Re: [asa] Level of certainty in science

Janice's accusation against NASA may seem to be correct at first
glance but a closer scrutiny of the issue reveals that in proper
context, NASA's announcement was indeed correct in establishing the
link of CFCs to the ozone depletion. However, the statement that there
are no natural sources of HF is incorrect, although the relevant data
suggest that there are no relevant natural sources to stratospheric
HF. It is the combination of findings from the NASA data which formed
the smoking gun.

First let's look at the NASA press release on December 19, 1994 which
has been the cause of so much confusion:
http://www.nasa.gov/home/hqnews/1994/94-215.txt

Notice that the presence of HF is but part of the evidence, it's the
presence of CFC's in the stratosphere which showed that CFC's indeed
make it into the stratosphere. So what about HF? In doing my research,
it seems that NASA and scientists were aware that there are natural
sources of HF, mostly volcanic and that the evidence had suggested to
them that volcanic HF is an insignificant stratospheric source because
most of the HF is quickly removed.

In 1994 the following paper observed that:

"Balloon-Borne Observations of Mid–Latitude Hydrofluoric Acid" reports that

"For almost a decade the importance of measurements of molecular
concentration of stratospheric hydrofluoric acid (HF) has been
emphasized for its distinctive role in the chemistry of halogenated
gases and their effect in the depletion of stratospheric ozone
[Alcl?llroy and Salawitch, 1989]. Gaseous HF in the Earth's
stratosphere is believed to be entirely due to photodissociation of
cholorofluorocarbons (CFC'S), Therefore, the abundance of gaseous HF
can be viewed as an indicator of the past rate of CFC'S photolysis in
the stratosphere, HF's primary path of destruction is its slow
diffusion into the troposphere and eventual rainout. Its stability in
the atmosphere and its dominance in the stratospheric fiuorine
abundance was believed to prevent any catalytic reactions involving
fluorine-species leading to ozone destruction [Stolamki and Rundel,
1975]. However, this could change as additional kinetic data of
fluorine species containing CF~ groups become available for
evaluation of their impact on stratospheric ozone [Ko et al., 1994]
</quote>

So what happens? Halogenated gasses released into the atmosphere by
humans will eventually dissociate and one of the side products is HF,
in other words, finding HF and low ozone correlations is one
prediction, the other is the presence of CFC's in the stratosphere.
Since the reactions involved are reasonably well known, one can thus
proceed to do inventory calculations to see if all adds up. In other
words, the chlorine, fluoride inventories need to balance as function
of altitude.

The balloon data, mentioned in the above paper showed that

<quote>
Since its presence was first confirmed by Zander et al. [1977],
stratospheric HF has been extensively studied because of its
dominance in the stratospheric fluorine budget.

....

The HF volume mixing ratio for the four flights at specific Nz O
levels is summarized in Table 2. Also tabulated is an estimate of the
annual linear increase in HF. The estimated small increase in
stratospheric Nz O (~0.5%/year) has been subtracted from the computed
linear trend in HF. While the average increase in mid-stratosphere HF
vmr of 3.0 + 0.4%/year is consistent with estimations of total
fluorine compound loading of the stratosphere [R-inn, 1988], our
results
show a clear increase with altitude, </quote>

The NASA press release may have caused some concern and the phrasing
certainly justifies a certain skepticism but reading the press release
in its full context and understanding the issues at that time reveal
that the NASA press release indeed presented a smoking gun, and HF was
just a minor part of it.

As far as HF and volcanoes, there is indeed a source of HF to be found
in the Antarctic in the form of a high altitude volcano called Mount
Erebus, however data suggest that little HF of this relatively low
activity volcano makes it into the stratosphere. In other words, the
smoking volcano, remains smoldering.

See for instance Volcanic gas emissions from Mount Erebus and their
impact on the Antarctic environment. J. Geophysical Research, vol. 102
no. b7, pp. 15,039-15,055

ZREDA-GOSTYNSKA, G., KYLE, P.R., FINNEGAN, D. 1993. Chlorine,
fluorine, and sulfur emissions from Mount Erebus, Antarctica and
estimated contributions to the Antarctic atmosphere. Geophysical
Research Letters 20, 1959-1962.

Contribution of Cl- and F-bearing gases to the atmosphere by volcanoes
Nature (ISSN 0028-0836), vol. 334, Aug. 4, 1988, p. 415-418.
NSF-supported research.

Equilibrium thermodynamics is used here to predict the speciation of
Cl and F in volcanic gases and provide new estimates of the global
emission rates to the atmosphere of these gases, which catalyze the
destruction of ozone in the stratosphere. The calculations show that
HCl and HF are the dominant species of Cl and F in volcanic gases, at
least several orders of magnitude more abundant than all other
species. The annual global volcanic fluxes of HCl and HF is estimated
to be 0.4-11 Tg and 0.06-6 Tg respectively. On average, less than 10
percent of these emissions come in large explosive eruptions that
transmit them efficiently to the stratosphere. Although they are
infrequent, large volcanic eruptions may inject significant amounts of
HCl and HF into the stratosphere. Passively degassing volcanoes are
also a major source of tropospheric HF, although sea salt is the main
source of tropospheric HCl.

On 2/5/07, Janice Matchett <janmatch@earthlink.net> wrote:
>
> NASA did this nation a serious disservice by making false statements about
> "the ozone hole". "It is a classic example of why Americans have become
> disillusioned about self-serving bureaucracies that hurt the citizens they
> are supposed to serve."
> http://www.discerningtoday.org/ozone_depl_twilight_.htm
>

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Received on Wed Feb 7 12:15:58 2007