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.