PMEL-JISAO Trace Gas Program

Program description
Summary of research accomplishments
Summary of available data

PMEL-JISAO Trace Gas Program Description

The goals of the PMEL-JISAO Trace Gas program are to determine:

• the regional and seasonal distribution of surface seawater and atmospheric concentrations of climatically important gas phase species,
• the importance of the ocean as a source of these species to the atmosphere, and
• the factors controlling the atmospheric concentrations and the oceanic production and emission of these species to the atmosphere.

The ship-based transects yield ocean-scale distributions of key species which can be used to test and validate trace gas sources, sinks and transport in GCMs. The program to date has focused on CO, CH₄, OCS and O₃. CO and CH₄ data are available for a series of 8 cruises throughout the Pacific Ocean (1987- 1994) and can be accessed via this server.

PMEL-JISAO Trace Gas Program Accomplishments

• Ozone concentrations in the atmospheric boundary layer of the Pacific and Indian Oceans were measured on four separate oceanographic cruises (July 1986, May to June 1987, July to August 1987, April to May 1988). These measurements show a distinct zone of near zero (< 3 ppb) ozone concentrations in the central equatorial Pacific in April-May, with ozone increasing in this region over the next four months. The measurements confirm a latitudinal distribution with higher ozone in the northern mid-latitudes, a pronounced equatorial minimum, and increasing ozone levels from the equator southward. The seasonal observed change in the latitudinal gradient of ozone is consistent with previous ozone measurements at Hilo and Samoa and predictions from an atmospheric general circulation study. A significant diurnal cycle of ozone was found in almost all locations with a maximum near sunrise, a minimum in the late afternoon, and a peak-to-peak amplitude of 1 to 2 ppb (10-20%), similar to that predicted by a photochemical model in the low NO_x limit (Johnson et al., 1990).
  
  
• Methane is a potent greenhouse gas and plays a major role in both tropospheric and stratospheric chemistry. Better estimates of the sources and sinks of methane are needed to understand the temporal trends in atmospheric methane concentrations, to predict future concentrations and climatic impacts, and to provide a basis for strategies to reduce emissions of greenhouse gases. Data from the PMEL Pacific Ocean meridional transects (1987-1994) were used to reevaluate the open-ocean source of methane to the atmosphere (Bates et al., 1996). The combined seasonal and zonal fluxes calculated from this study result in a total global ocean-to-atmosphere flux of 0.4 Tg methane per year which is an order of magnitude less than previous estimates and less than 0.1% of the total global source to the atmosphere (IPCC, 1994).
  
  
• The equatorial Pacific Ocean is a source of both sulfur and carbon to the atmosphere. In February and March of 1990, as part of the SAGA-3 Expedition, dimethylsulfide (DMS), methane (CH4), carbon monoxide (CO), and carbon dioxide (CO₂) partial pressures were determined in both surface seawater and the overlying atmosphere of the central equatorial Pacific Ocean (15 °N to 10 °S, 145 °W). The partial pressures were used to calculate the net flux of these gases from the ocean to the atmosphere. The average regional DMS and CO fluxes were similar, 7.1 and 4.2 (µmol/m²/day, respectively. The mixing ratio of CH4 in surface seawater was close to equilibrium with the overlying atmosphere and hence the average flux was only 0.39 (µmol/m²/d. The flux of CO₂ clearly dominated the air-sea carbon exchange with an average regional flux of 5.4 mmol/m²/day (Bates et al., 1993).
  
  
• The magnitude of the oceanic source of CO to the atmosphere depends on the concentration of CO in ocean surface waters. To ascertain the relative importance of the processes controlling this concentration, depth profiles of CO concentrations in the oceanic mixed layer and upper thermocline were made at two time series stations in the Tropical Pacific Ocean in 1993. The results from this study (Johnson and Bates, 1996) showed that the local microbial oxidation rate constant, not the local production rate, determines the surface concentration of CO at these two stations. Therefore, CO emissions to the atmosphere are linked to the surface ocean microbial ecology.
  
  
• Carbon monoxide (CO) is the major sink of the hydroxyl radical in the troposphere and hence strongly affects the oxidizing capacity of the atmosphere and the concentrations of the other trace gases. In order to be able to predict future atmospheric concentrations of radiatively important trace gas and aerosol species, climate-chemistry models must include accurate estimates of the sources of CO to the atmosphere. Data from the PMEL Pacific Ocean meridional transects (1987-1994) were used to assess the regional and seasonal oceanic emissions of CO to the atmosphere (Bates et al., 1995). Although the ocean is ubiquitously supersaturated with CO, the total annual emission from the ocean to the atmosphere (13 Tg) is small compared to current estimates from both natural and anthropogenic terrestrial sources of 2400 Tg(CO)/year (IPCC, 1994).
  
  
• Carbonyl sulfide (OCS) is the major non-volcanic source of stratospheric sulfate aerosols. These aerosols influence the Earth's climate by interacting with incoming solar radiation and enhancing ozone depletion through heterogeneous chemistry with chlorine and nitrogen species. Although there has been no detectable change in the average atmospheric concentration of OCS since 1977, budget estimates of sources exceed sinks by a factor of two. Data from PMEL cruises were used to quantify the OCS photo-production rates in surface seawater (Weiss et al., 1995b) and to reevaluate the open ocean source of OCS to the atmosphere (Weiss et al., 1995a). The calculated fluxes show that the open ocean is not a source of OCS to the atmosphere but instead is small net sink.

PMEL-JISAO Trace Gas Program Available Data

The data can be accessed HERE.

Cruise Descriptions

The data compiled here include carbon monoxide and methane measurements made by PMEL-JISAO in the surface ocean and overlying atmosphere on eight cruises in the Pacific and Indian Oceans between 1987 and 1994:

The cruises include:

• R87 Aboard the NOAA ship Oceanographer (RITS87) from Townsville, Australia (July 13, 1987) to Dutch Harbor, Alaska (August 21, 1987) and Seattle, Washington (August 29, 1987).
• R88 Aboard the NOAA ship Oceanographer (RITS88) from Dutch Harbor, Alaska (April 6, 1988) to American Samoa (May 5, 1988).
• R89 Aboard the NOAA ship Discoverer (RITS89) from Manzanillo, Mexico (February 13, 1989) to Easter Island (March 1, 1989), 60 S (March 15, 1989), Tahiti (April 2-7, 1989) and Seattle (April 20, 1989). The legs from 60 S to Seattle include only atmospheric data.
• R90 Aboard the Soviet ship Akademic Korolev as part of the SAGA-3 expedition from Hilo Hawaii (February 13, 1990) to American Samoa (March 12, 1990).
• R92 Aboard the University of Southern California's research vessel Vickers (RITS92) as part of the IGAC-MAGE cruise from Los Angeles, California (February 21, 1992) to Nuku Hiva in the Marquesas Islands (March 11-12, 1992) and back to Los Angeles (March 25, 1992).
• R93 Aboard the NOAA ship Surveyor (RITS93) from Punta Arenas, Chile (March 20, 1993) to Palmer Station, Antarctica (March 24-25, 1993), Tahiti (April 15-19, 1993) and Seattle, Washington (May 7, 1993).
• R94 Aboard the NOAA ship Surveyor (RITS94) from Seattle, Washington (November 20, 1993) to Tahiti (December 13-17, 1993), Palmer Station (January 3-4, 1994), and Punta Arenas, Chile (January 7, 1994).

Methods

Sampling

Gas partial pressures were measured continuously in surface seawater along the cruise tracks using an equilibrator system designed to partition dissolved gases into a vapor phase for sampling. The equilibrator was fed with seawater pumped directly to the laboratory area from an intake located at approximately 5 m depth near the bow of the ship. The equilibrator was constructed from Plexiglass and consisted of a 20 L enclosed headspace continuously showered with 15-20 L/min of seawater. Approximately every hour, 2 ml of gas were withdrawn from the headspace for analysis.

Air samples were pulled from the bow of the ship, 10 m above the sea surface, to the oceanographic laboratory (approximately 40 m) through plastic coated aluminum tubing (Dekoron) at a flow rate of 10 L/min. Dekoron tubing was also used to connect the equilibrator to the analytical system.

Analysis

Gas partial pressures were measured with an automated, temperature controlled, gas chromatographic system containing both a flame ionization detector (FID) and a mercury bed detector (MBD). Air samples from the air sampling line, the equilibrator, or a standard stream were dried using phosphorous pentoxide or potassium permanganate and drawn into a 2 ml sample loop connected to an automated sample valve. The gases were separated using a series of five columns with CO valved to the MBD and CH4 valved to the FID. The system was automated with a Carle Series 400 Controller and ran unattended with alternating injections of air, standard, and equilibrated air with the series repeated approximately each hour.

Data Reduction

The raw signals were first visually filtered to eliminate any episodes of ship contamination or instrument malfunction. CO and CH4 mixing ratios in both air and equilibrator samples were then computed based on peak height (CH4) or peak area (CO) and a six-hour running-mean single or dual-point standard. The CH4 data were further smoothed using a 12 hour weighted regression. The dry-air mixing ratios were then binned into hourly values based on the measurements made 30 minutes before and after the hour. The partial pressures measured in the equilibrator samples were corrected for warming during transit from the water intake to the equilibrator using a ratio of the seawater solubilities at sea surface temperature and the equilibrator temperature. The warming values were derived from the regression of hourly warming on sea surface temperature.

Standards were dried, whole-air mixtures contained in aluminum cylinders and were calibrated by NOAA/CMDL. CO and CH4 mixing ratios are referenced to the CMDL and Rasmussen scales, respectively. The accuracy of the standards, as determined by CMDL is ± 2% and ± 1.5% for CO and CH4, respectively. During these cruises the instrument precision, as determined by the average percent standard deviation of the standard response over a six hour period, was approximately ± 1.5% and ± 1.8% for CO and CH4, respectively.

Data Summary

The data from each cruise are available in ascii form in 16 columns:

• Decimal Day of Year (DOY) in GMT
• Latitude (south is negative)
• Longitude (west is negative)
• SST - Sea surface temperature ((C)
• Teq - Temperature measured in the equilibrator ((C)
• Salinity (ppt)
• Atmospheric Pressure (mbar)
• Air Temperature
• Wind Direction (degrees)
• Wind Speed (m/s)
• Methane partial pressure in the atmosphere (ppb)
• Methane partial pressure as measured in the equilibrator headspace (ppb)
• Methane partial pressure in seawater calculated from the equilibrator measurements and gas solubilities (ppb)
• Carbon monoxide partial pressure in the atmosphere (ppb)
• Carbon monoxide partial pressure as measured in the equilibrator headspace (ppb)
• Carbon monoxide partial pressure in seawater calculated from the equilibrator measurements and gas solubilities (ppb)

The gas data are the mole fraction dry gas concentrations. Missing data are denoted with a -9.

The data can be accessed HERE.

Last update: 13 November 1996