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13 How is chlorine distributed in the stratosphere?


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

13 How is chlorine distributed in the stratosphere?

Over the past 20 years an enormous effort has been devoted to
identifying sources and sinks of stratospheric chlorine. The
concentrations of the major species have been measured as a
function of altitude, by "in-situ" methods ( e.g. collection
filters carried on planes and balloons) and by spectroscopic
observations from aircraft, balloons, satellites, and the Space
Shuttle. From all this work we now have a clear and consistent
picture of the processes that carry chlorine through the stratosphere.

Let us begin by asking where inorganic chlorine is found. In the
troposphere, the HCl mixing ratio decreased markedly with increasing
altitude. In the stratosphere, on the other hand, it _increases_ with
altitude, rapidly up to about 35 km, and then more slowly up to 55km
and beyond. This was noticed as early as 1976 [Farmer et al.]
[Eyre and Roscoe] and has been confirmed repeatedly since. Chlorine
Nitrate (ClONO2), the other important inorganic chlorine compound in
the stratosphere, also increases rapidly in the lower stratosphere, and
then falls off at higher altitudes. These results strongly suggest
that HCl in the stratosphere is being _produced_ there, not drifting
up from below.

Let us now look at the organic source gases. Here, the data show
that the mixing ratios of the CFC's and CCl4 are _nearly independent
of altitude_ in the troposphere, and _decrease rapidly with altitude_
in the stratosphere. The mixing ratios of the more reactive
hydrogenated compounds such as CH3CCl3 and CH3Cl drop off somewhat
in the troposphere, but also show a much more rapid decrease in
the stratosphere. The turnover in organic chlorine correlates
nicely with the increase in inorganic chlorine, confirming the
hypothesis that CFC's are being photolyzed as they rise high enough
in the stratosphere to experience enough short-wavelength UV. At
the bottom of the stratosphere almost all of the chlorine is
organic, and at the top it is all inorganic. [Fabian et al. ]
[Zander et al. 1987, 1992, 1996] [Penkett et al.]

Finally, there are the stable reaction intermediates, COF2 and
COFCl. These have been found in the lower and middle stratosphere,
exactly where one expects to find them if they are produced from
organic source gases and eventually react to give inorganic chlorine.

For example, the following is extracted from Tables II and III of
[Zander et al. 1992]; they refer to 30 degrees N Latitude in 1985.
I have rearranged the tables and rounded some of the numbers, and
the arithmetic in the second table is my own.

Organic Chlorine and Intermediates, Mixing ratios in ppbv

Alt.,  CH3Cl CCl4 CCl2F2 CCl3F CHClF2 CH3CCl3 C2F3Cl3  ||  COFCl
12.5  .580  .100  .310  .205  .066     .096    .021    ||  .004
15    .515  .085  .313  .190  .066     .084    .019    ||  .010
20    .350  .035  .300  .137  .061     .047    .013    ||  .035
25    .120   -    .175  .028  .053     .002    .004    ||  .077
30     -     -    .030   -    .042      -       -      ||  .029
40     -     -     -     -     -        -       -      ||   -

Inorganic Chlorine and Totals, Mixing ratios in ppbv

Alt., HCl  ClONO2   ClO  HOCl   ||   Total Cl,  Total Cl,  Total Cl
                                ||   Inorganic   Organic
km                              ||
12.5   -     -       -     -    ||       -       2.63        2.63
15    .065   -       -     -    ||     0.065     2.50        2.56
20    .566  .212     -     -    ||     0.778     1.78        2.56
25   1.027  .849    .028  .032  ||     1.936     0.702       2.64
30   1.452 1.016    .107  .077  ||     2.652     0.131       2.78
40   2.213 0.010    .234  .142  ||     2.607       -         2.61

I have included the intermediate COFCl in the Total Organic column.
It should be noted that COFCl was not measured directly in this
experiment, although the related intermediate COF2 was.

This is just an excerpt. The original tables give results every 2.5km
from 12.5 to 55km, together with a similar inventory for Fluorine.
Standard errors on total Cl were estimated to be 0.02-0.04 ppbv.
[Zander et al. 1996] provide a similar inventory for the year 1994;
once again the total chlorine at any altitude is approximately
constant, but at ~3.5 ppbv instead of ~2.6 ppbv, indicative of
the increase in anthropogenic halocarbons between 1985 and 1994.

Notice that the _total_ chlorine at any altitude is nearly constant
at ~2.5-2.8 ppbv. This is what we would expect if the sequence of
reactions that leads from organic sources to inorganic reservoirs
was fast compared to vertical transport. Our picture, then, would be
of a swarm of organic chlorine molecules slowly spreading upwards
through the stratosphere, being converted into inorganic reservoir
molecules as they climb. In fact this oversimplifies things -
photolysis pops off a single Cl atom which does reach its final
destination quickly, but the remaining Cl atoms are removed by a
sequence of slower reactions. Some of these reactions involve
compounds, such as NOx, which are not well-mixed; moreover,
"horizontal" transport does not really take place along surfaces of
constant altitude, so chemistry and atmospheric dynamics are in fact
coupled together in a complicated way. These are the sorts of issues
that are addressed in atmospheric models. Nevertheless, this simple
picture helps us to understand the qualitative trends, and quantitative
treatments confirm the conclusions [McElroy and Salawich]
[Russell et al. 1996].

We conclude that most of the inorganic chlorine in the stratosphere
is _produced_ there, as the end product of photolysis of the organic
chlorine compounds.


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