Table 1. Optical properties commonly used as indicators of carbon composition.


[Abbreviations: DOC, dissolved organic cargon; DOM, dissolved organic matter; L, liters; mg, milligrams; m, meters; nm, nanometers; ex, excitation; em, emission; –, not applicable]


Parameter	Description	Calculated	Units
Specific Ultraviolet Absorbance (SUVA)1	Positively correlated with aromatic carbon content.	Absorbance at 254 nm normalized to DOC concentration	(L/mg-m)1
Spectral Slope (SX-Y) 2,3,4,5	Higher values indicate low molecular weight DOM and (or) decreasing aromaticity; lower values indicate DOM with a higher molecular weight and (or) higher aromatic content.	Nonlinear fit (exponential function) of the absorption spectrum over a specified wavelength range from X to Y nm.	nm-1
Spectral Slope Ratio (SR) 4,6	Positively correlated to DOM molecular weight; generally increases upon irradiation.	Ratio of spectral slopes: S275-295 divided by S350-400	–
Fluorescence Index (FI) 7,8	Higher values associated with microbial sources such as extracellular release and leachate from bacteria and algae; lower values associated with terrestrially-derived soil and plant organic matter; typical values range from 1.3–1.9.	The ratio of em wavelengths at 470 nm to 520 nm, obtained at ex 370 nm	–
Humification Index (HIX) 9,10,11	Indicator of humic substance content or extent of humification; based on the idea that the emission spectra of fluorescing molecules will shift toward longer wavelengths due to lower H:C ratios as humification of DOM proceeds.	Area under the em spectra 435–480 nm divided by the peak area 300–345 nm, at ex 254 nm	–

¹Weishaar and others (2003).


²Blough and Del Vecchio (2002).


³Obernosterer and Benner (2004).


⁴Helms and others (2008).


⁵Twardowski and others (2004).


⁶Spencer and others (2009).


⁷McKnight and others (2001).


⁸Cory and McKnight (2005).


⁹Zsolnay and others (1999).


¹⁰Ohno (2002).


¹¹Fellman and others (2010).

First posted February 11, 2013