This file describes the following data set:

GLOBALLY AVERAGED ATMOSPHERIC CFC-11 CONCENTRATIONS: MONTHLY AND ANNUAL DATA
FOR THE PERIOD 1975-1992. CDIAC DB-1010

M.A.K. Khalil and R. A. Rasmussen
Oregon Graduate Institute of Science and Technology
Portland, Oregon 97291 U.S.A.


CONTENTS
   I.   Overview
   II.  Sampling and Analysis Methods
   III. File Description
          "cfc11.dat" (globally averaged CFC-11 concentrations, monthly and
	   annual)
   IV.  References


I. OVERVIEW

This data set presents globally averaged atmospheric concentrations of
chlorofluorocarbon 11, known also as CFC-11 or F-11 (chemical name:
trichlorofluoromethane; formula: CCl3F). The monthly global average data are
derived from flask air samples collected at eight sites in six locations over
the period 8/80 to 7/92. The sites (with locations given in parentheses) are:
Barrow (Alaska); Cape Meares (Oregon); Cape Kumukahi and Mauna Loa (Hawaii);
Cape Matatula (American Samoa); Cape Grim (Tasmania); Palmer Station and the
South Pole (Antarctica). At each collection site, monthly averages were
obtained from flask samples collected every week in triplicate. Each location
was assigned a latitudinal band for which the CFC-11 concentration values for
that location were considered representative. The monthly global averages were
then obtained by calculating a weighted average of the CFC-11 concentrations at
the six locations as follows:

     Concentration (global) = (Sum of [A(i)*C(i)]) / (Sum of A(i)),

where C(i) is the monthly CFC-11 concentration and A(i) is the area of the
associated latitudinal band for a particular site "i". The latitudinal bands
were chosen in such a way that the sum of the areas of all the bands equals the
total area of the earth's surface.

In addition to the monthly global averages available for 1980-1992, this data
set also contains annual global average data for 1975-1985. These annual global
averages were obtained from sampling only at the South Pole and in the Pacific
Northwest of the United States (Washington state and Oregon coast). To obtain
the annual values, large numbers of samples collected in Januaries were
analyzed and averaged. The averaged data from the two locations (i.e., the
South Pole and the Pacific Northwest) were then corrected to reflect global
means. The correction was determined by comparing the January data for the
overlap period (1980-1985) obtained by the two-location global averaging with
those obtained by the six-location global averaging. The corrected two-location
January global averages were then used to obtain middle of the year (annual
average) concentrations, calculated as the average of the two January values
one year apart.

Users who wish to use these data in specific model exercises or research
exercises should first refer to Khalil and Rasmussen (1993), which is the
primary reference for this data set. 


II. SAMPLING AND ANALYSIS METHODS

Detailed information concerning the methods of obtaining samples and measuring
CFC-11 concentrations is given in Rasmussen and Khalil (1980). A brief summary
of these methods is given below.

The sampling containers used in the field were thin-walled, stainless steel,
high-pressure vessels with internal volumes of 0.8, 1.6, and 35.7 liters [at
standard temperature and pressure (STP)]. The vessels were internally
electropolished using the SUMMA (TM) process, which produces an inert, passive
surface. These containers were of all-welded construction, with no Teflon (TM)
pipe-thread tape, and the valves were Nupro SS 4H4 bellow seal types. The
containers were designed to provide a flow-through purge of the entire vessel
before pressurization. All of the low pressure (i.e., less than 40 pounds per
square inch) air samples were collected using a Metal Bellows Corp. Model
MB-158 pump.

Air samples [50 or 1000 liters (STP)] were alternatively collected without a
pump by a cryogenic collection method. The cryogenic method involved the
liquification of ambient air inside a container immersed in liquid nitrogen.
The condensation of the air inside the sampling vessel provided the partial
vacuum necessary to drive the system. The liquid air was then allowed to come
to room temperature after the sampling valve was closed.

Analyses were conducted in the field whenever possible or practical. To
accomplish these in situ field analyses, special sample preparation techniques
were used, including removal of the oxygen and nitrogen from the samples by
vacuum methods, isothermal distillations at -78 degrees Celsius in dry ice-
acetone baths, matrix isolation on cold-finger traps, and enrichment of the
analyzed gases on solid adsorbents.

Analyses for CFC-11 were made using a Perkin-Elmer 3920B dual electron capture-
gas chromatograph (EC-GC). One channel used a 10 foot X 1/4 inch (outside
diameter) column packed with 10% SP2100 (80-100 mesh), while the second channel
used a 10 foot X 1/8 inch column packed with Porasil B (80-100 mesh). The oven
temperature was 70 degrees Celsius, the temperature of the EC detectors was 350
degrees Celsius, with a standing current of 3.5 nA, and the carrier gas was 95%
argon/5% methane. Concentrations of CFC-11 were determined from automated
electronic measurements of chromatographic peak heights. Measurements were
referenced to a primary calibration standard prepared by the authors and
subjected to an interlaboratory comparison. Overall, results between
laboratories agreed within +-4%, and the absolute accuracy of the measurements
was estimated at +-5%.


III. FILE DESCRIPTION

"cfc11.dat"

This is a 1.8 kB data file containing the monthly global average CFC-11
concentration data, derived from weekly flask measurements carried out at eight
sites in six locations from August 1980 to July 1992. In addition, the file
contains annual global average data, derived from January measurements at two
locations (values represent calculated middle of the year CFC-11 levels). Each
data record in the file is presented in the following two-line format:

YEAR  JAN FEB MAR APR MAY JUN JUL AUG SEP
  OCT NOV DEC ANN

where:

YEAR           is the year of the record.

JAN - DEC      are the monthly global average atmospheric CFC-11 concentrations
	       derived from weekly measurements at six locations. Values are
	       expressed in parts per trillion by volume (pptv). Missing
	       values are denoted by -99.9.

ANN            is the annual global average (i.e., mid-year level) atmospheric
	       CFC-11 concentration derived from January measurements at two
	       locations. Values are expressed in parts per trillion by volume
	       (pptv). Data are available for 1975-1985 only. Missing values are 
               denoted by -99.9.


The file can be read by using the following FORTRAN code:

C FORTRAN data retrieval routine to read the file named "cfc11.dat".
C
C Unit 1 is input.
C
      INTEGER YEAR
      REAL JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC,
     2     ANN
      OPEN (UNIT=1, FILE='cfc11.dat')
      READ (1, 1) YEAR, JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP,
     2            OCT, NOV, DEC, ANN
    1 FORMAT (5(/)I4,9(2X,F5.1)/3(2X,F5.1),3X,F5.1)
   10 READ (1, 2, END=99) YEAR, JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG,
     2                    SEP, OCT, NOV, DEC, ANN
    2 FORMAT (I4,9(2X,F5.1)/3(2X,F5.1),3X,F5.1)
      GO TO 10
   99 STOP
      END

The data can also be read by using the following SAS code:

* SAS data retrieval routine to read the file named "cfc11.dat";
*;
DATA CFC;
  INFILE 'cfc11.dat';
  IF _N_=1 THEN INPUT #6 YEAR 1-4 @5 (MONTH1-MONTH9)(+2 5.1) /
    @1 (MONTH10-MONTH12)(+2 5.1) @25 ANN 5.1; 
  ELSE INPUT YEAR 1-4 @5 (MONTH1-MONTH9)(+2 5.1) /
    @1 (MONTH10-MONTH12)(+2 5.1) @25 ANN 5.1;
RUN;


IV. REFERENCES

Khalil, M.A.K., and R.A. Rasmussen. 1993. The environmental history and
probable future of fluorocarbon 11. Journal of Geophysical Research 98(D12):
23,091-23,106.

Rasmussen, R.A., and M.A.K. Khalil. 1980. Atmospheric halocarbons:
Measurements and analyses of selected trace gases. pp. 209-231. In A.C. Aikin
(ed.), Proceedings of the NATO Advanced Study Institute on Atmospheric Ozone:
Its Variations and Human Influences. Department of Transportation, Washington,
D.C.