Mass‐specific light absorption coefficients of natural aquatic particles in the near‐infrared spectral region

Abstract

The extent of light absorption in the near‐infrared spectral region (NIR; 700–900 nm) of natural suspended particles was investigated by determining the absorption and mass‐specific absorption coefficients of samples from different environments: river, coastal waters, tropical lagoon, and oceanic waters. Large amounts of sample were collected onto glass‐fiber filters and measured inside the integrating sphere of a spectrophotometer. The absorption coefficient of particle suspension was also determined for visible wavelengths with a point‐source integrating cavity absorption meter. Measurable nonzero particulate absorption in the NIR was determined in all samples, even in algal cultures. It was highest in the river samples (e.g., 1.7 m−1 at 850 nm), reaching values similar to the NIR absorption of pure water—a strong NIR absorber. Lowest values were in oligotrophic waters and in algal cultures. Ratios of absorption at 750 nm to absorption at 442 and 672 nm varied between 2% to 30% and 3% to 80%, respectively. Mass‐specific absorption in the NIR at 850 nm was also highest in the river (0.012 m2 g−1) and lowest in oligotrophic waters (0.002–0.003 m2 g−1). The observed NIR absorption can partly be explained by absorption of minerogenic particles, whereas the contribution of organic detritus to the NIR absorption is still mostly unknown.