Article (refereed) -postprint Freshwater Dom Quantity and Quality from a Two-component Model of Uv 3 Absorbance 4

28 We present a model that considers UV-absorbing dissolved organic matter (DOM) to consist 29 of two components (A and B), each with a distinct and constant spectrum. Component A 30 absorbs UV light strongly, and is therefore presumed to possess aromatic chromophores and 31 hydrophobic character, whereas B absorbs weakly and can be assumed hydrophilic. We 32 parameterised the model with dissolved organic carbon concentrations, [DOC], and 33 corresponding UV spectra for c. 1700 filtered surface water samples from North America 34 and the United Kingdom, by optimising extinction coefficients for A and B, together with a 35 small constant concentration of non-absorbing DOM (0.80 mg DOC L). Good unbiased 36 predictions of [DOC] from absorbance data at 270 and 350 nm were obtained (r=0.98), the 37 sum of squared residuals in [DOC] being reduced by 66% compared to a regression model 38 fitted to absorbance at 270 nm alone. The parameterised model can use measured optical 39 absorbance values at any pair of suitable wavelengths to calculate both [DOC] and the 40 relative amounts of A and B in a water sample, i.e. measures of quantity and quality. Blind 41 prediction of [DOC] was satisfactory for 9 of 11 independent data sets (181 of 213 42 individual samples). 43 44 45