Comprehensive characterization of humic-like substances in smoke PM_2.5 emitted from the combustion of biomass materials and fossil fuels

Abstract. Humic-like substances (HULIS) in smoke fine particulate matter (PM_2.5) emitted from the combustion of biomass materials (rice straw, corn straw, and pine branch) and fossil fuels (lignite coal and diesel fuel) were comprehensively studied in this work. The HULIS fractions were first isolated with a one-step solid-phase extraction method, and were then investigated with a series of analytical techniques: elemental analysis, total organic carbon analysis, UV–vis (ultraviolet–visible) spectroscopy, excitation–emission matrix (EEM) fluorescence spectroscopy, Fourier transform infrared spectroscopy, and ¹H-nuclear magnetic resonance spectroscopy. The results show that HULIS account for 11.2–23.4 and 5.3 % of PM_2.5 emitted from biomass burning (BB) and coal combustion, respectively. In addition, contributions of HULIS-C to total carbon and water-soluble carbon in smoke PM_2.5 emitted from BB are 8.0–21.7 and 56.9–66.1 %, respectively. The corresponding contributions in smoke PM_2.5 from coal combustion are 5.2 and 45.5 %, respectively. These results suggest that BB and coal combustion are both important sources of HULIS in atmospheric aerosols. However, HULIS in diesel soot only accounted for  ∼  0.8 % of the soot particles, suggesting that vehicular exhaust may not be a significant primary source of HULIS. Primary HULIS and atmospheric HULIS display many similar chemical characteristics, as indicated by the instrumental analytical characterization, while some distinct features were also apparent. A high spectral absorbance in the UV–vis spectra, a distinct band at λ_ex∕λ_em ≈  280∕350 nm in EEM spectra, lower H ∕ C and O ∕ C molar ratios, and a high content of [Ar–H] were observed for primary HULIS. These results suggest that primary HULIS contain more aromatic structures, and have a lower content of aliphatic and oxygen-containing groups than atmospheric HULIS. Among the four primary sources of HULIS, HULIS from BB had the highest O ∕ C molar ratios (0.43–0.54) and [H–C–O] content (10–19 %), indicating that HULIS from this source mainly consisted of carbohydrate- and phenolic-like structures. HULIS from coal combustion had a lower O ∕ C molar ratio (0.27) and a higher content of [Ar–H] (31 %), suggesting that aromatic compounds were extremely abundant in HULIS from this source. Moreover, the absorption Ångström exponents of primary HULIS from BB and coal combustion were 6.7–8.2 and 13.6, respectively. The mass absorption efficiencies of primary HULIS from BB and coal combustion at 365 nm (MAE₃₆₅) were 0.97–2.09 and 0.63 m² gC⁻¹, respectively. Noticeably higher MAE₃₆₅ values for primary HULIS from BB than coal combustion indicate that the former has a stronger contribution to the light-absorbing properties of aerosols in the atmospheric environment.