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 PM_2.5 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.

为了区分有机成分在华北平原（NCP）经常发生的霾发作中的作用，在四川农村地区冬季PM _2.5中腐殖样物质（HULIS）的分子组成，来源和光学性质。NCP进行了调查。于2017年11月4日至12月29日白天和晚上从河北省望都县（NCP的典型农村地区）采集了55 pm _2.5样品，以及HULIS-C，OC / EC，水溶性有机物同时分析了碳（WSOC），水溶性离子和左旋葡聚糖。HULIS-C的平均浓度为5.75±2.73μgm ^-3和7.39±3.35μgm ^-3在白天和夜间，分别为WSOC贡献了59 wt％和55 wt％。生物质燃烧，燃煤和次生形成被确定为HULIS的主要来源。365纳米（MAE ₃₆₅）的Ångström吸收指数（AAE）和质量吸收效率的值与其他地区和主要来源（生物质燃烧和煤炭燃烧）相比，HULIS处于中等水平。根据激发发射矩阵（EEM）与并行因子模型（PARAFAC）组合，HULIS的三个荧光成分被分类为：C1（低氧腐殖质样物质），C2（高氧腐殖质样物质）和C3（蛋白质-像物质）。皮尔逊相关系数表明，C2可能是影响HULIS吸光性能的主要物质，而C3对总荧光强度的贡献最大。FT-ICR MS分别为轻度污染和重度污染的样品鉴定出HULIS的1839和2777种化合物，而CHO / CHNO化合物是ESI（-）模式下HULIS的主要组成部分。