Protocol for the Assessment and Correction of Surface Water and Air pCO2 Measurements

EA : Protocol for the Assessment and Correction of Surface Water and Air pCO2

Measurements As part of the Endurance Array, surface buoy partial pressure CO2 (pCO2) measurements are made using the Pro- Oceanus CO2-Pro Atmosphere pCO2 sensor. This sensor measures the partial pressure of CO2 gas in both surface water and air, allowing for surface flux calculations. Both the air and water measurements, and the flux estimates, are available through the OOI Data

Portal (https:// ooinet.oceanobservatories.org).

Wingard et al. (2020) reported on the surface water and air pCO2 data returned for the first four years of

Endurance Array operations and described a protocol based on cross-comparisons to independent, shipboard (both CTD samples and underway flow-thru systems) pCO2 measurements, data available from the Lamont-

Doherty Earth Observatory (LDEO) V2018 pCO2 database (Takahashi et al., 2019), data from the NOAA

Earth System Research Laboratory (ESRL) Global

Monitoring Division (GMD) Carbon Cycle Greenhouse

Gases (CCGG) data repositories (NOAA ESRL Global

Monitoring Division, 2016), and a high-resolution monthly pCO2 climatology for the coastal ocean (Laruelle et al., 2017).

Use of independent sources of pCO2 data, such as the discrete sample data and the LDEO V2018 database, are critical to validating the surface water pCO2 measurements. The overlapping deployment structure 75 employed by OOI (newly refurbished and calibrated instruments deployed next to older instruments)

The comparison shows overall quality of the surface water pCO2 measurements collected by the OOI Endurance Array is very high. Beyond a simple smoothing filter and application of human in the loop flags, no further corrections are required to the data. OOI Endurance Array data provide a novel source of high-resolution (hourly), near-continuous surface water pCO2 measurements in the coastal regions off the Oregon and Washington coasts. While there are multiple gaps in the air pCO2 measurements (greater than 33% of the total record), the potential exists for the creation of a common, combined daily averaged air pCO2 record that could help to address these gaps. This assessment gives confidence to the data on the portal as well as the export of OOI Endurance Array data via the NANOOS Visualization System (NVS, http://nvs.nanoos.org/) and the Global Ocean Acidification Observing System Data Portal (GOA-ON, http://portal.goa-on.org/).