Abstract
Current protocols prescribe 0.2 µm filters for absorption measurements of Coloured Dissolved Organic Matter (a(CDOM)). However, a large historical dataset of a(CDOM) was obtained over 0.7 µm filters. Since small particles may pass 0.7 µm filters, a(CDOM) was probably overestimated because of residual scattering. Furthermore, current protocols require suspended matter to be filtered over 0.7 µm. So the 0.2 µm - 0.7 µm fraction is missing from the particles and attributed to enhanced a(CDOM), potentially leading to poor optical closure. In this paper natural water samples from a eutrophic lake were analysed with both filter pore sizes and two different absorption determination methods. Besides a standard bench spectrometer with a cuvette a calibrated Point-Source integrating-Cavity Absorption Meter (PSICAM) was used to gauge the absorption measurements. After filtration over 0.7 µm we found a systematic 8% overestimation of a(CDOM) measured in the cuvette caused by residual scattering. By filtering over 0.2 µm the a(CDOM) cuvette values are 6% underestimated because also a significant fraction of absorption was removed from the sample. As a consequence we recommend testing for coloured residues on the 0.2 µm filter after prime filtration with a glass-fibre (GF/F) filter. In case the 0.2 µm filter shows colouration, the method presented in this paper is a fast and simple check to correct the CDOM measurements.
Highlights
The colour of natural surface water is determined by the spectral characteristics and the concentrations of dissolved and suspended coloured compounds
The ρ was determined by calibration of the Point-Source integrating-Cavity Absorption Meter (PSICAM) using a series of solutions of Aldrich Humic acid with known absorption coefficients [13]
The τ is measured against a reference of Milli-Q water and the CDOM absorption spectrum is fitted to an exponential function with an offset (Eq (6))
Summary
The colour of natural surface water is determined by the spectral characteristics and the concentrations of dissolved and suspended coloured compounds. The establishment of quantitative relationships between the water colour and the concentrations of these compounds is essential for accurate derivation of the concentration of optical active constituents from handheld and satellite spectral observations. Reflection spectra are due to the light scattering of particles and water molecules that reflect upward part of the solar irradiance. A(CDOM) is the absorption by the dissolved humic substances in the water (Gilvin or Coloured Dissolved Organic Matter). This three-component model is applied in most optical modelling studies and is minimally required to successfully simulate natural waters [2]
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