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www.sea-technology.com October 2017 / st 11 absorbance is additive (the Beer-Lambert Law), and the presence of other absorbing species in seawater interferes with the nitrate measurement. In other words, the absor- bance measured by the SUNA is the sum of the absorbance of nitrate, seawater and any additional material found in the sample volume. In SUNA calibration and processing, the ef- fects of interfering species on measured absorbance spectra are considered to accurately determine nitrate concentra- tions. The absorbance in each wavelength channel is deter- mined from the measured light intensity of the in-situ sam- ple, a reference deionized water sample measured in the laboratory and dark intensities. Dark intensities are a zero- point calibration measured with the light source turned off. Even at zero light, the spectrometer intensity is not zero due to intrinsic properties of the circuit. By measuring the zero- point and subtracting it, the intensity of the amount of light incident on the sensor is determined. In the wavelength range resolved by the SUNA spectrom- eter, bromide, nitrite and bisulfide are compounds com- monly found in seawater having quantifiable absorbance spectra that overlap with nitrate. Laboratory calibrations performed at Sea-Bird are used to remove the baseline ab- sorbance spectra. To differentiate nitrate from overlapping absorbance spectra, only the 35 spectrometer channels spanning the 217- to 240-nm wavelength range are used to estimate nitrate concentration. This narrow band is cho- sen to minimize the effects of bromide and bisulfide whose peaks fall outside this range. The nitrite absorbance spec- trum is similar to nitrate and fall within this range, but nitrite concentrations in seawater are typically smaller than the accuracy of the SUNA. The impact of nitrite on the nitrate absorbance spectrum is therefore treated as negligible. After the absorbance spectrum for nitrate is determined, nitrate concentrations can then be calculated. The absor- bance spectrum is fit to a model using multivariate linear regression, from which molar nitrate is calculated and can then be converted to a nitrate concentration. Freshwater/Saltwater Calibrations, Use To remove baseline water absorbance spectra, SUNAs are calibrated with nitrate-spiked water, as well as nitrate- free water. The default class-based calibration uses coeffi- cients generated from a library of historical calibrations. For greater accuracy, SUNA sensors are individually calibrated at Sea-Bird Scientific for use in fresh or seawater. Due to dif- ference in absorptivity characteristics of seawater and fresh- water, the calibration data for each of these applications is distinct. Calibrations for instruments to be used in freshwater are determined from aqueous nitrate standards of deionized water with 40- and 4,000-µM nitrate concentrations and pure deionized water. The second calibration at 4,000 µM is performed for accuracy in terrestrial applications where nitrate concentrations are large. It is important to note that SUNA units configured for use in freshwater do not give reliable nitrate readings in seawater, as the interference from bromide will not be accounted for without seawater absorp- tion data. Instruments calibrated for use in seawater use nitrate standards of seawater with 40-µM nitrate concentrations and nitrate-free seawater to determine the baseline seawater absorbance spectrum. The use of seawater enables the cor- rection for the UV absorption of bromide and other sea salt components. Instruments calibrated for use in seawater can be used in freshwater applications (by checking a box), but the accuracy will be decreased in regions of high nitrate concentration because the instrument is calibrated for the smaller range of nitrate concentrations typically found in seawater. Precision, Accuracy SUNA accuracy for instruments with a 10-mm length is 10 percent of the reading, or 2 µM when 10 percent is less than 2 µM for instruments. For instruments with a 5-mm path length designed for use in turbid conditions, accuracy is 10 percent of the reading, or 4 µM when 10 percent is less than 4 µM. However, this accuracy is dependent on data fidelity and corrections that are applied. The precision of the nitrate sensors is 0.3 µM in fresh- water and in seawater with the T-S correction applied; 2.4 µM in seawater without the T-S correction. Precision is de- termined by how much light from the source is returned to the sensor and the number of wavelength channels that are (Left) Vertical profile (downcast) of temperature (red), salinity (blue) and nitrate concentration (black) in the water column at Jefferson Head Station on April 4, 2017. (Right) Vertical profile (upcast in blue and downcast in red) of nitrate in the water column at Jefferson Head Station on April 4, 2017. Discrete nitrate concentrations taken during the upcast are shown as green circles.