Measurement uncertainties in PSICAM and reflective tube absorption meters

Abstract

Abstract:The nature and magnitude of measurement uncertainties (precision and accuracy) associated with two approaches for measuring absorption by turbid waters (b(532 nm) ranging from 0.20 m −1 to 22.89 m −1 ) are investigated here: (a) point source integrating cavity absorption meters (PSICAM), and (b) reflective tube absorption meters (AC-9 and AC-sboth WET Labs Inc., USA). Absolute measurement precision at 440 nm was quantified using standard deviations of triplicate measurements for the PSICAM and de-trended, bin averaged time series for the AC-9/s, giving comparable levels (< 0.006 m −1 ) for both instruments. Using data collected from a wide range of UK coastal waters, PSICAM accuracy was assessed by comparing both total non-water absorption and absorption by coloured dissolved organic material (CDOM) measured on discrete samples by two independent PSICAMs. AC-9/s performance was tested by comparing total non-water absorption measured in situ by an AC-9 and an AC-s mounted on the same frame. Results showed that the PSICAM outperforms AC-9/s instruments with regards to accuracy, with average spread in the PSICAM total absorption data of 0.006 m −1 (RMSE) compared to 0.028 m −1 for the AC-9/s devices. Despite application of a state of the art scattering correction method, the AC-9/s instruments still tend to overestimate absorption compared to PSICAM data by on average 0.014 m −1 RMSE (AC-s) and 0.043 m −1 RMSE (AC-9). This remaining discrepancy can be largely attributed to residual limitations in the correction of AC-9/s data for scattering effects and limitations in the quality of AC-9/s calibration measurements. 709-722 (1977). 3. S. Sathyendranath and T. Platt, "Computation of aquatic primary production -extended formalism to include effect of angular and spectral distribution of light," Limnol. Oceanogr. 34(1), 188-198 (1989). 4. A. Morel, "Optical Modeling of the upper ocean in relation to its biogenous matter content (Case-1 waters)," J.Geophys. Res. Oceans 93(C9), 10749-10768 (1988). 5. L. Prieur and S. Sathyendranath, "An optical classification of coastal and oceanic waters based on the specific spectral absorption curves of phytoplankton pigments, dissolved organic-matter and other particulate materials," Limnol. Oceanogr. 26(4), 671-689 (1981). 6. H. R. Gordon, "Diffuse reflectance of the ocean: influence of nonuniform phytoplankton pigment profile," Appl.Opt. 31(12), 2116-2129 (1992). 7. J. R. V. Zaneveld, R. Bartz, and J. C. Kitchen, "A reflective-tube absorption meter," Proc. SPIE 1302, 124-136 (1990 5294-5309 (2006). 37. M. S. Twardowski, J. M. Sullivan, P. L. Donaghay, and J. R. V. Zaneveld, "Microscale quantification of the absorption by dissolved and particulate material in coastal waters with an ac-9," J. Atmos. Ocean. Technol. 16(6), 691-707 (1999). 38. C. Moore, J. R. V. Zaneveld, and J. C. Kitchen, "Preliminary results from an insitu spectral absorption meter,"

Hyperspectral optical absorption closure experiment in complex coastal waters

“…The PSICAM was used to measure a nw and a CDOM , following the set-up, calibration and protocol described in Röttgers et al (2005Röttgers et al ( , 2007Röttgers et al ( , and 2018 and Lefering et al (2018). The set up uses a broad band light source coupled to an optical fiber that delivers light to the diffuse point source in the middle of a 9-cm diameter spectralon cavity.…”

“…Reported measurement uncertainties for PSICAM data are on average 0.006 m À1 (8%) for a nw and 0.002 m À1 (13%) in the case of a CDOM , varying with wavelength. This reflects a combination of underlying wavelength dependencies in lamp output, wall reflectivity, detector sensitivity, and sample absorption (Lefering et al 2018). Measurement uncertainties increase toward the extremities of the visible spectrum where they can well exceed these average levels.…”

Development of a bio-optical model for the Barents Sea to quantitatively link glider and satellite observations

“…AC-9 measurements were calibrated using purified water in the laboratory before the first cruise and later during cruises on-board the ship. Average measured offsets were subtracted from absorption and attenuation spectra in post-cruise processing [37]. Data were further corrected for the effects of salinity and temperature using simultaneously collected CTD data (Seabird, SBE19Plus v. 2).…”

Preliminary results of new calibration approach and uncertainty assessment for PSICAM

“…Since the absorption should have a similar magnitude to the samples one is interested in, it would be ideal to use multiple dilutions of the dye, however the spectral shape of absorption for nigrosine dye varies greatly with pH. The need to regularly determine absolute absorption of the nigrosine standard in the field has led to the common practice of using liquid capillary waveguide (LCW) spectrometers to allow standard absorption reference measurements with long pathlengths in a very compact, portable form factor 6,10 . LCW systems, however, can have ambiguous errors from small bubbles and high frequency oscillations observed spectrally; resulting artifacts are difficult to quantify and correct.…”