This dataset is licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) License. Christopher Hunt University of New Hampshire (UNH)

Durham NH 03824 United States

6038621348 chunt@unh.edu 0000-0001-8061-4560 Douglas Vandemark Ocean Process Analysis Laboratory, University of New Hampshire

Morse Hall Room 142 8 College Road Durham NH 03824 United States of America

Shawn Shellito Ocean Process Analysis Laboratory, University of New Hampshire

Morse Hall Room 142 8 College Road Durham NH 03824 United States of America

Christopher Hunt University of New Hampshire

Morse Hall Room 142 8 College Road Durham NH 03824 United States of America

603-862-1348 chunt@unh.edu 0000-0001-8061-4560 This archival package contains the discrete (Niskin bottle and CTD profile) data collected near the UNH/PMEL CO2 Buoy, located off the coast of New Hampshire in 2023. All data were collected aboard the UNH R/V Gulf Challenger. Additionally, an underway pCO2 measurement system was operated on some of these cruises, and surface underway pCO2 data are included in this archival package. 2025-01-13 2025-07-14 -70.543 -70.542 43.022 43.020 R/V Gulf Challenger 33GC research vessel University of New Hampshire USA Hunt, Christopher, William, Vandemark, Douglas Salisbury, Joseph Dissolved Inorganic Carbon DIC IN-SITUOBSERVATION MEASURED profile umol/kg 0.1% Apollo SciTech AS-C2 Discrete samples were collected from Niskin bottles and transferred to sample bottles through silicone tubing to prevent bubbling. Samples for pH, total alkalinity, and DIC were transferred without bubbling into 60-ml or 125-ml glass BOD bottles with greased stoppers. These were filled to leave less than 1 % headspace in the bottle, preserved with 0.1 ml of saturated mercuric chloride solution, and immediately cooled. Samples for DIC were generally stored for several weeks before analysis, allowing for settling of particulate material, with the supernatant sample drawn for analysis. DIC of unfiltered water was determined using an AD-D1 automated analyzer built by Apollo SciTech. Immediately after opening the sample bottle, a digital syringe withdrew an aliquot of sample (9.0 mL), acidified it with 10 % phosphoric acid and subsequently measured the evolved CO2 with a Picarro G2131-i cavity ringdown mass spectrometer. Certified seawater reference materials from Dr. A. Dickson were used to determine DIC concentration by preparing a calibration curve covering the range of DIC from 200 to 2000 umol/kg, with a resulting average precision of 0.1 % (2 umol/kg ). daily Dickson Lab, Scripps various HgCL2 0.1 mL saturated HgCl2 Total Alkalinity Talk IN-SITUOBSERVATION MEASURED profile umol/kg 0.1% Custom Titrator, Based on Metrohm Dosimat Discrete samples were collected from Niskin bottles and transferred to sample bottles through silicone tubing to prevent bubbling. Samples for pH, total alkalinity, and DIC were transferred without bubbling into 300-ml or 500-ml glass BOD bottles with greased stoppers. These were filled to leave less than 1 % headspace in the bottle, preserved with 0.03% v/v saturated mercuric chloride solution, and immediately cooled. Samples for total alkalinity/pH and DIC were generally stored for several weeks before analysis, allowing for settling of particulate material, with the supernatant sample drawn for analysis. TAlk was measured by Gran titration (Gran 1952) with 0.1 N HCl using an automated titrator. This method adds an initial aliquot of acid to the sample in an open cell, generally lowering the sample pH below 3.5, and then adds subsequent aliquots of titrant until the pH changes linearly with the volume of acid added. The TAlk endpoint is then obtained from linear regression of the change of pH against the volume of acid added, according to a Gran transformation Multiple analyses of the Dickson reference material resulted in a calculated precision of this method of about 0.1 % (or �2 umol kg-1 ). The accuracy of the TAlk automated system, also calibrated with multiple batches of the Dickson CRM as discussed above, was +/-3 umol kg-1. daily Dickson Lab, Scripps various HgCl2 0.1 mL saturated HgCl2 OPEN Gran pH pH IN-SITUOBSERVATION MEASURED 0.001 Douglas and Byrne 2017, Marine Chemistry 197: 64-69 Niskin bottle Agilent 8454 UV-Vis spectrophotometer with custom software Discrete samples were collected from Niskin bottles and transferred to sample bottles through silicone tubing to prevent bubbling. Samples for pH were transferred without bubbling into 125-ml or 300-mL glass BOD bottles with greased stoppers. These were filled to leave less than 1 % headspace in the bottle, preserved with 0.1-0.3 ml of saturated mercuric chloride solution, and immediately cooled. Samples for pH were generally stored for several weeks before analysis, allowing for settling of particulate material, with the supernatant sample drawn for analysis. Samples were brought to 25 degrees C in a water bath, which was also used to maintain the temperature of the spectrophotometer sample cell holder. Sample pH was measured spectrophotometrically using an Agilent technologies Cary UV-Vis spectrophotometer. 10-cm cells were filled with sample via a siphon tube, and the sample was maintained at 25 degrees C in a customized temperature-controlled box. After a blank measurement, 0.1 mL of metacresol purple (mCP) was added to each sample, and the R ratio of absorbances at 578 nm and 434 nm were used to determine the pH, as described by Douglas and Byrne 2017. To correct for pH effect of mCP, a second mCP addition and pH measurement were made, and the resulting two pH measurements used to correct to a pH with zero mCP. TOTALSCALE(T) 25 25 pco2 (fco2) autonomous pCO2a IN-SITUOBSERVATION MEASURED surface underway uatm Gas equilibrator Li-cor LI840 "Flow to the shipboard flow-through system was pumped to an equilibrator, similar to that described by Wanninkhof and Thoning (1993), but consisting of three Plexiglas chambers instead of a single chamber. Equilibrated air was drawn out of the third chamber, while ambient air was drawn into the first chamber and passed through the second and third chambers, equilibrating with the pumped water supply at each step. The LI840 was calibrated with ultra high-purity nitrogen (0 CO2) and a CO2/N2 mixture standardized against a certified CO2/N2 mixture provided by NOAA ESRL. daily Scott-Marrin, references to NOAA ESRL various Li-cor 840 in-situ 1 6 vented 10 not measured not measured Nafion selectively permeable membrane (Perma Pure, Toms River NJ) with a counterflowing stream of dry nitrogen, which dried the sample gas stream of water vapor