Optical indicators of DOM photoreactivity: principles, applications, and possible improvements

Sunlit waters transform dissolved organic matter (DOM) from a passive carbon pool into a source of reactive chemistry. Optical indicators derived from ultraviolet-visible absorption and fluorescence spectroscopy are commonly used to characterize this reactivity, but their mechanistic meaning is often matrix-dependent. This review critically evaluates relationships between optical indicators and the formation of photochemically produced reactive intermediates (PPRIs), including triplet-state DOM ( 3 DOM*), singlet oxygen ( 1 O 2 ), and hydroxyl radicals (·OH). Absorbance metrics such as SUVA 254 , E 2 : E 3 , and spectral slopes reflect light-harvesting capacity, aromaticity, and molecular-size distribution, whereas fluorescence indices describe humic-like, microbial, and protein-like fractions. These indicators often provide useful information on the TMP-based apparent formation efficiency of oxidizing 3 DOM* and on the apparent quantum yield of 1 O 2 in natural waters. However, their predictive power weakens in engineered or disturbance-affected matrices, where source mixing, asynchronous component changes, structural masking, and quenching decouple optical properties from reactive precursor pools. ·OH remains particularly difficult to predict because it forms through multiple DOM-mediated and non-DOM pathways. Improved use of optical indicators requires domain-specific calibration and validation, systematic evaluation of matrix effects, integrated absorbance-fluorescence information, molecular-level interpretation, and process-informed multivariate models. These approaches can improve DOM photoreactivity assessment and optical monitoring of contaminant attenuation and DOM transformation across natural and engineered water matrices.