Light absorption properties of HULIS in primary particulate matter produced by crop straw combustion under different moisture contents and stacking modes

Crop straw combustion is a significant source for humic-like substances (HULIS) in fine particles (PM2.5), which have strong light absorption in the near ultraviolet and visible light region. In this study, the relative abundances, light absorption properties and fluorescence characteristics of HULIS produced by three crop straws (wheat, rice and corn straws) combusted under different moisture contents and stacking modes were comprehensively characterized. The results show that the moisture contents and stacking modes can affect the ratio of carbon content in HULIS to water-soluble organic carbon (HULIS-C/WSOC) and the ratio of HULIS to PM2.5 (HULIS/PM2.5). The absorption Angstrom exponent (AAE) and absorptivity ratios between 250 and 365 nm (E2/E3) for HULIS increased either under high moisture or stacking conditions for all three crop straws, while mass absorption efficiencies at 365 nm (MAE(365)) decreased under high moisture or stacking conditions. Four fluorescent components of HULIS were identified based on the characterization of the excitation-emission matrix combined with a parallel factor analysis model (EEM-PARAFAC): protein-like organic matter (Cl and C4), highly oxygenated humic-like substances (C3) and less-oxygenated humic-like substances (C2). A correlation analysis of the fluorescent components and light absorption indicators showed that the components C4, (C2, C3) and (C1, C2) determined the variations in the light absorption properties of HULIS derived from wheat, rice and corn straw burning, respectively. We also found that the flame burning of crop straw could reduce the contribution of light absorption to HULIS per unit mass of crop straw.