Xue Long Master Readme
The information presented in this file is applicable to all the data sets
collected on the Xue Long. Any changes in this information and deviation
from procedures are noted in the readme files for the individual
expeditions.
Class of Data: Surface ocean carbon dioxide concentrations
Statement of how to cite dataset:
These data are made available to the public and the scientific community in
the belief that their wide dissemination will lead to greater understanding
and new insights. The availability of these data does not constitute
publication of the data. We rely on the ethics and integrity of the user to
insure that the Third Institute of Oceanography, Xiamen, China; the University
of Georgia; and the AOML ocean carbon group receives fair credit for its work.
Please consult with us prior to use so we can insure that the quality and
limitations of the data are accurately represented.
Cruise Information:
The Ocean Carbon Group at NOAA's Atlantic Oceanographic and Meteorological
Laboratory (AOML) in cooperation with the University of Georgia (UGa), the
Third Institute of Oceanography (TIO) and the Polar Research Institute of
China (PRIC) installed an instrument to measure CO2 levels in surface water and air
on the Chinese Icebreaker, Xue Long. The Xue Long visits Antarctica each year
during the austral summer and regularly visits the Arctic Ocean. This
cooperative effort presents a rare opportunity to collect CO2 data at high
latitudes and along the long transects from China to the polar regions.
The ship underwent a major reconfiguration in 2007. Kevin Sullivan
(AOML) and Yongchen Wang (UGa) arrived at Shanghai in October 2007 to
install the major components of the CO2 instrument with the assistance of
Yuanhui Zhang (TIO). The final water connections were done during the
shakedown cruise in early November by Yongchen Wang and Yuanhui Zhang.
In July 2008, a faulty sample distribution valve was replaced by Denis
Pierrot (AOML) while the Xue Long visited Nome, Alaska.
Scientists responsible for the operations of this pCO2 system on the
Xue Long:
Professor Chen Liqi
Yuanhui Zhang
Hongmei Lin
Key Laboratory of Global Change and Marine-Atmospheric Chemistry (GCMAC)
Third Institute of Oceanography (TIO), SOA
178 Daxuelu
Xiamen 361005, P.R. CHINA
lqhen@soa.gov.cn
Scientists responsible for the reduction of this pCO2 dataset:
Rik Wanninkhof and Denis Pierrot
NOAA/AOML/Ocean Chemistry Division
4301 Rickenbacker Causeway
Miami, FL 33149
Rik.Wanninkhof@noaa.gov
Denis.Pierrot@noaa.gov
Contact persons for this dataset:
Kevin Sullivan and Denis Pierrot
NOAA/AOML/Ocean Chemistry Division
4301 Rickenbacker Causeway
Miami, FL 33149
Kevin.Sullivan@noaa.gov
Denis.Pierrot@noaa.gov
Yuanhui Zhang
Hongmei Lin
Key Laboratory of Global Change and Marine-Atmospheric Chemistry (GCMAC)
Third Institute of Oceanography (TIO), SOA
178 Daxuelu
Xiamen 361005, P.R. CHINA
INSTRUMENT DESCRIPTION and CONFIGURATION:
The general principle of operation of the instrument can be found in
Wanninkhof and Thoning (1993), Ho et al. (1995), Feely et al. (1998), and
Pierrot et al. (2009). Seawater flows through an equilibrator chamber where
CO2 exchanges between water and the air above it. An overflow pipe that is
loosely covered with an inverted cup maintains the continuously-flushed pool
of seawater (~750 ml). Small changes in seawater CO2 concentration are
rapidly translated into changes in CO2 concentration in the air of the
chamber (~850 ml of enclosed headspace). The mole fraction of CO2 in the
headspace gas is measured using a non-dispersive infrared (NDIR) analyzer
(LI-840 from LICOR®).
The analyzer compensates for the presence of water vapor in the sample gas.
However, these corrections are minimized by drying the gas before it reaches
the NDIR analyzer. The water is first condensed out of the gas stream by
cooling to ~5?C and further removed using Nafion® gas dryers. The sample
gases typically contain less than 3 millimoles/mole of water when they flow
through the analyzer.
During the cruise the NDIR analyzer is calibrated regularly using four
standard gas cylinders from NOAA's Earth System Research Laboratory (ESRL),
Global Monitoring Division in Boulder, CO. The analyzer is regularly zeroed
using nitrogen and spanned with the most concentrated standard.
STANDARD CYLINDER# CONCENTRATION(ppm)
STD1 CA03928 244.32
STD2 CA04455 546.80
STD3 CA02174 420.53
STD4 CA05559 366.87
The system also measures the CO2 content of the outside air, which is drawn
from an inlet on the instrument tower above the bridge. Atmospheric air was
constantly being pulled (6 liters/min maximum flow) through ~200 feet of
tubing (3/8" OD Dekoron) to the analytical system located many decks below
and aft. The seawater is drawn from the ship's flowing seawater line. The
ship has a thermosalinograph (TSG) and a remote temperature sensor. There is
a TSG (Sea-Bird SBE45) next to the CO2 instrument.
A 'Deck Box' containing a high precision pressure transducer, a GPS and
Iridium satellite modem is located outside and several decks above the system
on part of the ship's superstructure protected from severe weather. The
instrumental system via the deck box records the atmospheric pressure and the
position of the ship. The measured pressure is corrected for the height of
the barometer above the sea surface with the addition of dgh/u - where d is
atmospheric density (1.2 kg/m3), g is gravitational acceleration (9.8 m/sec2),
h is height of the barometer above the sea surface, and u is the conversion
factor from pascals to desired pressure units. The estimated height of 15
meters resulted in applying an increase of 1.8 mbar in the measured barometric
pressoure to correct the pressure at sealevel.
The sequence of continuous analyses was:
STEP TYPE REPETITIONS
1 Zero gas 1
2 Span gas 1
3 Standards (all four) 1
4 ATM 5
5 EQU 60
6 Loop to STEP 3 5
7 Standards (all four) 1
8 Loop to STEP 1 1
The amount of time between analyses depends on whether the analyses are of
the same type of gas (e.g., STD, ATM, EQU) or not. When switching between
different gases, the connecting tubes and analyzer are flushed for an initial
interval called the 'PRE-FLUSH' time plus an interval called the 'REGULAR
FLUSH' time. Between successive measurements of the same type of gas, the
system is flushed for only the 'REGULAR FLUSH' time. The gas flow is then
stopped. After the 'STOP FLOW' time interval, which is 10 seconds for all
analyses, the output of the NDIR analyzer is read. The pre-flush time is set
to 180 seconds and the regular flush time is set to 60 seconds for standard
and air analyses. Both the pre-flush and regular flush times are 120 seconds
for equilibrator headspace analyses. With these settings, a complete set of
standards and the atmospheric analyses are done every 3 hours and a full day
contains about 480 analyses of the equilibrator headspace.
CALCULATIONS:
The measured xCO2 values are linearly corrected for instrument response using
the standard measurements (see Pierrot et al., 2009).
For ambient air and equilibrator headspace the fCO2_ATM or fCO2_SW is calculated
assuming 100% water vapor content:
fCO2 = xCO2 P (1-pH2O) exp[(B11+2d12)P/RT]
where fCO2 is the fugacity in ambient air or equilibrator, pH2O is the water vapor
pressure at the sea surface or equilibrator temperature, P is the equilibrator or
outside atmospheric pressure (in atm), T is the SST or equilibrator temperature
(in K) and R is the ideal gas constant (82.057 cm^3·atm·deg^-1·mol^-1). The
exponential term is the fugacity correction where B11 is the first virial
coefficient of pure CO2
B11 = -1636.75 + 12.0408 T - 0.0327957 T^2 + 3.16528E-5 T^3
and
d12 = 57.7 - 0.118 T
is the correction for an air-CO2 mixture in units of cm^3·mol^-1 (Weiss, 1974).
The fugacity as measured in the equilibrator is corrected for any temperature
difference between sea surface temperature and equilibrator chamber using the
empirical correction outlined in Takahashi et al. (1993).
fCO2(SST) = fCO2(teq)exp[0.0423(SST-teq)]
where fCO2(SST) is the fugacity at the sea surface temperature and fCO2(teq) is
the fugacity at the equilibrator temperature. SST and teq are the sea surface
and equilibrator temperatures in degrees C, respectively.
REFERENCES:
Feely, R. A., R. Wanninkhof, H. B. Milburn, C. E. Cosca, M. Stapp and P. P.
Murphy (1998). A new automated underway system for making high
precision pCO2 measurements onboard research ships. Analytica Chim.
Acta 377: 185-191.
Ho, D. T., R. Wanninkhof, J. Masters, R. A. Feely and C. E. Cosca (1997).
Measurement of underway fCO2 in the Eastern Equatorial Pacific on NOAA
ships BALDRIGE and DISCOVERER, NOAA data report ERL AOML-30, 52 pp.,
NTIS Springfield.
Pierrot, D., C. Neill, K. Sullivan, R. Castle, R. Wanninkhof, R., H. Lüger,
T. Johannessen, A. Olsen, R. A. Feely, C. E. Cosca, 2009.
Recommendations for autonomous underway pCO2 measuring systems and
data reduction routines, Deep Sea Res II, 56: 512-522.
Wanninkhof, R. and K. Thoning (1993) Measurement of fugacity of CO2 in
surface water using continuous and discrete sampling methods. Mar.
Chem. 44(2-4): 189-205.
Weiss, R. F. (1970). The solubility of nitrogen, oxygen and argon in water
and seawater. Deep-Sea Research 17: 721-735.
Weiss, R. F. (1974). Carbon dioxide in water and seawater: the solubility of
a non-ideal gas. Mar. Chem. 2: 203-215.
Takahashi, T., J. Olafsson, J. G. Goddard, D. W. Chipman, and S. C.
Sutherland (1993). Seasonal variation of CO2 and nutrients in the
high-latitude surface oceans: a comparative study, Global Biogeochem.
Cycles, 7, 843-878.
METADATA:
List of variables included in this dataset:
COL HEADER EXPLANATION
1. Group_Ship AOML_XUE
2. Cruise_ID dependent upon expedition's name
3. JD_GMT Decimal year day
4. DATE_UTC_ddmmyyyy UTC Date
5. TIME_UTC_hh:mm:ss UTC Time
6. LAT_dec_degree Latitude in decimal degrees (negative
values are in southern hemisphere)
7. LONG_ dec_degree Longitude in decimal degrees (negative
values are in western latitudes)
8. xCO2_EQU_ppm Mole fraction of CO2 in the equilibrator
headspace (dry) at equilibrator
temperature, in parts per million
9. xCO2_ATM_ppm Mole fraction of CO2 in outside air (dry),
in parts per million
10. xCO2_ATM_interpolated_ppm xCO2 in outside air associated with each
water analysis. These values are
interpolated between the bracketing
averaged good xCO2_ATM analyses, in parts
per million
11. PRES_EQU_hPa Barometric pressure in the equilibrator
headspace, in hectopascals (1 hPa = 1 millibar)
12. PRES_ATM@SSP_hPa Pressure measured by outside barometer,
corrected to sea level, in hectopascals
13. TEMP_EQU_C Water temperature in equilibrator, in
degrees centigrade
14. SST_C Sea surface temperature from the ship's
remote temperature sensor, in degrees centigrade
[interpolated, see note below]
15. SAL_permil Salinity from the thermosalinograph
(SBE45), on the Practical Salinity Scale
16. fCO2_SW@TEMP_EQU Fugacity of CO2 in sea water, in
Microatmospheres (100% humidity)
17. fCO2_ATM_interpolated_uatm Fugacity of CO2 in air corresponding to the
interpolated xCO2, in microatmospheres
(100% humidity)
18. dfCO2_uatm Sea water fCO2 minus interpolated air fCO2,
in microatmospheres
19. WOCE_QC_FLAG Quality control flag for fCO2 values
(2 = good value, 3 = questionable value)
20. QC_SUBFLAG Quality control sub flag for fCO2 values
provides explanation for atypical data,
when QC_FLAG = 3
The quality control flags are provided as an aid to the interpretation of the CO2 data.
The amount of time between the sea water entering the ship and the water flowing through
the equilibrator is estimated before assigning an SST value to each analysis. The
patterns in the temperature records for the equilibrator and for SST over time are
compared, and a time offset that optimizes the match of these patterns is determined.
The time offset is applied to the SST measurements. A linear interpolation between
the time-adjusted SST data yields the SST value assigned to each CO2 analysis and
used in the fugacity calculations.
