Correlation between molecular absorption spectral slope ratios and fluorescence humification indices in characterizing CDOM

Ultraviolet-visible absorption spectral slope ratios S-R (slope in 275-295 nm divided by slope in 350-400 nm) and humification index (HIX, integrated fluorescence emission in 435-480 nm divided by that in 435-480 and 300-345 nm) were compared when characterizing chromophoric dissolved organic matter (CDOM) in three humic acids and 44 whole water samples. HIX increased with increasing pH for humic acids, while their S-R showed much more complicated dependencies on pH. There was a negative correlation between S-R and HIX. S-R increased in the order terrestrial coal/peat < terrestrial soil/river < seawater, while HIX increased in the order seawater < terrestrial soil/river < terrestrial coal/peat. The comparative study in this work indicates that terrestrially derived CDOM has higher HIX and lower S-R than marine CDOM. Investigators may potentially use these two indices to compare qualitatively the character of CDOM in different sources (e.g., terrestrial vs. marine).