Dissolved organic matter (DOM) plays an integral role in aquatic systems, and monitoring its concentration and composition informs carbon flux and reactivity in natural and engineered systems. Monitoring DOM has also been used as a surrogate for trace organic contaminants. Absorbance and fluorescence spectroscopy are two of the most frequently used methods to study the source, processing, and reactivity of DOM. Of all the laboratory characterization techniques, optical techniques are arguably the most amenable to implement as online, in situ sensors. However, there are many optical metrics measured in the laboratory (e.g., specific UV absorbance (SUVA) and fluorescence index (FI)) that are correlated with one another and occasionally yield contradictory interpretations. Sensor develop efforts need to prioritize metrics that are most informative by avoiding correlations with other monitored parameters. To address these knowledge gaps, we analyzed paired absorbance and fluorescence data from approximately 1000 DOM samples of diverse origin, isolation process, and treatment history. We will discuss which combinations of optical parameters are strongly correlated, which correlations provide limited or even contradictory information, and how these trends vary across different contexts. For example, the ratio of spectral slopes (SR) and absorbance ratio (E2:E3) are strongly correlated across size fractionated samples, where high ratios for both SR and E2:E3 indicate smaller molecular size. However, this relationship is not upheld for DOM treated with ozone, which is known to reduce DOM size. For scenarios with strong correlations between optical parameters (e.g., FI vs. freshness index), we discuss strategies to discern when multiple metrics may or may not be informative of unique chemical characteristics. Lastly, we critically assess how instrumental limitations may impact the sensitivity of optical parameters to quantify DOM characteristics across the wide range of gradients in natural and engineered systems.

Metanalysis of optical properties for the characterization of dissolved organic matter for online sensor development

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