Molecular composition and optical property of humic-like substances (HULIS) in winter-time PM2.5 in the rural area of North China Plain

Abstract

In order to distinguish the role of organic composition in haze episodes frequently happening in the North China Plain (NCP), the molecular composition, sources and optical property of humic-like substances (HULIS) in winter-time PM2.5 in a rural area in NCP were investigated. Fifty-five p.m.2.5 samples were collected in the day and night time from Wangdu County (a typical rural area in NCP) in Hebei province from November 4 to December 29 in 2017, and HULIS-C, OC/EC, water-soluble organic carbon (WSOC), water soluble ions and levoglucosan were simultaneously analyzed. The average concentrations of HULIS-C were 5.75 ± 2.73 μg m−3 and 7.39 ± 3.35 μg m−3 in daytime and nighttime, contributing 59 wt% and 55 wt% to WSOC, respectively. Biomass burning, coal combustion and secondary formation were identified as the main sources of HULIS. The values of Ångström absorption exponent (AAE) and mass absorption efficiencies at 365 nm (MAE365) of HULIS were in a moderate level compared with other areas and primary source (biomass burning and coal combustion). Three fluorescent components of HULIS were classified according to the excitation-emission matrix (EEM) combined with parallel factor model (PARAFAC): C1 (less oxygenated humic-like substance), C2 (highly oxygenated humic-like substance) and C3 (protein-like substance). Pearson's correlation coefficient indicated that C2 may be the dominant substance affecting the light absorption properties of HULIS and C3 contributed most to the total fluorescence intensity. FT-ICR MS identified 1839 and 2777 compound species of HULIS for lightly polluted sample and heavily polluted samples, respectively, and CHO/CHNO compounds were the main parts of HULIS in the ESI (−) mode. Lignin-like (64% and 62%) and protein/amino sugars (17% and 18%) species accounted higher proportions of HULIS.