PREFACE

Determination of surface water partial pressure/fugacity of carbon dioxide (pCO₂/fCO₂) is of great interest as it provides a means to determine flux of CO₂ between the ocean and atmospheric and offers the opportunity to measure changes in surface water CO₂ levels in support of ocean acidification research. Surface water CO₂ levels are steadily increasing due to increasing atmospheric CO₂ levels from anthropogenic sources, primarily burning of fossil fuels, and resulting increases in air-sea CO₂ fluxes. However, the ocean CO₂ sinks and surface water CO₂ increases are not homogeneous in time of space. On a climatological scale the lower latitude oceans emit CO₂ (CO₂ sources) and higher latitude oceans sequester CO₂ (CO₂ sinks). Patterns change by season and are modulated by biological and physical processes. Multi-year anomalies are becoming apparent in the observational databases often linked to large-scale climate reorganizations such as the ENSO.

The surface water CO₂ effort aims to quantify CO₂ uptake and surface water CO₂ changes on seasonal scales for the world's ocean to determine the sequestration of anthropogenic CO₂, and elucidate the mechanisms of changing uptakes. This is in support of assessments of the fate of CO₂ in the earth system and to determine impacts of rising and changing surface CO₂ levels on ocean ecosystems. The observations are done from a variety of platforms with automated instrumentation. Currently most measurements are done on ships of opportunity (SOOP-CO₂) and surface moorings utilizing similar systems that measure the CO₂ content in the headspace of a chamber (equilibrator) that had seawater flowing through it such that headspace and water have the same CO₂ concentrations. The infrared detectors used are calibrated at frequent intervals with traceable atmospheric standard gases providing a key link between surface water and atmospheric CO₂ levels.

New instrumentation and new platforms are coming on line that will enhance the surface water CO₂ observing network. These include smaller instruments with different calibration and equilibration procudures, and different detectors such as spectrophotometers. Autonomous surface vehicles utilizing wave, solar and wind power are being tested and deployed. Using pH measurements [on profiling floats] to calculate pCO₂ is being investigated. In order to include these measurements into the surface water CO₂ data holdings, careful tests and intercomparisons must be accomplished.