Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
NAT particles ranging from 0.02 ppbv at 60°N to 0.12 ppbv at 85°N at 20 km in January. These values provide an increase of 0.08 mm 2 cm -3 in PSC SAD at 20 km
and 85°N, assuming a unimodal distribution of PSC particles with diameter of 1 mm. The increase in spatial extent of PSCs both vertically and latitudinally is small in
the model. PSC increases are very sensitive to background temperature, H 2 O, and HNO 3 values and will differ considerably among models. The increases are not
likely to significantly alter ozone changes in polar winter because the SAD increases are much less than typical values of 1-10 mm 2 cm -3 calculated for PSC events,
and satellite data observations show that the probability of PSC formation below 14 km in the Arctic is generally very low (< 1%) (Poole and Pitts, 1994).
An important caveat related to the assessment of additional PSC formation as a result of aircraft emissions is that plume processes are not included. Global models
generally assume that aircraft emissions are homogeneously distributed in a model grid box that is much larger than an aircraft plume. The consequences of this
assumption have not yet been fully evaluated. In one model study, reactions on PSCs did not affect ozone chemistry in a subsonic plume at northern mid-latitudes in
April (Danilin et al., 1994). A further caveat is that estimated PSC changes from aircraft emissions have not accounted for projected cooling of the stratosphere, which
may enhance PSC formation.
The chemical implications of increased PSC formation for ozone chemistry and atmospheric composition are further discussed in Chapter 2. The effects of future
aircraft fleets on additional PSC formation and subsequent ozone response are presented in Chapter 4.
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