Humic-like substances in fresh emissions of rice straw burning and in ambient aerosols in the Pearl River Delta Region, China

Abstract. HUmic-LIke Substances (HULIS) are an abundant unresolved mixture of organic compounds present in atmospheric samples. Biomass burning (BB) has been recognized as an important primary source of HULIS, but measurements of HULIS in various fresh BB particles are lacking. In this work, HULIS in emissions of rice straw burning was measured in a number of field and chamber experiments. The average HULIS/OC ratio was 0.34±0.05 in μg/μgC, showing small variance among emissions under different burning conditions. The influence of BB on ambient HULIS levels was investigated by examining the spatial and temporal variation of HULIS and other aerosol constituents and interspecies relations in ambient PM_2.5. The PM_2.5 samples were collected at an urban and a suburban location in the Pearl River Delta (PRD), China over a period of one year. The HULIS concentrations in the ambient PM_2.5 were significantly higher in air masses originating from regions influenced by BB. Significant correlations between HULIS and water-soluble K⁺ concentrations at both sites further support that BB was an important source of HULIS. Ambient concentrations of HULIS also correlated well with those of sulfate, oxalate, and oxidant (the sum of O₃ and NO₂). The HULIS/OC ratios in BB-influenced ambient aerosols (~0.6) were much higher than those in the fresh BB emissions (0.34), implying that secondary formation was also an important source of HULIS in the atmosphere. The annual average HULIS concentrations were 4.9 μg m⁻³ at the urban site and 7.1 μg m⁻³ at the suburban site while the annual average concentrations of elemental carbon were 3.3 μg m⁻³ and 2.4 μg m⁻³, respectively. The urban-suburban spatial gradient of HULIS was opposite to that of elemental carbon, negating vehicular exhaust as a significant primary emission source of HULIS.