Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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2.2.2. Observing Changes in Atmospheric Ozone from Present Aviation
2.2.2.1. Observed Ozone Trends in the Upper Troposphere and Lower Stratosphere
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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 chlorinecontaining
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.
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