CARBON-13 ISOTOPIC ABUNDANCE AND CONCENTRATION OF ATMOSPHERIC METHANE FOR       
BACKGROUND AIR IN THE SOUTHERN AND NORTHERN HEMISPHERES FROM 1978 TO 1989.      
CDIAC NDP-049 (1995)                                                            
                                                                                
C.M. Stevens, Chemical Technology Division, Argonne National Laboratory,        
  Argonne, Illinois, U.S.A.                                                     
                                                                                
                                                                                
CONTENTS                                                                        
                                                                                
   I.    Acknowledgments                                                        
   II.   Background and Source Information                                      
   III.  Methodology                                                            
   IV.   Limitations and Restrictions                                           
   V.    File Descriptions                                                      
           "ndp049.dat" (atmospheric methane concentrations and carbon-13       
            isotopic abundances at globally distributed sites from 1978 through 
            1989)                                                               
           "ndp049.for" (FORTRAN 77 data retrieval program to read and write    
            the atmospheric methane concentration and carbon-13 isotopic        
            abundance data)                                                     
           "ndp049.sas" (SAS data retrieval program to read and write the       
            atmospheric methane concentration and carbon-13 isotopic abundance  
            data)                                                               
   VI.   Data Checks Performed by CDIAC                                         
   VII.  How to Obtain the Data and Documentation                               
   VIII. References                                                             
                                                                                
                                                                                
I. ACKNOWLEDGMENTS                                                              
                                                                                
     Measurements were carried out in the Chemistry Division, Argonne National  
Laboratory, Argonne, IL and supported by the Office of Basic Energy Sciences,   
Division of Mathematical and Geosciences, U.S. Department of Energy, Contract   
W-31-109-Eng-38 and the Interdisciplinary Research Program in Earth Sciences,   
National Aeronautics and Space Administration, Order No. W-16188.               
                                                                                
                                                                                
II. BACKGROUND AND SOURCE INFORMATION                                           
                                                                                
     Atmospheric methane (CH4) may become an increasingly important contributor 
to global warming in future years. Its atmospheric concentration has risen,     
doubling over the past several hundred years, and additional methane is thought 
to have a much greater effect on climate, on a per molecule basis, than         
additional CO2 at present day concentrations (Shine et al., 1990). The causes   
of the increase of atmospheric CH4 have been difficult to ascertain because of  
a lack of quantitative knowledge of the fluxes (i.e., net emissions) from the   
numerous anthropogenic and natural sources. The goal of CH4 isotopic studies is 
to provide a constraint (and so reduce the uncertainties) in estimating the     
relative fluxes from the various isotopically distinct sources, whose combined  
fluxes must result in the measured atmospheric isotopic composition, after the  
fractionating effect of the atmospheric removal process is considered. In       
addition, knowledge of the spatial and temporal changes in the isotopic         
composition of atmospheric CH4, along with estimates of the fluxes from some of 
the major sources, makes it possible to calculate growth rates for sources      
whose temporal emissions trends would be difficult to measure directly. A       
detailed discussion of the use of carbon isotopic data to elucidate CH4 source  
fluxes and growth rates is given in Stevens (1993), a reprint of which is       
included as Appendix B of the hard copy documentation that accompanies this     
data package.                                                                   
                                                                                
     The atmospheric CH4 concentration and carbon-13 isotopic abundance data    
presented in this package are derived from air samples collected over the       
period 1978-1989 at globally distributed clean-air sites. The data set          
comprises 201 records, 166 from the Northern Hemisphere and 35 from the         
Southern Hemisphere. The air samples were collected mostly in remote rural or   
marine locations, far from large sources of CH4, and are considered             
representative of tropospheric background conditions. Sampling locations in the 
Northern Hemisphere included 32 Pacific Ocean sites and 11 land-based sites. In 
the Southern Hemisphere, locations included 18 Pacific Ocean sites and 4        
land-based sites. At many locations, air samples were collected by C.M. Stevens 
and his colleagues. For some Southern Hemisphere and Pacific Ocean              
measurements, and all measurements at Cape Meares, Oregon, samples were         
obtained from stored air cylinders contributed by R.A. Rasmussen from the       
sample bank at the Oregon Graduate Institute of Science and Technology          
(formerly the Oregon Graduate Center), Portland, Oregon, U.S.A.                 
                                                                                
     These data represent one of the earliest records of carbon-13 isotopic     
measurements for atmospheric methane at globally distributed clean-air sites.   
Additional information, including measurements of the isotopic composition of   
CH4 sources, estimates of the atmospheric lifetime of CH4, measurements of      
carbon-14 abundances in atmospheric CH4, and some CH4 source flux estimates     
derived from emissions inventories, are provided in Stevens (1993). Together    
with these additional data, the atmospheric 13-CH4 data presented in this       
package have been used to refine estimates of the global CH4 fluxes from        
combined rice and cattle production and from biomass burning, to calculate      
growth rates in CH4 fluxes from biomass burning in the Southern Hemisphere and  
from natural wetlands in the Northern Hemisphere, and to provide evidence of    
changes in the rate of the atmospheric removal process (Stevens, 1993).         
                                                                                
                                                                                
III. METHODOLOGY                                                                
                                                                                
Sampling Techniques and Conditions                                              
                                                                                
     The sampling in both hemispheres occurred in two main phases:              
                                                                                
Phase 1: Samples collected during 1978-83 were, with few exceptions, obtained   
from the air sample bank of the Oregon Graduate Center in collaboration with    
R.A. Rasmussen. The samples in the bank had been stored from 1 to 5 years in    
cleaned and treated stainless steel cylinders of various sizes (approximately 2 
to 30 liters) and at various high pressures (up to 25 atmospheres). These       
cylinders were shipped to Argonne National Laboratory and then transferred by   
expansion to atmospheric pressure into 33-liter evacuated stainless steel       
cylinders (untreated WWII-surplus oxygen cylinders) of accurately known volume  
and analyzed within 1 to 2 days. The air in these stored samples had been       
collected either at sea or at land stations upwind of any urban or              
anthropogenic sources of CH4. During this period, some of the Northern          
Hemisphere samples were collected at the Cape Meares station of the Oregon      
Graduate Center; the others were taken at sea in the Pacific. Beginning in      
1983, samples were also collected in Northern Illinois. The average delta 13-C  
values of 36 samples collected in Northern Illinois during 1983 were compared   
with the average of seven samples collected at Cape Meares during the same year 
and were found to agree within 0.01 per mil.                                    
                                                                                
Phase 2: From 1984 through 1989 all samples were collected in Argonne 33-liter  
evacuated stainless steel cylinders (untreated WWII-surplus oxygen cylinders),  
either at rural sites in Northern Illinois or, in the case of the Southern      
Hemisphere samples, at rural sites upwind of Canberra (Australia), or at the    
shore near the NOAA observatory in American Samoa. A special set of analyses    
was carried out in 1988 on four samples which had been collected in 1978 and    
analyzed in 1983, in order to compare results after another five years of       
storage. The results of these reanalyses are found in the data set (see data    
for samples collected at Cape Meares, dated 4-4-78, 4-7-78, 10-5-78, and        
10-5-78).                                                                       
                                                                                
     The Illinois samples were collected mostly in the afternoon on days with   
winds in excess of 10 mph. Collections in the morning were avoided because of   
overnight temperature inversions. Typically, only one flask sample was          
collected at each site. An exception occurred on 9-8-88, when six different     
samples in six different flasks were collected simultaneously and analyzed at   
intervals over a six week period in order to ascertain the integrity of samples 
stored in Argonne cylinders. Multiple samples were combined only when it was    
necessary to combine air stored in several small cylinders (obtained from the   
Oregon Graduate Institute) in order to make enough for the 33 liters required   
for isotopic analysis. Multiple analyses from a single flask occurred in        
several cases, as in the samples with collection times denoted 11-00-81         
(Tasmania) and 05-00-82 (Pacific). (Days denoted as 00, as in 11-00-81,         
indicate that the exact day of sampling is not known.)                          
                                                                                
Analytical Methods                                                              
                                                                                
     Air samples were processed by the oxidation of the CH4 in the air to CO2   
after quantitative removal of atmospheric CO2 and CO2 from oxidation of         
atmospheric CO by Schutze reagent. (The experimental apparatus used to carry    
out this procedure is shown schematically in Figure 1 of the hard copy          
documentation that accompanies this package.) The CO2 from CH4 oxidation was    
quantitatively separated from the air by distillation and analyzed              
isotopically. Details of the experimental procedures are as follows.            
                                                                                
     The air pressure in the sampling flask was first measured, then the flask  
was connected to the air intake line and air was passed through a               
low-efficiency, high-flow liquid N2 trap to remove water, most of the           
atmospheric CO2, and non-methane hydrocarbons. Next, the samples were passed    
through molecular sieve 13X and another liquid N2 trap of high efficiency to    
further remove any CO2 present. The air stream was then passed through a        
granular Schutze reagent made up of I2O5 on silica gel, by which the            
atmospheric CO was oxidized to CO2. Another liquid N2 trap then removed this    
CO2. The gas was then passed through an electrically heated quartz combustion   
tube containing platinized silica, which quantitatively oxidizes the CH4 to CO2 
and H2O, which are then trapped in high-efficiency, liquid N2 traps. The air in 
the original cylinder was passed through this treatment until the pressure was  
reduced to approximately 5% of atmospheric pressure. The flow rate was          
maintained at a constant flow of approximately 0.5 liters/min by adjustment of  
the valve on the cylinder until the valve was wide open and the pressure was    
reduced to approximately 25% of atmospheric pressure; the flow then gradually   
decreases until terminated at 5% of atmospheric pressure. The line beyond the   
CH4 combustion section was pumped to a good vacuum and then isolated. The trap  
with the CO2 and H2O from CH4 oxidation was warmed with a hot water bath, and   
the CO2 and H2O were transferred with a controlled helium flow through a trap   
cooled by dry ice/methanol to remove the H2O. The CO2 was condensed in a trap   
in the micromanometer section, which was then pumped to a good vacuum and       
isolated. The trap containing the CO2 from CH4 oxidation was then warmed, and   
the CO2 pressure was measured with a precision of +-0.3%. Finally, the CO2 was  
distilled to a sample tube for isotopic analysis on a Consolidated-Nier         
isotope-ratio mass spectrometer. The concentration of CH4 was calculated        
stoichiometrically with a precision of +-0.5%. The delta 13-C values of the CO2 
samples were measured with a precision of +-0.05 per mil. A correction of -0.11 
per mil was made for an impurity of N2O produced in the combustion train. The   
overall uncertainties in the analyzed values of CH4 concentration and isotopic  
composition are estimated to be 0.02 parts per million (ppm) for the            
concentration values, 0.2 per mil for isotopic values prior to 1985, and 0.1    
per mil for isotopic values after 1985.                                         
                                                                                
                                                                                
IV. LIMITATIONS AND RESTRICTIONS                                                
                                                                                
     The isotopic results for the samples of stored air from 1978 through 1983  
were all analyzed during 1982-83 in random order and using a laboratory         
isotopic standard which had been accurately measured at -24.8 per mil. This     
standard had been used for many years and was replenished without fractionation 
2-3 times per year. The order of processing and analysis of samples was random  
for both the hemisphere of origin and the chronology of collection; therefore,  
it is unlikely that there are any systematic errors in the set of samples from  
this period. The same cannot be said for the samples from 1984 through 1989.    
During this period, new isotopic working standards were started, having a value 
nearly the same as the isotopic composition of the CH4 samples in order to      
reduce the mass spectrometer error resulting from measuring large differences,  
sometimes called the "memory" effect. There may have been some drift of the     
value of the isotopic standard due to fractionation while it was consumed       
during the analysis process, as these standards were not prepared in large      
amounts as had been the case for the standard used for the earlier samples. The 
repeat analyses done in 1988 on second aliquots of four of the stored air       
samples showed an average difference of +0.3 per mil compared to the first      
analysis done five years earlier using the -24.8 per mil isotopic standard.     
This difference is attributed to background CH4 contamination from the cylinder 
during storage. The analyses of all stored air samples were therefore corrected 
by -0.067 per mil per year of storage time between the collection and analysis  
dates (see column 9 of the data set). Another check of the last isotopic        
standard used for the 1988-89 samples indicated a possible drift of about +0.5  
per mil.                                                                        
                                                                                
     The most important feature of the isotopic data is the measured            
interhemispheric trend, which is independent of the individual hemispheric      
trends because the analyses were done with the same standard at more or less    
the same time period. Mathematical analysis of the hemispheric trends shows     
that the difference of the hemispheric trends is the most important term in the 
calculation of the trends of the fluxes in each hemisphere. The average trend   
of each hemisphere is of secondary importance in the analysis. Hence,           
systematic errors such as fractionation of the isotopic standard would have     
only a small effect on the final application of the results.                    
                                                                                
     Additional uncertainties may affect the ability to compare isotopic ratios 
in atmospheric CH4 from clean-air sites with isotopic ratios from the various   
source types for the purpose of resolving uncertainties in the global methane   
budget. Perhaps the largest uncertainty is the determination by inventory       
estimates of the globally averaged isotopic ratio of individual sources such as 
biomass burning or methane from ruminants. This uncertainty arises because the  
CH4 from these sources is made up of contributions of uncertain relative        
amounts globally from metabolically distinct plant types (i.e., C-3 and C-4     
plants), which have large isotopic variations. The globally averaged isotopic   
composition of CH4 sources from coal mining and natural gas losses are subject  
to large uncertainties of +-4 per mil because of large variations among the     
many individual sources.                                                        
                                                                                
                                                                                
V. FILE DESCRIPTIONS                                                            
                                                                                
Filename: ndp049.dat                                                            
                                                                                
     This 21.5 kB file presents 201 records (166 records from Northern          
Hemisphere sites and 35 records from Southern Hemisphere sites) of atmospheric  
methane concentrations and carbon-13 isotopic abundances from background        
(clean-air) samples collected at globally distributed sites from 1978 through   
1989.                                                                           
                                                                                
     Each data record presents the following variables: sample collection date; 
number of samples combined for analysis; sampling location (site name,          
latitude, and longitude); analysis date; CH4 concentration; carbon-13 isotopic  
abundance (uncorrected and corrected); and flag codes to indicate outliers,     
repeated analyses of a sample, and other information. The file can be read by   
using the following FORTRAN code:                                               
                                                                                
C FORTRAN data retrieval routine to read and write the file named               
                                                                                
C "ndp049.dat".                                                                 
C                                                                               
C Unit 1 is input.                                                              
C                                                                               
      CHARACTER NSAMP, LOCN*27, LAT*9, LON*11, AFLAG, OFLAG, AVCODE*2           
      INTEGER CMONTH, CDAY, CYEAR, AMONTH, ADAY, AYEAR                          
      REAL CONC, C13MEAS, C13CORR                                               
      OPEN (UNIT=1, FILE='ndp049.dat')                                          
      READ (1,10) CMONTH, CDAY, CYEAR, NSAMP, LOCN, LAT, LON, AMONTH,           
     2            ADAY, AYEAR, AFLAG, CONC, C13MEAS, OFLAG, C13CORR,            
     3            AVCODE                                                        
   10 FORMAT (//////I2,1X,I2,1X,I2,2X,A1,2X,A26,2X,A9,2X,A11,2X,I2,1X,          
     2              I2,1X,I2,1X,A1,2X,F5.3,2X,F6.2,1X,A1,2X,F6.2,1X,A2)         
   20 READ (1,30,END=99) CMONTH, CDAY, CYEAR, NSAMP, LOCN, LAT, LON,            
     2                   AMONTH, ADAY, AYEAR, AFLAG, CONC, C13MEAS,             
     3                   OFLAG, C13CORR, AVCODE                                 
   30 FORMAT (I2,1X,I2,1X,I2,2X,A1,2X,A26,2X,A9,2X,A11,2X,I2,1X,I2,1X,          
     2        I2,1X,A1,2X,F5.3,2X,F6.2,1X,A1,2X,F6.2,1X,A2)                     
      GO TO 20                                                                  
   99 STOP                                                                      
      END                                                                       
                                                                                
The data can also be read by using the following SAS code:                      
                                                                                
* SAS data retrieval routine to read and write the file named "ndp049.dat";     
*;                                                                              
DATA NDP049;                                                                    
  INFILE 'ndp049.dat';                                                          
  IF _N_=1 THEN INPUT ////// CMONTH 1-2 CDAY 4-5 CYEAR 7-8 NSAMP $ 11           
    @14 LOCN $CHAR26. @42 LAT $CHAR9. @53 LON $CHAR11. AMONTH 66-67             
    ADAY 69-70 AYEAR 72-73 AFLAG $ 75 @78 CONC 5.3 @85 C13MEAS 6.2              
    OFLAG $ 92 @95 C13CORR 6.2 AVCODE $ 102-103;                                
  ELSE INPUT CMONTH 1-2 CDAY 4-5 CYEAR 7-8 NSAMP $ 11 @14 LOCN $CHAR26.         
    @42 LAT $CHAR9. @53 LON $CHAR11. AMONTH 66-67 ADAY 69-70 AYEAR 72-73        
    AFLAG $ 75 @78 CONC 5.3 @85 C13MEAS 6.2 OFLAG $ 92 @95 C13CORR 6.2          
    AVCODE $ 102-103;                                                           
RUN;                                                                            
                                                                                
                                                                                
where                                                                           
                                                                                
CMONTH        is the numeric month of the year in which the air sample was      
              collected;                                                        
                                                                                
CDAY          is the numeric day of the month on which the air sample was       
              collected;                                                        
                                                                                
CYEAR         is the final two digits of the year (since 1900) in which the air 
              sample was collected;                                             
                                                                                
NSAMP         is the number of samples combined from different locations for a  
              single analysis;                                                  
                                                                                
LOCN          is a descriptive character string consisting of (1) the location  
              of the sampling site and, in some cases, (2) the sample           
              number(s), denoted as one or more numbers following a "#" sign    
              and referring to the identity of the stored air sample(s)         
              contributed from the sample bank at the Oregon Graduate Institute 
              of Science and Technology, Portland, Oregon, U.S.A., by  R.A.     
              Rasmussen;                                                        
                                                                                
LAT           is the estimated latitude (or range of latitudes) of the sampling 
              site(s), given in decimal degrees;                                
                                                                                
LON           is the estimated longitude (or range of longitudes) of the        
              sampling site(s), given in decimal degrees;                       
                                                                                
AMONTH        is the numeric month of the year in which the air sample was      
              analyzed;                                                         
                                                                                
ADAY          is the numeric day of the month on which the air sample was       
              analyzed;                                                         
                                                                                
AYEAR         is the final two digits of the year (since 1900) in which the     
              sample was analyzed;                                              
                                                                                
AFLAG         is a one-character flag code providing additional information     
              about the sample analysis: 'R' -- entry represents one of two or  
              more repeated analyses carried out on the same air sample; 'C' -- 
              identifies samples where the actual date of analysis is not given 
              but was within one week of the date of collection;                
                                                                                
CONC          is the CH4 concentration in the air sample, given in parts per    
              million (1 * 10**6) by volume;                                    
                                                                                
C13MEAS       is the measured del C-13 (per mil), corrected by +0.10 per mil    
              for N2O contamination but uncorrected for water vapor             
              contamination;                                                    
                                                                                
OFLAG         is a one-character flag code denoting values of C13MEAS that are  
              considered as outliers and not included in any subsequent         
              averages; the symbol for the flag code is "*";                    
                                                                                
C13CORR       is a corrected value of del C-13 (per mil), calculated only for   
              samples stored for a considerable time before analysis; values    
              represent an addition of -0.067 per mil per year for background   
              contamination (from the cylinder walls) that accumulated between  
              the collection and analysis dates;                                
                                                                                
AVCODE        is a two-character code denoting an entry whose C13CORR value is  
              an average of those of the previous 2 to 5 entries [i.e., the     
              immediately preceding entries containing AFLAG values of 'R'      
              (excluding outliers, denoted by OFLAG="*")]: A2, A3, and A5       
              signify entries representing averages of 2, 3, and 5 previous     
              entries, respectively;                                            
                                                                                
                                                                                
Stated in tabular form, the contents include the following.                     
                                                                                
____________________________________________________________________________    
                                                                                
               Variable         Variable         Starting             Ending    
Variable*        type            width**          column              column    
____________________________________________________________________________    
                                                                                
CMONTH         Numeric              I2               1                    2     
CDAY           Numeric              I2               4                    5     
CYEAR          Numeric              I2               7                    8     
NSAMP          Character            A1              11                   11     
LOCN           Character           A26              14                   39     
LAT            Character            A9              42                   50     
LON            Character           A11              53                   63     
AMONTH         Numeric              I2              66                   67     
ADAY           Numeric              I2              69                   70     
AYEAR          Numeric              I2              72                   73     
AFLAG          Character            A1              75                   75     
CONC           Numeric            F5.3              78                   82     
C13MEAS        Numeric            F6.2              85                   90     
OFLAG          Character            A1              92                   92     
C13CORR        Numeric            F6.2              95                  100     
AVCODE         Character            A2             102                  103     
____________________________________________________________________________    
                                                                                
* Missing values for numeric variables are represented as follows -- CMONTH,    
CDAY, CYEAR, AMONTH, ADAY, AYEAR: 00; CONC: 9.999; C13MEAS, C13CORR: 999.99.    
Missing values for character variables (NSAMP, AFLAG, OFLAG, AVCODE) are        
represented as blanks.                                                          
                                                                                
** Values for variable width are entered as FORTRAN 77 format codes.            
                                                                                
                                                                                
Filename: ndp049.for                                                            
                                                                                
     This file contains a FORTRAN 77 data retrieval routine to read and write   
the file "ndp049.dat".  The following is a listing of this program. For         
additional information regarding variable definitions and format statements,    
please see the file description for "ndp049.dat".                               
                                                                                
C FORTRAN data retrieval routine to read and write the file named               
C "ndp049.dat".                                                                 
C                                                                               
C Unit 1 is input.                                                              
C Unit 2 is output.                                                             
C                                                                               
      CHARACTER NSAMP, LOCN*27, LAT*9, LON*11, AFLAG, OFLAG, AVCODE*2           
      INTEGER CMONTH, CDAY, CYEAR, AMONTH, ADAY, AYEAR                          
      REAL CONC, C13MEAS, C13CORR                                               
      OPEN (UNIT=1, FILE='ndp049.dat')                                          
      OPEN (UNIT=2, FILE='output')                                              
      READ (1,10) CMONTH, CDAY, CYEAR, NSAMP, LOCN, LAT, LON, AMONTH,           
     2            ADAY, AYEAR, AFLAG, CONC, C13MEAS, OFLAG, C13CORR,            
     3            AVCODE                                                        
   10 FORMAT (//////I2,1X,I2,1X,I2,2X,A1,2X,A26,2X,A9,2X,A11,2X,I2,1X,          
     2              I2,1X,I2,1X,A1,2X,F5.3,2X,F6.2,1X,A1,2X,F6.2,1X,A2)         
   20 WRITE (2,30) CMONTH, CDAY, CYEAR, NSAMP, LOCN, LAT, LON, AMONTH,          
     2             ADAY, AYEAR, AFLAG, CONC, C13MEAS, OFLAG, C13CORR,           
     3             AVCODE                                                       
   30 FORMAT (I2.2,1X,I2.2,1X,I2.2,2X,A1,2X,A26,2X,A9,2X,A11,2X,I2.2,           
     2        1X,I2.2,1X,I2.2,1X,A1,2X,F5.3,2X,F6.2,1X,A1,2X,F6.2,1X,           
     3        A2)                                                               
      READ (1,40,END=99) CMONTH, CDAY, CYEAR, NSAMP, LOCN, LAT, LON,            
     2                   AMONTH, ADAY, AYEAR, AFLAG, CONC, C13MEAS,             
     3                   OFLAG, C13CORR, AVCODE                                 
   40 FORMAT (I2,1X,I2,1X,I2,2X,A1,2X,A26,2X,A9,2X,A11,2X,I2,1X,I2,1X,          
     2        I2,1X,A1,2X,F5.3,2X,F6.2,1X,A1,2X,F6.2,1X,A2)                     
      GO TO 20                                                                  
   99 CLOSE (1)                                                                 
      CLOSE (2)                                                                 
      STOP                                                                      
      END                                                                       
                                                                                
                                                                                
Filename: ndp049.sas                                                            
                                                                                
     This file contains a SAS data retrieval routine to read and write the      
file "ndp049.dat". The following is a listing of this program. For additional   
information regarding variable definitions and format statements, please see    
the file description for "ndp049.dat".                                          
                                                                                
* SAS data retrieval routine to read the file named "ndp049.dat";               
*;                                                                              
DATA NDP049;                                                                    
  INFILE 'ndp049.dat';                                                          
  IF _N_=1 THEN INPUT ////// CMONTH $ 1-2 CDAY $ 4-5 CYEAR $ 7-8                
    NSAMP $ 11 @14 LOCN $CHAR26. @42 LAT $CHAR9. @53 LON $CHAR11.               
    AMONTH $ 66-67 ADAY $ 69-70 AYEAR $ 72-73 AFLAG $ 75                        
    @78 CONC 5.3 @85 C13MEAS 6.2 OFLAG $ 92 @95 C13CORR 6.2                     
    AVCODE $ 102-103;                                                           
  ELSE INPUT CMONTH $ 1-2 CDAY $ 4-5 CYEAR $ 7-8 NSAMP $ 11                     
    @14 LOCN $CHAR26. @42 LAT $CHAR9. @53 LON $CHAR11.                          
    AMONTH $ 66-67 ADAY $ 69-70 AYEAR $ 72-73 AFLAG $ 75                        
    @78 CONC 5.3 @85 C13MEAS 6.2 OFLAG $ 92 @95 C13CORR 6.2                     
    AVCODE $ 102-103;                                                           
  FILE 'output';                                                                
  PUT CMONTH 1-2 CDAY 4-5 CYEAR 7-8 NSAMP 11 @14 LOCN $CHAR26.                  
    @42 LAT $CHAR9. @53 LON $CHAR11. AMONTH 66-67 ADAY 69-70                    
    AYEAR 72-73 AFLAG 75 @78 CONC 5.3 @85 C13MEAS 6.2 OFLAG 92                  
    @95 C13CORR 6.2 AVCODE 102-103;                                             
RUN;                                                                            
                                                                                
                                                                                
VI. DATA CHECKS PERFORMED BY CDIAC                                              
                                                                                
     The Carbon Dioxide Information Analysis Center (CDIAC) endeavors to        
provide quality assurance (QA) of all data before their distribution. To ensure 
the highest possible quality in the data, CDIAC conducts extensive reviews for  
reasonableness, accuracy, completeness, and consistency of form. While having   
common objectives, the specific form of these reviews must be tailored to each  
data set; this tailoring process may involve considerable programming efforts.  
The entire QA process is an important part of CDIAC's effort to assure          
accurate, usable data for researchers.                                          
                                                                                
     For the atmospheric methane concentration and carbon-13 isotopic abundance 
data, the QA procedure consisted of the following:                              
                                                                                
     1.  The format of all information was checked to ensure consistency        
         throughout each data record.                                           
                                                                                
     2.  Data values were examined for reasonableness, absence of typographical 
         errors, and absence of outliers.                                       
                                                                                
                                                                                
     No errors or inconsistencies of the types described above were found in    
the atmospheric methane concentration and carbon-13 isotopic abundance data     
received by CDIAC. All data values in the file distributed by CDIAC are         
identical to those received from C.M. Stevens. However, in order to enhance     
their value and their ease of use, the data records were reformatted and        
appended in the following way:                                                  
                                                                                
     1.  The latitude and longitude of each sampling site, which were absent    
         for most locations in the original data set, were obtained from C.M.   
         Stevens and appended to each data record.                              
                                                                                
     2.  The conventions for missing values and repeated analyses were altered  
         in order to create a consistent format for all data records. New flag  
         codes were created to identify any of the following: (1) uncertainties 
         in the analysis date, (2) records that represent repeated measurements,
         and (3) records that represent averages of two or more previous        
         measurements.                                                          
                                                                                
                                                                                
VII. HOW TO OBTAIN THE DATA AND DOCUMENTATION                                   
                                                                                
     The data and hard copy documentation described herein are available from:  
                                                                                
               Carbon Dioxide Information Analysis Center                       
               Oak Ridge National Laboratory                                    
               Post Office Box 2008                                             
               Oak Ridge, TN 37831-6335, U.S.A.                                 
               Telephone (615) 241-4851                                         
                                                                                
     The following citation should be used for referencing this archive and/or  
the documentation report:                                                       
                                                                                
Stevens, C.M. 1995. Carbon-13 Isotopic Abundance and Concentration of           
  Atmospheric Methane in Background Air in the Southern and Northern            
  Hemispheres from 1978 to 1989. ORNL/CDIAC-80, NDP-049. Carbon Dioxide         
  Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge,        
  Tennessee, U.S.A.                                                             
                                                                                
                                                                                
VIII. REFERENCES                                                                
                                                                                
Rust, F., and C.M. Stevens. 1980. Carbon kinetic isotope effect in the          
oxidation of methane by hydroxyl. International Journal of Chemical Kinetics    
12:371-77.                                                                      
                                                                                
Shine, K.P., R.G. Derwent, D.J. Wuebbles, and J-J. Morcrette. 1990. Radiative   
forcing of climate. pp. 41-68. In J.T. Houghton, G.J. Jenkins, and J.J.         
Ephraums (eds.), Climate Change: the IPCC Scientific Assessment. Cambridge      
University Press, Cambridge.                                                    
                                                                                
Stevens, C.M. 1993. Isotopic abundances in the atmosphere and sources. pp.      
62-88.  In M.A.K. Khalil (ed), Atmospheric Methane: Sources, Sinks, and Role in 
Global Change, Proceedings of the NATO Advanced Research Workshop on the        
Atmospheric Methane Cycle: Sources, Sinks, Distributions, and Role in Global    
Change, held at Mt. Hood near Portland, OR, U.S.A., October 7-11, 1991. NATO    
ASI Series I: Global Environmental Change, Vol. 13. Springer-Verlag, Berlin and 
Heidelberg.                                                                     
                                                                                
Stevens, C.M. 1988. Atmospheric methane. Chemical Geology 71:11-21.             
                                                                                
Stevens, C.M., and A. Engelkemeir. 1988. Stable carbon isotopic composition of  
methane from some natural and anthropogenic sources. Journal of Geophysical     
Research 93(D1):  725-33.                                                       
                                                                                
Stevens, C.M., and A. Engelkemeir. 1985. Causes of increasing methane fluxes    
based on carbon isotopic studies. Special Environmental Report No. 16, WMO      
647.  WMO Technical Conference on Observation and Measurement of Atmospheric    
Contaminants, World Meteorological Organization, Geneva.