Aviation and the Global Atmosphere

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5.4.2. Results

5.4.2.1. Ozone Column Changes

Figure 5-6: Calculated ozone and UVery averaged between 65S and 65N for July and referred to the calculated background values for 1970.

Figure 5-7: Calculated ozone and UVery at 45N in January referred to the calculated background values for 1970.

Figure 5-8: Calculated ozone and UVery at 45N in July referred to the calculated background values for 1970.

Figure 5-9: Percent changes in UVery as a function of latitude for July and October for a number of scenarios.

Figure 5-5 shows calculated ozone column changes as a function of latitude for July and October and for a range of scenarios. The top panel shows the change in columns attributable to subsonic fleets over the period 1992-2050 relative to a background atmosphere of the same year. The corresponding calculations for 1970 are not shown, but the effects attributable to aircraft for this period were very much smaller than any shown in the top panel of Figure 5-5 (see Figures 5-6, 5-7, and 5-8). The middle panel shows a corresponding calculation for the hybrid (subsonic+supersonic) fleets for 2015 and 2050, and the bottom panel shows the changes in background between 2050 and 1970, between 2015 and 1970, and between 1992 and 1970. The major features of the aircraft impact on ozone have been discussed in Chapter 4. For the background atmospheres (bottom panel), the calculations predict a systematic increase in ozone after 1992. For the Southern Hemisphere, this outcome is largely because of the expected decrease in bromine and chlorine. The additional increase in ozone in the Northern Hemisphere relative to the Southern Hemisphere is because the projected release of NOx at the surface is greater in the Northern Hemisphere.

5.4.2.2. Calculated Changes in Ozone Compared to Changes in UV

An alternative to the method of presentation of results used in Figure 5-5 is to show the evolution of changes in ozone and UV as a function of time, as in Figure 5-6. Here, calculated ozone and UVery averaged between 65S and 65N for July and for a range of scenarios are referred to calculated background values for 1970. When averages are taken over a broad latitude band such as that in Figure 5-6, seasonal behavior is removed, to a large extent. However, a particular latitude may exhibit significant seasonal behavior in the calculated departure from background values for 1970. For example, Figure 5-7 shows calculated changes for 45N for January, and Figure 5-8 shows the corresponding calculations for July. Comparison of Figures 5-7 and 5-8 shows that, although the calculated ozone and UVery for the background in 2050 for January are little different from the corresponding values for 1970, there are significant departures in July. This result is almost certainly because of the increased ozone calculated for the upper troposphere in 2050 resulting from the increase in NOx released from the surface of the Earth relative to 1970.

The method of presentation shown in Figures 5-6, 5-7, and 5-8 has the advantage that the predicted impact of either the hybrid fleet or the subsonic fleet on the background atmosphere at the corresponding time can easily be determined from the diagram; in addition, the departure for any of the scenarios from the calculated value for the background atmosphere in 1970 is readily determined. However, this method of presentation does not show changes as a function of latitude; that information is provided in Figure 5-9, which shows percentage UVery changes for July and October corresponding to ozone changes in Figure 5-5. The labels on the diagram refer to the atmospheres defined in Table 5-2.

The three panels in Figure 5-9 show the calculated changes in UVery for subsonics (top), subsonics+supersonics (middle), and changes in ground emissions (bottom). Several trends are clear from Figure 5-9. First, the impact of subsonic aviation is significantly greater in 2015 and 2050 than in 1992 (top panel); at northern mid-latitudes, the impact of the subsonic fleet is roughly proportional to the levels of aircraft emissions assumed. Second, when the effect of the hybrid fleet is compared to the corresponding background, there is an increase in UVery relative to the case where a pure subsonic fleet is considered (compare the top and middle panels). Third, calculated changes in UVery relative to 1970 for present and future background atmospheres show an increase in 1992 and a systematic decrease thereafter (bottom panel). This behavior is directly related to the levels of bromine and chlorine assumed for the stratosphere and the amount of NOx assumed to be released from the surface of the Earth.

5.4.2.3. Treatment of Uncertainties in Ozone Change and UVery Change Calculations

The calculations reported in Sections 5.4.2.1 and 5.4.2.2 are based on information provided in Chapter 4. In that chapter, calculations were carried out for a number of scenarios corresponding to the years 1992, 2015, and 2050. For 1992, the calculated impact of the subsonic fleet on atmospheric ozone was estimated using a 3-D CTM. Similar calculations were performed to assess the expected impact of subsonic aircraft in 2015 and 2050. In addition, the impact of hybrid fleets of subsonic and supersonic aircraft for 2015 and 2050 were obtained using a combination of 2-D and 3-D chemical transport calculations. The results for the hybrid fleet in Sections 5.4.2.1 and 5.4.2.2 are based on calculations provided in Chapter 4 for the AER model for scenarios S1k-D for 2015 and S9h-D9 for 2050. These scenarios are defined in Tables 4-10, 4-11, and 4-12. In addition to performing these calculations, the authors of Chapter 4 set uncertainty limits on these results. For ozone changes resulting from the impact of the subsonic fleets, the uncertainty was taken to be a factor of two times the difference obtained by the Oslo 3-D model for scenarios (B-A), (D-C), and (F-E) defined in Table 4-4. The quoted uncertainties are taken as the 67% likelihood range. We believe that uncertainty in the calculation of ozone percentage changes is by far the greatest uncertainty in the determination of percentage 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.

Chapter 4 considered three components in assessing the uncertainty for the impact of the hybrid fleet on ozone: 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 HSCTs) would be in the range of -3.5 to +1% when compared with the impact of the subsonic fleet and that the best estimate is-1% given by the AER 2-D model. The uncertainty range again represents the 67% likelihood range with a confidence in this

uncertainty range of "fair." As with estimated subsonic impacts, we believe that the uncertainty in the change in ozone caused by the hybrid fleet is much greater than any other uncertainties in the calculation of changes in UVery. Chapter 4 has provided only an annually averaged Northern Hemisphere value because it is not possible at present to assign uncertainty factors as functions of latitude, altitude, and season. The large variations in ozone changes predicted by a range of models as functions of latitude, altitude, and season (see Figures 4-6c, 4-6d, 4-12a, and 4-12b) are clear indications of shortcomings inherent in current models.

Given that the predicted change in UVery is primarily a function of changes in the ozone column and is less sensitive to the exact altitude dependence of the ozone change, an estimate of the uncertainty for changes in ozone columns as a function of latitude and season should be sufficient. This chapter uses the subjective estimates of uncertainties from Chapter 4 and defines the 67% likelihood range for changes in ozone columns at each latitude and season as follows. If the column change predicted by the AER 2-D model at latitude (Q) and time of year (T) is a(Q,T)%, the uncertainty range in percent column ozone change at that latitude and time of year is given by [a(Q,T) -3]% to [a(Q,T) +2]%. The same definition will be adopted for fleet sizes of either 500 or 1,000 HSCTs. These estimates are very subjective; although the confidence attached to these uncertainties for the tropics is "good," outside the tropics the confidence can be considered only "fair." In the absence of a rigorous method of obtaining uncertainties, this chapter also assumes that there is a 5% range in estimated uncertainties for change in UVery; this range is given by [b(Q,T) -2]% to [b(Q,T) +3]%, where b(Q,T)% is the percent change in UVery corresponding to a(Q,T)% change in ozone column. In summary, uncertainties in the impact of the subsonic fleet on UVery may be obtained from any of the diagrams in this chapter that report this change simply by doubling or halving the change shown in the diagram. For example, in the top panel of Figure 5-9, the change in UVery shown for the subsonic impact for July of 2015 at 45N is -0.9%. Accordingly, the 67% likelihood range for this particular change is-1.8 to -0.5%; because it is a calculation for 2050, our confidence in this result is "poor" as prescribed by Chapter 4. Similarly, in the middle panel of Figure 5-9, the impact of the hybrid fleet at the equator in July of 2050 is shown as -0.2%. This impact is based on a calculation of ozone changes by the AER 2-D model. From the discussion above, we determined the 67% likelihood range for this impact to be -2.2 to +2.8%, and our estimate of the confidence is "good." At 65N in July of 2050 (middle panel of Figure 5-9), the calculated impact of the hybrid fleet on UVery is +0.5%, which gives-1.5 to +3.5% as the 67% likelihood range with an estimated confidence of "fair."

As discussed in Section 5.4.1.1, the calculations shown in Figures 5-6 to 5-9 include percent changes in UVery for the background atmosphere using 1970 as the reference year. At 45N in July, these changes are +8, +3, and -3% for 1992, 2015, and 2050, respectively. At 45S in January, the calculated changes are +9, +4.5, and 0% for 1992, 2015, and 2050, respectively. For comparison, the computed change due to observed ozone depletion at 35-50N in July is about 4% over the period 1970-1992 (WMO, 1999). The corresponding change for 35-50S in January is 8%.

5.4.2.4. Contribution of Persistent Contrails, Cirrus Clouds, and Aerosols to the Impact of Aviation on UV

The geographic and temporal variability associated with clouds and aerosols complicates attempts to make general statements concerning their effects on UV irradiance. Nonetheless, this section considers some highly simplified scenarios to place bounds on the influence of altered contrails, cirrus, and aerosol amounts. There are three issues to address: The effect of regional, persistent contrails; the effect of observed trends in cirrus that may result from a variety of processes, including aviation; and the effect of aviation-produced aerosols.

To estimate the effects of persistent contrails on ground-level UV, calculations assume a 5% area coverage, appropriate to the local maximum over the eastern United States of America (see Section 3.4.3), and a contrail optical thickness of 0.3. For latitude 45N summer, local noon, this scenario leads to a reduction in UVery of 0.2% relative to clear skies. If persistent contrail coverage were to increase to 10%, the corresponding reduction in UVery would be 0.4%.

The observed trends in cirrus presented in Chapter 3 include the effects of aviation, as well as any other influences that may be operative. The estimated response of UVery to trends in the cirrus background is based on the following assumptions: The cirrus have a scattering optical thickness of 0.3, and initially 23% of the land area is covered by cirrus at an altitude of 11 km. Starting from this condition, an increase of 3.5% per decade in the land area covered by cirrus, appropriate to the United States of America in spring (Figure 3-19), leads to a decline in UVery of approximately 0.1% per decade for local noon at latitude 45N. If changes in persistent contrails and cirrus, especially on regional scales, differ substantially from the estimates adopted above, the resulting changes in UVery would have to be modified accordingly. The numbers given here point to the magnitude of reasonable changes, but they should not be viewed as predictions of the future.

The aircraft-related perturbation to atmospheric optical depth in the UV from soot and sulfate aerosols is very small compared to the natural background opacity. The values presented in Table 3-4 imply enhanced aerosol optical depths for soot and sulfates of less than 10-3. The decrease in UVery associated with such perturbations is less than 0.1%. Calculations indicate that, although an increase in aerosols in the background atmosphere between 1970 and 2050 from sources other than aviation would result in a reduction of UVery relative to clear skies, the percent change in UVery attributable to aviation is relatively insensitive to background aerosol loading.



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