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Measuring coastal and oceanic absorption coefficients of dissolved and particulate matter in the visible domain usually requires a methodology for amplifying the natural signal because conventional spectrophotometers lack the necessary sensitivity. The WET Labs ac-9 is a recently developed in situ absorption and attenuation meter with a precision better than ±0.001 m−1 in the raw signal, which is sufficient to make these measurements in pristine samples. Whereas the superior sensitivity of the ac-9 has been well documented, the accuracy of in situ measurements for bio-optical applications has not been rigorously evaluated.

Obtaining accurate results with an ac-9 requires careful attention to calibration procedures because baselines drift as a result of the changing optical properties of several ac-9 components. To correct in situ measurements for instrument drift, a pressurized flow procedure was developed for calibrating an ac-9 with optically clean water. In situ, micro- (cm) to fine- (m) scale vertical profiles of spectral total absorption, at(λ), and spectral absorption of dissolved materials, ag(λ), were then measured concurrently using multiple meters, corrected for drift, temperature, salinity, and scattering errors and subsequently compared. Particulate absorption, ap(λ), was obtained from at(λ) − ag(λ). CTD microstructure was simultaneously recorded. Vertical profiles of ag(λ), at(λ), and ap(λ) were replicated with different meters within ±0.005 m−1, and spectral relationships compared well with laboratory measurements and hydrographic structure.