---------------------------------------------------------------------


Atmospheric Carbon Monoxide Dry Air Mole Fractions from 
the NOAA ESRL Carbon Cycle Cooperative Global Air Sampling 
Network, 1988-2020

Reference scale:  WMO CO_X2014A

Version: 2020-08-31
--------------------------------------------------------------------
CONTENTS

1.       Data source and contacts
2.       Use of data
2.1      Citation
3.       Reciprocity 
4.       Warnings
5.       Update notes
6.       Introduction
7.       DATA - General Comments
7.1      DATA - Sampling Locations
7.2      DATA - File Name Description
7.3      DATA - Event with single parameter
7.4      DATA - Event with multiple parameters
7.5      DATA - QC Flags
7.6      DATA - Monthly Averages
8.       Data retrieval
9.       References

--------------------------------------------------------------------
1. DATA SOURCE AND CONTACTS

National Oceanic and Atmospheric Administration (NOAA)
Earth System Research Laboratories (ESRL)
Global Monitoring Laboratory (GML)
Carbon Cycle Greenhouse Gases (CCGG)

Correspondence concerning these data should be directed to:

Gabrielle Petron
NOAA ESRL Global Monitoring Laboratory
325 Broadway, GML-1
Boulder, Colorado, 80305 USA

Telephone: 303 497-4890

Electronic Mail: gabrielle.petron@noaa.gov

--------------------------------------------------------------------
2. USE OF DATA

These data are made freely available to the public and the
scientific community in the belief that their wide dissemination
will lead to greater understanding and new scientific insights.
The availability of these data does not constitute publication
of the data.  NOAA relies on the ethics and integrity of the user to
ensure that GML receives fair credit for their work.  If the data 
are obtained for potential use in a publication or presentation, 
GML should be informed at the outset of the nature of this work.  
If the GML data are essential to the work, or if an important 
result or conclusion depends on the GML data, co-authorship
may be appropriate.  This should be discussed at an early stage in
the work.  Manuscripts using the GML data should be sent to GML
for review before they are submitted for publication so we can
ensure that the quality and limitations of the data are accurately
represented.

2.1 CITATION

Please reference these data as 

   G. Petron, A.M. Crotwell, M.J. Crotwell, E. Dlugokencky, 
   M. Madronich, E. Moglia, D. Neff, S. Wolter, J.W. Mund (2020), 
   Atmospheric Carbon Monoxide Dry Air Mole Fractions 
   from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 
   1988-2020, Version: 2020-08, https://doi.org/10.15138/33bv-s284
   

--------------------------------------------------------------------
3. RECIPROCITY

Use of these data implies an agreement to reciprocate.
Laboratories making similar measurements agree to make their
own data available to the general public and to the scientific
community in an equally complete and easily accessible form.
Modelers are encouraged to make available to the community,
upon request, their own tools used in the interpretation
of the GML data, namely well documented model code, transport
fields, and additional information necessary for other
scientists to repeat the work and to run modified versions.
Model availability includes collaborative support for new
users of the models.

--------------------------------------------------------------------
4. WARNINGS

Every effort is made to produce the most accurate and precise
measurements possible.  However, we reserve the right to make
corrections to the data based on recalibration of standard gases
or for other reasons deemed scientifically justified.

We are not responsible for results and conclusions based on use
of these data without regard to this warning.


--------------------------------------------------------------------
5. UPDATE NOTES

+++++++++++++++++++++++++++++++
Lab-wide notes:

2011-10-07

We introduced the term "measurement group", which identifies
the group within NOAA and Institute of Arctic and Alpine Research (INSTAAR)
University of Colorado Boulder that made the measurement.  We can 
now have multiple groups measuring some of the same trace gas species 
in our discrete samples.  

Measurement groups within NOAA and INSTAAR are 

  ccgg:  NOAA Carbon Cycle Greenhouse Gases group (CCGG)
  hats:  NOAA Halocarbons and other Atmospheric Trace Species group (HATS)
  arl:   INSTAAR Atmospheric Research Laboratory (ARL)
  sil:   INSTAAR Stable Isotope Laboratory (SIL)
  curl:  INSTAAR Laboratory for Radiocarbon Preparation and Research (CURL)

We also changed the file naming convention (see section "DATA - FILE 
NAME DESCRIPTION").

+++++++++++++++++++++++++++++++
Project-specific notes:

2020-07-16

Sample elevation was revised for ALT.  All prior entries
were set to the correct elevation.

2019-09-09

Sample elevation was corrected for AMY.  All prior entries
were set to the correct elevation.
 
2017-07-27

Method codes and sample locations were edited for accuracy.
Edited sites are: ZEP, BKT, OXK, CGO, RPB, ASC, CHR, ICE,
KEY, KUM, and TAP

2016-07-07

Incorrect sample dates from Ulaan Uul, Mogolia (UUM) from
20 Aug. 2013 through 30 Sept. 2015 were corrected on 
13 May 2016.

2016-07-07

Since 24 Jan 2015, air samples from Negev Desert, Isreal (WIS)
are collected at 29.9731N, 35.0567E, 156 masl; the old location 
was 30.8595N, 34.7809E, 482 masl.

2016-07-07

Since 03 Dec 2015, sampling in Natal, Brazil (NAT) was moved
from 5.5147S, 35.2603W, 20 masl to 5.7952S, 35.1853W, 87 masl.

2015-11-20

Updated the content and format of event files to include elevation in 
meters above sea level (masl) and sample collection intake height in 
meters above ground level (magl). Elevation plus collection intake 
height equals altitude, which has always been included in the NOAA 
distribution. In adding these 2 fields, the event number column
has moved. The new format is described in Sections 7.3 and 7.4.

Users may find minor changes (from earlier distributions) to reported 
monthly mean values for the beginning months of data records.  These 
changes are due to minor corrections to our curve fitting methods as
first described by Thoning et al. (JGR, 1989).

2013-08-27

Coordinates of some of the sample locations were revised 
to reflect improved information. Changes are minor. 

2011-10-01

The data file format has been modified to include the measurement group
and, additionally, the sample collection and analysis times 
now include seconds (e.g., 2011 03 15 23 06 12).  See section 7.3 
for details.

2010-10-01

The format of the NOAA GML data records has been changed to include
an estimate of the uncertainty associated with each measurement.  The
determination of the estimate is trace gas specific and described in
section 6 (INTRODUCTION).

+++++++++++++++++++++++++++++++
Parameter-specific notes:

The carbon monoxide data contained in these files supercede 
all previous measurement results. Data are referenced to the 
most recent WMO scale - WMO CO_X2014A.

Released in December 2015, the new scale was developed using the
calibration results of secondary standards against six sets 
of primary standards. Three of these were not previously 
available. The additional primary standards allow better 
estimation of drift in the secondary standards.  

The conversion of previous results to the new scale used the
archived instrument signals from the sample and standard
measurements. The original calibration curves used for the GC-HgO 
instruments were reconstructed using the raw instrument signals 
of the standards with their new CO assignements on the X2014A 
scale. The sample mole fractions were recalculated against the 
new calibration curves. 

A description of CO_X2014A and the revision of previous measurements
are available at: http://www.esrl.noaa.gov/gmd/ccl/co_scale.html

Until the 2017 data release, if a flask pair differed 
by less than 3 ppb the samples were accepted, otherwise they were 
flagged and not used in further analysis (Novelli et al., 1998). 
Given the VUV measurement noise in recent years and the variability 
in atmospheric CO, we use a CO pair agreement 
criteria of 4 ppb for some pairs analyzed between 2016 and 2019. 
This allowed us to retain more sample pairs while still requiring 
a reasonable agreement between paired flasks. Data from the new 
TILDAS instrument use a strict 3 ppb pair agreement criteria.

At a few sites with in-situ measurement systems, a flask pair is collected 
using the in-situ sampling line ("through analyzer" or method "S" flasks) 
in addition to the flask pair collected with the portable sampling unit (PSU).
By default, the S flasks have been flagged "S.." for CO results. SMO S flask 
CO data are now available from 2009-06-05 onwards.

One member of the flask pair from the Cape Grim Observatory (CGO) is first 
analyzed by CSIRO (the comparison flask). For several years, we often noticed 
large pair differences for CO, with the comparison flask being higher 
than its mate. This suggested a potential contamination 
in the comparison flask for CO. 
A significant number of CGO CO flask pairs have been flagged prior to mid 2019 
due to large pair differences. To retain valid samples and measurements, 
only the CGO comparison flasks with suspected contamination were flagged 
"N.." (or "N.I") for CO. The CGO CO results for flasks analyzed solely at NOAA 
have been retained (first column "." flag) if all other data quality criteria 
(besides flask pair agreement) were met. The CGO comparison flask issue 
was resolved by CSIRO in early 2019.

The flask data will be reprocessed and uncertainty estimates
will be available with the next CO calibration scale revision, 
which we hope to complete soon.

--------------------------------------------------------------------
6. INTRODUCTION

For the past three decades CO has been measured in discrete samples
of air collected as part of the NOAA Collaborative Global Air
Sampling Network. Three analytical methods were used.  From 1988 to 
2008 instruments based on gas chromatography and HgO reduction detection
were used (GC-HgO, instruments from Trace Analytical Inc.)  
The instruments had a nonlinear response over the range 
of the remote troposphere (Novelli et al., 1991).  Response curves 
composed of 4-8 standards defined instrument response (Novelli et al., 
1994, 1998).

In 2008 an analyzer based on CO fluorescence in the vacumm ultra violet
(VUV, Gerbig et al.,1996) replaced one of the two GCs on the measurement 
system. The remaining GC and the VUV instrument were calibrated using a 
common set of reference gases, ranging from 50 to 350 ppb. In 2010 the 
last GC was replaced by a VUV instrument.  

Since August 2019, all samples are analyzed for CO by Tunable 
Infrared Laser Direct Absorption Spectroscopy. The TILDAS instrument 
is calibrated regularly using 11 standards, ranging from 23 to 486 ppb.
In this range, the instrument response is linear.

The air samples were collected on site in evacuated glass flasks 
or using a portable air sampling pump package. Two air samples were 
collected in series nearly simultaneously, constituting a pair (Lang 
et al., 1989 a,b). Quality of the air samples is evaluated by the 
difference between the two flasks. For most pairs, if they differed 
by < three ppb the samples were accepted, otherwise they are flagged 
and not used in further analysis (Novelli et al., 1998). If one or both 
flasks have a sampling or analysis issue, the corresponding data 
is flagged with N or A in the flag's first column and it should not 
be used. Data flags are described in details in Section 7.4.

A review of the four sets of primary standards prepared between 
1989-2000 suggested our working standards were drifting upward at 
rates of ~0.5-1 ppb year. A time dependant correction was applied to 
all air samples measured through 12/2000 (Novelli et al., 2003). It 
is now believed primary standards prepared 1999/2000 were biased.  
They were assigned new mole fractions based on their calibrations
versus primary standards prepared in 2006, 2011 and 2015 and the 
flask air measured against these standards were re-calculated. 

Instruments used in the analysis are identified in the data string
by their ID.  The GC-HgO instruments were R2,CS, R5 and R6.  
              The VUV instruments are V2, V3 and V4.
              The TILDAS instrument is AR2.

The flask data will be reprocessed when the CO calibration scale is updated 
and uncertainty estimates will be provided.

-------------------------------------------------------------------
7. DATA - GENERAL COMMENTS

Carbon monoxide mixing ratios in these files are reported 
in units of nmol/mol (10^-9 mole CO per mole of dry air 
or as part per billion by mole fraction (ppb)) relative
to the NOAA/WMO CO scale (Novelli et al., 1991, Novelli 
et al., 2003).  The reproducibility of the measurements, 
estimated from repeated analysis of air contained in a 
high-pressure cylinder, is ~1-2 ppb using GC-HgO, 0.5 
ppb or better for the VUV instruments, and 0.1 ppb
for the TILDAS.

Two air samples are collected on site nearly simultaneously 
in glass flasks and constitute a pair. Samples are typically
collected using a semi-automated unit which incorporates a
powerful battery, a high capacity pump, ~5 m intake line,
and a back pressure regulator to control the pressure of 
air in the flasks.  Two air samples are collected in series         
and pressured to ~1.5 atm.  At a few sites air is sampled 
in evacuated glass flasks. Over time several versions of glass 
flasks and automatic samplers have been used; they are 
indentified in the data string.  

Sampling frequencies are approximately weekly for land sites.
For close to three decades ships transecting the Pacific Ocean 
collected one sample pair every three weeks per five degree 
latitude zone. Cruise tracks for the Pacific sampling program 
ran between the US west coast and New Zealand or Australia. 
In the South China Sea one bin sample pair was collected 
per week per 3 degree latitude. 

Details of the Cooperative Air Sampling Program can be found 
at http://www.esrl.noaa.gov/gmd/ccgg/flask.html.                                             

Pacific Ocean Cruise (POC, travelling between the US west coast
and New Zealand or Australia) data were merged and grouped into 
5 degree latitude bins.  For the South China Sea cruises (SCS) the 
data are grouped in 3 degree latitude bins.

Sampling intervals are approximately weekly for the fixed sites
and average one sample every 3 weeks per latitude zone for POC and
about one sample every week per latitude for SCS.

Historically, samples have been collected using two general methods:
flushing and then pressurizing glass flasks with a pump, or opening a
stopcock on an evacuated glass flask; since 28 April 2003, only the
former method is used.  During each sampling event, a pair of flasks 
is filled.

-------------------------------------------------------------------
7.1 DATA - SAMPLING LOCATIONS

For a summary of sampling locations, please visit

http://www.esrl.noaa.gov/gmd/dv/site/site_table.html.

IMPORTANT NOTES: 
1.  Data may not be available for all species at all sites listed 
in the table.
2.  The exact locations of a sampling sites recorded in our database
may change or become better defined over time.  The latitude,
longitude, and altitude of a sample event is based on the best
information available at the time of sample collection.  Differences
in sample position associated with a particular site may be due
to the site moving or changes in technology that permit a more
accurate location determination.

To view near real-time data, manipulate and compare data, and create
custom graphs, please visit

http://www.esrl.noaa.gov/gmd/dv/iadv/.

-------------------------------------------------------------------
7.2 DATA - FILE NAME DESCRIPTION

Encoded into each file name are the parameter (trace gas identifier); sampling 
site; sampling project; laboratory ID number; measurement group; and optional 
qualifiers that further define the file contents.

All file names use the following naming scheme:

         1      2         3               4                   5            
[parameter]_[site]_[project]_[lab ID number]_[measurement group]_[optional 

         6           7
qualifiers].[file type]


1. [parameter]

   Identifies the measured parameter or trace gas species.

   (ex)
   co2      Carbon dioxide
   ch4      Methane
   co2c13   d13C (co2)
   merge    more than one parameter

2. [site]

   Identifies the sampling site code.

   (ex)
   brw
   pocn30
   car
   amt

3. [project]
   
   Identifies sampling platform and strategy.

   (ex)
   surface-flask
   surface-pfp
   surface-insitu
   aircraft-pfp
   aircraft-insitu
   tower-insitu

4. [lab ID number]

   A numeric field that identifies the sampling laboratory (1,2,3, ...).
   NOAA ESRL is lab number 1 (see http://www.esrl.noaa.gov/gmd/ccgg/obspack/labinfo.html).

5. [measurement group]

   Identifies the group within NOAA or INSTAAR that makes the actual measurement.
   See Section 5 (UPDATE NOTES) for details.

   (ex)
   ccgg
   hats
   arl
   sil

6. [optional qualifiers]

   Optional qualifier(s) may indicate data subsetting or averaging.
   Multiple qualifiers are delimited by an underscore (_).  A more detailed
   description of the file contents is included within each data file.

   (ex)
   event         All measurement results for all collected samples (discrete (flask) data only).
   month         Computed monthly averages all collected samples (discrete (flask) data only).
   hour_####     Computed hourly averages for the specified 4-digit year (quasi-continuous data only)
   HourlyData    Computed hourly averages for entire record (quasi-continuous data only)
   DailyData     Computed daily averages for entire record (quasi-continuous data only)
   MonthlyData   Computed monthly averages for entire record (quasi-continuous data only)

7. [file type]
   
   File format (netCDF, ASCII text). 

   (ex) 

   txt           ASCII text file
   nc            netCDF4 file

-------------------------------------------------------------------
7.3 DATA - EVENT WITH SINGLE PARAMETER

The event data files in ftp://aftp.cmdl.noaa.gov/data/trace_gases/co/flask/surface/ 
use the following naming scheme (see Section 7.2):

     [parameter]_[site]_[project]_[lab ID number]_[measurement group]_[optional qualifiers].txt

(ex) CH4_pocn30_surface-flask_1_ccgg.txt contains CH4 ccgg measurement
     results for all surface flask samples collected on the Pacific 
     Ocean Cruise sampling platform and grouped at 30N +/- 2.5 degrees.

(ex) CO2_brw_surface-flask_1_ccgg.txt contains CO2 ccgg measurement 
     results for all surface flask samples collected at Barrow, Alaska.

The data files contain multiple lines of header information followed by one 
record for each atmospheric measurement of a single parameter or trace gas species.

Fields are defined as follows:

Field 1:    [SITE CODE] The three-character sampling location code (see above).

Field 2:    [YEAR] The sample collection date and time in UTC.
Field 3:    [MONTH]
Field 4:    [DAY]
Field 5:    [HOUR]
Field 6:    [MINUTE]
Field 7:    [SECOND]

Field 8:    [FLASK ID] The sample container ID.

Field 9:    [METHOD] A single-character code that identifies the sample 
             collection method.  The codes are:

             P - Sample collected using a portable, battery
                 powered pumping unit.  Two flasks are
                 connected in series, flushed with air, and then
                 pressurized to 1.2 - 1.5 times ambient pressure.

             D - Similar to P but the air passes through a
                 condenser cooled to about 5 deg C to partially
	              dry the sample.

             G - Similar to D but with a gold-plated condenser.

             T - Evacuated flask filled by opening an O-ring sealed       
                 stopcock.

             S - Flasks filled at NOAA GML observatories by sampling
                 air from the in situ CO2 measurement air intake system.

             N - Before 1981, flasks filled using a hand-held
                 aspirator bulb. After 1981, flasks filled using a
                 pump different from those used in method P, D, or G.

             F - Five liter evacuated flasks filled by opening a
                 ground glass, greased stopcock.

Field 10:   [TRACE GAS NAME] Gas identifier (e.g., co2, co2c13).

Field 11:   [MEASUREMENT GROUP] Identifies the group within NOAA and INSTAAR 
	     making the actual measurement (e.g., ccgg, hats, arl).  

Field 12:   [MEASURED VALUE] Dry air mole fraction or isotopic composition.  
             Missing values are denoted by -999.99[9].

Field 13:   [ESTIMATED UNCERTAINTY] Estimated uncertainty of the reported
             measurement value.  Missing values are denoted by -999.99[9].

Field 14:   [QC FLAG] A three-character field indicating the results of our 
             data rejection and selection process, described in section 7.5.

Field 15:   [INSTRUMENT] A 2-character code that identifies the instrument 
             used for the measurement.

Field 16:   [YEAR] The measurement date and time in LT.
Field 17:   [MONTH]
Field 18:   [DAY]
Field 19:   [HOUR]
Field 20:   [MINUTE]
Field 21:   [SECOND]

Field 22:   [LATITUDE] The latitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Southern Hemipshere).

Field 23:   [LONGITUDE] The longitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Western Hemisphere).

Field 24:   [ALTITUDE] The altitude of the sample inlet (masl). The reported altitude
            is the surface elevation plus sample intake height.

Field 25:   [ELEVATION] Surface elevation (masl).

Field 26:   [INTAKE HEIGHT] Air sample collection height above ground level (magl).

Field 27:   [EVENT NUMBER] A long integer that uniquely identifies the sampling
             event.

Fields in each line are delimited by whitespace.

(ex)
   BRW 1971 10 19 01 20 00 48-71 N co2 CCGG 319.520 -999.990 N.. L1 1971 12 15 08 46 
   00 71.3200 -156.6000 13.00 11.00 2.00 26142

-------------------------------------------------------------------
7.4 DATA - EVENT WITH MULTIPLE PARAMETERS

On special request we can distribute a "merged" file, which
includes for each sampling event, measurement results for muliple 
parameters or trace gas species.  A merged file does not include all 
information found in a single parameter data file.  For example,
merged files exclude measurement uncertainty, analysis instrument 
ID and date and time for each parameter.  Thus, the single parameter
data file is our most comprehensive data archive. 

The format of a merged file is slightly different from a single parameter event file.
A "merged" file will have the word "merge" in the parameter field of the file name.  
The file name does not inform on the number of parameters included in the file.

Merged data files use the following naming scheme (see Section 7.2):

     merge_[site]_[project]_[lab ID number]_[measurement group]_[optional qualifiers].txt

(ex) merge_pocn30_surface-flask_1_ccgg.txt contains ccgg measurement results for two or
     more parameters for all surface flask samples collected on the Pacific Ocean Cruise 
     sampling platform and grouped at 30N +/- 2.5 degrees.

(ex) merge_brw_surface-flask_1_ccgg.txt contains ccgg measurement results for two or more
     parameters for all surface flask samples collected at Barrow, Alaska.

The data files contain multiple lines of header information followed by one 
record for each atmospheric measurement of a single parameter or trace gas species.

Fields are defined as follows:

Field 1:    [SITE CODE] The three-character sampling location code (see above).

Field 2:    [YEAR] The sample collection date and time in UTC.
Field 3:    [MONTH]
Field 4:    [DAY]
Field 5:    [HOUR]
Field 6:    [MINUTE]
Field 7:    [SECOND]

Field 8:    [FLASK ID] The sample container ID.

Field 9:    [METHOD] A single-character code that identifies the sample 
             collection method.  The codes are:

             P - Sample collected using a portable, battery
                 powered pumping unit.  Two flasks are
                 connected in series, flushed with air, and then
                 pressurized to 1.2 - 1.5 times ambient pressure.

             D - Similar to P but the air passes through a
                 condenser cooled to about 5 deg C to partially
	              dry the sample.

             G - Similar to D but with a gold-plated condenser.

             T - Evacuated flask filled by opening an O-ring sealed       
                 stopcock.

             S - Flasks filled at NOAA GML observatories by sampling
                 air from the in situ CO2 measurement air intake system.

             N - Before 1981, flasks filled using a hand-held
                 aspirator bulb. After 1981, flasks filled using a
                 pump different from those used in method P, D, or G.

             F - Five liter evacuated flasks filled by opening a
                 ground glass, greased stopcock.

Field 10:   [LATITUDE] The latitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Southern Hemipshere).

Field 11:   [LONGITUDE] The longitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Western Hemisphere).

Field 12:   [ALTITUDE] The altitude of the sample inlet (masl). The reported altitude
            is the surface elevation plus sample intake height.

Field 13:   [ELEVATION] Surface elevation (masl).

Field 14:   [INTAKE HEIGHT] Air sample collection height above ground level (magl).

Field 15:   [EVENT NUMBER] A long integer that uniquely identifies the sampling
             event.


There is a group of 4 fields for each parameter and measurement group included in the 
merge file.

Field ##+1: [TRACE GAS NAME] Gas identifier (e.g., co2, co2c13).

Field ##+2: [MEASUREMENT GROUP] Identifies the group within NOAA and INSTAAR 
	     making the actual measurement (e.g., ccgg, hats, arl).

Field ##+3: [MEASURED VALUE] Dry air mole fraction or isotopic composition.  
             Missing values are denoted by -999.99[9].

Field ##+4: [QC FLAG] A three-character field indicating the results of our 
             data rejection and selection process, described in section 7.5.

Fields in each line are delimited by whitespace.

(ex)

   BRW 2015 11 02 20 05 00 4827-99 P 71.3230 -156.6114 16.00 11.00 5.00 397570 
   co2 CCGG 400.480 ... co CCGG 105.340 ...

-------------------------------------------------------------------
7.5 QC FLAGS

NOAA ESRL uses a 3-column quality control flag where each column
is defined as follows:

column 1    REJECTION flag.  An alphanumeric other
            than a period (.) in the FIRST column indicates
            a sample with obvious problems during collection
            or analysis.  This measurement should not be interpreted.

column 2    SELECTION flag.  An alphanumeric other than a
            period (.) in the SECOND column indicates a sample
            that is likely valid but does not meet selection
            criteria determined by the goals of a particular
            investigation.

column 3    INFORMATION flag.  An alphanumeric other than a period (.) 
            in the THIRD column provides additional information 
            about the collection or analysis of the sample.

            WARNING: A "P" in the 3rd column of the QC flag indicates
            the measurement result is preliminary and has not yet been 
            carefully examined by the PI.  The "P" flag is removed once 
            the quality of the measurement has been assessed.


-------------------------------------------------------------------
7.6 DATA - MONTHLY AVERAGES

The monthly data files in ftp://aftp.cmdl.noaa.gov/data/trace_gases/co/flask/surface/ 
use the following naming scheme (see Section 7.2):

     [parameter]_[site]_[project]_[lab ID number]_[measurement group]_month.txt

(ex) CH4_pocn30_surface-flask_1_ccgg_month.txt contains CH4 ccgg monthly
     mean values for all surface flask samples collected on the Pacific
     Ocean Cruise sampling platform and grouped at 30N +/- 2.5 degrees.

(ex) CO2_brw_surface-flask_1_ccgg_month.txt contains CO2 ccgg monthly
     mean values for all surface flask samples collected at Barrow, Alaska.

Monthly means are produced for each site by first averaging all
valid measurement results in the event file with a unique sample
date and time.  Values are then extracted at weekly intervals from 
a smooth curve (Thoning et al., 1989) fitted to the averaged data 
and these weekly values are averaged for each month to give the 
monthly means recorded in the files.  Flagged data are excluded from the
curve fitting process.  Some sites are excluded from the monthly
mean directory because sparse data or a short record does not allow a
reasonable curve fit.  Also, if there are 3 or more consecutive months
without data, monthly means are not calculated for these months.

The data files contain multiple lines of header information 
followed by one line for each available month.

Fields are defined as follows:

Field 1:    [SITE CODE] The three-character sampling location code (see above).

Field 2:    [YEAR] The sample collection year and month.
Field 3:    [MONTH]

Field 4:    [MEAN VALUE] Computed monthly mean value

-------------------------------------------------------------------
8. DATA RETRIEVAL

To transfer all files in a directory, it is more efficient to 
download the tar or zipped files.  

To transfer a tar file, use the following steps from the ftp prompt:

   1. ftp> binary                    ! set transfer mode to binary
   2. ftp> get filename.tar.gz       ! transfer the file
   3. ftp> bye                       ! leave ftp

   4. $ gunzip filename.tar.gz       ! unzip your local copy
   5. $ tar xvf filename.tar         ! unpack the file

To transfer a zipped file, use the following steps from the ftp prompt:

   1. ftp> binary                    ! set transfer mode to binary
   2. ftp> get filename.zip          ! transfer the file
   3. ftp> bye                       ! leave ftp

   4. $ unzip filename.zip           ! uncompress your local copy

-------------------------------------------------------------------
9. REFERENCES

Lang, P.M., L.P. Steele, R.C. Martin, and K.A. Masarie,
  Atmospheric methane data for the period 1983-1985 from
  the NOAA/GMCC global cooperative flask sampling network,
  NOAA Technical Memorandum ERL CMDL-1, 1990a.

Lang, P.M., L.P. Steele, and R.C. Martin, Atmospheric
  methane data for the period 1986-1988 from the NOAA/CMDL
  global cooperative flask sampling network, NOAA Technical
  Memorandum ERL CMDL-2, 1990b.

Gerbig, C., S. Schmitgen et al., An improved fast-response
  vacuum-UV resonance fluorescence CO instrument, J. Geophys.
  Res., 104, 1699-1704, 1999.

Novelli, P.C., J.E. Elkins, and L.P. Steele, The development
  and evaluation of a gravimetric reference scale for
  measurements of atmospheric carbon monoxide, J. Geophys.
  Res., 96, 13,109-13,121, 1991.

Novelli, P.C., L.P. Steele, and P.P. Tans, Mixing ratios of
  carbon monoxide in the troposphere, J. Geophys. Res., 97,
  20,731-20,750, 1992.

Novelli, P.C., J.E. Collins, Jr, R.C. Myers, G.W. Sachse,
  and H.E. Scheel, Reevaluation of the NOAA/CMDL carbon
  monoxide reference scale and comparisons to CO reference
  gases at NASA-Langley and the Fraunhofer Institute, 99,
  12,833- 12,839, 1994.

Novelli, P.C., K.A. Masarie, and P.M. Lang, Distributions
  and recent changes in carbon monoxide in the lower
  troposphere, J. Geophys. Res., 103, 19,1015- 19,033, 1998.

Novelli, P.C., K.A. Masarie, P.M. Lang, B.D. Hall, R.C. Myers,
  and J.W. Elkins, Re-analysis of tropospheric CO trends:
  Effects of the 1997-1998 wild fires, J. Geophys. Res., 108,
  D15 : 4464, doi:10.1029/2002JD003031, 2003.

-------------------------------------------------------------------