Metadata Report for BODC Series Reference Number 1049299

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

Data Access Policy

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

Narrative Documents

RRS Challenger 127 CTD Instrument Documentation

Instrument Summary

The CTD package used on Challenger 127 was a self logging Sea Bird SBE25. The package included a temperature and conductivity probe, 0-500 db. pressure sensor, SeaTech 10cm path length transmissometer, SeaTech fluorometer, pump, battery pack and data logger. The instrumentation was pressure hardened to 600m but maximum operational depth was determined by the pressure sensor fitted.

Sea-Bird SBE 25 SEALOGGER CTD

The SBE 25 SEALOGGER is a research-quality CTD profiling system used for coastal, estuarine and, can also be a practical option, for deep-water work. It is easily configured in the field for a wide range of auxiliary sensors. The SEALOGGER is self-powered, requires no conductive cable, and is designed for use up to 6800 meters (10,000 psia). It uses the SBE3F temperature and SBE4 conductivity sensors as well as having an external strain gauge pressure sensor. It provides pump-controlled T-C ducted flow, samples at 8 Hz, records internally and provides simultaneous real-time data via its built-in RS-232 interface.

The standard CTD unit comes with a plastic housing (rated to 600 m), although this can be replaced by aluminium housing for depths up to 6800 m.

Specifications

Options and accessories

Additional sensors can be attached to the CTD, including:

• Dissolved Oxygen (SBE 43 DO Sensor)
• pH (SBE 18 pH Sensor or SBE 27 pH/ORP Sensor)
• fluorescence
• radiance (PAR)
• light transmission
• optical backscatter (turbidity)

The SBE 5T titanium pump can be used in place of SBE 5P pump. Further details can be found in the manufacturer's SBE 25 instrument specification.

SeaTech fluorometer S131

This fluorometer is designed to measure in situ chlorophyll-a fluorescence and provide high resolution data for assessment of phytoplankton biomass and monitoring of primary productivity in fresh or marine waters. It's versatility allows the instrument to be deployed on a mooring or in profiling mode. It is not sensitive to ambient light, permitting laboratory calibration with normal room lighting, and field measurements to be made at the water surface.

Specifications

*FWHM- Full-Width Half-Maximum

Further details can be found in the manufacturer's manual.

SeaTech Transmissometer

Introduction

The transmissometer is designed to accurately measure the the amount of light transmitted by a modulated Light Emitting Diode (LED) through a fixed-length in-situ water column to a synchronous detector.

Specifications

• Water path length: 5 cm (for use in turbid waters) to 1 m (for use in clear ocean waters).
• Beam diameter: 15 mm
• Transmitted beam collimation: <3 milliradians
• Receiver acceptance angle (in water): <18 milliradians
• Light source wavelength: usually (but not exclusively) 660 nm (red light)

Notes

The instrument can be interfaced to Aanderaa RCM7 current meters. This is achieved by fitting the transmissometer in a slot cut into a customized RCM4-type vane.

A red LED (660 nm) is used for general applications looking at water column sediment load. However, green or blue LEDs can be fitted for specilised optics applications. The light source used is identified by the BODC parameter code.

Further details can be found in the manufacturer's Manual.

RRS Challenger CH127 CTD Data Documentation

Operational Procedures

The CTD package was deployed on the hydrographic wire on the starboard midships 'A' frame. The instrument was switched on and then deployed as quickly as possible to a depth of 4-5m, the depth of inlet to the ship's non-toxic sea water supply. It was held here for several minutes to allow a salinity sample to be taken from the non-toxic supply and sufficient time to elapse for the salt water switch to activate the pump, fluorometer and transmissometer.

The package was then lowered to the required depth at a rate of 0.5 ms^-1. As the primary purpose of the CTD casts was to study trace gases in the euphotic zone, most of the casts covered the top 100m of the water column. For stations of intermediate depth on the shelf break deeper casts were made to look at the deeper hydrography. Close (>10-20m) approaches to the sea floor were not attempted to avoid risk of damage to the instrument package. As soon as the desired depth had been reached, the package was recovered at 0.5 ms^-1.

Once on board, the package was detached from the hydrographic wire and brought into the laboratory for the data from the logger to be downloaded onto a laptop PC. Once this was done, all plumbing was flushed through and filled with deionised water.

Most CTD casts were followed by a hydrographic bottle station to collect water for trace gas analysis. However, occasionally operational pressures on wire time forced the bottle string to be attached to the hydrographic wire during the CTD cast resulting in pauses both during the downcast and the upcast whilst bottles were attached and removed. The casts operated in this mode were CTD1, CTD2, CTD11 and CTD18.

Shipboard Data Processing

The SBE25 sampled at 8 Hz which was averaged to 2 Hz by the on board data logger. The raw data were converted to the following engineering units using the Sea Bird Seasoft package:

The resulting 2 Hz data files were transferred to BODC on board via floppy disk. Two of the profiles (CTD2 and CTD3) had 5 scan averaging applied by the Seasoft software giving a sampling interval of 2.5 seconds.

Shipboard Calibration Procedures

The CTD used had been serviced and calibrated by the manufacturer in February 1996. The calibration certificates quoted a temperature T value of zero and a conductivity slope correction factor of unity. The transmissometer calibration quoted an air voltage of 4.645 (blocked 0.001) compared to the manufacturer's voltage of 4.840 (blocked 0.001). An air correction based on these data was included in the Seasoft processing.

In addition to the manufacturer's calibration, the following procedures were followed during the cruise to ensure accuracy was maintained.

On every cast a salinity sample was taken from the ship's non-toxic sea water supply whilst the CTD package was being held level with the inlet. The bottle salinities were measured during the cruise on a Guildline Autosal bench salinometer.

A one-off calibration/intercalibration exercise was carried out. On Challenger's starboard deck there is a large tank plumbed into the non-toxic sea water supply. This is normally used to house the underway fluorometer (a Chelsea Instruments Aquatracka) and transmissometer (25cm path length SeaTech). By removing the baffle plates it was possible to immerse both the Sea Bird CTD package and the underway instruments.

A calibrated (on 20/5/94) SIS digital reversing thermometer was placed in the tank as close as possible to the temperature sensor of the CTD. This was read in continuous mode every 2 minutes over a half hour period. The transmissometer light path was blocked simultaneously to provide a clear event marker in the CTD record. A single salinity bottle sample was taken from the water in the tank at the start of the exercise. The underway transmissometer and fluorometer were in the tank and being logged throughout.

Chlorophyll samples for the fluorometer calibration were collected from 1 litre Knudsen bottles deployed on the hydrographic wire either during or shortly after the CTD cast.

Post-cruise Data Processing

The Seasoft output files were reformatted into PXF (the BODC internal format) using the well established BODC Transfer System. The software detected casts where pressure had been converted to depth by interrogating the header and reverted the values to decibars using the UNESCO algorithm (Fofonoff and Millard 1983) and the true station latitude.

• The percentage transmission was converted to attenuance using the equation:

• Attenuance = -10.0 * ln (% transmission/100)

  The scaling factor of 10 is the inverse of the path length in metres.

• A natural log transform was applied to the raw fluorometer data to conform with the data processing software, which was designed for the logarithmic output from an Aquatracka fluorometer rather than the linear output of the Sea-Bird system.

Once in PXF the each cast was inspected using the SerPlo interactive flag editing package. The start and end of the downcast were delimited and any suspect values flagged. The data generally looked good with no series requiring heavy flagging on any parameter except for the saturation of the fluorometer on the first two casts.

This inspection revealed significant differences between the upcasts and downcasts for many of the profiles. Some of the offsets were clearly due to internal wave activity in the upper water column. However, others were not. On many casts the attenuance values were considerably greater on the upcast than the downcast. This difference increased steadily from the bottom of the cast to the surface reaching a magnitude of approximately 0.1 per metre. Not all casts were affected and the problem was more prevalent in clearer water indicating that this very short path length instrument is pushing its operational limit.

The salinity profiles of a number of casts exhibited a symmetrical hysterisis as the CTD passed through well developed thermoclines with values enhanced on the downcast and suppressed on the upcast. These were attributed to a partial success by the Seasoft algorithm correcting for the response time differences between the temperature and conductivity sensors. Where these deviations were deemed significant (>0.02 PSU) the downcast values have been flagged suspect.

The following table shows how the variables within the originator's files were mapped to appropriate BODC parameter codes:

The downcasts were extracted and loaded into a relational database. Calibration equations were determined as detailed in the next section and applied to the data. The final archive data set was derived by averaging into 2 db bins (1 db if the maximum pressure was less than 100 decibars). All data flagged suspect were excluded from the averages. Data gaps were set null unless the gap was 3 bins or less in which case a linear interpolation was applied.

These data have since (2011) been extracted from the relational database and transferred into QXF (BODC internal format) files for distribution as series (complete profiles).

Post-cruise Calibrations

Pressure

The pressure in air was determined for each cast and gave a mean value of -0.2 db (standard deviation 0.13). A pressure correction of 0.2 db has been applied.

Temperature and Salinity

The temperature calibration exercise in the non-toxic tank showed the mean difference between the CTD and digital thermometer was -0.001C (standard deviation 0.003). This was not deemed significant so no post-cruise back calibration has been applied to temperature.

The salinity calibration showed the CTD to be reading consistently lower than the salinometer. A mean correction of 0.004 PSU (standard deviation 0.002) was determined and has been applied.

Attenuance

Observation of the attenuance data highlighted that the values were higher than would be expected for this part of the northeast Atlantic. In particular, the last cast (CTD24) was taken in water that was visually observed to be exceptionally clear, yet the attenuance value recorded by the CTD transmissometer was approximately 0.6 per m. This prompted an intercalibration between the CTD instrument and the underway instrument. The result of this showed the CTD instrument to be reading high by an average of 0.241 per m (standard deviation 0.041 per m).

One possible solution was to apply a universal correction of -0.241 to the CTD attenuance data set. However, subsequent investigation revealed that this would have resulted in a significant proportion of the data being reduced to values in the range 0.33 to 0.35 which is below the value for particle-free sea water. This is almost certainly due to instrumental drift to which a 10cm instrument in such clear water is inevitably prone.

The option of determining a separate correction for each cast was investigated but proved impractical because calibration data were not available for all casts. Finally, it was decided to apply a compromise correction of -0.2 to all the CTD attenuance data. The resulting data set is still systematically high, but the accuracy is now believed to match the precision () observed through comparison of upcasts and downcasts.

Chlorophyll

A subset of the water bottles deployed had samples taken for extracted chlorophyll determination. However, the resulting data did not seem in keeping with visual observations made on the cruise. The values from water that was clearly green were less than 1 mg/m3. Consequently, it was decided not to use these data for calibration of the CTD fluorometer.

Instead, the CTD fluorometer was intercalibrated against the underway fluorometer which had been calibrated against the UEA extracted chlorophyll data. This exercise went surprisingly well and gave the following calibration equation:

Note that some of the chlorophyll profiles show clear evidence of near surface quench effects. As no light meter was fitted to the CTD package it has not proved possible to correct for this. However, as the underway fluorometer calibration included a quench term, a manual quench correction (assuming a fixed chlorophyll value in the upper mixed layer) was included in the calibration for those profiles that showed no evidence of a sub-surface chlorophyll maximum.

Operational Problems

On the first two casts, the fluorometer was set to its maximum sensitivity causing the instrument to saturate. Where this has occurred, the data have been flagged suspect which has resulted in large gaps in the surface data.

The transmissometer fitted to the CTD package had a short (10 cm) path length. This has limited both the absolute accuracy and precision of the attenuance data to ±0.5 per m.

Correction of the salinity data for differences in response times of the temperature and conductivity sensors fell short of perfection. This has resulted in minor data loss in the regions of sharp thermoclines due to flagging of the affected data.

Conclusions

The CTD data from this cruise are of acceptable quality. Temperature and salinity accuracy significantly exceed the JGOFS protocol standard (0.02) but do not reach the WOCE standards of 0.005 for salinity and 0.002 for temperature. Attenuance data are believed accurate within ±0.5 per m.

References

Fofonoff, N.P. and Millard, R.C. Jr. (1983). Algorithms for the computation of fundamental properties of sea water. UNESCO Tech. Pap. Mar. Sci. 44, 53 pp.

Project Information

ACSOE Eastern Atlantic Experiment (EAE)

The Eastern Atlantic Experiment was a part of the Marine Aerosol and Gas Exchange (MAGE) component of the Atmospheric Chemistry Studies in the Oceanic Environment (ACSOE) project.

The aims of the experiment were:

• To quantify input of DMS into a parcel of air
• To examine the oxidation of DMS and its reaction with nitrogen species with time
• To investigate the formation of new particles as a result of these transformations
• To discriminate between the natural and anthropogenic fractions of sulphur and nitrogen using isotopic measurements

The experiment included two campaigns in the spring seasons of 1996 and 1997, each of which incorporated three elements:

• A land-based site at Mace Head (at the seaward end of Galway Bay)
• A research vessel operating off the west coast of Ireland (RRS Challenger)
• Research aircraft overflights to link shipborne and land-based measurements

The primary measurements made during the campaigns were concentrations of DMS in the atmosphere and the water column, but a wide range of additional measurements were made including:

• Atmospheric ozone and nitrogen species
• Atmospheric particulates and their chemistry
• Atmospheric nitrogen and sulphur isotopic composition
• Oceanic temperature, salinity, attenuance and chlorophyll
• Meteorology

The fieldwork was supported by modelling work with a zero-dimensional time-dependent photochemical box model of an air mass in the marine boundary layer.

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

No Fixed Station Information held for the Series

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle: