Bias-corrected high-resolution vertical nitrate profiles from the CTD rosette-mounted submersible ultraviolet nitrate analyzer

Abstract

Measurements by the submersible ultraviolet nitrate analyzer (SUNA) can be used to derive high-resolution in situ nitrate concentration with reliable accuracy and precision. Here we report our operational practices for SUNA deployment (including pre-cruise instrument preparation and in-cruise instrument maintenance) and detailed post-cruise nitrate quality control procedures for SUNA integrated onto the CTD rosette. This work is based on experiences and findings from over 500 individual SUNA casts collected from 24 cruises (of which 14 cruises have been quality controlled so far) over the past 5 yr. After applying previously published spectral corrections for temperature, salinity, and pressure effects, we found residual biases in SUNA nitrate estimates compared to independently measured discrete samples. We further develop and assess a new two-step procedure to remove remaining biases: (1) a general temperature-dependent adjustment at low-nitrate concentrations; and (2) a cruise-specific full-range bias correction. Our final quality-controlled SUNA nitrate data achieve an accuracy of 0.34–0.78 μM, with a precision of 0.08–0.21 μM, at a vertical resolution of 1 m. Additional comparisons between the nitrate and density data confirm the high quality of the quality-controlled SUNA data. Although applying spectral correction algorithms increases the accuracy and precision of the instrument-output nitrate concentration, we emphasize that additional constraints of SUNA measurements against other independent sources (e.g., bottle data, temperature, and density) are irreplaceable to ensure the accuracy of final nitrate data.