Assessing uncertainties in scattering correction algorithms for reflective tube absorption measurements made with a WET Labs ac-9

Abstract

In situ absorption measurements collected with a WET Labs ac-9 employing a reflective tube approach were scatter corrected using several possible methods and compared to reference measurements made by a PSICAM to assess performance. Overall, two correction methods performed best for the stations sampled: one using an empirical relationship between the ac-9 and PSICAM to derive the scattering error (ε) in the near-infrared (NIR), and one where ε was independently derived from concurrent measurements of the volume scattering function (VSF). Application of the VSF-based method may be more universally applicable, although difficult to routinely apply because of the lack of commercially available VSF instrumentation. The performance of the empirical approach is encouraging as it relies only on the ac meter measurement and may be readily applied to historical data, although there are inevitably some inherent assumptions about particle composition that hinder universal applicability. For even the best performing methods, residual errors of 20% or more were commonly observed for many water types. For clear ocean waters, a conventional baseline subtraction with the assumption of negligible near-IR absorption performed as well or better than the above methods because propagated uncertainties were lower than observed with the proportional method.

Measurement uncertainties in PSICAM and reflective tube absorption meters

“…However, the tendency to overestimate absorption can most likely be attributed to residual scattering artefacts in AC-9/s data. This suggests that further work is required to develop a robust scattering correction for a wide range of water types, supporting findings of a recent study showing remaining discrepancies between PSICAM and AC-9 absorption data corrected with the semi-empirical correction [17]. Analysis of measurement uncertainties for each of these instruments raises a philosophical problem with respect to calibration approach.…”

“…Sampling sites covered a wide range of different water types from sediment-dominated estuaries (Bristol Chanel with an maximum b(532nm) of 22.89 m −1 ) to relatively clear waters north of Scotland (b(532 nm) = 0.20 m −1 ). Details on the data set and the optical properties of the waters samples can be found in several previous publications [17,33,34]. Water samples were collected using Niskin bottles, mainly at depths close to the surface (top 10 m) plus a few at greater depths, up to 85 m. On board, samples were divided into 4 subsamples for independent PSICAM absorption measurements of both total absorption and coloured dissolved organic matter (CDOM) absorption by teams from the Helmholtz-Zentrum Geesthacht (HZG) and the University of Strathclyde (Strath).…”

“…However, both absorption and attenuation measurements suffer from significant scattering errors: the reflective tube absorption measurement fails to collect all scattered photons, while the attenuation measurement does not exclude all scattered light due to the relatively large collection angle (0.9 degrees in water [14]). To correct for these measurement artefacts, a number of AC-9/s scattering corrections have been developed over the years, correcting either just the absorption [15][16][17] or both absorption and attenuation measurements [18].…”

“…Uptake of the technology by the optical oceanography community has been limited, however, largely because calibration and measurement protocols are labour intensive and require very careful sample handling. The potential value of investing time and effort into PSICAM measurements has recently been demonstrated through applications where PSICAM data was used to establish the performance of various AC-9/s scattering corrections [17,18] and to establish new methods to correct filter pad absorption data for path length amplification and scattering offset errors [31].…”

Assessment of Scattering Error Correction Techniques for AC-S Meter in a Tropical Eutrophic Reservoir

“…These methods were originally developed for marine waters and regions of temperate climate [28][29][30][31][32][33]. Overall, open ocean waters exhibit much lower a(λ) than inland waters due to lower primary productivity and sediments coming from the watershed.…”