ALBERT

Description: As ocean acidification (OA) sensor technology develops and improves, in situ deployment of such sensors is becoming more widespread. However, the scientific value of these data depends on the development and application of best practices for calibration, validation, and quality assurance as well as on further development and optimization of the measurement technologies themselves. Here, we summarize the results of a 2-day workshop on OA sensor best practices held in February 2018, in Victoria, British Columbia, Canada, drawing on the collective experience and perspectives of the participants. The workshop on in situ Sensors for OA Research was organized around three basic questions: 1) What are the factors limiting the precision, accuracy and reliability of sensor data? 2) What can we do to facilitate the quality assurance/quality control (QA/QC) process and optimize the utility of these data? and 3) What sort of data or metadata are needed for these data to be most useful to future users? A synthesis of the discussion of these questions among workshop participants and conclusions drawn is presented in this paper.

Description: The air-sea gas transfer velocity (K-660) is typically assessed as a function of the 10-m neutral wind speed (U-10n), but there remains substantial uncertainty in this relationship. Here K-660 of CO2 derived with the eddy covariance (EC) technique from eight datasets (11 research cruises) are reevaluated with consistent consideration of solubility and Schmidt number and inclusion of the ocean cool skin effect. K-660 shows an approximately linear dependence with the friction velocity (u*) in moderate winds, with an overall relative standard deviation (relative standard error) of about 20% (7%). The largest relative uncertainty in K-660 occurs at low wind speeds, while the largest absolute uncertainty in K-660 occurs at high wind speeds. There is an apparent regional variation in the steepness of the K-660-u* relationships: North Atlantic 〉= Southern Ocean 〉 other regions (Arctic, Tropics). Accounting for sea state helps to collapse some of this regional variability in K-660 using the wave Reynolds number in very large seas and the mean squared slope of the waves in small to moderate seas. The grand average of EC-derived K-660 ( - 1.47 + 76.67 u * + 20.48 u *(2) o r 0.36 + 1.203 U-10n + 0.167 U (2)(10n) ) is similar at moderate to high winds to widely used dual tracer-based K-660 parametrization, but consistently exceeds the dual tracer estimate in low winds, possibly in part due to the chemical enhancement in air-sea CO2 exchange. Combining the grand average of EC-derived K-660 with the global distribution of wind speed yields a global average transfer velocity that is comparable with the global radiocarbon (C-14) disequilibrium, but is similar to 20% higher than what is implied by dual tracer parametrizations. This analysis suggests that CO2 fluxes computed using a U-10n (2) dependence with zero intercept (e.g., dual tracer) are likely underestimated at relatively low wind speeds.