Comprehensive characterizations of HULIS in fresh and secondary emissions of crop straw burning

The humic-like substances (HULIS) in PM2.5 in fresh and secondary emissions of crop straw burning will result in strong light absorption of atmospheric aerosols in the near-ultraviolet and visible light region. Herein, the relative carbon contents, light absorption properties, fluorescence characteristics, FTIR and H-1 NMR spectra of fresh and secondary HULIS in crop straws (wheat, rice, and corn) burning were comprehensively characterized. The results showed that the ratios of carbon contents in HULIS to water-soluble organic carbon and organic carbon (HULIS-C/WSOC and HULIS/OC) of HULIS in secondary emissions of crop straw burning were higher than that in fresh emissions. The MAE365 (mass absorption efficiencies at 365 nm) of secondary HULIS in crop straws burning were larger than that in fresh emissions. Excitation-emission matrix (EEM) spectra indicated that fresh HULIS contained phenol-like substances, which can form light-absorbing substances through secondary conversion. The spectrum of FTIR and H-1-nuclear magnetic resonance (H-1 NMR) showed that chemical structures of secondary HULIS in crop straws combustion were more complicated and contained more absorbing aromatic groups (e.g. aromatic rings, carbonyl groups, and aliphatic chains). Therefore, the secondary aerosol conversion may cause more aromatic structures in HULIS from crop straws burning, resulting in stronger light absorption abilities.

Automated summary

HULIS in PM2.5 from fresh and secondary emissions of crop straw burning have strong light absorption abilities, with higher carbon content, larger MAE365, and more absorbing aromatic structures in the secondary emissions. The conversion of secondary aerosols from crop straw burning can lead to the formation of light-absorbing substances with complex chemical structures.