Department of Marine System Engineering, Osaka Metropolitan University Graduate School of Engineering, 1-1, Gakuencho, Naka-ku, Sakai-shi, Osaka, 599-8531, Japan.
Material and Equipment Department, Nippon Kaiji Kyokai, 3-3, Kioicho Chiyoda-ku, Tokyo, 102-0094, Japan.
J Environ Manage. 2023 Oct 15;344:118689. doi: 10.1016/j.jenvman.2023.118689. Epub 2023 Aug 6.
Various methods have been proposed for in situ measurement of nitrate concentrations from the ultraviolet (UV) absorbance spectrum of seawater with stable salinity and constituents. However, salinity and temperature affect the UV absorption spectrum of seawater. In sea areas with large variability in salinity and water temperature, accurate nitrate ion concentration measurements remain challenging. We performed in situ measurements of nitrate, chloride, and bromide in estuarine seawater with different salinity compositions and applied water temperature compensation. First, the impact of water temperature on the UV absorbance of chloride, bromide, and nitrate was experimentally investigated and represented in a mathematical model. Next, chloride, bromide, and nitrate concentrations were estimated by suppressing the impact of residual components from the UV absorbance spectra of seawater using principal component regression (PCR). Hence, the chloride, bromide, and nitrate concentrations were determined by measuring the UV absorbance spectrum of seawater alone, without measuring water temperature and electrical conductivity. The proposed method was more accurate (±1.39 μM below 100 μM and ±0.90 μM below 20 μM) than the conventional method (±2.35 μM below 100 μM and ±1.88 μM below 20 μM) and PCR without water temperature compensation (±3.67 μM). In a field study, an in situ UV spectrophotometer with water temperature compensation was used to measure depth profiles of nitrate concentrations in estuarine seawater. We successfully measured the depth profiles of low chloride and high nitrate concentrations in the surface layer as well as high chloride and low nitrate concentrations in the lower layer. The proposed method enables in situ measurements of nitrate concentrations in waters with either stable or highly variable salinity and composition. Unlike conventional chemical analysis, our method can describe detailed spatiotemporal variations in nitrate concentrations.
已提出了多种方法，可从稳定盐度和成分的海水中的紫外线（UV）吸收光谱原位测量硝酸盐浓度。然而，盐度和温度会影响海水的 UV 吸收光谱。在盐度和水温变化较大的海域，准确测量硝酸盐离子浓度仍然具有挑战性。我们对不同盐度组成和应用水温补偿的河口海水中的硝酸盐、氯和溴进行了原位测量。首先，实验研究了水温对氯、溴和硝酸盐的 UV 吸收的影响，并将其表示在数学模型中。接下来，通过使用主成分回归（PCR）从海水的 UV 吸收光谱中抑制残余成分的影响来估计氯、溴和硝酸盐的浓度。因此，通过单独测量海水的 UV 吸收光谱，而无需测量水温和电导率，即可确定氯、溴和硝酸盐的浓度。与传统方法（低于 100 μM 时为±1.39 μM，低于 20 μM 时为±0.90 μM）和没有水温补偿的 PCR（低于 100 μM 时为±3.67 μM，低于 20 μM 时为±1.88 μM）相比，该方法更准确。在现场研究中，使用带有水温补偿的原位 UV 分光光度计测量了河口海水中硝酸盐浓度的深度分布。我们成功地测量了表层低氯和高硝酸盐浓度以及下层高氯和低硝酸盐浓度的深度分布。该方法可用于原位测量稳定或变化较大盐度和组成的水中的硝酸盐浓度。与传统化学分析不同，我们的方法可以描述硝酸盐浓度的详细时空变化。
