A decreasing trend in stratospheric ozone has been a pivotal diagnostic in the assessment of anthropogenic halocarbon release. Because the bulk of global ozone resides in the stratosphere, measurements of total column ozone-which can be made quite accurately-have served as a proxy for stratospheric ozone abundance. Downward trends in total ozone are now well-established throughout all seasons and all latitudes, except in the tropics (WMO-UNEP, 1999). Broad agreement on the magnitude of the total ozone trend exists between ground-based and satellite observational databases and model predictions based on chlorine-catalyzed ozone destruction. However, as discussed in Section 2.1, aircraft engine emissions may induce changes of different magnitude and/or sign in tropospheric and stratospheric ozone densities. Therefore, to observe possible effects of aviation on the ozone layer, one is likely to have to focus on trends in the vertical ozone profile rather than overall column abundance.
Natural phenomena such as volcanic eruptions and seasonal and interannual climate variations may affect ozone density variations in the UT and LS. The time constants associated with these phenomena range from months (in the case of short-term climate variation) to years (for the occasional volcanic eruption) to possibly decades (for long-term climate change). Because extensive observational data on ozone are limited to the past several decades, it is not possible to completely deconvolute the impacts of various natural phenomena. The data record is sufficiently long, however, to allow characterization of periodic phenomena occuring on shorter time scales. Most of the anthropogenic forcings have been increasing secularly during the period of observation. Consequently, attempts to discriminate trend components can be carried out only with the aid of model predictions for each forcing.
Trend analyses of vertical ozone profiles have become possible only during the 1980s and 1990s as a result of data from the ground-based (Umkehr technique) and ozonesonde networks, and satellite-borne solar backscatter ultraviolet spectrometer (SBUV) and Stratospheric Aerosol and Gas Experiment (SAGE) I/II instruments (Logan, 1994; Miller et al.,1995; WMO-UNEP, 1995; Fortuin and Kelder, 1997; Harris et al., 1997; WMO, 1998; see also Figure 2-4). The middle stratospheric trends derived from different data sets show broad agreement with each other. The negative trend peaking at ~40-km altitude and extending from 30 to 50 km in middle latitudes is ascribed to the simple Cl-ClO catalytic cycle of ozone destruction from enhanced atmospheric chlorine loading. A significant negative trend is also discerned in the lowermost stratosphere (i.e., between the troposphere and approximately 20-km altitude), where increased heterogeneous conversion of chlorine-containing reservoir species to reactive radical forms has been suggested as a factor in ozone destruction through catalytic cycles involving the ClO+ClO and ClO+BrO reactions.
Figure 2-6: (a) Median NOx mixing ratios measured between 9 and 12 km during a number of in situ aircraft campaigns (compiled in Emmons et al., 1997); (b) NOx concentration field in the altitude regions between 300 and 190 hPa obtained by the measurements of NOxAR.
Trends in UT ozone for the time period encompassing large growth in aircraft fuel consumption (1970 to the present) are available from a number of ozonesonde stations. All of the sonde stations at middle and high latitudes of the Northern Hemisphere show a stratospheric decrease at altitudes between the tropopause and ~24 km for the period 1970-96. Upper tropospheric trends vary substantially among stations, with increases of 10-20%/decade over Europe, decreases of 5-10%/decade over Canada and the eastern United States, and no trend over Japan (WMO-UNEP, 1999). Neither aircraft nor surface NOx emissions-both showing little geographical variation in their European, North American, and Asian trends (Logan, 1994)-are consistent with observed ozone trend variations.
Detailed time-series analysis of ozone trend data in an area of heavy aircraft traffic (i.e., west-central Europe) reveals that air traffic growth is unlikely to be a primary factor in the observed upper tropospheric trend. For example, the sonde observations at Hohenpeissenberg, Germany, clearly show a mean increase in ozone of ~10%/decade below ~9-km altitude between 1970-96 (Figure 2-5). However, most of the increase occurred before 1985 (WMO-UNEP, 1999), even though air traffic growth remained steady, and the ozone trend for the period 1980-96 is slightly negative in the UT. The lack of growth in ozone after 1985 mimics the lack of growth of surface emissions of NOx (Logan, 1994). Decreases in the amount of ozone transported to the UT from the LS, because of reductions in stratospheric ozone abundance and/or weakening of dynamical transport, may also be a factor in the observed trend during the 1980s and 1990s.
In situ aircraft sampling of ozone in the 9-13 km region that has occurred sporadically over the past 20 years provides complementary data sets for use in understanding ozone climatology in the tropopause region. In 1994, a focused effort to collect climatological ozone data from aircraft platforms was initiated as the MOZAIC program (Marenco et al., 1999; Thouret et al., 1999). This database is now sufficiently long to address a number of important issues related to tropopause heights and seasonal variations, although it cannot yet address the issue of long-term trends.
As discussed in previous sections, ambient levels of NOx and soot are likely to be affected by aircraft to a greater extent than ozone. Accordingly, a comprehensive set of NOx and aerosol measurements taken over a wide range of locations and over the period of the past 20 years could provide a basis for evaluating aircraft impacts on these ozone-related species. Compared to the historical record for ozone, however, the available information on NOx and aerosol is sparser and was obtained only by in situ sampling from aircraft and balloons (Hofmann, 1993; Blake and Kato, 1995; Emmons et al., 1997). Satellite data are available for lower stratospheric aerosol, but the data record is relatively short and heavily influenced by recent volcanic eruptions. Analysis of NOx and aerosol trends would be exceedingly difficult to interpret because the aircraft source would be convolved with many other increasing sources of anthropogenic NOx and aerosol. In addition, the high degree of air mass variability in the troposphere places severe constraints on the atmospheric sampling strategy one would have to adopt to collect representative data.
In principle, aircraft signatures could be discerned from observation of the distribution of NOx because the aircraft source is geographically distinct. In situ aircraft sampling efforts have begun to provide a global map of NOx in the UT (Emmons et al., 1997; see also Figure 2-6a). During the past few years, field campaigns have been performed specifically to investigate aircraft flight corridors. For example, observations in air traffic have been made by Schlager et al. (1996, 1997). The observation area was the major flight route in the eastern North Atlantic, and the parameters observed were NOx, SO2, and particles; observations were made perpendicular to flight tracks. Under special meteorological conditions associated with a stagnant anticyclone, measured data indicated a large-scale accumulation of NOx and particles from aircraft emissions.
Approximately 4,000 hours of NOx measurements were collected from a B-747 platform during the Nitrogen Oxides and ozone measurements along Air Routes (NOxAR) project between spring 1995 and spring 1996, as shown in Figure 2-6b (Brunner, 1998). The NOxAR measurements demonstrated that, in addition to aircraft emissions, NOx produced by lightning and NOx emitted at the surface and transported upward by convection make large contributions to the NOx abundance in the UT. These contributions were largest over and downstream of continents in summer. Finally, the recently completed SONEX and POLINAT II campaigns were designed specifically to quantify various NOx sources in the UT. The findings of these latest studies are just now being reported.
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