| Summary: | An automated underway pH system was operated in the hydro lab of the R/V Ronald H. Brown during the CLIVAR/GO-SHIP Repeat Section A13.5 cruise in 2010. pH was measured every 30 seconds using a Durafet III pH combination electrode and an Orion Chloride Ion Selective Electrode (Cl-ISE). This combination allows for the calculation of pH using two reference electrodes: the internal reference electrode of the Durafet (pHint) and the Cl-ISE directly submersed in seawater (pHext). pH is reported on 30 second intervals to provide the maximum flexibility for assessing precision and capturing small scale features in the ocean. Seawater continually flowed through a flowcell that housed the Durafet and Cl-ISE, then a SBE45 Micro Thermosalinograph. The temperature inside the flowcell was assumed to be the same as the SBE45. The salinity measurements from the SBE45 were not reliable on this cruise (based on comparisons with discrete bottle samples), therefore salinity from the SBE45 located next to the underway fCO2 system [Wanninkhof, 2012] was interpolated onto the pH time stamp. Latitude, longitude, and sea surface temperature (SST) were also interpolated from the underway fCO2 data set. All pH is reported on the total scale, and at SST. All data reported here are considered good. We recommend the use of pHext, as we believe this is more accurate than pHint.
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