Spectral Characteristics and Source Analysis of WSOC of PM2.5 in Winter of Xi'an

Abstract

The spectral characteristics and sources of water-soluble organic compounds (WSOC) in PM2.5 in winter were studied by using UV-vis absorption spectroscopy, three-dimensional fluorescence spectroscopy, parallel factor analysis, and backward trajectory model. The results showed that the concentration of WSOC in PM2.5 was 4.66-14.75 μg ·m-3. The values of E2/E3, E3/E4, S275-295, SUVA254, AAE, and MAE365 of WSOC were, respectively, in the range of 2.85-4.32, 2.21-3.56, 0.0099-0.0127 nm-1, 2.35-3.89 m2 ·g-1, 2.66-4.60, and 1.51-2.60 m2 ·g-1. The E2/E3, E3/E4, S275-295, and AAE values of WSOC at the sampling site in the southern suburb of Xi'an, China (Xi'an University of Architecture and Technology) were higher than those at the sampling site in the northern suburb (sports park), while the values of SUVA254 and MAE365 were lower. There were four fluorescent components in WSOC identified by the EEMs-PARAFAC model: C1 and C2 were fulvic acid-like and protein-like, respectively, and C3 and C4 were humus-like components. The fluorescence intensities and the sum of the fluorescent components were positively correlated with the concentrations of PM2.5, OC, WSOC, and A254 value (P<0.01). The fluorescence index (FI), biological source index (BIX), and humic index (HIX) values of WSOC were 1.75-2.12, 1.14-1.46, and 1.18-2.06, respectively. During the monitoring period, the air mass transmission trajectory was dominated by the local southwest of short-distance transmission, and its trajectory accounted for more than 50%. The pollutant emissions from Xinjiang, Inner Mongolia, and Gansu also made significant contributions to the air pollution levels in Xi'an in winter. There was a small difference in the carbon component content of PM2.5 in the northern and southern suburbs of Xi'an. The molecular weight, humification degree, and light absorption capacity of WSOC at the southern suburb sampling site were lower than those in the northern suburb where the wavelength dependence of light absorption intensity was relatively stronger. The WSOC mainly originated from biological sources or both from biological and terrestrial sources. Local transmission had the most significant contribution to PM2.5 and WSOC in winter.