Derivation of a series of annual values for the equivalent CFC-11
concentration of all halogenated species except CFC-12

The series is derived from the radiative forcing computed from the
annual concentrations of all the halogenated species except CFC-12
specified in Climate Change 2001: The Scientific Basis (http://www.grida.no/climate/ipcc_tar/wg1/index.htm)
- Annex II.2.10 (http://www.grida.no/climate/ipcc_tar/wg1/540.htm) 
for CFCs
- Annex II.2.4   (http://www.grida.no/climate/ipcc_tar/wg1/534.htm) 
for PFCs, SF6 and HFCs


Concentrations are at 5-yearly resolution are taken from the report: 
UNEP/WMO Scientific Assessment of Ozone Depletion: 1998 Chapter 11:
Halocarbon scenarios for the future ozone layer and related consequences,
Guus Velders (RIVM) and Sasha Madronich (NCAR), Version 5, June 3, 1998,
Calculations by John Daniel and Guus Velders Guus.Velders@rivm.nl &
jdaniel@al.noaa.gov

For CFC-11, CFC-113 and CCl4 use is made of the annual values described in:
Walker, S. J., R. F. Weiss and P. K. Salameh, Reconstructed histories of
        the annual mean atmospheric mole fractions for the halocarbons
        CFC-11, CFC-12, CFC-113 and carbon tetrachloride. Journal of 
        Geophysical Research, 105, C6, 14,285-14,296, 2000
(http://gaslab.ucsd.edu/pub/cfchist/)

The CF4 preindustrial concentration is taken from reference:
Harnish J, R Borchers, P Fabian, HW Gäggeler, U Schotterer, 1996:
Effect of natural tetrafluoromethane, Nature 384, 32. 
The annual values of CF4 and C2F6 have been derived using
the aluminium production data given in:
Weston RE, 1996: Possible greenhouse effects of tetrafluoromethane and
carbon dioxide emitted from aluminum production. Atmos Env 30, 2901-2910

Concentrations are derived at annual resolution using cubic spline
interpolation inside the xmgrace package. The resulting series have
been edited to suppress small oscillations around zero that can be
generated by the cubic spline interpolation.

The computations of the radiative forcing are made within the Excel 
spreadsheet GHGs_1850-2000.xls using the values of the radiative 
forcing for each gas given in table 1-6 of the  WMO/UNEP 
Scientific Assessment of Ozone Depletion: 2002 
(http://www.wmo.ch/web/arep/reports/o3_assess_rep_2002_front_page.html)

The total radiative forcing is then converted back into the concentration
of CFC-11 (called CFC-11*) that would give the same radiative forcing 
as the sum of the radiative forcing of all the halogenated specied

(To make it clear that the series represent annual mean,
that is central values, for the year in question, 0.49 is added to
the year before writing the series. In this way a rounding of the year
when writing it in integer form rounds the year to the correct integer)