ORNL/CDIAC-114
NDP-067

THE INTERNATIONAL INTERCOMPARISON EXERCISE OF UNDERWAY fCO2
SYSTEMS DURING THE R/V METEOR CRUISE 36/1 IN THE
NORTH ATLANTIC OCEAN

Contributed by
Arne Koertzinger, Ludger Mintrop, and Jan C. Duinker
Department of Marine Chemistry
Institute of Marine Research 
University of Kiel
Kiel, Germany

Other data contributors
Kenneth M. Johnson, Craig Neill, Douglas W. R. Wallace,
Bronte Tilbrook, Philip Towler, Hisayuki Inoue, Masao Ishii,
Gary Shaffer, Rodrigo Torres, Eiji Ohtaki, Eiji Yamashita, 
Alain Poisson, Christian Brunet, Bernard Schauer, 
Catherine Goyet, and Greg Eischeid

Prepared by 
Alexander Kozyr 
Carbon Dioxide Information Analysis Center
Oak Ridge National Laboratory
Oak Ridge, Tennessee, U.S.A.

Environmental Sciences Division
Publication No. 4844

Date Published: March 1999

Prepared for the
Environmental Sciences Division
Office of Biological and Environmental Research
U.S. Department of Energy
Budget Activity Numbers KP 12 04 01 0 and KP 12 02 03 0

Prepared by the
Carbon Dioxide Information Analysis Center
OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37831-6335
managed by
LOCKHEED MARTIN ENERGY RESEARCH CORP.
for the
U.S. DEPARTMENT OF ENERGY
under contract DE-AC05-96OR22464

ABBREVIATIONS

AT   total alkalinity
BMBF Bundesministerium f r Bildung, Wissenschaft, Forschung und
     Technologie
     (German Ministry for Education, Science, Research and
     Technology)
BNL  Brookhaven National Laboratory
CMDL Climate Monitoring and Diagnostics Laboratory
CSIRO Commonwealth Scientific and Industrial Research
      Organisation
CT   total dissolved inorganic carbon (synonyms: DIC, TCO2, ?CO2)
CTD  conductivity-temperature-depth probe
DFG  Deutsche Forschungsgemeinschaft (German Research Foundation)
DOE  U.S. Department of Energy
fCO2 fugacity of CO2
GMT  Greenwich mean time
GPS  global positioning system
IfMK Institut f r Meereskunde Kiel (Institute of Marine Research
     at the University of Kiel)
IOC  Intergovernmental Oceanographic Council
JGOFS  Joint Global Ocean Flux Study
MRI  Meteorological Research Institute
NBI  Niels Bohr Institute for Astronomy, Physics and Geophysics
NDIR nondispersive infrared
NDP  numeric data package
NOAA National Oceanic and Atmospheric Administration
OU   Okayama University
pCO2 partial pressure of CO2
pH   pH value
PSS  Practical Salinity Scale
RF   Reedereigemeinschaft Forschungsschiffahrt GmbH
R/V  Research Vessel
SCOR Scientific Committee on Oceanic Research
SIO  Scripps Institution of Oceanography
SOMMA     single-operator multiparameter metabolic analyzer
UP&MC     Universit  Pierre et Marie Curie
UTC  universal time coordinated
WHOI Woods Hole Oceanographic Institution
WMO  World Meteorological Organization
XBT  expendable bathythermograph
xCO2 mole fraction of CO2

ACKNOWLEDGMENTS

The authors would like to thank the following people for their
cooperation and participation in this exercise. Their motivation
and active involvement made this study possible. Our special
thanks are addressed to those who joined the scientific party on
board R/V Meteor for their willingness to cope with all the
little problems such an exercise inevitably provides. 

* Douglas Wallace, Kenneth Johnson, and Craig Neill, Brookhaven
National Laboratory, Department of Applied Science, Upton, Long
Island, New York, U.S.A.
* Bronte Tilbrook and Philip Towler, Commonwealth Scientific and
Industrial Research Organisation, Division of Oceanography,
Hobart, Tasmania, Australia
* Joanna Waniek, Susanne Schweinsberg, and Frank Malien, Institut
fuer Meereskunde Kiel (Institute of Marine Research), Kiel,
Germany
* Hisayuki Inoue and Masao Ishii, Meteorological Research
Institute, Tsukuba, Japan
* Gary Shaffer, Niels Bohr Institute for Astronomy, Physics and
Geophysics, University of Copenhagen, Copenhagen, Denmark
* Rodrigo Torres, Goteborg University and Chalmers University of
Technology, Department of Analytical and Marine Chemistry,
Goteborg, Sweden
* Eiji Ohtaki and Eiji Yamashita, Okayama University, Okayama,
Japan
* Andrew Dickson and Justine Parks, Scripps Institution of
Oceanography, Marine Physical Laboratory, La Jolla, California,
U.S.A.
* Alain Poisson, Christian Brunet, and Bernard Schauer,
Universite Pierre et Marie Curie, Laboratoire de Physique et
Chimie Marines, Paris, France
* Catherine Goyet and Greg Eischeid, Woods Hole Oceanographic
Institution, Department of Marine Chemistry and Geochemistry,
Woods Hole, Massachusetts, U.S.A.

In addition, we wish to thank the chief scientist of Meteor
Cruise 36/1, Detlev Schulz-Bull, for perfect cooperation and for
giving this exercise the high priority it needed. We appreciate
the valuable comments and helpful advice from Andrew Dickson
during the early stages of the exercise. Our thanks to Douglas
Wallace and Andrew Watson for reviewing the manuscript and making
helpful comments.

We also express our appreciation to the Reedereigemeinschaft
Forschungsschiffahrt GmbH and the crew of R/V Meteor for their
extremely helpful technical support before and throughout the
cruise.

On behalf of the participants of this intercomparison exercise,
we extend special thanks to all national funding agencies for
providing the participants with the necessary funding.
Last but not least, the authors thank the Bundesministerium f r
Bildung, Wissenschaft, Forschung und Technologie (BMBF) for
funding this exercise through German Joint Global Ocean Flux
Study (JGOFS) funds as well as the Deutsche
Forschungsgemeinschaft (DFG) for making it possible to invite the
international participants on this cruise. 

ABSTRACT

Koertzinger, A., L. Mintrop, J.C. Duinker, K. Johnson, C. Neill, 
D.W.R. Wallace, B. Tilbrook, P. Towler, H. Inoue, M. Ishii, G. Shaffer, 
R. Torres, E. Ohtaki, E. Yamashita, A. Poisson, C. Brunet, B. Schauer, 
C. Goyet, G. Eischeid, and A. Kozyr (ed.). 1999. The International 
Intercomparison Exercise of Underway fCO2 Systems During the R/V Meteor 
Cruise 36/1 in the North Atlantic Ocean. ORNL/CDIAC-114, NDP-067. 
Carbon Dioxide Information Analysis Center, Oak Ridge National 
Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, U.S.A.

Measurements of the fugacity of carbon dioxide (fCO2) in surface
seawater are an important part of studies of the global carbon
cycle and its anthropogenic perturbation. An important step
toward the thorough interpretation of the vast amount of
available fCO2 data is the establishment of a database system
that would make such measurements more widely available for use
in understanding the basin- and global-scale distribution of fCO2
and its influence on the oceanic uptake of anthropogenic CO2.
Such an effort, however, is based on knowledge of the
comparability of data sets from different laboratories.
Currently, however, there is not much known about this subject.
In the light of the aforementioned situation, an International
Intercomparison Exercise of Underway fCO2 Systems was proposed
and carried out by the Institut fuer Meereskunde Kiel (IfMK)
(Institute of Marine Research at the University of Kiel), Kiel,
Germany, during the R/V Meteor Cruise 36/1 from Hamilton,
Bermuda, to Las Palmas, Gran Canaria, Spain. Nine groups from six
countries (Australia, Denmark, Germany, France, Japan, and the
United States) participated in this ambitious exercise, bringing
together 15 participants with 7 underway fCO2 systems, 1 discrete
fCO2 system, and 2 underway pH systems, as well as discrete
systems for alkalinity and total dissolved inorganic carbon. This
report presents only the results of the underway measurements of
fCO2.

The main idea of the exercise was to compare surface seawater
fCO2 synchronously measured by all participating instruments
under identical conditions. This synchronicity was accomplished
by providing the infrastructure during the exercise, such as a
common seawater and calibration gas supply. Another important
issue was checks of the performance of the calibration procedures
for CO2 and of all equilibrator temperature sensors. Furthermore
a common procedure for the calculation of final fCO2 was applied
to all data sets. All these measures were taken in order to
reduce the largest possible amount of controllable sources of
error.

In this report we will demonstrate that the results of three of
the seven underway systems agreed to within  2 atm throughout
the cruise. This was not only the case for seawater fCO2
measurements but also for measurements of theatmospheric mole
fraction of CO2 (xCO2). One system was in good agreement (+/-2
atm) for most of the time but showed a considerable positive
offset of up to 9 atm for about 40 h. However, it was found that
significant offsets of up to 10 atm occurred in underway fCO2
measurements for three systems under typical and identical field
work conditions. Although at least in one case this may be a
consequence of a technical failure, it is an indication of
significant systematic differences in other cases. Finally, the
discrete fCO2 system measurements agreed within its nominal
accuracy of 1% with the three most consistent underway fCO2
systems data sets.

On the basis of a detailed comparison and evaluation of this
large intercomparison data set we try to come up with general
conclusions and recommendations for underway fCO2 work. These may
serve as background information for a successful preparation of a
coherent database of surface ocean fCO2 values. The results of
this exercise certainly underline the need to address carefully
the important issue of the inter-laboratory comparability of fCO2
data.

PART 1

OVERVIEW

1.  INTRODUCTION
1.1  SCIENTIFIC BACKGROUND OF THE EXERCISE

Currently marine scientists are applying different concepts to
quantify the oceanic uptake of CO2. These efforts are being
undertaken in the light of theatmospheric CO2 perturbation and
its possible impact on the earth's climate. One important concept
is based on the determination of the partial pressure difference
of CO2 (delta pCO2) between the surface seawater and the overlying
air, which is the thermodynamic driving force for any net
exchange of CO2. By means of a transfer coefficient, a measured
(pCO2 can be converted into a momentary net flux of CO2 across
the air-sea interface. Given the strong spatial and temporal
variability of pCO2 in the ocean, this concept faces the
challenge of coming up with representative mean delta pCO2 values on a
global grid. If this concept is to be successful in pinning down
the present oceanic uptake of CO2 reliably, the combined efforts
of research groups all over the world are necessary. The
Intergovernmental Oceanographic Council (IOC)/Scientific
Committee on Oceanic Research (SCOR) Carbon Dioxide Advisory
Panel recently established an international inventory of pCO2
measurements that have been identified so far
(http://cdiac.esd.ornl.gov/oceans/pco2inv.html). One important
requirement in this context is a good inter-laboratory
comparability of the data sets, which were generated by quite
different types of analytical systems. While the analytical
precision of the various systems in use is mostly of the order of
1 atm or better, not much is known presently about the
comparability between different laboratories.

As a first important step to assess the current state of this
parameter, an international shore-based intercomparison exercise
of underway fugacity of CO2 (fCO2) systems was carried out by
Andrew Dickson in June 1994 at the Scripps Institution of
Oceanography, Marine Physical Laboratory, La Jolla, California,
U.S.A. (http://www-mpl.ucsd.edu/people/adickson/CO2_QC) on behalf
of the Joint IOC/SCOR CO2 Advisory Panel. However, the general
consensus in the scientific community was that a necessary second
step would be an at-sea intercomparison under more typical and
identical operation conditions. Such an exercise, to be carried
out during the R/V Meteor cruise 36/1, was proposed by the Kiel
CO2 group in June 1995 and received very positive feedback within
the scientific community. For a number of reasons the proposed
cruise leg was perfectly suited for such an exercise. Funding of
the exercise came through the German Joint Global Ocean Flux
Study (JGOFS) program. More than fifteen research groups,
representing a fairly good geographical distribution, were
contacted and invited to participate in the exercise, nine of
which were finally able to do so (Koertzinger et al. 1996a).
 
1.2  THE PRINCIPAL DESIGN OF THE EXERCISE

The basic idea of the exercise was to operate as many underway
fCO2 systems simultaneously for as much time as possible.
Combined with in situ salinity and temperature as well as
navigational and meteorological data, this combined underway fCO2
data set is the mainstay of the exercise. Whereas shore-based
intercomparison exercises allow researchers to devise special
experiments that reflect extreme situations, ship-based exercises
have to rely fully on the conditions that are provided by the
ocean. The chosen cruise track reflects the attempt to
include-within the limits of a single and comparatively short
cruise-extreme oceanic regimes. Whereas the situation was very
stable in the Eastern North Atlantic with not much variability in
surface seawater temperatures and salinities and likewise fCO2,
the North Atlantic Drift region off Newfoundland provided extreme
variability with steep gradients. The overall temperature range
during the exercise was from 6.0C to 25.1C, while the salinity
varied between 32.3 and 37.0. In the western part our cruise
track hit warm and cold ring features. Associated with these
rings were steep frontal gradients with changes of up to 15 C and
more than 3 in salinity over a few nautical miles. 

These different regimes provide different information about the
performance and comparability of the participating systems. The
stable situation during the second half of the exercise allows
the detection of systematic offsets between the data sets, thus
providing the basic information about the inter-laboratory
comparability. In contrast to this, the strong gradient regime
mimics to some extent the step experiments of shore-based
intercomparison exercises. The fast change between two "batches"
of seawater, which are characterized by different fCO2 values,
reveals the different time constants of the analytical systems.
Fast responding systems are able to follow the signal much more
closely than the more slowly responding ones. So, even if there
are no systematic differences between two systems, the systems
may have quite different response times, which translates into
different spatial resolution in underway work.

Right from the beginning, it was regarded as high priority to
measure as many parameters [i.e., pH, fCO2, total dissolved
organic carbon (CT), and total alkalinity (AT)] of the marine CO2
system as possible rather than restricting the exercise to mere
fCO2 measurements. For this purpose, we followed two different
sampling strategies  (i.e., underway sampling and discrete
sampling). As all participating fCO2 systems (CSIRO, IfMK, MRI,
NBI, OU, UP&MC, WHOI; see Sect. 2.1.2 for a list of participating
institutions) were operated in an underway mode on the same
seawater source, it was highly desirable to back up these fCO2
measurements with additional underway measurements of other CO2
parameters. This was accomplished by underway pH measurements
with two different spectrophotometric systems (SIO, WHOI) as well
as underway CT measurements (BNL/IfMK) with a newly modified
single-operator multiparameter metabolic analyzer (SOMMA)
coulometric titration system (Johnson et al. 1998), all of which
were hooked up to the seawater pumping system. Discrete sampling
was carried out for discrete measurements of fCO2 (BNL), CT
(BNL/IfMK), AT (IfMK), and salinity (IfMK) as well as nutrients
(IfMK) in samples taken regularly from the same seawater pumping
system. 

By measuring more than two parameters of the CO2 system in
seawater, the system is overdetermined, as all parameters can be
calculated from any combination of two measured parameters and
knowledge of the thermodynamic relationships involved. This was
the case for both sampling strategies. Overdetermination will
therefore allow for consistency checks on the data sets. It may
also provide additional information in the question of the best
set of thermodynamic constants for the CO2 system. The broad CO2
database furthermore serves as valuable background information
and will strongly enhance further interpretation of the results.
The exercise also included checks on ancillary measurements, such
as temperature and barometric pressure, as performed by most of
the analytical systems. All temperature sensors were compared
against a calibrated Pt-100 reference thermometer. The barometric
pressure readings were also referenced against a high-quality
digital barometer. In many cases, these checks revealed offsets
and miscalibrations, which, if not corrected for, would have led
to significant biases of the final fCO2 values. These checks
helped to identify the error contribution from these sources.
They also allowed us to correct all fCO2 measurements for these
effects to reveal any systematic differences that cannot be
attributed to the quality of temperature and pressure
measurements. 

Further checks were carried out with the calibration gases. The
suite of calibration gases supplied by the organizer covered a
range of CO2 concentrations between 250 and 500 ppmv with nominal
values of 250, 300, 350, 400, 450, and 500 ppmv. While every
group required one or more of these calibration gases for their
calibration procedure, they measured all other concentrations as
unknown samples on their systems. The results provide information
on the quality and reliability of the calibration procedures over
the whole range from 250 to 500 ppmv. As the infrared detectors
used by all groups generally show nonlinear response functions,
the calibration procedure is a crucial point. 

2.  DESCRIPTION OF THE EXERCISE
2.1  THE CRUISE
2.1.1  R/V Meteor, Technical Details and Brief History

The R/V Meteor is owned by the Federal Republic of Germany,
represented by the Ministry for Education, Science, Research and
Technology (BMBF), which financed its construction. It is
operated by the German Research Foundation (DFG), which provides
about 70% of its operating funds (the remainder is supplied by
the BMBF). The Senate Commission for Oceanography of the DFG
plans expeditions from the scientific viewpoint and appoints
cruise coordinators and chief scientists. The Operations Control
Office of the University of Hamburg is responsible for
management, logistics, execution, and supervision of ship
operations. These functions are exercised by direct cooperation
with expedition coordinators and the managing owner, the
Reedereigemeinschaft Forschungsschiffahrt GmbH (RF). The latter
is responsible for hiring, provisioning, and coordinating ship
maintenance. Designed as a multipurpose vessel for living and
nonliving resources and worldwide operation, the R/V Meteor
routinely carries scientists from many different countries. The
basic technical details are

Port of registration     Hamburg
Call sign                DBBH
Classification           GL + 100 A4 E2 + MC Auto
Operator                 University of Hamburg, Institute for Marine Research
Managing owner           RF Reedereigemeinschaft Forschungsschiffahrt
                         GmbH, Bremen
Built                    1985/86 at Schlichting Werft, Travem nde, Germany
Basic dimensions:
     Gross registered tonnage 4280 t
     Net registered tonnage   1284 t
     Displacement   4780 t
     Length overall 97.50 m
     Beam 16.50 m
     Draught max.   5.60 m
     Service speed  12 kn
Personnel Crew:          32, Scientists: 28, German Weather Service: 2
Main engine              4 ( Mak 6 M 332 = 4 ( 1000 kW at 750 rpm
Propulsion               Diesel-electrical, tandem motor = 2 ( 1150 kW
Maneuvring propulsion devices:     Special rudder with flap, type
                                   Becker FKSR
                          Omnithruster-bowthruster 919 kW, 10 t thrust
                          thwartships, 
Fuel consumption          About 12 t IFO 80 per day at service speed
Maximum cruise duration   60 days
Nautical equipment        Integrated navigation system with data
                          transfer to position computer, echo sounder synchronization and
                          supervision, data processing facility
Science quarters          20 laboratories on the main deck with
                          approximately 400 m2 working space for multidisciplinary
                          research. Air chemistry lab above the wheelhouse. About 400m2 of
                          free deck working area, mainly with timber planking. Very little
                          vibration and noise achieved by special construction. 

Meteor (I) was built in 1915 in Danzig as a gunboat for the
German navy. However, it never reached completion as such and
remained in an unfinished state until 1925, when it was converted
in Wilhelmshaven to the first German research and survey vessel
of that name. The steel-hull ship Meteor (I) had a length overall
of 71.15 m, a displacement of 1179 t, and carried a crew of 122
plus 11 scientists. One of its first expeditions was the German
Atlantic Ocean Expedition of 1925-27, which was organized by the
Institute for Marine Research in Berlin. Thereafter, the vessel
was used until 1934 for German physical, chemical, and
microbiological marine investigations and for navy surveying as
well as fishery protection duties. 

Meteor (II) was carefully planned after the 1950s; it was jointly
operated by the German Research Foundation (DFG) in Bonn and the
German Hydrographic Institute (DHI) in Hamburg. With a length
overall of 82.10 m and a displacement of 3054 t, the second
Meteor carried 52 in crew and 24 scientists. Commissioned in
1964, Meteor (II) participated in the International Indian Ocean
Expedition. During 73 voyages between 1964 and 1985, the Meteor
(II) sailed a total distance of about 650,000 nm to all parts of
the world's oceans.

Meteor (III), used during the intercomparison exercise described
here, was completed in 1986, replacing Meteor (II). Based in
Hamburg, it is used for German marine research worldwide and for
cooperative efforts with other nations in this field. The vessel
serves scientists of all marine disciplines in all of the world's
oceans.

2.1.2  R/V Meteor, Cruise 36/1 Information

Ship name        Meteor
Cruise/leg       36/1
Location         Hamilton, Bermuda, to Las Palmas, Gran Canaria, Spain
Dates            June 6-19, 1996
Chief scientist  D. Schulz-Bull, Institute of Marine Research,
                 Kiel
Master           M. Kull

Institutions Participating in the Exercise

BNL     Brookhaven National Laboratory, Department of Applied
        Science, Upton, Long Island, New York, U.S.A.
CSIRO   Commonwealth Scientific and Industrial Research
        Organisation, Division of Oceanography, Hobart, Tasmania,
        Australia
IfMK    Institut fuer Meereskunde Kiel (Institute of Marine Research
        at the University of Kiel), Kiel, Germany
MRI     Meteorological Research Institute, Tsukuba, Japan
NBI     Niels Bohr Institute for Astronomy, Physics and Geophysics,
        University of Copenhagen, Copenhagen, Denmark
OU      Okayama University, Okayama, Japan
SIO     Scripps Institution of Oceanography, Marine Physical
        Laboratory, La Jolla, California, U.S.A.
UP&MC   Universit  Pierre et Marie Curie, Laboratoire de
        Physique et Chimie Marines, Paris, France
WHOI    Woods Hole Oceanographic Institution, Department of Marine
        Chemistry and Geochemistry, Woods Hole, Massachusetts, U.S.A.

Parameters measured                     Institution         Principal investigators

Conductivity-temperature-depth (CTD),       IfMK          J. Waniek
salinity, expendable bathythermograph 
(XBT)

Nutrients                                   IfMK          D. Schulz-Bull, A. Koertzinger

Oxygen                                      IfMK          D. Schulz-Bull, A. Koertzinger

Total dissolved inorganic carbon (CT)       BNL, IfMK     K. Johnson, A. Koertzinger

Alkalinity (AT)                             IfMK          L. Mintrop

PH                                          WHOI          C. Goyet
                                            SIO           A. Dickson
                                            BNL           D. Wallace

Fugacity of CO2 (fCO2)                      CSIRO         B. Tilbrook
                                            IfMK          A. Koertzinger
                                            MRI           H. Inoue
                                            NBI           R. Torres
                                            OU            E. Ohtaki
                                            UP & MC       A. Poisson
                                            WHOI          C. Goyet

2.1.3  Brief Cruise Summary

After completion of the previous cruise 35/4, the R/V Meteor
reached Hamilton, Bermuda, on June 4, and Detlev Schulz-Bull
(IfMK) relieved Dieter Meischner (University of Gottingen,
Germany) as chief scientist. A reception for invited officials of
governmental and scientific institutions as well as private
companies was held on board the Meteor on June 4. The scientific
party of cruise 36/1 embarked on June 5. Equipment setup began on
the same day. The R/V Meteor departed Hamilton at 9:00 a.m. local
time on June 6, 1996.

The cruise track of cruise 36/1 (Fig. 1 in hard copy) ran on straight lines
from Bermuda to the Flemish Cap off Newfoundland, Canada, and
then to Gran Canaria, Spain. The turning point was located at 46
40' N, 41 54' W. All seven underway pCO2 systems were operated
simultaneously for most of the time between June 7 and June 17.
Small technical problems that occurred to some of the systems
only caused short interruptions. Only one system suffered heavy
damage in the infrared gas analyzer and it had to quit
measurements by June 14. The two underway spectrophotometric pH
systems were operated throughout the cruise. The newly modified
coulometric SOMMA system for underway determination of CT was
tested successfully at sea and contributed about 450 high-quality
underway CT measurements along the cruise track (Johnson et al.
1998). Synchronized with the XBT survey, a total of 57 discrete
samples were taken from the seawater supply and were analyzed for
pH, CT, and AT. The discrete fCO2 measurements could not be
carried out on the same schedule; samples were taken for this
parameter only at about 17 stations. 

In addition to the various surface measurements (whether
continuous or discrete), five hydrographic stations were occupied
during the cruise. Samples were drawn for measurements of all
four CO2 system parameters (pH, fCO2, CT, AT) thus yielding the
highest possible overdetermination of the marine CO2 system. The
R/V Meteor arrived at Las Palmas, Gran Canaria, Spain, on June
19, 1996. Weather and sea conditions had been excellent
throughout the cruise allowing for uninterrupted scientific work.

2.2  TECHNICAL ASPECTS OF THE EXERCISE

2.2.1  The Underway Pumping System

The intercomparison exercise was almost entirely based on
continuous underway sampling of surface seawater. All
participating groups operated their underway fCO2 systems
simultaneously on the same seawater pumping system. Like most
up-to-date research vessels, the R/V Meteor provides a special
seawater pumping system for scientific purposes. However, from
experience, it is known that the use of this kind of pumping
system for measurements of dissolved gases may be hampered by a
number of problems. Pump action may cause cavitation when
underpressure is applied to the water flow, thus making
undisturbed gas measurements nearly impossible. Because of the
location of the seawater intake close to the bow on R/V Meteor,
air bubbles are introduced into the water lines in a rough sea.
This again possibly biases the concentration of dissolved gases
or even makes seawater sampling technically impossible in such
cases. Furthermore the unavoidable warming of seawater during its
travel from the bow intake to the user may be quite significant.
In the case of the fCO2 intercomparison exercise, it was
desirable to keep the temperature change as small as possible.
As a result of the sluggish exchange of CO2 between the gas phase
and the water phase, sampling for CO2 measurements (e.g., fCO2,
pH, CT) is less susceptible to biases caused by inadequate
pumping techniques than is sampling of reactive gases like
oxygen. Nevertheless a careful sampling technique was an
important aspect of the exercise. For this reason, a simple and
reliable underway pumping system (see also Koertzinger et al.
1996b) was designed for use in the "moon pool" of R/V Meteor. The
system consisted of a small CTD probe (ECO type, ME
Meerestechnik-Elektronik GmbH, Trappenkamp, Germany) for
measuring in situ seawater temperature and salinity at the intake
as well as a submersible pump, both of which were installed in
the shell plating at the bottom of the "moon pool." Figure 2 in hard copy
shows a schematic drawing of this underway pumping system. The
system also includes a separate Global Positioning System (GPS)
receiver (GPS 120, Garmin/Europe Ltd., Romsey, Hampshire, U.K.).
Navigational data from the GPS system as well as CTD data were
continuously logged on a computer.

The moon pool of R/V Meteor is specially designed for sampling
purposes so that no cooling or wastewaters are emitted ahead of
it and even at full speed or in a very rough sea no air bubbles
reach it. Seawater was pumped through the moon pool from below
the ship by means of a large submersible pump (multivane impeller
pump, type CS 3060, ITT Flygt Pumpen GmbH, Langenhagen, Germany)
at a pumping rate of about 350 L/min-1 (pump head approx. 12 m).
The CTD probe was installed next to the submersible pump. All
underway fCO2 systems were assembled in the geology lab of R/V
Meteor (see Sect. 2.2.2). Two seawater supply lines (port and
starboard) were teed-off from the main bypass and laid through
the lab. All underway systems were hooked-up to these supply
lines which delivered the necessary flow rates of seawater to
each system (approx. 
1-15 L/min-1). 

The wastewater from the systems was collected in three 100-L
carboys and from there was disposed of continuously through the
floor drains of the geology lab. In case of (occasionally
observed) clogging of the lab's floor drains as a result of rough
sea conditions, small submersible pumps (multivane impeller pump,
type GS 9565, ITT Flygt Pumpen GmbH, Langenhagen, Germany) were
at hand to pump the wastewater actively out of the lab. These
pumps did not have to be used during this cruise, however.

2.2.2  The Laboratory Setup

During cruise 36/1 four labs were reserved for the
intercomparison exercise (Fig. 3 in hard copy). All underway systems were
assembled side by side in the geology lab (no. 16) of R/V Meteor.
This lab is the largest lab on the R/V Meteor. It is located on
the main deck, starboard side, with direct access to the working
deck. Adjacent to the geology lab is the universal lab (no. 15,
not shown in Fig. 3), where the dynamic transformer was
installed. The SOMMA coulometric analyzer for CT and the
alkalinity titration system were installed in the clean lab (no.
4, not shown in Fig. 3) on the port side of the ship. The moon
pool is located in the hold (Lab 17) for the CTD rosette. This
lab is very close to the main lab of the exercise (Lab 16) thus
allowing for short water lines of the seawater pumping system.

2.2.3  Other Infrastructure of the Exercise

In addition to the common seawater line (Sect. 2.2.1), a common
supply of calibration gases was regarded a key requirement for
the exercise, as otherwise systematic errors most likely would
have been introduced. We therefore provided a whole suite of
calibration gases. Fifteen cylinders with precisely known amounts
of CO2 in natural dry air covering a nominal concentration range
from 250 ppmv to 500 ppmv were purchased from the National
Oceanic and Atmospheric Administration (NOAA) Climate Monitoring
Diagnostics Laboratory (CMDL) in Boulder, Colorado, U.S.A. Before
final filling, all aluminum standard cylinders (Scott Specialty
Gases Inc., Plumsteadville, Pennsylvania, U.S.A.) undergo a
conditioning period of at least one week with clean ambient air.
To prepare the standards, the cylinders are filled with ambient
air at Niwot Ridge, Colorado. The air is dried using magnesium
perchlorate and either scrubbed with Ascarite or spiked with a
10% CO2-in-air mixture to obtain mixing ratios below or above
ambient levels, respectively (Zhao et al. 1997). Six cylinders of
this consistent suite of 
gases were used during the exercise by all groups for calibrating
their instruments. Additionally nitrogen (purity 99.999%) was
used by some groups for zeroing their gas analyzers.
The mixing ratios of CO2 in the cylinders were calibrated in the
NOAA/CMDL Carbon Cycle Group laboratories on three separate days
over a period of 2-3 weeks. The results of these calibrations are
summarized in Table 1. The CO2 mixing ratios are reported as
micromoles per mole (mol/mol-1 = ppmv) of dry air in the World
Meteorological Organization (WMO) X85 mole fraction scale,
traceable to primary standards at the Scripps Institution of
Oceanography (SIO). The NOAA/CMDL calibrations are done by
comparison on a nondispersive infrared CO2 analyzer against four
tertiary standards with assigned mixing ratios traceable to SIO
(Thoning et al. 1987; Zhao et al. 1997). The uncertainty of the
assigned values for the tertiary standards is approximately 0.06
ppmv. The tertiary set of standards used ranges between 250 and
450 ppmv CO2. The repeatability of the NOAA/CMDL calibrations
depends on the stability of the CO2 mixing ratio in the cylinder
and the fit of the analyzer response to the known tertiary
standards. For cylinders that are stable and within the range of
standards, the repeatability is on the order of 0.01 ppmv. The
overall uncertainty associated with precision is therefore about
0.06 ppmv. When calibrating cylinders at the extremes of the
tertiary standards or extrapolated outside the range, the
reproducibility decreases. For mixing ratios above 450 ppmv, the
reproducibility is on the order of (0.3 ppmv and further
decreases with the interpolation away from the tertiary
standards. The absolute accuracy of the assigned mixing ratios is
determined by the accuracy of the SIO standards (Keeling et al.
1986, and references therein).

Table 1. Summary of calibration results for six cylinders with
CO2 in natural dry air The measurements were carried out at the
NOAA/CMDL Carbon Cycle Group Laboratory in Boulder, Colorado,
U.S.A. These six cylinders constitute the suite of calibration
gases used by all participating groups during the exercise.

------------------------------------------------------------------
Cylinder    Date of     Measured CO2    Average CO2      Standard
   #      measurement  concentration   concentration    deviation
          (MM/DD/YY)      (ppmv)          (ppmv)          (ppmv)
------------------------------------------------------------------
2178       03/06/96        252.42

2178       03/11/96        252.46

2178       03/14/96        252.45         252.44           0.02

1996       02/22/96        298.43

1996       03/07/96        298.47

1996       03/18/96        298.42         298.44           0.03

2172       02/05/96        349.53

2172       03/14/96        349.52

2172       03/18/96        349.51         349.52           0.01

1980       03/05/96        403.85

1980       03/11/96        403.84

1980       03/13/96        403.87         403.85           0.02

2186       02/27/96        450.69

2186       02/29/96        450.68

2186       03/13/96        450.73         450.70           0.03

2112       04/10/96        511.28

2112       04/15/96        511.61

2112       04/19/96        511.60         511.50           0.19
-------------------------------------------------------------------

According to the different power requirements of the analytical
systems, the ship provided three different power sources, the
standard 220V/50Hz system as well as two additional systems for
220V/50Hz (static transformer) and 220V/60Hz (dynamic
transformer).

3.  METHODS AND PROCEDURES

3.1  PRINCIPLE OF MEASUREMENT OF THE FUGACITY OF CO2

The principle of the measurement of the fugacity of CO2 (fCO2) in
seawater is based on the determination of the CO2 mixing ratio in
a gas phase that is in equilibrium with a seawater sample at
known temperature and pressure. The CO2 mixing ratio can either
be measured with a nondispersive infrared analyzer (NDIR) or with
a gas chromatograph (GC) with flame ionization detector after
catalytic conversion of the CO2 into methane. Whereas the GC
approach has a few advantages (e.g., small sample volume and the
ability to measure additional trace gases), the more rugged
infrared technique has shown better suitability for use at sea
and allows measurements in a truly continuous fashion. 
Depending on the sampling strategy (discrete or continuous), two
different families of analytical systems have been developed. For
the determination of the fCO2 in air that is in equilibrium with
a discrete sample, a known amount of seawater is isolated in a
closed system containing a small known volume of air with a known
initial CO2 mixing ratio. For the determination of the fCO2 in
air that is in equilibrium with a continuous flow of seawater, a
fixed volume of air is equilibrated with seawater that flows
continuously through an equilibrator.

Continuous (or underway) fCO2 systems are more widely used in
marine CO2 research. They provide important information about the
saturation state of seawater at the air-sea interface when
operated on board research vessels with a continuous flow of
seawater usually obtained by means of a shipborne pumping system. 

3.2  PARTICIPATING UNDERWAY fCO2 SYSTEMS

Throughout this report we present technical details as well as
the results of the participating systems in a semi-anonymous
fashion. The main reason for this is the fact that the results of
the exercise cannot easily be extrapolated to the performance of
any participating system in general. Strictly they are only
representative for this single cruise. To avoid the erroneous
association in the scientific community of the performance of a
particular system during this exercise with the general
performance of this system, we choose to report in this
semi-anonymous fashion.

Seven underway fCO2 systems, all of which are based on NDIR
detection of CO2, participated in this exercise. Most of these
systems have received detailed descriptions in the literature,
which can therefore be omitted here. Where such publications are
available, they are reprinted at the end of hard copy report (Appendix
B). For two systems, however, this is not the case. One is the
fCO2 system of CSIRO which features a slightly smaller Weiss-type
equilibrator and is otherwise quite similar to the other systems.
The second one is a system that is manufactured commercially by a
U.K. company (Challenger Oceanic, Haslemere, Surrey, U.K.). For
details about the latter system, further information is available
through the company's internet site 
http://www1.btwebworld.com/challengeroceanic/index.html 

Whereas most of the underway fCO2 systems are similar in the
general design and principle of measurement, they are
considerably different in detail. For quick reference, the main
features of all underway fCO2 systems are summarized in Table 2.
All different equilibrator design principles (i.e., showerhead,
bubbler, and thin film type) were represented by at least one
system, with the majority being of the showerhead type. In most
systems (except "D" and "F") these equilibrators are vented to
theatmosphere and thus operated at ambient pressure. The volumes
of water and air in the equilibrators cover a wide range from a
few milliliters to 15 liters. This is also true for the flow
rates of water (0-15 L/min) and air (0.17-0.8 L/min) through
the equilibrators.

Table 2. Summary of main features of the underway fCO2 systems
"A" through "G" that participated in the exercise

------------------------------------------------------------------------------------------------
                   "A"        "B"         "C"        "D"          "E"       "F"        "G"
------------------------------------------------------------------------------------------------                 
Equilibrator
 Design         Showerhead   Bubbler   Showerhead  Thin film*  Showerhead  Bubbler   Showerhead
 Total volume     1000 mL    1400 mL     13.1 L     119 mL       11.0 L     36 mL    1200 mL
 Water volume      500 mL    1000 mL      2.3 L      21 mL       10.0 L     18 mL     ~75 mL
 Air volume        500 mL     400 mL     10.8 L      98 mL        1.0 L     18 mL     500 mL
 Water flow rate  4-6 L/min 2.0 L/min   8.0 L/min  2.0 L/min  10-15 L/min  0 L/min**  1.2 L/min
 Air flow rate    0.2 L/min 0.8 L/min   0.5 L/min  2.0 L/min    0.5 L/min 0.17 L/min 0.18 L/min
   Vented?          Yes        Yes         Yes       No***         Yes       No        Yes

CO2 measurement
 Method            NDIR       NDIR        NDIR       NDIR         NDIR       NDIR      NDIR
 Wet/dry?           Wet        Wet         Wet        Dry          Dry        Dry       Wet

Analyzer calibration
 No. of st. gases     2          2           2          2            4          2****     2
 Zero gas?           No        Yes          No         No           No        Yes        No

Measurement cycle   
Calibr. freq.       6-8 h      6 h         6 h       4-6 h        1.5 h      15 min      2 h
Air meas. freq.     6-8 h      1 h         6 h       4-6 h        0.5 hr      n/a       7 min
Interrogat. interv. 6 sec      6 sec     1 sec      10 sec       0.1 sec     15 min    0.33 sec
Averaging interv.   1; 3 min   1 min     4 min       5 min         1 min      n/a       1 sec
Data points/interv. 10; 30     10          240        33           600         1        3
-----------------------------------------------------------------------------------------------
     *Film thickness approximately 0.75 mm.
    **Semicontinuous approach.
   ***Vented only every 20 min.
  ****Standard gas generator is initially calibrated using all
      six calibration gases; linearity checks are carried out for every
      sample with only two calibration gases.

A further distinction can be made in whether the sample gas is
measured dry or wet. The traditional procedure is based on NDIR
measurement of the dried sample gas ("D," "E," and "F"). However,
in four systems ("A," "B," "C," and "G") the sample gas is not
dried prior to NDIR measurement. This is feasible on the basis of
the LI-6262 CO2/H2O gas analyzer (Li-Cor Inc., Lincoln, Nebraska,
U.S.A.) which is a dual-channel instrument that simultaneously
measures CO2 and H2O mole fractions of the sample gas and
provides internal algorithms for correction of the diluting and
pressure-broadening effects of water vapor on the CO2 measurement
(McDermitt et al. 1993).

All NDIR instruments were calibrated with the NOAA/CMDL CO2
standards provided by the organizer (Table 1). Because of the
individual calibration procedures, different numbers of gases (2
to 4) were required. Some systems also required a zero gas
(nitrogen, purity 99.999%) for calibration purposes or as a
reference gas.

Whereas underway fCO2 systems "A" through "E" and "G" are similar
in that fCO2 is calculated from the CO2 mixing ratio in a gas
phase that is in equilibrium with a constantly renewed seawater
phase, system "F" is of a principally different design. Here, for
every fCO2 measurement, five aliquots of a discrete seawater
sample (semicontinuous mode) are equilibrated with five different
standard gases bracketing the observed range of seawater fCO2.
For each equilibration run, changes with time in the standard gas
CO2 concentration as a result of CO2 exchange with the sample
aliquot are recorded in terms of positive or negative deviations
from the standard's initial CO2 concentration. If flow conditions
during these five equilibration runs are kept identical, the
heights of the resulting deviation peaks are proportional to the
concentration difference between the carrier gas and a gas that
is in equilibrium with the sample. If peak heights are plotted
versus the initial xCO2 of the standard gases, the equilibrium
xCO2 can be found where a linear regression to the five data
points intersects the x-axis.

Participating groups were asked to operate their systems
according to their typical operation profile (i.e., frequency of
calibration and air measurements, interrogation, and averaging
intervals, etc.). This strategy was chosen to ensure that all
systems were operated in modes to which they have been optimized
in the field and in which their operators have gained the highest
confidence. The consequence, however, was quite different
averaging and/or reporting intervals for the different groups. In
particular, the averaging intervals between 1 and 5 minutes have
certain implications that need to be taken into account when the
data are being compared. This inherent discrepancy of the whole
data set represents a certain limitation for the temporal
resolution to which the interpretation can be extended. This is
discussed in more detail in the results section.

3.3  PARTICIPATING DISCRETE fCO2 SYSTEM

The only discrete fCO2 system ("H") involved in this
intercomparison exercise is based on a batch-equilibration,
static-headspace technique that requires a small sample volume of
60 mL and has an average analysis time of only 2 min per sample.
It includes closed-system equilibration of a headspace in a
shaking water bath, followed by analysis of the CO2 mole fraction
in the water-saturated equilibrated headspace by gas
chromatography with flame-ionization detection (GC-FID). The
method has been described in detail by Neill et al. (1997). This
paper is also reprinted in Appendix B at the end of hard copy report.
It should be noted that this method is not specifically designed
for work in surface seawater but for full-depth profiling. The
equilibration temperature (i.e., the water bath temperature) was
changed two times during the exercise, from 17C (June 10) to
20C (June 11-13, first sample) and, finally, to 25C (from June
13, second sample). The magnitude of the correction of fCO2 from
the temperature of equilibration to the in situ temperature was
44-133 atm (mean: 89 atm) for the samples presented here.

3.4  CHECKS AND CALCULATION ROUTINES

The main idea of the exercise was to compare the surface seawater
fCO2 data as measured by all participating instruments under
identical conditions. This was to some extent accomplished by
providing the infrastructure during the exercise, such as a
common seawater and calibration gas supply (Sect. 2.2). The same
care, however, that had been taken on the side of the logistical
infrastructure was also advisable with respect to ancillary
measurements as well as the calculation procedures involved in
the computation of final fCO2 values. This issue was addressed in
different ways. In the following sections we describe the results
of two different experiments: (1) a check of the performance of
the calibration procedures for CO2 (Sect. 3.4.1), and (2) a check
of all temperature sensors that were used to measure the seawater
temperature in the equilibrators (Sect. 3.4.2). We also describe
the common procedure of the calculation of final fCO2 values
(Sect. 3.4.3) and of the synchronization of the final fCO2
profiles (Sect. 3.4.4).

3.4.1  Check of CO2 Calibration Performance

In order to check the performance of the individual calibration
procedures, every group measured between one and four NOAA/CMDL
CO2 standards in the nominal concentration range of 250-500 ppmv
as "unknown samples." Depending on the individual calibration
procedure, different CO2 standards were measured. Figure 4 in hard copy
shows the results of this exercise. It should be pointed out that this
check was carried out on the last day of the exercise (June 17).
Therefore no data are available for system "A," which had to
prematurely quit the exercise on June 13 because of major
technical problems. For system "C" only one standard could be
measured because the measurement range had been fixed to an upper
limit of 400 ppmv, which was slightly exceeded by the relevant
NOAA/CMDL standard (403.85 ppmv). System "F" required all six
standards for initial calibration, which could therefore not be
measured as "unknown samples."

The results show that essentially all checked NDIR instruments
were calibrated to an accuracy on the order of 1 ppmv or better
over the whole concentration range of 250 to 500 ppmv. Only
system "G" shows deviations of 3 to 6 ppmv. This is indicative of
a systematic problem associated with the calibration of the CO2
analyzer or with the measurement itself. Such deviations are
clearly not tolerable and need to be addressed thoroughly. System
"B" makes use of the factory calibration of the LI-COR LI-6262
instrument, which only requires the adjustment of "zero" (with a
CO2-free gas) and "span" (with a single CO2 standard). It appears
that some accuracy is lost by this somewhat crude calibration
technique,1 and the "classical" approach using at least two CO2
standards spanning the range of anticipated CO2 mixing ratios is
preferred. 

3.4.2  Check of Equilibrator Temperature Sensors

In marine applications, fCO2 results are generally reported at in
situ seawater temperature (T in situ). As the seawater temperature
in the equilibrator during measurement (T eq) usually deviates
from T in situ, a temperature correction needs to be applied. The
size of this correction obviously depends on the choice of the
parameterization (Sect. 3.4.3) and the size of the temperature
deviation itself. As the fCO2 strongly varies with temperature,
measurements of T in situ and T eq have to be made rather
accurately. An error of 0.1C in the resulting temperature
deviation (Teq-Tin situ) is equivalent to an error of about 0.4 %
or 1.5 atm in fCO2 (at 350 atm).

During the exercise, T in situ was measured with a CTD installed
at the seawater intake in the bottom plate of the "moon pool".
The CTD had been calibrated just before the
exercise to an accuracy of 0.05C. These in situ temperature
readings were used in the calculation of all fCO2 data. In order
to exclude possible errors contributed by inaccurate measurements
of T eq, all groups had their equilibrator temperature probes
referenced against a recently calibrated platinum resistance
thermometer (Pt-100) provided by WHOI. For every comparison,
equilibrator probe and reference probe were kept together in a
water bath until the readings had stabilized. In most cases this
was done at three temperatures between 0C and up to 30C. Based
on the deviation from the reference, an individual linear
correction was calculated for every system and applied to its
measurements of T eq. In one case (lab "F"), the temperature probe
could not be removed easily from the water bath surrounding the
equilibrator and the reference probe had to be installed next to
it in the bath thus yielding only a single measurement which was
then treated as a uniform offset. 

Figure 5 in hard copy shows measured deviations and the resulting correction
lines of only those temperature sensors which were used in the
calculation of final fCO2 values (some systems feature up to
three equilibrator temperature sensors). The observed deviations
are roughly between -0.5C and +0.1C with a clear tendency
towards negative values and a negative slope of the linear
correction line. If this inconsistency of the temperature
measurements is not accounted for, differences of up to 2% or
about 7 atm (at 350 atm) in the final fCO2 values are caused as
an artifact entirely the result of inaccurate temperature
measurements. 

Even though the CTD as well as the WHOI reference thermometer may
themselves have been affected by some degree of miscalibration,
the present procedure of referencing all measurements to these
two temperature sources removes the incompatibility of all
temperature readings to better than 0.1C or 1.5 atm. It should
be pointed out that the observed deviations of up to 0.5C are
clearly above a tolerable level. Temperature readings have to be
carried out with an accuracy of at least 0.1C. Ideally they
should be checked for consistency with the temperature probe used
to measure in situ temperature. 

3.4.3  Calculation of fCO2 Results

The calculation of final fCO2 values from the raw voltage
readings of an NDIR analyzer involves a number of steps that are
only briefly described here. More detail of the calculation
procedure can be found in Appendix A and in the reprints of the
pertinent literature section (Appendix B).

The NDIR detector signal depends on the number of CO2 molecules
in the optical path which, in turn, is mainly a function of
pressure and temperature for a given CO2 mixing ratio. The
calculation procedure, therefore, requires temperature and
pressure corrections to account for any fluctuations in these
parameters as well as a calibration function. First the raw
voltage readings are corrected to a standard pressure of one
atmosphere (p^0) to account for fluctuations of the NDIR cell
pressure. This requires continuous monitoring of the pressure in
the cell. It has been found empirically (Welles and Eckles 1991)
that pressure p affects the voltage signal v of NDIR analyzers in
a linear fashion:

v'=v(p^0/p)

NDIR instruments are calibrated using standard gases with known
CO2 mixing ratios in dry air. The mixing ratio of a component gas
(like CO2) in a mixture of gases (like air) is equivalent to its
mole fraction (xCO2), assuming ideal behavior. The CO2 mixing
ratio of the standard gases should closely bracket the expected
range of the sample xCO2. Although the response of NDIR analyzers
is considerably nonlinear, the use of a simple linear calibration
function is generally justified over a small concentration range
of 100-200 ppmv. The error incurred by this approximation is
typically on the order of a few tenths of a ppmv. Furthermore any
deviation of the NDIR cell temperature T from the calibration
temperature T^0 has to be accounted for. Welles and Eckles (1991)
have shown that the mole fraction xCO2* is scaled linearly with
the inverse of the absolute temperature:

xCO2=xCO2*(T/T^0)

The resulting CO2 mole fraction xCO2 in dry air is temperature
and pressure corrected. The latter because the sample gas is
either measured dry (i.e., after full removal of water vapor) or
has been arithmetically corrected for the diluting and
pressure-broadening effects of water vapor based on simultaneous
wet xCO2 and xH2O measurements. As the air at the air-sea
interface can be assumed to be at 100% humidity, a correction has
to be applied to account for the increase of the CO2 mole
fraction that is the result of the (actual or arithmetical)
removal of water vapor prior to the infrared measurement. Here
the saturation water vapor pressure of seawater at equilibrator
temperature was calculated using an equation by Weiss and Price
(1980), which is valid over the temperature range 273-313 K and
the salinity range 0-40 (see Appendix A, Part 1). 

For very accurate interpretations the non-ideal behavior of CO2
should be taken into account (i.e., fugacity has to be used
instead of partial pressure). As the results are to be used later
for consistency checks, together with other parameters of the CO2
system in seawater, we decided to use fCO2. The calculation of
the fCO2 at equilibrator temperature from the measured mole
fraction (xCO2) in dry air is described in detail in Appendix A
(Part 2). The fugacity coefficient (i.e., the ratio between
fugacity and partial pressure of CO2), is on the order 0.996 to
0.997 under typical conditions (p = 1atm, T = 270-300 K, pCO2 =
350 atm). Barometric pressure readings from the shipborne
meteorological sensor were used for all calculations of final
fCO2 data. 

Because the fCO2 in seawater strongly varies with temperature,
the final step in the calculation of fCO2 (in situ) requires a
correction to compensate for any difference between the
equilibration temperature and the in situ seawater temperature.
Different equations have been proposed for the temperature
dependence of CO2 partial pressure/fugacity in seawater (e.g.,
Gordon and Jones 1973; Weiss et al. 1982; Copin-Montegut 1988,
1989; Goyet et al. 1993; Takahashi et al. 1993). Because
temperature deviations were typically on the order of a few
tenths of a degree for all systems during the exercise, the
correction is rather small and the choice from the above suite of
equations is not critical. We have chosen the equation based on
temperature and salinity given by Weiss et al. (1982), which is
valid for ranges of 0 to 36C in temperature, 30 to 38 in
salinity, and 80 to 2000 atm in fCO2 (see Appendix A, Part 3.).
All temperature corrections of the fCO2 measurements during this
exercise are based on this equation.

3.4.4  Synchronization of Surface Measurements

Profiles of in situ temperature and salinity in surface seawater,
as measured by the CTD probe at the seawater intake, and the
different seawater fCO2 profiles had to be matched and
synchronized. Given the strong gradients in surface seawater
temperature encountered during some periods of the cruise, this
was very important for a reliable estimation of the differences
between equilibrator and in situ temperatures. At the beginning
of the exercise, all systems were switched to Universal Time
Coordinated (UTC) time. The UTC time readings of all measurements
are therefore the primary criterion for matching the data sets.
However, UTC time alone would not produce a proper
synchronization of the profiles for two reasons. The first is the
different time the water travels from the seawater intake (where
its temperature and salinity are being measured) to a given
equilibrator (where the equilibration temperature is determined).
This depends mainly on the individual flow rate of water and to
some extent also on the location of the equilibrator in the
supply line. By running two separate supply lines (port and
starboard side line), which were kept at roughly the same total
flow rate, we tried to make the supply flow characteristics
comparable for all systems. With a single supply line, the ratio
of water consumed for analyses to the water bypassing through
this supply line would have changed more strongly en route with
unknown implications for the water characteristics (such as the
temperature deviation). With the chosen setup (see also Fig. 3),
we tried to make the supply similar for all systems. The second,
rather trivial, reason for an insufficient synchronization of the
profiles based on UTC readings alone is that there are errors in
the UTC readings themselves, which in some cases appear to
account for 1 to 2 min during the course of the exercise. 

The final matching of the profiles is based on the assumption
that the profiles of in situ and equilibrator temperature should
be connected by a fixed daily temperature offset. This is a
first-order approximation, because the offset certainly depends
on the stability of the water flow rate and the difference
between seawater and ambient temperature. Flow rates were usually
kept constant during the course of the exercise. The change in
seawater temperature was significant, but its effect was
minimized by matching the profiles on a daily basis. The matching
procedure involved correcting every fCO2 profile with daily time
lags in 1-min steps until the standard deviation of the
difference between in situ and equilibrator temperature reached a
minimum. This could always be achieved by time lags of < or = 3 min. In
other words the fCO2 profiles were shifted minute-wise backwards
in time against the CTD readings until the two temperature
profiles showed the best match with the smallest standard
deviation of the resulting offset. 

This procedure proved very necessary. Even a mismatch of 2 min
could cause a bias in the calculated temperature difference of up
to 1C and more (i.e., > or = 10 atm) in the strong gradient regime.
In the more stable regime, toward the end of the exercise the
effect of this synchronization procedure is less pronounced or
even negligible. On the other hand, the profiles could not be
synchronized to better than 1 min, which still allows errors of
the order of several atm in some cases only because of temporal
mismatch. This is an important aspect which restricts the
interpretability of the results during passage of the very strong
gradients. 

Even after correction of all equilibrator temperature readings
and after this synchronization procedure, the remaining
uncertainty is on the order of 2 atm for the largest portion of
the cruise. To put it the other way round, any differences of < or =
2 atm between the final fCO2 profiles are not significant under
the circumstances of this exercise. During passage of the
strongest gradients, the overall uncertainty is definitely higher
than 2 atm, at least for short periods, and may account for a
mismatch of up to 5 atm.

4.  RESULTS

4.1  SURFACE TEMPERATURE AND SALINITY

As said before, the cruise track of the R/V Meteor during the
exercise (Fig. 1) was chosen in order to provide the largest
possible range of surface temperatures and salinities in the
whole area of the North Atlantic Ocean accessible during this
rather short cruise. It was assumed that this track should likely
provide more stable conditions in the eastern part as well as a
highly variably situation at the northern turning point near the
Flemish Cap off Newfoundland. This assumption was later verified
to a full extent by the encountered ranges of surface temperature
and salinity.

Figure 6 shows the large observed ranges in surface temperature
and salinity as measured during the course of the exercise:
Surface seawater temperatures ranged from 6.2C to 25.1C while
surface salinities covered a range from 32.6 to 37.0. This is
equivalent to a span of 19C in temperature as well as 4.4 in
salinity. It should be pointed out that the observed temperature
and salinity drop around 51 W was as large as 2.9C/min-1 and
0.4/min-1 for salinity, which is equivalent to 4.2C/km-1 and
0.9/km-1, respectively. Such gradients can be regarded as extreme
situations that represent a "worst case scenario" for any kind of
intercomparison rather than a typical open ocean situation.
Toward the eastern part of the cruise track, a more typical
regime was found that represents the standard case for at-sea
operation. In order to provide the hydrographic background for
the fCO2 data, measurements of surface temperature and salinity
are given as 1-min averages in daily figures in hard copy 
(Figs. 7-9) for the period June 8-16.

4.2  COMPARISON OF ATMOSPHERIC xCO2 DATA

Measurements of theatmospheric xCO2 were carried out by all
underway fCO2 systems except system "F" (see Table 2 in Sect.
3.2). As will be shown, the atmospheric xCO2 data-while not
immediate focus of this exercise-may still provide additional
information for identifying likely sources of error in the
surface fCO2 profiles. All xCO2 data are given (in ppmv) for dry
air and shown in Fig. 10 in hard copy.

Of the six data sets, four show good agreement to within 1 ppmv
throughout the exercise: Profiles "C" and "D" show virtually
identical values (except for a few data points), whereas profile
"E" tends to values that are lower by -0.5 to -1 ppmv. Profile
"B" is characterized by a somewhat variable behavior: For most of
the time, "B" is in very good agreement with "E." However, from
June 10, 08:30 UTC, to June 11, 14:30 UTC, "B" shows a positive
offset of about 1 ppmv from "E," hereby agreeing perfectly with
"C" and "D." In contrast, "B" deviates by -0.5 to -1.0 ppmv from
"E" during the period from June 14, 14:30 UTC, until June 13,
18:00 UTC, which is equivalent to an offset of -1.0 to -1.5 ppmv
with respect to profiles "C" and "D." We have calculated a mean
xCO2 (air) of 366.21  0.72 ppmv for the period of the exercise
where data from all six systems are available from the means of
profiles "B" through "E" which is shown in Fig. 10 (red line).
Only results from this restricted period are used in the
following comparison.

Two profiles ("A" and "G") are characterized by a much larger
scatter which obscures the pattern of theatmospheric CO2
contained in the other four profiles. This scatter is not a real
property of the sampled air, as proved by profiles "B" through
"E," and thus indicates an analytical problem associated with
these systems. All air intakes were located on the same spot
above the wheelhouse of R/V Meteor approximately 20 m above sea
level, thus making differences in the properties of the sampled
air very unlikely. The majority of measurements of "G" show a
positive deviation of up to 8 ppmv which is consistent with the
rather large positive offset of 3 to 6 ppmv determined during the
checks of the CO2 calibration performance (Sect. 3.4.1). The mean
of "G" (368.27 ppmv) is 2.06 ppmv higher than the combined mean
of "B" through "E" (366.21 ppmv). With a mean value of 362.89
ppmv, the xCO2 measurements of "A" are clearly marked by a
negative offset of 3.32 ppmv with respect to the mean of "B"
through "E."
Figure 11 in hard copy shows the individual mean and standard deviation of
each data set as well as an overall mean calculated from the mean
of profiles "B" through "E," all for the restricted period of
time only. The individual standard deviations reflect the
averaging interval in the case of laboratories "B" through "E," where the
smaller scatter is associated with the longer averaging intervals
of 4 to 5 min (laboratories "C" and "D") and the somewhat larger
scatter reflects averaging intervals of 1 min (laboratories "B"
and "E"). In the case of laboratories "A" and "G" the scatter is
no obvious function of the averaging interval but an expression
of an analytical problem. 

In the comparison of surface fCO2 data in Sect. 4.3, these
results, sometimes referred to as the general trends of agreement
or disagreement between the xCO2 (air) data sets, will largely be
retained in the fCO2 data. The combination of both results
provides much of the argument for the discussion of the overall
results. We will demonstrate that the three laboratories "C,"
"D," and "E" show the same high degree of agreement in surface
fCO2 data as they do in xCO2, and a strong case will be made that
these systems represent the "best" values of xCO2 (air) and fCO2.

4.3  COMPARISON OF SURFACE fCO2 DATA

As described in Sect. 3.4, the following main steps in the
calculation of final fCO2 values constitute the general procedure
that was applied identically to all underway fCO2 data sets:

- Calculation of xCO2 in dry sample air (final data product
  received from every group)

- Synchronization of daily CTD and equilibrator profiles based on
  standard deviation of the temperature offset

- Calculation of fCO2 in equilibrator (at T eq, 100% humidity)

- Correction of fCO2 to in situ seawater temperature based on
  corrected temperature readings

In order to gain better interpretability of any differences in
the final fCO2 data sets, we tried to exclude as many
controllable sources of error as possible. This was accomplished
by carefully addressing the following points:

- Temperature readings can be a significant source of error as
  shown in Sect. 3.4.2. However, on the basis of the checks of the
  temperature probes against a reference probe we were able to
  remove this error and assure consistent temperature measurements.

- The choice of the parameterizations for calculating the
  saturation water vapor pressure and for the temperature
  correction of fCO2 also introduces some kind of uncertainty,
  which, however, in our case seems to be rather small compared
  with the errors of the temperature measurements. Again, the
  common calculation procedure (Sect. 3.4.3) excludes
  inconsistencies based on the use of different equations. 

- Finally, the common infrastructure (i.e., the seawater and
  calibration gas supply) assured a physically identical background
  for all systems (Sect. 2.2).

It should be emphasized that none of these consistent conditions
are usually present in typical fCO2 measurements in the field
(i.e., temperature probes are sometimes used uncalibrated or at
least not calibrated to the same standard; calculation procedures
vary; calibration gases are of different origin and likely
quality, too; the seawater sources may be quite different or even
inadequate for gas measurements etc.).

In the interpretation of the results, any differences of >2 atm
(up to >5 atm in the highly variable regime) in the final fCO2
data can be attributed either to differences in the equilibration
process itself and/or to differences in the subsequent
measurement of CO2. A tool to separate these two possible sources
of error is measurements of the atmospheric xCO2, which were
discussed in some detail in Sect. 4.2. Unlike the calibration
gases, atmospheric air is comparable to the seawater equilibrated
air in that it has a wet sample matrix. Thus atmospheric air
undergoes the same procedure of (physical or arithmetical)
drying. If, for example, differences between seawater fCO2 data
from two systems were also present in the atmospheric xCO2 data,
this is indicative of problems associated with the infrared CO2
measurement and/or the drying procedure. If, in contrast, the
atmospheric xCO2 data turned out to be identical while seawater
fCO2 was different, the source of error must be attributed to the
equilibration process and/or the way of handling of the seawater
equilibrated air. 

Whereas these reasons add to the interpretability of the results,
it should be pointed out that any observed differences cannot per
se be attributed to a particular data set or system. As a
superior reference method was not available, the "true" fCO2
values are simply not known. Given the still remaining
uncertainty about the valid set of dissociation constants of
carbonic acid in seawater, even consistency checks based on the
other three parameters of the CO2 system in seawater (i.e., CT,
AT, pH)-although to be carried out later on-will not provide an
unambiguous means of finding "true" fCO2 values. 

However, we found three data sets (systems "C," "D," and "E") to
be very close in seawater fCO2 and atmospheric xCO2 values
throughout the cruise, whereas the other data sets show variable
offsets to these three profiles and some of them are also
associated with significantly larger scatter. Because the general
design of the three systems "C," "D," and "E" is significantly
different (showerhead equilibrator-thin film equilibrator, small
equilibrator volume-large equilibrator volume, small flow
rates-large flow rates, equilibrator vented-equilibrator not
vented, wet CO2 measurement-dry CO2 measurement, etc.), this
agreement cannot simply be attributed to an essentially identical
design. This is by no means a sufficient argument to regard the
three consistent fCO2 profiles as the "truth," although we feel
that this marked agreement is at least a strong indication of
this. However, with the lack of a superior method, this sort of
discussion is to some extent futile and cannot be solved here. 

When preparing the following figures, we wanted to discuss the
fCO2 results not only as absolute numbers but also as deviations
from a reference. Because a superior method was not available and
the choice of a single "true" fCO2 profile was not feasible, we
decided to calculate the deviation of every single fCO2 data
point from an 11-min running mean calculated from the three most
consistent profiles (labs "C," "D," and "E"). In the light of the
arguments given in the foregoing discussion, this choice remains
arbitrary, but it nevertheless seems to be the most reasonable
choice. However, it should be kept in mind that these deviations
are, of course, dependent on the choice of the reference and are
therefore not independent results. We are fully aware that this
is a somewhat critical step in the interpretation, which seems
only justified by the better visualization of the differences and
the enhanced interpretability of the data set.

A further problem associated with calculating deviations from an
11-min running mean stems from the fact that this reference
represents a strongly smoothed profile, whereas the original fCO2
data represent significantly smaller averaging intervals (minimum
1 min). Thus, all temporal variability on the minute-scale as
contained in the fCO2 data with higher temporal resolution (e.g.,
profiles "B" and "E") translates into a larger scatter than that
of the deviations from the smoothed reference profile. This
artifact has to be kept in mind, because it is of a different
magnitude for the various fCO2 data sets. This effect is more
strongly obvious in the strong gradient regime (e.g., June 10).
The main message of these deviation figures therefore has to be
the general offset rather than the scatter of a profile. 

Table 3 provides an overview of the minima, maxima, and
differences of measured in situ temperature, salinity, and fCO2
(11-min running mean from profiles "C," "D," and "E") on a daily
basis. As intended with the choice of the cruise track and as
already documented in the daily profiles of temperature and
salinity (Sect. 4.1), the encountered conditions of the surface
waters along the cruise track varied between a smooth regime with
low variability during the second half of the cruise and a strong
gradient regime with much higher variability in the area close to
the northern turning point off Newfoundland (marked in yellow)
during the first half. 

Table 3. Overview of minimum, maximum, and difference of measured
values of temperature T (C), salinity S, and the fugacity of CO2
(fCO2, 11-min running mean from profiles "C," "D," and "E"). The
strong gradient regime is shaded.

--------------------------------------------------------------------------------------
         June 8  June 9  June 10  June 11  June 12  June 13  June 14  June 15 June 16
--------------------------------------------------------------------------------------
Tmin      20.3     6.0     6.4     10.1     12.9     15.3     16.8      18.4   19.6
Tmax      25.1    24.5    16.4     16.6     16.9     17.1     19.8      20.6   21.0
del T      4.8    18.5    10.0      6.5      4.0      1.8      3.0       2.2    1.4
Smin     36.38   32.57   32.90    33.88    34.66    35.98    35.97     36.11  36.35
Smax     36.81   36.61   36.24    36.32    36.29    36.25    36.25     36.46  36.96
del S     0.43    4.04    3.34     2.44     1.63     0.27     0.28      0.35   0.61
fCO2min  315.6   270.2   264.6    281.5    276.9    303.8    306.7     332.2  338.6
fCO2max  340.7   339.2   321.0    327.9    327.3    326.4    346.4     359.3  355.7
del fCO2  25.1    69.0    56.4     46.4     50.4     22.6     39.7      27.1   17.1
--------------------------------------------------------------------------------------

4.3.1  Underway Profiles

Figures 12-20 in hard copy show the final underway fCO2 profiles "A" through
"G" as well as the discrete fCO2 data of laboratory "H" (top) and
the deviations of all fCO2 data from the 11-min running mean
calculated from profiles "C," "D," and "E" (bottom). It should be
noted that the top figures show variable scaling of the y-axis
(see 5 atm bar indicator), while the bottom figure is always at
the same scale. The latter also includes two horizontal lines,
one at +2 and one at -2 atm deviation which is about the limit
of interpretation.

4.3.2  Discussion of Profiles

Following is a brief day-by-day description of the major features
contained in Figs. 12-20. We tried to identify the most important
results and to point to some major trends and changes. Again we
would like to emphasize that the scatter of the bottom figures is
mainly an artifact of the referencing procedure. This can be
readily observed in Fig. 13 (June 9, 1996): Between 08:00 and
19:00 UTC the seawater exhibits low variability resulting in very
little scatter in the bottom figure. Immediately before and after
this period the seawater was much more variable which translates
into the high scatter of the deviation figure. The observed
offsets discussed here can therefore only be identified in the
trends and have to be regarded as rough approximations.

June 8, 1996

Missing data - "A": 04:00 to 10:00 UTC, "B": before 18:30 UTC
(start delayed because of sample gas leakage), "F": 03:00 to
10:30 UTC, "H": no samples measured. Agreement to within 2 atm
- "B," "C," "D," and "E". Positive offset - "F": 3 atm. Negative
offset - "G": 6 atm. Variable offset - "A": -3 to +3 atm.
Comment - "G" starts with a marked negative offset, which turns
slowly into a positive offset during the next days and then
disappears toward the end of the exercise. However, the large
scatter of "G" seen in theatmospheric xCO2 readings is not
visible here, which points toward problems with the handling of
atmospheric air within this system (e.g., leakage in air pump,
valves, or tubing).

June 9, 1996

Missing data - "A": 10:00 to 12:00 UTC, "C": 13:00 to 24:00 UTC,
"H": no samples measured. Agreement to within 2 atm - "B," "C,"
"D," and "E." Negative offset - "G": 6 atm. Variable offset -
"A": within  2 atm (before 07:00 UTC), -5 to 
-8 atm after 12:00 UTC; "F": within 2 atm (09:00 to 19:00
UTC), +5 to +10 atm (before 09:00 UTC and after 19:00 UTC).
Comment - "A" shows a sudden change around 12:00 UTC from good
agreement to a negative offset of the order of 5 atm. This
offset remained until the end of the exercise. The scatter of
profile "A" (3-min intervals) is significantly larger than in the
1-min averages of "B" as can be seen in the smooth period (13:00
to 17:00 UTC). This is contradictory to what one would expect and
may be related to the rather large scatter observed in the
atmospheric xCO2 readings of "A." Interestingly, the offset of
the latter showed up from the beginning of the exercise (i.e.,
before June 9, 12:00 UTC when it suddenly appeared in seawater
fCO2 readings). This is indicative of different reasons for the
offsets observed inatmospheric xCO2 and seawater fCO2 readings
of "A."

June 10, 1996

Missing data - "A": after 12:00 UTC, "C": before 13:00 UTC, "G":
12:00 to 23:00 UTC. Agreement to within 2 atm - "C," "D," and
"E." Positive offset - "B": 3 to 9 atm, "F": 4 to 10 atm.
Negative offset - "A": 5 atm, "G": 5 atm. Variable offset -
"H": within 2 atm at 13:22 and 21:09 UTC, +7 atm at 06:12 UTC.
Comments - "B" immediately started to develop a positive offset
which more or less remained until about 18:00 UTC of the
following day. This offset is not seen inatmospheric xCO2
measurements of "B." "F" also lost its good agreement and started
to develop a positive offset which stabilized toward the end of
the exercise. Interestingly these offsets of "B" and "F" show up
at about the same time and with a very similar pattern over the
2-day period. Furthermore, the 06:12 UTC data point of "H" has
the same positive offset as "B" and "F." Whether this is pure
coincidence or an expression of something real is not known.
These three systems, however, are very different in their
principle of measurement and the location in the seawater supply
line so that a common systematic error can be ruled out. Also
their common offset seems to be inversely correlated with
seawater temperature (see Fig. 7). On the basis of this
observation it also has to be questioned whether a systematic
offset may be present in the "reference" profiles "C," "D," and
"E." This puzzle, however, cannot be solved here. 

June 11, 1996

Missing data - "A": before 05:30 UTC, "B": 17:00 to 19:00 UTC,
"C": 12:00 to 16:00 UTC, "F": 20:30 to 23:30 UTC. Agreement to
within 2 atm - "C," "D," "E," and "H." Positive offset - "F": 2
to 8 atm, "G": 2 to 9 atm. Negative offset - "A" 3 to 8 atm.
Variable offset - "B": within 2 atm (after 18:00 UTC), +3 to +9
atm (00:00 to 17:00 UTC). Comment - Positive offsets of "B,"
"F," and "G" are essentially parallel throughout the day (see
also comment of the previous day). Between 12:00 and 19:00 UTC
the positive offset of "B" slowly disappears while at the same
time a negative offset in theatmospheric xCO2 measurements of
"B" develops.

June 12, 1996

Missing data - "C": before 16:00 UTC. Agreement to within 2 atm
- "C," "D," and "E." Positive offset - "F": 3 to 9 atm. Negative
offset - "A": 3 to 6 atm. Variable offset - "B": 0 to +3 atm,
"G" -2 to +8 atm, "H": +1 to -6 atm. Comments - The top figure
shows nice parallel patterns of all fCO2 profiles even in this
strongly variable environment. The bottom figure heavily suffers
from the artificial scatter but nevertheless reveals the general
offsets and trends. The negative offset of "H" at 17:29 UTC is
likely due to this artifact because "H" is in very good agreement
with "B" which does not show a general offset here. 

June 13, 1996

Missing data - "A": after 18:00 UTC, "B": after 22:00 UTC, "F":
16:00 to 20:00 UTC, "G": 03:00 to 05:00 UTC. Agreement to within
2 atm - "B," "C," "D," "E," and "G." Positive offset - "F": 6
to 9 atm. Negative offset - "A": 5 to 6 atm. Variable offset -
"H": -2 and -5 atm. Comment - This is about the beginning of the
"smooth regime" with comparatively low variability in surface
water which persisted for the rest of the exercise. The kind of
agreement seen in this figure continues to exist in the following
figures with very little alteration. In contrast to the highly
variable situation encountered earlier this cruise, this
situation is probably more representative of typical oceanic
conditions in underway fCO2 field work. Again, the negative
offset of "H" at 00:31 UTC is likely an artifact as it follows
the profile of "B" which itself is in good agreement with "C,"
"D," and "E." System "A" had to quit the exercise at about 18:00
UTC because of a technical problem associated with the NDIR
instrument, and no more data from this system are available
beyond this point.

June 14, 1996

Missing data - "A": no data available, "B": 17:00 to 22:00 UTC,
"G": before 09:00 UTC. Agreement to within 2 atm - "B," "C,"
"D," "E," "G," and "H." Positive offset - "F": 7 to 10 atm.
Comment - Whereas the general agreement of all profiles except
"F" is rather good, even among them slight trends toward positive
("H") or negative ("B" and "G") offsets can be identified that
persist for the rest of the exercise. 

June 15, 1996

Missing data - "A": no data available. Agreement to within 2
atm - "C," "D," and "E." Positive offset - "F": 5 to 10 atm.
Negative offset - "B": 3 atm, "G": 4 atm. Variable offset -
"H": +1.5 to +4.5 atm. Comment - See comment for previous day. 

June 16, 1996

Missing data - "A": no data available, "F": 17:00 to 21:00 UTC.
Agreement to within 2 atm - "C," "D," "E," and "H." Positive
offset - "F": 5 to 8 atm. Negative offset - "B": 2 to 3 atm,
"G": 2 to 4 atm. Comment - As the hydrographic conditions have
become much less variable, the overall picture of agreement among
the various systems is very consistent for the last three days of
the exercise.

Overview

The overall picture of agreement is characterized by a very good
agreement of profiles "C," "D," and "E" essentially throughout
the cruise. While also in good agreement for most of the time,
profile "B" shows a 2-day period with a marked positive offset.
Two profiles show a more or less constant sign of deviation,
which is positive in the case of "F" and negative in the case of
"A." The reason for this could not be identified easily. However,
for system "A" we know of an instance of severe damage in the
NDIR instrument toward the end of the exercise, which may well
have started biasing the measurements in an early stage of the
exercise. With respect to system "F," which is of a principally
different design (see Sect. 3.2, Table 2), the question of
whether the different principle of measurement could be the
reason for the rather large observed offset should be addressed
carefully. Finally, system "G" shows anything from large negative
offsets over periods of good agreement to rather strong positive
offsets. These problems were also apparent inatmospheric xCO2
measurements and checks of the CO2 calibration performance
probably because of an improper calibration technique. The
calibration of the system appears to lack-at least during this
exercise-the necessary reproducibility (i.e., it may be good in
one case and bad in another one). This obvious problem of system
"G" also needs careful checks.

In addition to the daily figures (Figs. 12 to 20) representing
the full data set, we present three figures in hard copy (Figs. 21 to 23) 
with enlarged views of shorter periods. These were chosen because they
reveal more detail than is available in the daily figures.
Furthermore they also cover the whole range of situations, from
smooth to highly variable.

Figure 21 shows a 3-hour period of measurements on June 9 that
was characterized by very low variability in the surface seawater
fCO2 (Fig. 13) as well as temperature and salinity (Fig. 7). The
total change in fCO2 values during this period of time is about 6
atm. This is uniformly seen in all profiles, which are almost
perfectly parallel. Profiles "B," "D," "E," and "F" agree to
within 1 atm, while profiles "A" and "G" are characterized by a
negative offset of about 8 atm and 5 atm, respectively. The
scatter is smallest in profile "D" (averaging interval 5 min;
large time constant, as shown) and highest in profile "A"
(averaging interval 3 min; short time constant). The
comparatively small scatter in profile "B" with 1-min averaging
intervals shows that much of the scatter in profile "A" (also
seen in theatmospheric xCO2 data of "A") is not real and may
thus indicate again the existence of a technical problem. 

In contrast, Fig. 22 shows the much more variable situation of a
3.5-hour period of measurements during June 12. The total range
of fCO2 values covered during this period is about 35 atm with
gradients of up to 3 atm/min-1. Again, the agreement is good in
profiles "B," "D," and "E." Profiles "F" and "G" show positive
offsets, while profile "A" has a negative offset of a few atm.

Individual time constants involved in the equilibration process
going on in every system can be estimated rather precisely with
step experiments carried out under well-defined conditions in a
shore-based laboratory (Copin-Montegut 1988; Koertzinger et al.
1996b). This is definitely not the case in the present
intercomparison exercise. We therefore do not try make any
estimates of individual time constants. Nevertheless, in addition
to the examination of offsets we do try to gain insight into the
apparent time constants (i.e., we want to see whether there is
any indication of differences in kinetic aspects of the
equilibration processes). Because most time constants are on the
order of a few minutes, this analysis is only feasible where fCO2
was measured at rather short intervals of (5 min (only profiles
"A," "B," "C," and "D"), but even in these cases this is not a
sound approach. 

We have marked approximate relative minima and maxima observed in
the enlarged periods shown in Figs. 22 and 23. The pattern of
vertical lines observed in these groups is highly consistent:
Extrema always occur first and simultaneously in profiles "A" and
"B," while profiles "C" and "D" lag behind by 5 to 8 min and 2 to
5 min, respectively. The range is mainly a consequence of the
different averaging intervals. These time lags cannot be
attributed to a temporal mismatch of the profiles (see Sect.
3.4.4). They are, however, clearly related to differences in the
general design of these systems. Systems "A" and "B" are similar
with respect to volumes and flow rates of water and air. For
example, the total air volume of the equilibrator is exchanged
every 2.5 min and 0.5 min, respectively, hence the similar
equilibration times. In system "C" the large volume of air in the
equilibrator is only exchanged every 20 min, which explains the
more sluggish response seen in Fig. 23. System "D" is of the thin
film type (i.e., unlike in the other system no turbulent mixing
occurs in the equilibrator). It is known that this equilibration
concept is characterized by somewhat larger time constants.

We would like to point out that different time constants are no
quality criterion per se but rather must be seen in the context
of the application. A detailed process study would certainly
require high spatial and temporal resolution and hence an fCO2
system with rather small time constants to resolve small-scale
features. This is not equally the case in a basin-wide assessment
of the fCO2 in surface seawater, where the large-scale averaging
would eliminate the effect of different time constants. The main
point here is simply to show that these different characteristics
are clearly reflected in the fCO2 data set.

5.  CONCLUSIONS

We have provided a common infrastructure to all participating
groups in this exercise. We also have carried out several checks
to exclude possible sources of error. Furthermore all raw data
were run through the same calculation procedure. All these
measures were taken in order to reduce as much as possible
controllable sources of error. In this respect the exercise was
technically a full success, and as summarized in the following
discussion we also think that the exercise was a success
scientifically.

We have demonstrated that the results of three out of seven
underway systems agree to within about 2 atm throughout the
cruise. This is not only the case for underway seawater fCO2
measurements but also for measurements ofatmospheric xCO2.
Interestingly, these three systems represent differences in such
aspects as the design principle of the equilibrator, the volumes
and flow rates of water and air involved, and the choice of wet
or dry NDIR measurements. Thus this perfect agreement shows
that-at least for NDIR instruments-the variety of designs used in
the scientific community does not necessarily give rise to
comparability problems or, to put it the other way round, systems
of different design can produce reliable and consistent results.

We have also demonstrated that significant offsets of up to 10
atm can be found in underway fCO2 measurements under typical and
identical field work conditions. Although in at least one case
this may be a consequence of a technical failure, it is an
indication of significant systematic differences in other cases.
We certainly cannot claim that the observed differences are
representative for these fCO2 systems in general. They may also
be typical only for the specific conditions of this particular
cruise. There is, however, no indication that this cruise
provided in any way untypical circumstances that could be made
responsible for some of the observed deviations. 

Finally we were able to demonstrate that discrete fCO2
measurements agree with the results of the three most consistent
underway fCO2 systems. Therefore, measurements with these quite
different approaches can be made with sufficient consistency, and
the horizontal and vertical fCO2 profiles generated from these
different techniques can be expected to match in surface waters.
In conclusion, therefore, three main messages can be derived from
this exercise:

- Underway measurements of the CO2 fugacity in surface seawater
  and overlying air can be done to a high degree of agreement 
  (1 atm) with a variety of possible equilibrator and system designs.

- Even well-designed systems, which are operated without any
  obvious sign of malfunction, can show significant differences of
  up to 10 atm.

- The discrete fCO2 measurements are in good agreement with the
  three most consistent underway fCO2 data sets, at least to within
  its nominal accuracy of 1%. 

These results pose the important question of how fCO2 data sets
acquired from different groups can be combined into a common
database in light of possible incompatibilities of up to 10 atm.
Although the results of this exercise do not solve this problem,
they underline the importance of this aspect which must be taken
into account in the construction of a consistent global fCO2
database. Contributing to this dilemma is the fact that, in
contrast to this exercise, other sources of error (temperature
and pressure measurements, calibration gases etc.) further
contribute to this uncertainty in field data.

In addition to this more general outcome, some of the results in
more concrete terms follow. These may also serve as
recommendations for future fCO2 work in the ocean.

- The exercise shows no "best choice" for the type of the
  equilibrator (i.e., "showerhead," "bubbler," or "thin film") nor
  specifics on its dimensions and flow rates of seawater and air in
  regard to the achievable accuracy of the fCO2 system.

- In contrast, the equilibrator type and its flow rates of
  seawater and air are important aspects with respect to the time
  constant of the equilibration process.

- Wet measurements can be done on the basis of the LI-6262
  CO2/H2O gas analyzer (LI-COR Inc., U.S.A.) without necessary loss
  of accuracy when compared with traditional dry measurements.

- The factory calibration of the LI-COR LI-6262 CO2/H2O gas
  analyzer, which only requires the user to adjust "zero" and
  "span" of the instrument, seems to result in a loss of accuracy,
  which can easily be avoided by establishing an individual
  calibration curve on the basis of measurements of standard gases. 

- The importance of rather accurate measurements of in situ and
  equilibrator temperature does not seem to be addressed adequately
  in the community. The observed differences between temperature
  measurements are clearly above a tolerable level and
  contribute-if representative and usually left unaccounted
  for-inconsistencies of several atm (up to about 7 atm in the
  present exercise).

- Calibration gases are an important issue. Even with the
  provided suite of consistent calibration gases, the NDIR
  analyzers could only be calibrated to an accuracy of 0.5 to 1.0
  ppmv. We feel that this is about the tolerable limit. So any
  further error contribution from the calibrated standard
  concentrations worsens the situation. Use of calibration gases
  that are traceable to the same primary standards, such as the WMO
  primary standards maintained at SIO, would be desirable.

6.  DATA CHECKS AND PROCESSING PERFORMED BY CDIAC

An important part of the numeric data package (NDP) process at
the Carbon Dioxide Information Analysis Center (CDIAC) involves
the quality assurance (QA) of data before distribution. Data
received at CDIAC are rarely in a condition that would permit
immediate distribution, regardless of the source. To guarantee
data of the highest possible quality, CDIAC conducts extensive QA
reviews that involve examining the data for completeness,
reasonableness, and accuracy. Although they have common
objectives, these reviews are tailored to each data set and often
require extensive programming efforts. In short, the QA process
is a critical component in the value-added concept of supplying
accurate, usable data for researchers. 

The following information summarizes the data-processing and QA
checks performed by CDIAC on the underway data obtained during
the R/V Meteor Cruise 36/1 in the North Atlantic Ocean.

1. All data files were provided to CDIAC as 10 comma-separated
   files (9 for surface seawater underway measurements of fCO2 and 1
   for all marine air xCO2 measurements) by Dr. Arne Koertzinger of
   IfMK. A FORTRAN 77 retrieval program was written and used to
   reformat the original files into uniform ASCII-formatted "water"
   and "air" data files.

2. All underway "water" data are presented as 9 daily data
   files. These files can be merged into a single data file by
   request.

3. All data were plotted to check for obvious outliers.

4. Dates and times were checked for bogus values (e.g., values
   of MONTH < or >6, DAY <8 or >16, YEAR < or >1996, TIME <0000 or
   >2400.

5. The cruise track was plotted using the coordinates presented
   in data files and compared with the maps and cruise information
   supplied by A. Koertzinger.

 7.  HOW TO OBTAIN THE DATA AND DOCUMENTATION

This database (NDP-067) is available free of charge from CDIAC.
The data are available from CDIAC's anonymous file transfer
protocol (FTP) area via the Internet. Please note: your computer
needs to have FTP software loaded on it (this is built into most
newer operating systems). Commands used to obtain the database
are 

     >ftp cdiac.esd.ornl.gov or >ftp 128.219.24.36
     Login: "anonymous" or "ftp"
     Password: YOU@your internet address
     Guest login ok, access restrictions apply.
     ftp> cd pub/ndp067/
     ftp> dir
     ftp> mget (files)
     ftp> quit

The complete documentation and data may also be obtained from
CDIAC's Web site at the following URL:
http://cdiac.esd.ornl.gov/oceans/doc.html

For non-FTP data acquisitions (e.g., floppy diskette, 8-mm tape,
CD-ROM, etc.), users may order through CDIAC's online ordering
system (http://cdiac.esd.ornl.gov/pns/ how_order.html) or contact
CDIAC directly to request the data and choice of media.
For additional information, contact CDIAC.

Address:  Carbon Dioxide Information Analysis Center
          Oak Ridge National Laboratory
          P.O. Box 2008
          Oak Ridge, Tennessee 37831-6335
          U.S.A.

Telephone:     (423) 574-3645 (Voice)
          (423) 574-2232 (Fax)

Electronic mail: cdiac@ornl.gov

URL: http://cdiac.esd.ornl.gov/

8.  REFERENCES

Copin-Montegut, C. 1988. A method for the continuous
determination of the partial pressure of carbon dioxide in the
upper ocean. Mar. Chem. 17:13-21.

Copin-Montegut, C. 1989. A new formula for the effect of
temperature on the partial pressure of CO2 in seawater.
Corrigendum. Mar. Chem. 27:143-44.

Gordon, L. I., and L. B. Jones. 1973. The effect of temperature
on carbon dioxide partial pressure in seawater. Mar. Chem.
1:317-22.

Goyet, C., F. J. Millero, A. Poisson, and D. K. Shafer. 1993.
Temperature dependence of CO2 fugacity in seawater. Mar. Chem.
44:205-19.

Goyet, C., and E. Peltzer. 1994. Comparison of the
August-September 1991 and 1979 surface partical pressure of CO2
distribution in the Equatorial Pacific Ocean near 150  W. Mar.
Chem. 45:257-66.

Guggenheim, E. A. 1967. pp. 175-77 In: Thermodynamics,
North-Holland, Amsterdam.

Johnson, K. M., A. Koertzinger, L. Mintrop, J. C. Duinker, and D.
W. R. Wallace. 1998. Coulometric total carbon dioxide analysis
for marine studies: Measurement and internal consistency of
underway surface TCO2 concentrations. Mar. Chem. (in press).

Keeling, C. D., P. R. Guenther, and D. J. Moss. 1986. Scripps
Reference Gas Calibration System for Carbon Dioxide-in-Air
Standards: Revision of 1985. WMO Environmental Pollution
Monitoring and Research Programme No. 42, Technical Document
WMO/TD-125, World Meteorological Organization, Geneva,
Switzerland.

Koertzinger, A., L. Mintrop, and J. C. Duinker. 1996a. The
floating dripstone cave: CO2 teams compare underway systems. U.S.
JGOFS News 7(4):14-15.

Koertzinger, A., H. Thomas, B. Schneider, N. Gronau, L. Mintrop,
and J. C. Duinker. 1996b. At-sea intercomparison of two newly
designed underway pCO2 systems: Encouraging results. Mar. Chem.
52:133-45.

McDermitt, D. K., J. M. Welles, and R. D. Eckles. 1993. Effects
of Temperature, Pressure, and Water Vapor on Gas Phase Infrared
Absorption by CO2. Li-Cor Inc., Lincoln, Nebraska, U.S.A.

Neill, C., K. M. Johnson, E. Lewis, and D. W. R. Wallace. 1997.
Accurate headspace analysis of fCO2 in discrete water samples
using batch equilibration. Limnol. Oceanogr. 42:1774-83.

Ohtaki, E., E. Yamashita, and F. Fujiwara. 1993. Carbon dioxide
in surface seawaters of the Seto Inland Sea, Japan. J. Oceanogr.
49:295-304.

Poisson, A., N. Metzl, C. Brunet, B. Schauer, B. Bres, D.
Ruiz-Pino, and F. Louanchi. 1995. Variability of sources and
sinks of CO2 in the Western Indian and Southern Oceans during the
year 1991. J. Geophys. Res. 98(C12):22759-778.

Robertson, J. E., and A. J. Watson. 1992. Thermal skin effect of
the surface ocean and its implications for CO2 uptake. Nature
358:738-40.

Schluessel, P., W. J. Emery, H. Grassl, and T. Mammen. 1990. On
the bulk-skin temperature difference and its impact on satellite
remote sensing of sea surface temperature. J. Geophys. Res.
95:13341-356.

Takahashi, T., J. Olafsson, J. G. Goddard, D. W. Chipman, and S.
C. Sutherland. 1993. Seasonal variations of CO2 and nutrient
salts in the high latitude oceans: A comparative study. Global
Biogeochem. Cycles 
7:843-848.

Thoning, K. W., P. Tans, T. J. Conway, and L. S. Waterman. 1987.
NOAA/GMCC calibrations of CO2-in-air reference gases: 1979-1985,
NOAA Technical Memorandum ERL/ARL-150 Environmental Research
Laboratory, Boulder, Colorado.

Welles, J. M., and R. D. Eckles. 1991. Li-Cor 6262 CO2/H2O
Analyzer Operating and Service Manual. Publication No. 9003-59.
Li-COR Inc., Lincoln, Nebraska, U.S.A.

Weiss, R. F. 1974. Carbon dioxide in water and seawater: The
solubility of a non-ideal gas. Mar. Chem. 2:203-15.

Weiss, R. F., and B. A. Price. 1980. Nitrous oxide solubility in
water and seawater. Mar. Chem. 8:347-59.

Weiss, R. F., R. A. Jahnke, and C. D. Keeling. 1982. Seasonal
effects of temperature and salinity on the partial pressure of
CO2 in seawater. Nature 300:511-13.

Zhao, C. L., P. P. Tans, and K. W. Thoning. 1997. A high
precision manometric system for absolute calibrations of CO2 in
dry air. J. Geophys. Res. 102:5885-94.

PART 2

CONTENT AND FORMAT OF DATA FILES

9.  FILE DESCRIPTIONS

This section describes the content and format of each of the 13
files that make up this NDP (see Table 4). Because CDIAC
distributes the data set in several ways (e.g., via anonymous FTP
and on floppy diskette), each of the 13 files is referenced by
both a file number and an ASCII file name, which is given in
lowercase, bold faced type (e.g., ndp067.doc). The remainder of
this section describes (or lists, where appropriate) the contents
of each file.

Table 4. Content, size, and format of data files

--------------------------------------------------------------------------------
File number, name, and description    |   Logical records  | File size in bytes
--------------------------------------------------------------------------------
1. ndp067.txt:                               3,059                 122,515
A detailed description of the cruise 
network, the two FORTRAN 77
data-retrieval routines, and the ten 
oceanographic data files

2. fco2wat.for:                                 54                   2,005
A FORTRAN 77 data-retrieval routine 
to read and print all *w.txt
files (Files 4-12)

3. xco2air.for:                                 42                   1,264
A FORTRAN 77 data-retrieval routine to 
read and print xco2air.txt (File 13)

4. 080696w.txt:                              1,444                 218,336
Underway measurements of surface seawater 
fCO2 and hydrographic parameters during 
8 June 1996

5. 090696w.txt:                              1,442                 218,032
Underway measurements of surface seawater 
fCO2 and hydrographic parameters during 
9 June 1996

6. 100696w.txt:                              1,442                 218,032
Underway measurements of surface seawater 
fCO2 and hydrographic parameters during 
10 June 1996

7. 110696w.txt:                              1,442                 218,183
Underway measurements of surface seawater 
fCO2 and hydrographic parameters during 
11 June 1996

8. 120696w.txt:                              1,442                 218,032
Underway measurements of surface seawater 
fCO2 and hydrographic parameters during 
12 June 1996

9. 130696w.txt:                              1,442                 218,032
Underway measurements of surface seawater 
fCO2 and hydrographic parameters during 
13 June 1996

10. 140696w.txt:                             1,443                 218,184
Underway measurements of surface seawater 
fCO2 and hydrographic parameters during 
14 June 1996

11. 150696w.txt:                              1,443                 218,184
Underway measurements of surface seawater 
fCO2 and hydrographic parameters during 
15 June 1996

12. 160696w.txt:                              1,444                 218,336
Underway measurements of surface seawater 
fCO2 and hydrographic parameters during 
16 June 1996

13. xco2air.txt:                              1,744                 104,604
Underway measurements of air xCO2 during 
the entire expedition
----------------------------------------------------------------------------
Total                                        17,883               2,193,739
----------------------------------------------------------------------------

9.1  ndp067.txt (FILE 1)

This file contains a detailed description of the data set, the
two FORTRAN 77 data-retrieval routines, and the ten oceanographic
data files. It exists primarily for the benefit of individuals
who acquire this database as machine-readable data files from
CDIAC.

9.2  fco2wat.for (FILE 2)

This file contains a FORTRAN 77 data-retrieval routine to read
and print all *w.txt files. The following is a listing of this
program. For additional information regarding variable
definitions, variable lengths, variable types, units, and codes,
please see the description for *w.txt files in Sect. 9.4.

c****************************************************************
c* FORTRAN 77 data retrieval routine to read and print the files* 
c* named "*w.txt" (Files 4-12)                   *
c****************************************************************

c*Defines variables*

    CHARACTER date*10, time*8
    INTEGER course
    REAL latdcm, londcm, speed, temp, salt, press, fco2a, fco2b
    REAL fco2c, fco2d, fco2e, fco2f, fco2g, fco2h
    OPEN (unit=1, file='input.txt')
    OPEN (unit=2, file='output.txt')
    write (2, 5)
    
c*Writes out column labels*
   
 5   format (5X,'DATE',9X,'TIME',5X,'LATITUDE',3X,'LONGITUDE',
   1 3X,'SPEED',1X,'COURSE',4X,'TEMP',2X,'SALNTY',4X,
   2 'PRESS',4X,8('fCO2',4X),/,5X,'GMT',10X,'GMT',8X,
   3 'DEG N',6X,'DEG E',7X,'KN',3X,'DEG',5X,'DEG C',
   4 2X,'PSS-78',5X,'hPA',5X,8('atm',4X),/,
   5 2X,'DD.MM.YYYY',4X,'HH:MM:SS',66X,'LAB A',3X,
   6 'LAB B',3X,'LAB C',3X,'LAB D',3X,'LAB E',3X,
   7 'LAB F',3X,'LAB G',3X,'LAB H')
   
c*Sets up a loop to read and format all the data in the file*

     read (1, 6)
 6   format (////////////)
   
 7   CONTINUE  
     read (1, 10, end=999) date, time, latdcm, londcm, speed,
   1 course, temp, salt, press, fco2a, fco2b, fco2c, fco2d, 
   2 fco2e, fco2f, fco2g, fco2h
   
 10  format (2X, A10, 4X, A8, 4X, F7.4, 4X, F8.4, 4X, F4.1, 
   1 2X, I3, 5X, F5.2, 3X, F5.2, 3X, F6.1, 1X, F7.2, 1X, F7.2,
   2 1X, F7.2, 1X, F7.2, 1X, F7.2, 1X, F7.2, 1X, F7.2, 1X, F7.2)
   
     write (2, 20) date, time, latdcm, londcm, speed,
   1 course, temp, salt, press, fco2a, fco2b, fco2c, fco2d, 
   2 fco2e, fco2f, fco2g, fco2h
   
 20  format (2X, A10, 4X, A8, 4X, F7.4, 4X, F8.4, 4X, F4.1, 
   1 2X, I3, 5X, F5.2, 3X, F5.2, 3X, F6.1, 1X, F7.2, 1X, F7.2,
   2 1X, F7.2, 1X, F7.2, 1X, F7.2, 1X, F7.2, 1X, F7.2, 1X, F7.2)

     GOTO 7
999  close(unit=1)  
     close(unit=2)
     stop
     end

9.3  xco2air.for (FILE 3)

This file contains a FORTRAN 77 data-retrieval routine to read
and print xco2air.txt file. The following is a listing of this
program. For additional information regarding variable
definitions, variable lengths, variable types, units, and codes,
please see the description for xco2air.txt file in Sect. 9.5.

c****************************************************************
c* FORTRAN 77 data retrieval routine to read and print the files*
c* named "xco2air.txt" (File 13)                  *
c****************************************************************

c*Defines variables*

    CHARACTER date*10, time*8, lab*1
    REAL latdcm, londcm, xco2
    OPEN (unit=1, file='xco2air.txt')
    OPEN (unit=2, file='xco2air.dat')
    write (2, 5)
    
c*Writes out column labels*
   
 5   format (5X,'DATE',9X,'TIME',4X,'LATITUDE',2X,'LONGITUDE',
   1 2X,'xCO2_AIR',1X,'LAB',/,5X,'GMT',10X,'GMT',7X,
   3 'DEG N',6X,'DEG E',5X,'PPMV',4X,'NO',/,
   5 2X,'DD.MM.YYYY',4X,'HH:MM:SS')
   
c*Sets up a loop to read and format all the data in the file*

     read (1, 6)
 6   format (////////////)
   
 7   CONTINUE  
     read (1, 10, end=999) date, time, latdcm, londcm, xco2, lab
   
 10  format (2X, A10, 4X, A8, 4X, F6.3, 4X, F7.3, 4X, F6.2, 
   1 3X, A1)
   
     write (2, 20) date, time, latdcm, londcm, xco2, lab
   
 20  format (2X, A10, 4X, A8, 4X, F6.3, 4X, F7.3, 4X, F6.2, 
   1 3X, A1)

     GOTO 7
999  close(unit=1)  
     close(unit=2)
     stop
     end

9.4  *w.txt (FILES 4-12)

These 9 data files contain the underway measurements of surface
seawater fCO2 and hydrographic parameters made by participants in
the systems intercomparison exercise during the R/V Meteor Cruise
36/1 in the North Atlantic Ocean. All files have the same ASCII
format and can be read by using the following FORTRAN 77 code
[contained in fco2wat.for (File 2)]:

    
    CHARACTER date*10, time*8
    INTEGER course
    REAL latdcm, londcm, speed, temp, salt, press, fco2a, fco2b
    REAL fco2c, fco2d, fco2e, fco2f, fco2g, fco2h

    read (1, 10, end=999) date, time, latdcm, londcm, speed,
   1 course, temp, salt, press, fco2a, fco2b, fco2c, fco2d, 
   2 fco2e, fco2f, fco2g, fco2h
   
 10  format (2X, A10, 4X, A8, 4X, F7.4, 4X, F8.4, 4X, F4.1, 
   1 2X, I3, 5X, F5.2, 3X, F5.2, 3X, F6.1, 1X, F7.2, 1X, F7.2,
   2 1X, F7.2, 1X, F7.2, 1X, F7.2, 1X, F7.2, 1X, F7.2, 1X, F7.2)

Stated in tabular form, the contents include the following:
     
----------------------------------------------
Variable  Variable  Variable  Starting  Ending
             type    width     column   column
----------------------------------------------   
date     Character    10         3        12
time     Character     8        17        24
latdcm    Numeric      7        29        35
londcm    Numeric      8        40        47
speed     Numeric      4        52        55
course    Numeric      3        58        60
temp      Numeric      5        66        70
salt      Numeric      5        74        78
press     Numeric      6        82        87
fco2a     Numeric      7        89        95
fco2b     Numeric      7        97       103
fco2c     Numeric      7       105       111
fco2d     Numeric      7       113       119
fco2e     Numeric      7       121       127
fco2f     Numeric      7       129       135
fco2g     Numeric      7       137       143
fco2h     Numeric      7       145       151
---------------------------------------------     

The variables are defined as follows:

date      is the sampling date (day/month/year);

time      is the sampling time [Greenwich mean time (GMT)];

latdcm    is the latitude of the sampling location (decimal
          degrees; negative values indicate the Southern Hemisphere);

londcm    is the longitude of the sampling location (decimal
          degrees; negative values indicate the Western Hemisphere);

speed     is the speed of the ship during the measurements (kn);

course    is the course of the ship during the measurements
          (degrees);

temp      is the sea-surface temperature ((C);

salt      is the sea-surface salinity [on the Practical Salinity Scale
          (PSS)];

press     is theatmospheric pressure (hPA);

fco2a     is the underway fugacity of CO2 in surface seawater
          (atm) measured by the laboratory A;

fco2b     is the underway fugacity of CO2 in surface seawater
          (atm) measured by the laboratory B;

fco2c     is the underway fugacity of CO2 in surface seawater
          (atm) measured by the laboratory C;

fco2d     is the underway fugacity of CO2 in surface seawater
          (atm) measured by the laboratory D;

fco2e     is the underway fugacity of CO2 in surface seawater
          (atm) measured by the laboratory E;

fco2f     is the underway fugacity of CO2 in surface seawater
          (atm) measured by the laboratory F;

fco2g     is the underway fugacity of CO2 in surface seawater
          (atm) measured by the laboratory G;

fco2h     is the underway fugacity of CO2 in surface seawater
          (atm) measured by the laboratory H.

9.5  xco2air.txt (FILE 13) 

This data file contains the underway measurements ofatmospheric
xCO2 made during R/V Meteor Cruise 36/1 in the North Atlantic
Ocean. The data are presented in ASCII format and can be read by
using the following FORTRAN 77 code [contained in xco2air.for
(File 3)] in Sect. 9.3:

     CHARACTER date*10, time*8, lab*1
     REAL latdcm, longdcm, xco2

     read (1, 10, end=999) date, time, latdcm, londcm, xco2, lab
   
 10  format (2X, A10, 4X, A8, 4X, F6.3, 4X, F7.3, 4X, F6.2, 
   1 3X, A1)

Stated in tabular form, the contents include the following:
     
-----------------------------------------------
Variable  Variable  Variable  Starting   Ending
            type     width     column    column
-----------------------------------------------     
date     Character    10          3        12
time     Character     8         17        24
latdcm    Numeric      6         29        34
londcm    Numeric      7         39        45
xco2      Numeric      6         50        55
lab      Character     1         59        59
-----------------------------------------------     

The variables are defined as follows:

date      is the sampling date (day/month/year);

time      is the sampling time (GMT);

latdcm    is the latitude of the sampling location (decimal
          degrees; negative values indicate the Southern Hemisphere);

londcm    is the longitude of the sampling location (decimal
          degrees; negative values indicate the Western Hemisphere);

xco2      is the mole fraction ofatmospheric CO2 (ppmv) measured in
          dry air;

lab       is the laboratory identifier.