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Spectrophotometric Determination of Carbonate Ion Concentrations: Elimination of Instrument-Dependent Offsets and Calculation of In Situ Saturation States

This work describes an improved algorithm for spectrophotometric determinations of seawater carbonate ion concentrations ([CO32–]spec) derived from observations of ultraviolet absorbance spectra in lead-enriched seawater. Quality-control assessments of [CO32–]spec data obtained on two NOAA research cruises (2012 and 2016) revealed a substantial intercruise difference in average Δ[CO32–] (the difference between a sample’s [CO32–]spec value and the corresponding [CO32–] value calculated from paired measurements of pH and dissolved inorganic carbon). Follow-up investigation determined that this discordance was due to the use of two different spectrophotometers, even though both had been properly calibrated. Here we present an essential methodological refinement to correct [CO32–]spec absorbance data for small but significant instrumental differences. After applying the correction (which, notably, is not necessary for pH determinations from sulfonephthalein dye absorbances) to the shipboard absorbance data, w...

Authors:   Jonathan D. Sharp; Robert H. Byrne; Xuewu Liu; Richard A. Feely; Erin E. Cuyler; Rik Wanninkhof; Simone R. Alin
Journal:   Environmental Science & Technology
Year:   2017
DOI:   10.1021/acs.est.7b02266
Publication date:   04-Aug-2017
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