Laboratory evaluation of 4-channel irradiance radiometers suitable for observations on Argo floats

In this study, we evaluate the performance of three 4-channel irradiance radiometers suitable for the observations on Argo floats (Sea-Bird/Satlantic OCR-504, RBR quadrante, and SIAT SLIM-I4) through two laboratory experiments assessing their radiometric accuracy and stability, and their dark-signal stability and residual. Our findings highlight the importance of temperature correction for dark signals and provide insights into the aging drift effect over time. While all three sensors exhibit similar performance levels, the OCR-504 demonstrates high radiometric accuracy and stability but relatively low dark-signal stability; the quadrante offers excellent dark-signal stability, though with large dark-signal residual and relatively low radiometric accuracy and stability; and the SLIM-I4 excels in both radiometric and dark-signal performance. Additionally, we observed the aging drift on the dark signal of the OCR-504 over six years and found that the drift only affects the intercept term of dark signals, leaving the slope term unaffected. Based on these results, we recommend all the radiometers be equipped with an internal temperature sensor for more accurate temperature correction, and we propose three key technical criteria for irradiance radiometers on Argo floats: radiometric accuracy within ±3%, radiometric stability within 1% for irradiance and 0.1% for PAR, and dark-signal stability superior to 0.005 µW/cm²/nm for irradiance and 0.03 µmol/m²/s for PAR. These criteria are essential for ensuring reliable, consistent, and interoperable BGC-Argo datasets, as well as for facilitating the integration of new radiometers into the Argo program.