Measurement Details

General Comments

GMD carbon cycle data are freely available directly from NOAA and from the CDIAC and WMO WDCGG data archive centers. Before actual data are made available, they must undergo critical evaluation. Evaluation procedures ensure that (1) the standard reference gases used in making the measurements in Boulder and in the field are well characterized (i.e., calibrated before and after their use); (2) samples compromised during collection or analysis are identified, and (3) valid samples not representative of typical background conditions are identified. Quality control of the data requires considerable time and effort and is an essential part of the GMD operations.

Warning: Preliminary data include the this group's most up-to-date data and have not yet been subjected to rigorous quality assurance procedures. Preliminary data viewed from this site are "pre-filtered" using tools designed to identify suspect values. Filtering is performed each time a data set containing preliminary data is requested. Filtering, however, cannot identify systematic experimental errors and will not be used in place of existing data assurance procedures. Thus, there exists the potential to make available preliminary data with systematic biases. In all graphs, preliminary data are clearly identified. Users are strongly encouraged to contact Dr. Pieter Tans, Group Chief (pieter.tans@noaa.gov) before attempting to interpret preliminary data.

Learn more about GMD carbon cycle measurements from this Web site and from the literature.

Carbon Dioxide (CO2)

Carbon dioxide (CO2) in ambient and standard air samples is detected using a non-dispersive infrared (NDIR) analyzer. The measurement of CO2 in air is made relative to reference standards whose CO2 mixing ratio is determined with high precision and accuracy. Because detector response is non-linear in the range of atmospheric levels, ambient samples are bracketed during analysis by a set of reference standards used to calibrate detector response. Measurements are reported in units of micromol mol-1 (10-6 mol CO2 per mol of dry air or parts per million (ppm)). Measurements are directly traceable to the WMO CO2 mole fraction scale. Measurement accuracy determined from repeated analysis of CO2 in standard gas cylinders using an absolute manometric technique is ~0.2 micromol mol-1. Measurement precision determined from repeated analysis of the same air is ~0.1 micromol mol-1. Average pair agreement among network flasks sampled in series is ~0.2 micromol mol-1.

Methane (CH4)

Ambient and standard air samples are injected into the gas chromatograph (GC). Methane (CH4) is separated from other sample constituents using packed columns and detected using flame ionization (FID). This process is highly automated for field and laboratory operations. Instrument response of the sample must be compared to a standard of known CH4 content. Measurements are reported in units of nanomol mol-1 (10-9 mol CH4 per mol of dry air (nmol mol-1) or parts per billion (ppb)) relative to the NOAA GMD CH4 scale. Reproducibility of our measurements, based on repeated analysis of air from a high-pressure cylinder, has ranged from 1 to 3 nmol mol-1 over the period of our measurements.

Carbon Monoxide (CO)

Ambient and standard air samples are injected into the gas chromatograph (GC). Carbon monoxide (CO) and molecular hydrogen (H2) are separated from other sample constituents using dual columns. CO and H2 are reacted with hot HgO bed to produce mercury (Hg). Hg is then determined photometrically. The non-linear detector requires a multipoint calibration (we use 6 standards in the atmospheric range). This process is highly automated for field and laboratory operations. Measurements are reported in units of nanomol mol-1 (10-9 mol CO per mol of dry air (nmol mol-1) or parts per billion (ppb)) relative to the WMO CO scale. Reproducibility of our measurements, based on repeated analysis of air from a high-pressure cylinder, is 1 nmol mol-1 at 50 nmol mol-1 and 2 nmol mol-1 at 200 nmol mol-1 over the period of our measurements. The absolute accuracy of our CO scale, estimated by comparing results obtained various sets of standards (both in-house and from national laboratories) using two independent absolute analytical techniques, is ~1.5 nmol mol-1 at 50 nmol mol-1 and ~2 nmol mol-1 at 200 nmol mol-1.

Molecular Hydrogen (H2)

Ambient and standard air samples are injected into the gas chromatograph (GC). Carbon monoxide (CO) and molecular hydrogen (H2) are separated from other sample constituents using dual columns. CO and H2 are reacted with hot HgO bed to produce mercury (Hg). Hg is then determined photometrically. This process is highly automated for field and laboratory operations. Instrument response must be compared to reference gas of known H2 content. Measurements are reported in units of nanomol mol-1 (10-9 mol H2 per mol of dry air (nmol mol-1) or parts per billion (ppb)) relative to the NOAA GMD H2 scale. Reproducibility of our measurements, based on repeated analysis of air from a high-pressure cylinder, is <= 6 nmol mol-1 over the period of our measurements. The absolute accuracy of our H2 scale is unknown.

Nitrous Oxide (N2O)

Ambient and standard air samples are injected into the gas chromatograph (GC). Nitrous oxide (N2O) and sulfur hexafluoride (SF6) are separated from other sample constituents using porous polymer columns and detected using electron capture (ECD). This process is highly automated for field and laboratory operations. Instrument response of the sample must be compared to a standard of known N2O content. Measurements are reported in units of nanomol mol-1 (10-9 mol N2O per mol of dry air (nmol mol-1) or parts per billion (ppb)) relative to the GMD HATS N2O scale. Reproducibility of our measurements, based on repeated analysis of air from a high-pressure cylinder, is 0.2 nmol mol-1 over the period of our measurements. The absolute accuracy of our N2O scale is estimated as 1 nmol mol-1.

Sulfur Hexafluoride (SF6)

Ambient and standard air samples are injected into the gas chromatograph (GC). Nitrous oxide (N2O) and sulfur hexafluoride (SF6) are separated from other sample constituents using porous polymer columns and detected using electron capture (ECD). This process is highly automated for field and laboratory operations. Instrument response of the sample must be compared to a standard of known SF6 content. Measurements are reported in units of picomol mol-1 (10-12 mol SF6 per mol of dry air (pmol mol-1) or parts per trillion (ppt)) relative to the GMD HATS SF6 scale. Reproducibility of our measurements, based on repeated analysis of air from a high-pressure cylinder, is 0.04 pmol mol-1 over the period of our measurements. The absolute accuracy of our SF6 scale is estimated as 0.2 pmol mol-1.

Carbon isotope of carbon dioxide (δ13C of CO2)

Each analysis is carried out by first extracting CO2 from the sample and then measuring its isotopic composition relative to CO2 gas in a reference standard. The isotopic analysis (MS) is carried out simultaneously with the extraction of the next sample. The isotope ratios are reported in units of per mil where δ13C = [(13C/12Csample/ (13C/12Cstandard/)-1] x 1000. Measurement precision is 0.01 per mil. Measurement uncertainty is 0.02 per mil. The internationally accepted scale for reporting δ13C is PDB (Pee Dee Bellemnite), but the link of CO2-in-air standards to PDB differs among laboratories. Ongoing intercomparison experiments among laboratories making these measurements is essential for assessing the comparability of isotope measurements from one lab with another (Masarie et al., JGR, Vol. 106, D17, 2001; Allison et al., WMO/GAW Report No. 148, p.17-30, Geneva, 2003)). Measurement accuracy based on results from intercomparison experiments is 0.03 per mil. This system is maintained and operated by the University of Colorado/INSTAAR Stable Isotope Laboratory.

Oxygen isotope of carbon dioxide (δ18O of CO2)

Each analysis is carried out by first extracting CO2 from the sample and then measuring its isotopic composition relative to CO2 gas in a reference standard. The isotopic analysis is accomplished using a dual inlet mass spectrometer. The isotope ratios are reported in units of per mil where δ18O = [(18O/16Osample/ (18O/16Ostandard/)-1] x 1000. Measurement precision is 0.02 per mil. Measurement uncertainty is 0.07 per mil. The internationally-accepted absolute scale for δ18O of CO2 in air is V-PDB. Ongoing intercomparison experiments among laboratories making these measurements is essential for assessing the comparability of isotope measurements from one lab with another. The INSTAAR-NOAA scale is known to be 0.8 per mil different from other labs measuring the δ18O of CO2 in air (Masarie et al., JGR, Vol. 106, D17, 2001; Allison et al., WMO/GAW Report No. 148, p.17-30, Geneva, 2003). This system is maintained and operated by the University of Colorado/INSTAAR Stable Isotope Laboratory.

Reliability of δO18 of CO2 data: Warning for users of tropical data
Users of δO18 data should be aware that many flasks in the network are sampled without drying of the air. This can result in isotopic exchange between CO2 and H2O. (See Gemery et al, 1996). δO18 values depleted by as much as several per mil are typical in such flasks. While most of the problem flasks can be identified via poor flask pair agreement, the Stable Isotope Lab recommends using δO18 data at sites between 30oN and 30oS with caution. These sites are the warmest and most humid and have the highest chance of compromised δO18 data. Data from other sites are considered more reliable. NOAA is working to dry all air at wet sites prior to sampling. Air dried during sampling can be distinguished by referring to the sampling method field in the data files. For more information about sampling method symbols used in the data sets, please see the FTP site README file. Efforts are underway to better quantify the problem, find a selection method by which historical data can be used, and ultimately establish a sampling technique that will eliminate or minimize the problem altogether. Users of the δO18 data are encouraged to please contact James White (james.white@colorado.edu) prior to using the δO18 data for a current update.

Carbon isotope of methane (δ13C of CH4)

Approximately 115 cc of ambient and standard air samples are pulled by a vacuum pump through a mass flow controller (@15 sccm) into a chilled pre-column of HayeSep D (Alltech 2829) held at -130oC where carbon dioxide and methane collect. The chilled pre-column is flushed with helium at 30cc/m to remove other contaminants. The pre-column is then warmed to -20oC, releasing the trapped gases. A helium carrier gas flowing at ~1.5 scc/m carries the released gases -primarily methane and carbon dioxide- into the cryo-focus section, which is held at -130oC, trapping the gas again in a smaller section that serves to sharpen the peak. The cryo-focus section is then warmed to 50oC while the helium carrier gas moves the sample into a 50 meter Porabond Q column (Varian, part #CP7352) for chromatographic separation. The separated methane is then combusted at 1150oC and the resulting CO2 is fed into an Isoprime continuous flow mass spectrometer (GV Instruments) for analyses. The isotope ratios are reported in units of per mil where δ13C = [(13C/12Csample / (13C/12Cstandard) - 1] x 1000. Measurement uncertainty is +/- 0.06 per mil, reported relative to V-PDB (Pee Dee Bellemnite), the internationally accepted scale for reporting δ13C. Reference cylinders are tied to the V-PDB scale via inter-comparisons with other labs. During an analysis run of 20 samples, whole air standards from at least two high-pressure cylinders are introduced on the same manifold as the samples, providing 12 reference analyses along with the samples. This system is maintained and operated by the University of Colorado/INSTAAR Stable Isotope Laboratory.

Carbon isotope of carbon dioxide (δ14C of CO2)

The δ14CO2 value is determined by cryogenically extracting CO2 from whole air, reducing the CO2 to graphite with hydrogen over an iron catalyst, and packing into aluminium targets at INSTAAR, University of Colorado. Graphite targets are measured for 14C content by Accelerator Mass Spectrometry at either the University of California, Irvine, or Rafter Radiocarbon Laboratory, New Zealand. Typically, the available air from both flasks of a flask pair is combined to obtain a single δ14CO2 measurement. Results are corrected for background, isotopic fractionation and radioactive decay since the time of collection, and are reported as δ14C, following the conventions of Stuiver and Polach (1977). Uncertainties are determined as the larger of the statistical counting uncertainty for each measurement or the long-term repeatability of a standard air tank, and are typically 1.8-2.4 per mil.