Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
heterogeneous reaction between ozone and soot proceeds at the highest reported rate (the one used in the
Bekki calculation), namely g(O3 ) = 3.0 x 10-4 (Stephens et al., 1986; Fendel et al., 1995; Rogaski et al.,
1997), the time required to consume the mass of soot present in the stratosphere would be approximately 10
minutes. It is important to note that this result is independent of the soot surface-to-volume ratio; it depends
only upon the ozone concentration, which is taken to be 1011 cm-3 at 10-km altitude. At this reaction rate,
only 5 hours would be required to consume the entire soot particulate population at 10 km (assuming 5 x 107
carbon atoms cm-3 at a soot loading of 1 ng m-3 ) and cause the effective soot-ozone reaction rate to drop to
zero.
Laboratory evidence suggests that the soot-ozone reaction rate decreases after consumption and alteration
of the top surface carbon layer (Stephens et al., 1986). Small reported laboratory g values-on the order of 3.0
x 10 -5 -likely reflect the reaction rate associated with a "deactivated" laboratory soot surface. Using this value
for g, the time required to consume individual particles of soot would be less than a day, and the time to
consume all atmospheric soot would be lengthened to approximately 3 weeks. Although this change in
reaction rate extends the lifetime of atmospheric soot, it comes at the expense of the overall ozone
destruction rate. Accordingly, our analysis indicates that the massof soot in the UT and LS does not seem to
be sufficient to quantitatively affect ozone, regardless of the reaction rate chosen.
In support of this conclusion, we note that in situ measurements in the engine exhaust of a Concorde
supersonic aircraft in the LS have been used recently to constrain heterogeneous reaction rates on soot
particles in a plume model (Gao et al., 1998). Gao et al. inferred low reactivity for the ozone-soot interaction;
coupled with the measured abundance of the soot aerosol, Gao et al. reach the same conclusion that we
derived from our qualititative arguments. In summary, the few model calculations performed suggest that
subsonic aircraft sulfate and soot emissions in the stratosphere act as substrates for ozone depletion. The
magnitude of the depletions is likely to be small relative to ozone increases from NO x emissions, but the
depletions are highly uncertain because of the poorstate of understanding and model development relative to
aerosol chemistry.
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Figure 2-5: Annual trends for the periods 1970-96
and 1980-96 shown by ozonesonde measurements
at Hohenpeissenberg, Germany. 95% confidence
limits are shown [adapted from WMO (1998) by J.
Logan].
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