Improving the estimation of DOC concentrations and aromaticity from optical measurements

Obtaining real-time estimations of DOC concentration, reactivity and fate requires the ability to detect changes in the chemical composition of DOM. Optical sensors are increasingly used for this purpose but are typically incapable of distinguishing between variability caused by changes in the quantity versus quality of DOM.

This study developed a new fluorescence index for detecting changes in the composition of DOM. The aromaticity index (ARIX) links the fluorescence composition of aquatic dissolved organic matter to its SUVA aromaticity and predicts ratios between FT-ICR MS molecular formulae and between LC-OCD fractions. In datasets showing decoupling between DOC and absorbance due to biogeochemical processing, the correlation between DOC and absorbance measurements was significantly improved by accounting for interactions between absorbance and ARIX.

A meta-analysis spanning seven continents indicated a linear relationship tying SUVA to ARIX in bulk and extracted freshwater DOM. For DOM isolates, linearity extended into the oceans. These results provide new insights into the relationships between measurements obtained using different techniques for evaluating dissolved organic matter composition. They further have exciting implications for field studies involving water quality monitoring using optical sensors.