During R/V Maria S. Merian cruise MSM97/2 a sensor box (AddOn Box) was attached to a PocketFerryBox (4H Jena, Germany) in-line obtaining waters from the ships two autonomous measurement systems (RSWS). For the measurement of underway absorption spectra to calculate optical nitrate, the AddOn Box was equipped with two UV-process spectrophotometer (ProPS and OPUS, TriOS Mess- und Datentechnik GmbH, Germany). Data were recorded continuously in a ten-minute interval along the cruise track with an optical path length of 10 mm. The integration time was 256 ms. The ProPS photometer is equipped with a Deuterium lamp as light source whereas the OPUS photometer is equipped with a Xenon flash lamp. Baseline calibrations were done for both UV-photometer using ultrapure water. For the ProPS, the calibration was done in the laboratory before the cruise whereas the calibration for the OPUS was done by the manufacturer. The water inlet of the RSWS is allocated at approx. 6.5 m below the sea surface. While one box is measuring, the other box is being cleaned. The boxes switch after a user-defined measuring and cleaning interval. On MSM97/2, the boxes were alternating every 12 hours, including one short cleaning procedure during measurements after 4 hours. Absorption spectra obtained by the AddOn Box during the alternation process of the two RSWS, as well as during the internal cleaning procedure are not included. Spectra from station work are included. Within this dataset, absorption spectra were cut to a relevant wavelength range (from 189-360 nm (ProPS) and 199-360 nm (OPUS) to 210-260 nm). No further correction was done for the absorbance spectra in this processing step. After the cruise, gathered absorption spectra were used to improve current processing algorithms of optical nitrate detection for coastal and open ocean waters. Processing was performed according to Zielinski et al. (2011) and Frank et al. (2014) using MATLAB (R2018a). Detailed processing steps for the calculation of optical nitrate can be found on Zenodo (doi 10.5281/zenodo.4091420 ). Temperature and salinity data are necessary to compute nitrate values from derived UV-spectra and were taken from the attached PocketFerryBox using a SBE45 probe (Sea Bird Scientific, USA). A linear CDOM-offset correction was carried out. Nitrate reference spectra and salinity reference spectra were measured in the laboratory before the cruise. Corresponding nitrate reference samples using wet chemical analysis will be published separately.
