This article is from the Ozone Depletion: Stratospheric Chlorine and Bromine FAQ, by Robert Parson firstname.lastname@example.org with numerous contributions by others.
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 km 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 - - - - - - - || -
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