Removal of large outliers in ASVCO2-measured ocean pCO2 

Large outliers in the hourly ASVCO2-measured ocean pCO2 (pCO2-ASVCO2) were occasionally measured during the highest wind speeds and wave heights. We hypothesized this could be due to the ASVCO2 system seawater intake sampling a mixture of air and water, while the atmospheric pCO2 values remained stable. We identify and remove outliers in the hourly pCO2-ASVCO2 data by finding values greater than three standard deviations from the mean (the mean includes the outliers) and removing them. We repeat this process another time, which overall eliminates 4.8% of the pCO2-ASVCO2 data, with the largest gaps lasting up to 4 hours. 

Calculating ocean pCO2 from pH and quantifying its errors

Before each hourly pCO2 measurement, the ASVCO2 system performs a 2-point calibration with a zero sample (air scrubbed of CO2) and a standard span gas with a known CO2 (Sabine et al., 2020); therefore, we assume the absolute value of these data is correct. Because the pH sensor records measurements every 10 minutes and persisted about 24 hours beyond the failure of the ASVCO2 system, we leverage the pH observations to study high-resolution pCO2 variability. We first use the pH measured by the internal (int_10min_pH) and external (ext_10min_pH) reference electrodes on a Honeywell Durafet III and apply the corrections as described in Martz et al. (2010) (martz_10min_pH). Next, we calculate pCO2 from pH observations (pCO2-pH) and estimates of total alkalinity (TA) using the MATLAB version of CO2sys (Lewis & Wallace, 1998). We estimate TA using the regional TA-salinity (S) relationship for the North Atlantic:

TA = 670 + 47 * S

This relationship is based on data from the GLODAPv2.2021 release  (Lauvset et al., 2021) and one Gulf of Mexico East Coast Carbon (GOMECC) cruise (GOMECC-2; https://doi.org/10.3334/cdiac/otg.coastal_gomecc2; Wanninkhof et al., 2016), which was not included in the GLODAPv2.2021 release.
The following choices are selected in CO2sys: pH on the total scale, first and second dissociation constants (K1 and K2) of Lueker et al., (2000), the dissociation constant of sulfuric acid (KSO4) of Dickson et al., (1990), total borate of Uppström, (1974), and phosphate and silicate from the wintertime average of the World Ocean Atlas (Garcia et al., 2018). 

The pCO2-pH is offset from the pCO2-ASVCO2 because the pH sensor was not field calibrated and is susceptible to drift over the time during deployment, however the variability is similar between the two sensors (r = 0.65, RMSE = 196 µatm). To correct the pCO2-pH, we first compute the difference between the pCO2-pH and pCO2-ASVCO2. Gaps in the pCO2-ASVCO2 data arise from when the air intake froze at the very beginning of the mission and due to sensor failure at the end of the mission. The presence of these data gaps leads to corresponding gaps in the calculated offset between the pCO2-ASVCO2 and pCO2-pH which need to be filled to perform the correction. To do so, we take the average of this offset for the first 24 hours of the mission to fill the missing values from when the sensors were frozen. The gap at the end of the time series is filled using the average offset from the last 48 hours of the mission. Next, we calculate the 12-hour running mean of this offset and subtract it from the pCO2-pH. This step improves the correlation between the pCO2-ASVCO2 and pCO2-pH and reduces the error (r = 0.80, RMSE = 4.78 µatm). 

After applying this correction, the largest remaining discrepancies between the pCO2-pH and pCO2-ASVCO2 exist when the pH sensor measured large sporadic anomalies, which we assumed were errors. To address this, we compute the residual by subtracting the pCO2-ASVCO2 from the corrected pCO2-pH from the previous step. Differences of the residual that are greater than three standard deviations are removed. The resulting pCO2-pH has a higher correlation and lower error compared against pCO2-ASVCO2 (r = 0.87, RMSE = 3.85 µatm), and is the final version that is included in this file (pCO2_from_10minpH_corrected). 

References

Dickson, A. G., Wesolowski, D. J., Palmer, D. A., & Mesmer, R. E. (1990). Dissociation constant of bisulfate ion in aqueous sodium chloride solutions to 250°C. Journal of Physical Chemistry, 94(20), 7978–7985. https://doi.org/10.1021/j100383a042
Garcia, H., Weathers, K. W., Paver, C. R., Smolyar, I., Boyer, T. P., Locarnini, R. A., et al. (2018). World Ocean Atlas 2018. Volume 4: Dissolved Inorganic Nutrients (phosphate, nitrate and nitrate+nitrite, silicate). NOAA Atlas NESDIS 84, 84(July), 35.
Lauvset, S. K., Lange, N., Tanhua, T., Bittig, H. C., Olsen, A., Kozyr, A., et al. (2021). An updated version of the global interior ocean biogeochemical data product, GLODAPv2.2020. Earth System Science Data, 1–32. https://doi.org/10.5194/essd-12-3653-2020
Lewis, E., & Wallace, D. (1998). Program developed for CO2 system calculations. Ornl/Cdiac-105. Retrieved from http://cdiac.esd.ornl.gov/oceans/co2rprtnbk.html
Lueker, T. J., Dickson, A. G., & Keeling, C. D. (2000). Ocean pCO2 calculated from dissolved inorganic carbon, alkalinity, and equations for K1 and K2: Validation based on laboratory measurements of CO2 in gas and seawater at equilibrium. Marine Chemistry, 70(1–3), 105–119. https://doi.org/10.1016/S0304-4203(00)00022-0
Martz, T. R., Connery, J. G., & Johnson, K. S. (2010). Testing the Honeywell Durafet® for seawater pH applications. Limnology and Oceanography: Methods, 8(MAY), 172–184. https://doi.org/10.4319/lom.2010.8.172
Sabine, C., Sutton, A., McCabe, K., Lawrence-Slavas, N., Alin, S., Feely, R., et al. (2020). Evaluation of a new carbon dioxide system for autonomous surface vehicles. Journal of Atmospheric and Oceanic Technology, 37(8), 1305–1317. https://doi.org/10.1175/JTECH-D-20-0010.1
Uppström, L. R. (1974). The boron/chlorinity ratio of deep-sea water from the Pacific Ocean. Deep-Sea Research and Oceanographic Abstracts, 21(2), 161–162. 

Wanninkhof, Rik; Zhang, Jia-Zhong; Baringer, Molly O.; Langdon, Chris; Cai, Wei-Jun; Salisbury, Joseph E.; Byrne, Robert H. (2016). Partial pressure (or fugacity) of carbon dioxide, dissolved inorganic carbon, pH, alkalinity, temperature, salinity and other variables collected from discrete sample and profile observations using CTD, bottle and other instruments from NOAA Ship RONALD H. BROWN in the Gray's Reef National Marine Sanctuary, Gulf of Mexico and North Atlantic Ocean from 2012-07-21 to 2012-08-13 (NCEI Accession 0157619). [surface alkalinity and salinity]. NOAA National Centers for Environmental Information. Dataset.