Chemical characterization of humic-like substances (HULIS) in PM_2.5 in Lanzhou, China

Evaporative light scattering detection (ELSD) was applied to quantify HULIS (humic-like substances) for the first time in 2012 winter and 2013 summer at an urban site in Lanzhou. Water soluble organic carbon (WSOC), water soluble inorganic ions, and carbonaceous species (OC/EC) were also analyzed. The results show that OM (Organic Matter = OC × 1.6, constituting 45.8% to PM_2.5) was the most abundant species, followed by SNA (SO₄ ^2 − + NO₃ ⁻ + NH₄ ⁺, constituting 23.4% to PM_2.5). The chemical species were in the order of: OC > EC > SO₄ ^2 − > NO₃ ⁻ > NH₄ ⁺ > Cl⁻ > Ca^2 + > K⁺. The annual average concentration of HULIS was 4.70 μg/m^− 3 and HULISc (carbon content of HULIS) contributed 6.19% to PM_2.5 and 45.6% to WSOC, indicating that HULIS was the most important components of WSOC. The concentration of HULIS was 2.14 ± 0.80 μg/m³ in summer and 7.24 ± 2.77 μg/m³ in winter, respectively. The concentrations of HULIS were relatively low and stable in summer, while high and varied dramatically in winter. The abundance of HULISc in WSOC shows a more concentrated distribution in Lanzhou, with a range between 0.28–0.57. The ratios of HULIS/K⁺ were 6.25 ± 1.41 and 6.14 ± 1.96 in summer and winter, respectively, suggesting there were other significant sources in addition to biomass burning emissions. HULIS and WSOC exhibited similar seasonal variation and had a strong positive correlation. In addition to the good relationship (0.89) between HULIS and Cl⁻ in winter, the great enhancement of HULIS with significantly high Cl⁻ and relatively low K⁺ in winter indicated that residential coal burning was probably an important HULIS source in winter. Correlation and back trajectory analysis suggested that biomass burning and secondary formation were also important HULIS sources and the contribution of HULIS from dust could be neglected. Adverse meteorological conditions were also important factors for the accumulation of HULIS in winter.