Optical And Chemical Properties Of Humic-like Substances In PM_2.5 Produced By Crop Straw Burning

Humic-like substances(HULIS)have a significant impact on the environment and human health,and crop straw burning is one of its main primary sources.In this study,China's main crop straws(wheat,rice,corn)were selected for combustion in the laboratory,and PM2.5 samples under different combustion conditions(water contents and stacking mods)were collected.The content,optical and chemical characteristics of HULIS in PM2.5 were characterized.The emission characteristics of HULIS indicate that the emission factor and relative content of HULIS increase with increasing straw moisture content and usually stacking eombustion is greater than tiling.Under tiling burning conditions,the HULIS/PM2.5 of wheat,rice and com varied with water content range by 21.67±1.95%-30.65±0.44%,22.25 ±4.28%-27.18±2.16%and 15.84±3.01%-29.29±2.44%,respectively.Under stacking burning conditions,the HULIS/PM2.5 of wheat,rice and corn varied with water content range by 21.50±2.03%-32.46 ±0.71%,23.97±5.20%-38.47±5.10%,26.89±2.41%-29.29±2.55%,respectively.Therefore,crop straws burned in a tiling mod with a low moisture content will produce less HULISThe mass absorption efficiency at 365 nm(MAE365)of HULIS decreases with increasing straw moisture content,and the MAE365 values of HULIS during stacking were smaller than those during tiling.But the absorption Angstrom exponent(AAE)and the ratio of absorbance at the wavelengths of 250 and 365(E2/E3)increases with increasing water content,and the AAE and E2/E3 values during stacking were larger than those during tiling,which indicates that HULIS produced under low moisture content and tiling conditions have stronger light absorption abilities and have larger aromaticity and molecular weightBased on the characterization of the excitation-emission matrix combined with a parallel factor analysis model(EEM-PARAFAC),four fluorescent components of HULIS were identified in HULIS produced by crop straw combustion,which were Cl(CHO+protein-like organic matter),C4(CHxSOy+protein-like organic matter),C3(highly oxygenated HULIS)and C2(less-oxygenated HULIS).C4,(C2,C3)and(Cl,C2)may be the main substances affecting the change in the HULIS light absorption properties of wheat,rice and corn straws,respectively.Fourier infrared(FTIR)and nuclear magnetic resonance(NMR)spectroscopy showed that the HULIS produced by three straw burning contained functional groups such as alcohols,phenols,fats and aromatics,and HULIS produced by low water content straw burning contained more aromatics which leads to a stronger light absorbing ability.