08 What are the sources of chlorine in the troposphere?
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This article is from the Ozone Depletion: Stratospheric Chlorine and Bromine FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
08 What are the sources of chlorine in the troposphere?
Let us divide the chlorine-containing compounds found in the
atmosphere into two groups, "organic chlorine" and "inorganic
chlorine". The most important inorganic chlorine compound in the
troposphere is hydrogen chloride, HCl. Its principal source is
acidification of salt spray - reaction of atmospheric sulfuric and
nitric acids with chloride ions in aerosols. At sea level, this
leads to an HCl mixing ratio of 0.05 - 0.45 ppbv, depending strongly
upon location (e.g. smaller values over land.) However, HCl dissolves
very readily in water (giving hydrochloric acid), and condensation of
water vapor efficiently removes HCl from the _upper_ troposphere.
Measurements show that the HCl mixing ratio is less than 0.1 ppbv at
elevations above 7 km, and less than 0.04 ppbv at 13.7 km.
[Vierkorn-Rudolf et al.] [Harris et al.]
There are many volatile organic compounds containing chlorine, but
most of them are quickly decomposed by the natural oxidants in the
troposphere, and the chlorine atoms that were in these compounds
eventually find their way into HCl or other soluble species and are
rained out. The most important exceptions are:
ChloroFluoroCarbons, of which the most important are
CF2Cl2 (CFC-12), CFCl3 (CFC-11), and CF2ClCFCl2 (CFC-113);
HydroChloroFluoroCarbons such as CHClF2 (HCFC-22);
Carbon Tetrachloride, CCl4;
Methyl Chloroform, CH3CCl3;
and Methyl Chloride, CH3Cl (also called Chloromethane).
Only the last has a large natural source; it is produced
biologically in the oceans and chemically from biomass burning.
The CFC's and CCl4 are nearly inert in the troposphere, and have
lifetimes of 50-200+ years. Their major "sink" is photolysis by UV
radiation. [Rowland 1989, 1991] The hydrogen-containing halocarbons
are more reactive, and are removed in the troposphere by reactions
with OH radicals. This process is slow, however, and they live long
enough (1-20 years) for a large fraction to reach the stratosphere.
As a result of this enormous difference in atmospheric lifetimes,
there is more chlorine present in the lower atmosphere in
halocarbons than in HCl, even though HCl is produced in much larger
quantities. Total tropospheric organic chlorine amounted to
~3.8 ppbv in 1989 [WMO 1991], and this mixing ratio is very nearly
independent of altitude throughout the troposphere. Methyl Chloride,
the only ozone-depleting chlorocarbon with a major natural source,
makes up 0.6 ppbv of this total. Compare this to the tropospheric HCl
mixing ratios given above: < 0.5 ppbv at sea level, < 0.1 ppbv at 3 km,
and < 0.04 ppbv at 10 km.
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