage changes to apply to measured ozone columns; until the reasons for the variations in the range of models are clear, however, neither approach is clearly superior to the other.

The current limitations of multidimensional CTMs generate additional complications that must be addressed in assessing the impacts of aviation on the composition of the atmosphere. The impact of subsonic fleets on ozone, for the present and for 2015 and 2050, has been discussed in Chapter 4, based on a range of 3-D CTMs. In their present state, these predominantly tropospheric models are unable to take into account adequately changes in the chemistry of the stratosphere. Between 1992 and 2050, for example, these changes would be induced predominantly by changes in the concentrations of inorganic chlorine and bromine compounds in the stratosphere. If the principal concern were to evaluate the impact of aviation on ozone for a given year, these limitations would not be too severe. However, if in addition one wishes to compare the effects of aircraft emissions with those that can be attributed to changes in stratospheric processes over the period 1970 to 2050, then one requires more information than current 3-D CTMs can supply. Furthermore, it is expected that future fleets may contain a supersonic component. These supersonic aircraft will fly in the stratosphere; to calculate the effects of this hybrid subsonic/supersonic fleet, models capable of assessing stratospheric changes must be used.

Calculations of the effects of the hybrid subsonic/supersonic fleets were carried out in Chapter 4 in the following way. For a given year (2015 for example), a 3-D CTM calculation using the Oslo model was performed to determine the change in ozone concentration for the subsonic fleet relative to the background atmosphere for that year. A 2-D calculation was then carried out with the AER 2-D model to determine the change in mixing ratio for ozone between the hybrid fleet and subsonic-only fleet scenarios. For the latitudes and seasons shown in Table 5-1, these mixing ratio differences were added to the mixing ratio differences, calculated by the Oslo 3-D CTM, between the subsonic fleet and the background atmosphere. The scenarios used in Chapter 4 to provide the ozone differences for the subsonic impact were (A,B, 1992), (C,D, 2015), and (E,F, 2050), as defined in Table 4-4. The hybrid fleet impacts were obtained by comparing scenario S1k, defined in Table 4-11, with scenario D of Table 4-10 for 2015 and scenario S9h, defined in Table 4-11, with scenario D9 from Table 4-10 for 2050. Results from these sets of scenarios correspond to the calculated impacts of 500 HSCTs in 2015 and 1,000 HSCTs in 2050.

Figure 5-5: Ozone column changes for July and October.

Calculations designed to show the effects of aviation relative to changes in ozone or UV resulting from changes in the composition of the stratosphere require a slightly more convoluted approach. This approach is illustrated in Figure 5-4. The top panel shows the differences in background atmospheres calculated by the Oslo 3-D CTM for the years 2050 and 1970 (2050bg^-1970bg), 2015 and 1970 (2015bg^-1970bg), and 1992 and 1970 (1992bg^-1970bg). The middle panel shows the results obtained when these calculations are performed with the AER 2-D CTM. The background surface concentrations used in these calculations for 1992, 2015, and 2050 are those shown in Table 4-8. The background surface concentrations used for the 1970 calculation are those given in Table 6.3 of WMO (1994). It is clear from the top panel that the 3-D CTM does not provide any information about changes occurring in the atmosphere above about 18 km for the period from 1970 to 2050. The calculated increases for ozone shown below 18 km and

reaching peak values in the lower troposphere result from increased surface emissions, predominantly NOx, that are expected to occur over this time period. The calculations reported in the middle panel show quite different behavior. In this set of 2-D calculations, surface emissions of NOx were not increased between 1970 and 2050. The changes in ozone concentrations shown in this panel are directly attributable to expected changes in concentrations of inorganic chlorine and bromine compounds, as well as those for nitrous oxide.

The bottom panel in Figure 5-4 shows the result of the linear combination of the concentration changes calculated in the top and middle panels. Results such as those in the bottom panel are then used to derive the background atmospheres for 1970, 2015, and 2050 from the background atmosphere used for 1992. The procedure of combining the results of 3-D and 2-D CTM calculations in this manner is clearly questionable, but at the present stage of model development this compromise is necessary.

5.4.1.2. Method for Referring Calculated Changes to 1970

As discussed elsewhere in this chapter, the calculated impact of aviation on UV is to be compared with impacts on UV resulting from changes in atmospheric composition from other sources. For this purpose, 1970 has been taken as the reference year because it represents a period before the expected onset of substantial changes in the concentration of stratospheric ozone resulting from increases in the concentrations of stratospheric chlorine and bromine. The required ozone columns for 1970 have been obtained in the following manner: It is assumed that the amount of ozone in the lower troposphere of the background atmosphere has not changed between 1970 and 1992. Just as for 2015 and 2050, a 2-D calculation is performed to derive the 1970 background columns from corresponding values for 1992. As explained in Section 5.4.1.1, the background surface concentrations used for the 1970 calculation are those given in Table 6.3 of WMO (1994). It is further assumed that changes in tropospheric ozone due to aviation between 1970 and 1992 are proportional to the amount of NOx emitted. The amount of NOx emitted by aviation in 1970 is deduced from a linear extrapolation of the values given in Table 9-4; this amount is estimated to be 0.72 Tg.

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