College of Marine Science, University of South Florida , 140 Seventh Avenue South, St. Petersburg, Florida 33701, United States.
Pacific Marine Environmental Laboratory, NOAA , 7600 Sand Point Way NE, Seattle, Washington 98115, United States.
Environ Sci Technol. 2017 Aug 15;51(16):9127-9136. doi: 10.1021/acs.est.7b02266. Epub 2017 Aug 4.
This work describes an improved algorithm for spectrophotometric determinations of seawater carbonate ion concentrations ([CO]) derived from observations of ultraviolet absorbance spectra in lead-enriched seawater. Quality-control assessments of [CO] data obtained on two NOAA research cruises (2012 and 2016) revealed a substantial intercruise difference in average Δ[CO] (the difference between a sample's [CO] value and the corresponding [CO] 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 [CO] 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, we fit the combined-cruise data set to produce empirically updated parameters for use in processing future (and historical) [CO] absorbance measurements. With the new procedure, the average Δ[CO] offset between the two aforementioned cruises was reduced from 3.7 μmol kg to 0.7 μmol kg, which is well within the standard deviation of the measurements (1.9 μmol kg). We also introduce an empirical model to calculate in situ carbonate ion concentrations from [CO]. We demonstrate that these in situ values can be used to determine calcium carbonate saturation states that are in good agreement with those determined by more laborious and expensive conventional methods.
本工作描述了一种改进的算法，用于从富铅海水中的紫外吸收光谱观测中推导出海水碳酸根离子浓度([CO])。对两次 NOAA 研究航次（2012 年和 2016 年）获得的[CO]数据进行质量控制评估，发现平均Δ[CO]（样品[CO]值与根据 pH 和溶解无机碳的配对测量计算得出的相应[CO]值之间的差异）存在显著的航次间差异。后续调查确定，这种不一致是由于使用了两种不同的分光光度计造成的，尽管这两种分光光度计都经过了正确校准。在此，我们提出了一种基本的方法改进，可以校正[CO]吸收数据中的微小但显著的仪器差异。在将校正（值得注意的是，对于磺基内酞酣染料吸收的 pH 测定，这不是必需的）应用于船上的吸收数据后，我们拟合了联合航次数据集，以生成用于处理未来（和历史）[CO]吸收测量的经验更新参数。使用新程序，上述两次航次之间的平均Δ[CO]偏移从 3.7 μmol kg 降低到 0.7 μmol kg，这在测量的标准偏差内（1.9 μmol kg）。我们还引入了一个经验模型，用于根据[CO]计算现场碳酸根离子浓度。我们证明，这些现场值可用于确定碳酸钙饱和度状态，与更繁琐和昂贵的常规方法确定的饱和度状态非常吻合。
