Light absorption by natural aquatic particles in the near-infrared (700 900 nm) spectral region.

Light absorption by aquatic particle in the near-infrared spectral region is often assumed to be negligible. So, any observed attenuation in this spectral region was supposed to be due to errors induced by scattering, and a general practice to correct for this scattering error in absorption measurements is to subtract the attenuation of these NIR wavelengths from all other wavelengths. We used the sensitive quantitative filter technique to measure particulate absorption but placed the filter inside an integrating sphere. The positioning of the filter inside the sphere does strongly improve the precision of the whole measurement by making any scattering error negligible. This allowed us to perform very sensitive measurements in the NIR region of 700 to 900 nm. Samples from different environments (open ocean, coastal waters and a river) and geographical regions (North Sea, Baltic Sea and the Atlantic Ocean) were examined. In nearly all cases a significant NIR particulate absorption could be observed that was as well visible when the samples had been bleached. In many case the NIR particulate absorption was high, reaching even 40 % of the absorption at the red chlorophyll maximum (672 nm). A good correlation of NIR particulate absorption with back-scattering in the same spectral region and with the concentration of total suspended matter was observed. We conclude that near-infrared particulate absorption mainly by detritus and minerals is not negligible (but might sometimes be very low), and that a subtraction of the measured attenuation in this region might lead to a large underestimation of the real absorption at shorter wavelength. http://oceanopticsconference.org/abstracts/by_session/3