First steps to implement SOCONET-Tier 1 measurements and data delivery

This document describes the first operational steps to implement the reference-quality measurements of the Surface Ocean CO2 Observing Network (SOCONET-Tier 1). The network aims to coordinate groups making high-quality surface water fugacity1 of CO2 (fCO2w) and marine boundary dry mole fraction (xCO2) observations. SOCONET-Tier 1 will be the observational cornerstone for surface ocean fCO2w observations to quantify sea-air CO2 fluxes with sustained delivery of reference quality datasets in support of efforts such as the WMO Global Greenhouse Gas Watch (G3W). The requirements for SOCONET-Tier 1 are to perform high-quality observations with an accuracy of <2 µatm in fCO2w and <0.2 ppm (µmol mol-1) in mole fraction in dry air, xCO2a, and implementation of these measurements in a coordinated framework with benefits outlined in Table S1. This approach and these accuracies are necessary to constrain global annual sea-air CO2 fluxes to 0.2 Pg C (1015 g C) or uncertainties of less than 10% of the current annual global sea-air CO2 flux (Bender et al. 2002). Here we provide the practical considerations to establish uniform presentation, documentation, and (meta)data delivery. It incorporates ideas and common practices in place by established operators with the goal of providing a unified outward-facing presence of the entities providing high-quality measurements. Therefore, adoption of the steps by established groups is envisioned to be relatively straightforward. This document is focused on Tier 1 operations, but much of the practices will likely be adapted for non-Tier 1 measurements once experience is gained in incorporating the procedures by the established groups. This document is not an implementation plan for SOCONET, which would entail a holistic view of SOCONET and its role in a surface water CO2 observing system, including detailed network design and gap analysis of spatial and temporal coverage of measurements.