Measurements of optical absorption by chromophoric dissolved organic matter using a point‐source integrating‐cavity absorption meter

Abstract

The light absorption of 0.22‐µm filtered seawater samples from the North Sea and the Atlantic Ocean was measured by a point‐source integrating‐cavity absorption meter (PSICAM), and the results were compared with measurement in a spectrophotometer. The absorption coefficient of this Gelbstoff or chromophoric dissolved organic matter (CDOM) ranges from high values of 0.103 to 0.519 m−1 at 442 nm for samples from the North Sea to very low values of 0.004 to 0.046 m−1 for samples from the Atlantic Ocean. At these low values, the influence of sample temperature and salinity on water absorption was significant. Hence, instrument‐specific temperature and salinity coefficients were determined and, subsequently, used for correcting absorption data. Spectrophotometrically determined absorption was significantly higher at shorter wavelengths (< 442 nm) than absorption measured by the PSICAM, probably showing a significant influence of scattering by small particles remaining in the sample after filtration. At wavelengths between 442 and 500 nm, the correlation of PSICAM and spectrophotometer followed the 1:1 correspondence, whereas at longer wavelengths, the absorption was below the detection limit of the spectrophotometer. A similar agreement was obtained for the exponential slope of the CDOM absorption. The PSICAM matches the spectrophotometer results but is more sensitive and not affected by scattering. It will improve our ability to determine CDOM absorption in oligotrophic waters for the visible light spectral region.