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Three levels of observations are included in the CO2/CLIVAR hydrographic section program with selection criteria listed in parentheses:
• Level I core measurements (mandatory on all cruises; suggested standard for international collaborators; measured at highest spatial resolution practical; funded through the omnibus proposal across all cruises).
• Level II recommended measurements (highly desirable on subset of US cruises; may be collected on coarser station spacing; coordinated with the core effort but funded by separate proposals either on a cruise by cruise basis of by specific measurement).
• Level III ancillary measurements (on opportunity and space available basis; not to significantly interfere with Level I or II effort; may be regional or specific to individual cruise; extramural funding).

Rationale based on measurements required to directly quantify change in ocean carbon inventory, estimate anthropogenic CO2 empirically, characterize large-scale water mass ventilation rates, constrain horizontal heat, freshwater, C, N, and O2 transports and/or net divergence, and provide on-going basis for model evaluation.

• DIC dissolved inorganic carbon (DIC)
• Total Alkalinity (TAlk)
• CTD pressure, temperature, conductivity (salinity)
• CTD oxygen (sensor)
• Bottle salinity
• Nutrients by standard auto analyzer (NO3/NO2, PO4, SiO3)
• Dissolved oxygen (O2)
• Chlorofluoro carbon tracers CFC-11, -12, -113
• Tritium-3He
• Total organic carbon (TOC, DOC and/or POC)
• Total organic nitrogen
• Surface underway system: T, S, pCO2
• ADCP shipboard
• ADCP lowered

These measurements are related to large-scale carbon cycle and/or ventilation; specific rationale listed after measurement; possibly on coarser spatial resolution than Level I but on all cruises

• pH (internal carbonate system consistency)
• discrete pCO2
• (internal carbonate system consistency)
• 14C by AMS (bomb penetration; southern ocean circulation changes; may need to repeat only on 20 year time-scale)
• CCl4 and SF6
• (to extend range of age tracers further back in time (CCl4) and into the future (SF6)
• del 13C of DIC
• (independent measure of anthropogenic CO2 uptake and inventory changes)
• Fe/trace metals
• (others? Zn? Al for dust? There are three sampling approaches
• on "regular" rosette
• Teflon/plastic water sampler hanging below CTD
• Separate Kevlar wire; separate casts)
• CTD transmissometer
• (POC distribution; ambiguity as to calibration; regional?)
• More complete surface underway system: nutrients, O2, Chl, DIC, surface skin temperature

These measurements focus on upper ocean biogeochemistry and cycling.

• Chlorophyll
• Primary production (on deck incubations)
• HPLC pigments (phytoplankton community composition)
• PIC
• HCFC
• Experimental continuous analyzers (as they develop and can go full depth)
• del 15N NO3 (nutrient utilization)
• 32Si
• 18O of H2O
• NH4
• Low level nutrients
• Total organic phosphorus (difficult to measure??)
• Upper ocean optical profile (similar to AMT line; algorithm development/validation; short cast; needs to be coherent with biological sampling)
• del 17O of O2 (gross primary production)
• methyl halides (linkages to SOLAS)
• DMS (linkages to SOLAS)
• ADCP (multibeam) (optimized for (biological) particle enumeration)