Characterization of macromolecular organic matter in atmospheric dust from Guangzhou, China

Abstract

The chemical compositions of organic macromolecules in dust are very complex and have not yet been investigated in detail. In this paper, we study the organic macromolecules in 12 dust samples collected from an urban and suburban area of Guangzhou, China. Organic macromolecules in the dust were firstly separated into humic acids (HA), kerogen and black carbon (KB) and black carbon (BC) fractions by chemical treatment with NaOH, HCl/HF and K2Cr2O7/H2SO4, respectively. The isolated fractions were subsequently characterized using elementary analysis (EA), organic petrographic examination (OPE), scanning electron microscopy (SEM), and solid state 13C cross-polarization and magic angle spinning nuclear magnetic resonance spectroscopy (13C-CP/MAS NMR) etc. The results showed that the major organic macromolecules were KB, which accounted for 64.8%–95.8% of total organic carbon (TOC) in the dusts. Isolated HAs were characterized by high H/C, N/C and O/C atomic ratios, and high carboxyl, methoxyl and aliphatic carbon contents in the overall carbon structure; meanwhile, kerogens (K) showed lower H/C, N/C and O/C atomic ratios than those of HAs and had chemical compositions enriched in hydroxyl carbon; further, the optical features of K were similar to those of “vitrinite”. BC is formed from the combustion of biomass and fossil fuels, so that aromatic and aliphatic carbons are the most important components in its carbon skeleton. Under the microscope, BC appeared to belong to the petrographic groups, such as “semifusinte” and “fusinite”, and showed a distinct texture when observed by SEM. Compared with the HAs and K isolated from soils and sediments, HAs in dust were relatively lower in aromatic carbon and K in dust was always “type III”. These results are strongly consistent with the dust samples originating from the photochemical degradation of volatile organic compounds, in contrast to the geological HAs and K which are sourced from the biodegradation of lignin and algae or from bacterial activities in water and sediment.