PM2.5 Humic-like substances over Xi'an, China: Optical properties, chemical functional group, and source identification

Humic-like substances (HULIS) in fine particulate matter (PM2.5) were investigated during May 2015 to January 2016 in an urban environment in Xi'an, China. UV-VIS spectrometer and Fourier transform infrared spectroscopy (FTIR) were used to investigate optical properties and chemical structures of HULIS. Annual mean concentrations of HULIS-C (the carbon content of HULIS) was 2.9 +/- 2.1 mu g m(-3). On average, the contributions of HULIS-C to PM2.5, TC, and OC were 6.1%, 27.2%, and 34.5%, respectively. Seasonal average of HULIS-C concentration followed a decreasing order of winter, spring, summer, and autumn. The UV absorption intensity of PM2.5 HULIS showed the highest in winter and the lowest in summer. The chemical functional groups for PM2.5 HULIS were highlighted with the presence of aliphatic C-H, hydroxy, carbonyl, carboxyl and aromatic rings structures. SUVA(254) and SUVA(280) values in autumn and winter exhibited more dispersive distribution than those in spring and summer, which indicated sources of HULIS in autumn and winter samples were relative complicated. The E-2/E-3 ratio showed a summer maximum and a winter minimum, indicating greater conjugation and aromaticity of HULIS in winter. In addition, strong correlations between HULIS-C with K+ and OC1 + OP2 in spring, autumn and especially winter implied the important source of biomass burning to PM2.5 HULIS. The C-O stretching of COH, carbonyl groups (C=O) and O-H stretching of carboxylic acid were abundance both in winter and summer, implied that secondary organic aerosol (SOA) formation was found to be the dominant mechanism producing HULIS in Xi'an.