Regular papers

Assessment of the scattering by sub-micron particles in inland waters

¹ Institute for Environmental Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands
² Current address: Water Insight BV, Marijkeweg 22, 6709 PG Wageningen, The Netherlands
³ CSIRO Land and Water, Clunies Ross Street, Black Mountain ACT 2601, Australia

^* laanen@waterinsight.nl

Received: 1 June 2011

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.