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24 Will commercial supersonic aircraft damage the ozone layer?


This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.

24 Will commercial supersonic aircraft damage the ozone layer?

Short answer: Probably not. This problem is very complicated,
and a definitive answer will not be available for several years,
but present model calculations indicate that a fleet of high-speed
civil transports would deplete the ozone layer by < 2%. [WMO 1991, 1994]

Long answer (this is a tough one):

Supersonic aircraft fly in the stratosphere. Since vertical transport
in the stratosphere is slow, the exhaust gases from a supersonic jet
can stay there for two years or more. The most important exhaust gases
are the nitrogen oxides, NO and NO2, collectively referred to as "NOx".
NOx is produced from ordinary nitrogen and oxygen by electrical
discharges (e.g. lightning) and by high-temperature combustion (e.g. in
automobile and aircraft engines).

The relationship between NOx and ozone is complicated. In the
troposphere, NOx _makes_ ozone, a phenomenon well known to residents
of Los Angeles and other cities beset by photochemical smog. At high
altitudes in the troposphere, similar chemical reactions produce ozone
as a byproduct of the oxidation of methane; for this reason ordinary
subsonic aircraft actually increase the thickness of the ozone layer
by a very small amount.

Things are very different in the stratosphere. Here the principal
source of NOx is nitrous oxide, N2O ("laughing gas"). Most of the
N2O in the atmosphere comes from bacteriological decomposition of
organic matter - reduction of nitrate ions or oxidation of ammonium
ions. (It was once assumed that anthropogenic sources were negligible
in comparison, but this is now known to be false. The total
anthropogenic contribution is estimated at 8 Tg (teragrams)/yr,
compared to a natural source of 18 Tg/yr. [Khalil and Rasmussen].)
N2O, unlike NOx, is very unreactive - it has an atmospheric lifetime
of more than 150 years - so it reaches the stratosphere, where most of
it is converted to nitrogen and oxygen by UV photolysis. However, a
small fraction of the N2O that reaches the stratosphere reacts instead
with oxygen atoms (to be precise, with the very rare electronically
excited singlet-D oxygen atoms), and this is the major natural source
of NOx in the stratosphere; about 1.2 million tons are produced each
year in this way. This source strength would be matched by 500 of the
SST's designed by Boeing in the late 1960's, each spending 5 hours per
day in the stratosphere. (Boeing was intending to sell 800 of these
aircraft.) The Concorde, a slower plane, produces less than half as
much NOx and flies at a lower altitude; since the Concorde fleet is
small, its contribution to stratospheric NOx is not significant. Before
sending large fleets of high-speed aircraft into the stratosphere,
however, one should certainly consider the possible effects of
increasing the rate of production of an important stratospheric trace
gas by as much as a factor of two. [CIC 1975]

In 1969, Paul Crutzen discovered that NOx could be an efficient
catalyst for the destruction of stratospheric ozone: [Crutzen 1970]

       NO + O3  -> NO2 + O2
       NO2 + O -> NO + O2
 net:  O3 + O -> 2 O2

(For this and other contributions to ozone research, Crutzen,
together with Rowland and Molina, was awarded the 1995 Nobel Prize
in Chemistry. The official announcement from the Swedish Academy is
available at http://www.nobel.se/announcement95-chemistry.html .)
Two years later, Harold S. Johnston made the connection to SST
emissions. Until then it had been thought that the radicals H, OH,
and HO2 (referred to collectively as "HOx") were the principal
catalysts for ozone loss; thus, investigations of the impact of
aircraft exhaust on stratospheric ozone had focussed on emissions of
water vapor, a possible source for these radicals. (The importance of
chlorine radicals, Cl, ClO, and ClO2, referred to as - you guessed it
- "ClOx", was not discovered until 1973.) It had been argued -
correctly, as it turns out - that water vapor injection was
unimportant for determining the ozone balance. The discovery of
the NOx cycle threw the question open again.

Beginning in 1972, the U.S. National Academies of Science and
Engineering and the Department of Transportation sponsored an
intensive program of stratospheric research. [CIC 1975] It soon
became clear that the relationship between NOx emissions and the
ozone layer was very complicated. The stratospheric lifetime of
NOx is comparable to the timescale for transport from North to
South, so its concentration depends strongly upon latitude. Much
of the NOx is injected near the tropopause, a region where
quantitative modelling is very difficult, and the results of
calculations depend sensitively upon how troposphere-stratosphere
exchange is treated. Stratospheric NOx chemistry is _extremely_
complicated, much worse than chlorine chemistry. Among other
things, NO2 reacts rapidly with ClO, forming the inactive chlorine
reservoir ClONO2 - so while on the one hand increasing NOx leads
directly to ozone loss, on the other it suppresses the action
of the more potent chlorine catalyst. And on top of all of this, the
SST's always spend part of their time in the troposphere, where NOx
emissions cause ozone increases. Estimates of long-term ozone
changes due to large-scale NOx emissions varied markedly from year
to year, going from -10% in 1974, to +2% (i.e. a net ozone _gain_)
in 1979, to -8% in 1982. (In contrast, while the estimates of the
effects of CFC emissions on ozone also varied a great deal in these
early years, they always gave a net loss of ozone.) [Wayne]

The discovery of the Antarctic ozone hole added a new piece to the
puzzle. As described in Part III, the ozone hole is caused by
heterogeneous chemistry on the surfaces of stratospheric cloud
particles. While these clouds are only found in polar regions,
similar chemical reactions take place on sulfate aerosols which are
found throughout the lower stratosphere. The most important of the
aerosol reactions is the conversion of N2O5 to nitric acid:

N2O5 + H2O -> 2 HNO3 (catalyzed by aerosol surfaces)

N2O5 is in equilibrium with NOx, so removal of N2O5 by this
reaction lowers the NOx concentration. The result is that in the
lower stratosphere the NOx catalytic cycle contributes much less to
overall ozone loss than the HOx and ClOx cycles. Ironically, the
same processes that makes chlorine-catalyzed ozone depletion so
much more important than was believed 10 years ago, also make
NOx-catalyzed ozone loss less important.

In the meantime, there has been a great deal of progress in developing
jet engines that will produce much less NOx - up to a factor of 10 -
than the old Boeing SST. The most recent model calculations indicate
that a fleet of the new "high-speed civil transports" would deplete
the ozone layer by 0.3-1.8%. Caution is still required, since the
experiment has not been done - we have not yet tried adding large
amounts of NOx to the stratosphere. The forecasts, however, are
good. [WMO 1991, Ch. 10] [WMO 1994] Very recently, a new complication
has appeared: _in situ_ measurements in the exhaust plume of a
Concorde aircraft flying at supersonic speeds indicate that the
ground-based estimates of NOx emissions are accurate, but that the
exhaust also contains large amounts of sulfate-based particulates
[Fahey et al. 1995]. Since reactions on sulfate aerosols are believed
to play an important role in halogen-catalyzed ozone depletion, it may
be advisable to concentrate on reducing the sulfur content of the
fuels that are to be used in new generations of supersonic aircraft,
rather than further reducing NOx emissions.

_Aside_: One sometimes hears that the US government killed the SST
project in 1971 because of concerns raised by H. S. Johnston's work
on NOx. This is not true. The US House of Representatives had already
voted to cut off Federal funding for the SST when Johnston began
his calculations. The House debate had centered around economics and
the effects of noise, especially sonic booms, although there were
some vague concerns about "pollution" and one physicist had testified
about the possible effects of water vapor on ozone. About 6 weeks
after both houses had voted to cancel the SST, its supporters
succeeded in reviving the project in the House. In the meantime,
Johnston had sent a preliminary report to several professional
colleagues and submitted a paper to _Science_. A preprint of
Johnston's report leaked to a small California newspaper which
published a highly sensationalized account. The story hit the press
a few days before the Senate voted, 58-37, not to revive the SST.
(The previous Senate vote had been 51-46 to cancel the project. The
reason for the larger majority in the second vote was probably the
statement by Boeing's chairman that at least $500 million more would
be needed to revive the program.)


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