[Effect of pH perturbation on rainwater dissolved organic matter fluorescence characteristics]

The influence of pH variation on the fluorescence characteristics of rainwater dissolved organic matter (DOM) was studied via excitation emission-matrix spectroscopy. A rainwater sample (pH 5.7) collected in Xiamen (China) in the summer of 2009 was adjusted to different pH values ranging from 3.2 to 10.7. Parallel factor analysis (PARAFAC) decomposed the fluorescence matrices into four humic-like components (C1, C3, C4, and C5), one protein-like component (C2) and an unknown component (C6). As pH values of the sample increased from the initial value of 5.7 to 10.7, the fluorescence intensities of C1, C3, and C5 also increased, while those of C4 and C6 decreased gradually, resulting in an increasing contribution of humic-like components to total fluorescence. The opposite trend was observed when pH decreased to more acidic conditions and the unknown component (< 250, 330/386 nm) became dominant. The relative contribution of the protein-like component to the total fluorescence remained more or less constant as pH varied. In addition, the fluorescence index (FI), humification index (HIX) and the index of recent autochthonous contribution (BIX) were also affected by pH perturbation. It is therefore recommended that the pH of rainwater samples should be measured and reported in future studies to effectively evaluate such "pH effects" on rainwater DOM fluorescence characteristics.