Chemical and Isotopic Composition of Humic-Like Substances (HULIS) in Ambient Aerosols in Guangzhou, South China

Humic-like substances (HULIS) constitute a class of organic compounds identified in atmospheric samples that influence many properties of aerosols in the atmosphere. In this study, 6 HULIS samples were isolated from atmospheric total suspended particle (TSP) samples collected at 3 locations in Guangzhou of China, 1 each in summer and winter. On the basis of analyses by elemental analyzer, more detailed chemical species of elements were explored by proton nuclear magnetic resonance (H-1 NMR) spectroscopy for hydrogen and X-ray photoelectron spectroscopy (XPS) for carbon, oxygen, nitrogen, and sulfur. The sources of HULIS were identified by carbon isotopic techniques. The results show that HULIS made up an important component of water-soluble organic carbon (WSOC). Carbon and oxygen were the predominant components in the HULIS. The H-1 NMR spectra indicated that H functional groups consisted of aliphatic C-H, aromatic C-H, H-C-C=, and H-C-O groups. They were all characterized by the highest contents of aliphatic protons and the lowest contents of aromatic protons. The XPS analysis of the HULIS fraction isolated from winter samples at the urban site Wushan (WS) showed that C, O, N, and S have various chemical states. The oxidized carbon groups accounted for 30.5% of total carbon. The nitrogen species contained amide, quaternary, and nitrate groups, and the sulfur species consisted of thiophenes, thioesters, mercaptanes, sulfones, and sulfates. The delta C-13 values of HULIS showed no obvious differences among the 6 HULIS samples. The delta N-15 values of summer HULIS samples in Maofengshan Mountain Forest Park (MFS) and Universities Town (UT) were significantly higher than those of winter samples. It is noteworthy that C-14 data clearly indicate that even in urban areas biomass-derived carbon was the major component in the atmospheric HULIS samples.