Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
NO x emission to 0.5 Tg N yr -1 (Figure 2-3).
The original 2-D (altitude-latitude) model studies of the tropospheric ozone impacts of subsonic NO x emissions are close to the (mean + 1xsd) line from the entire
ensemble of studies. 2-D model estimates have declined steadily so that by 1992 they were close to the (mean - 1xsd) line. The original 1-D studies showed a large
amount of scatter and have not been used extensively for assessment purposes for the past decade or so. 2-D channel (altitude-longitude) models have been applied
to the aircraft assessment as an interim strategy prior to 3-D (altitude-latitude-longitude) model development. 2-D channel model results are toward the high end of the
range of results.
3-D models have been employed increasingly from 1994 onward, with the results lying toward the low end of the overall spread of model results. 3-D model results still
show a considerable amount of scatter, and there is no evidence that the move from 2-D to 3-D models or the selection of 3-D models in Chapter 4 has improved intermodel
consistency or overall predictive confidence. Regardless of how 2-D model results compare with zonally averaged 3-D model results, a crucial advantage of 3-D
models is that they can account for zonal asymmetries and, in principle, be evaluated more decisively against local observations. Consequently, more confidence can
be gained in their predictive capabilities for aircraft assessments (see Sections 2.3.1.1 and 2.3.1.2).
Based on all model results compiled for this assessment, our estimate of the likely ozone increase at its maximum in the principal traffic areas-9-13 km altitude, 30-60°
N latitude-from 1992 subsonic aviation and during summer is about 6%, or 8 ppb, for a global aircraft NOx emission of 0.5 Tg N yr-1 (1.65 Tg NOx as NO2 yr-1 ; see
Chapter 9).
Chapter 4 focuses attention on the development of future scenarios for subsonic aircraft NOx emissions; six 3-D global models have been employed to assess future
impacts of subsonic aircraft. Table 2-1 shows ozone changes calculated in some of these models for 1992 subsonic operations compared with model results presented
in previous studies. The latest model results in Table 2-1 appear to lie well within the central range of estimates from previous studies.
The estimates in Table 2-1 can be compared with previous assessments of the likely ozone impacts of NOx emissions from present aviation. The WMO-UNEP (1995)
assessment described model results available to the end of 1993 as preliminary and indicative of maximum ozone increases of 4-12% at around 10 km for 30-50°N.
The above estimates encompass a narrower range than that given in the WMO-UNEP (1995) assessment and suggest that the true value is at the lower end of
previous estimates. An assessment prepared for the European Commission (Brasseur et al., 1998) concluded that the current fleet of commercial aircraft should have
increased NOx abundances by 50-100 ppt near 200 hPa, with a corresponding increase in ozone concentrations of 5-9 ppb (i.e., 4-8%), during summertime. This latter
range is consistent with the range identified in the present study, based on the five selected 3-D models.
2.2.1.2. Effects of Aircraft NO x Emissions on Stratospheric Ozone
2-D models have been developed extensively over the past decade in response to the
need for credible predictions of CFC impacts on stratospheric ozone and more recently for
understanding the effects of future supersonic (high speed civil transport, or HSCT)
aircraft fleets. Emphasis in these models has been placed on ClOx , HOx , and NOx chemistry in the 20-40 km region of the atmosphere, where the CFC and HSCT effects
are expected to be manifested. Model calculations for the 1992 subsonic aircraft fleet
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