Evaluation and Improvement of an Iterative Scattering Correction Scheme for in situ Absorption and Attenuation Measurements

Abstract

The performance of several scattering correction schemes for reflecting-tube absorption and beam attenuation measurements is evaluated with data collected in European shelf seas. Standard scattering correction procedures for absorption measurements perform poorly because of nonzero absorption in the near infrared and wavelength-dependent scattering phase functions. A previously described iterative correction procedure based on Monte Carlo simulations of the Western Environmental Technologies Laboratories (WET Labs) ac-9 and independent estimates of particle backscattering initially performs poorly, but is greatly improved when realistic losses at flow-tube walls are incorporated into the model. The updated Monte Carlo scattering correction provides excellent agreement with independent absorption and attenuation measurements made with a point-source integrating-cavity absorption meter (PSICAM) and a Laser In Situ Scattering and Transmissometer (LISST, Sequoia Scientific), respectively. Implications for historic datasets and requirements for application to future datasets are discussed.

Measurement uncertainties in PSICAM and reflective tube absorption meters

“…The reflective tube technology is well established as an industry standard for in situ measurements of spectral absorption (NASA Protocols [9]). In light of recent improvements in scattering correction approaches for AC-9/s instruments [16,18], it seems timely to revise and quantify uncertainties associated with AC-9/s absorption measurements beyond scattering errors. The PSICAM technique, on the other hand, is much less widely used because its operation is laborious, despite its potential to produce absorption data that is effectively free from systematic scattering artefacts.…”

“…Although it is not possible to determine the true absorption coefficient of a sample, given its relatively high precision and absence of systematic scattering error, PSICAM data has previously been used as a reference for validating other absorption measurements [16,18,31]. Results showed that AC-9/s data typically overestimate PSICAM absorption across the representing average estimates of accuracy statistics for AC-9 and AC-s data respectively.…”

“…Most scattered light is redirected to a diffuse detector using total internal reflection at the external air-quartz interface of the flow tube [38,39]. However, a significant portion of scattered light (particularly beyond the critical angle for total internal reflection [18]) does not reach the detector which can result in overestimation of absorption coefficients of > 80% [16]. Absorption data measured in AC-9/s devices therefore have to be corrected for scattering errors by applying one of several correction methods which have been proposed over the years.…”

“…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].…”