First spectroscopic study on the structural features of dissolved organic matter isolated from rainwater in different seasons.

The complexity of rainwater dissolved organic matter (DOM) and the large percentage considered uncharacterized has made it difficult to determine the role of rainwater DOM in regional and global carbon budgets. Recent studies have concentrated on determining the structural characteristics of the bulk DOM in rainwater, but a comparison between the structural characteristics of rainwater DOM from different seasons is lacking. In this work, DOM was extracted from rainwater collected in different seasons by a procedure based on adsorption onto DAX-8 resin and a comparison between the spectroscopic characteristics of extracted DOM was performed using UV-visible, excitation-emission matrix (EEM) fluorescence and (1)H NMR spectroscopies. Similar structural characteristics were observed for extracted DOM from the different seasons: high content of aliphatic structures, hydroxy and alkoxy groups, carbonyl groups and unsaturated carbon atoms, and low content of aromatic structures when compared with aliphatic structures. The obtained results suggest a model of chemical structures for the extracted DOM from rainwater, as consisting mainly of aliphatic chains, with COOH, -CH(2)OH, -COCH(3), or -CH(3) terminal groups, and with only a minor aromatic component. Moreover, this study suggests that the DOM extracted from rainwater has higher aliphatic character and lower aromatic content than aquatic humic substances. Thus, the chemical characteristics of aquatic humic substances may not be good models for DOM extracted from rainwater.