Chemical and Absorption Characteristics of Water-soluble Organic Carbon and Humic-like Substances in Size-segregated Particles from Biomass Burning Emissions

Abstract

In this study, measurements of size-segregated particulate matter (PM) emitted from the combustion of rice straw, pine needles, and sesame stem were conducted in a laboratory chamber. The collected samples were used to analyze amounts of organic and elemental carbon (OC and EC), water-soluble organic carbon (WSOC), humic-like substances (HULIS), and ionic species. The light absorption properties of size-resolved water extracts were measured using ultraviolet-visible spectroscopy. A solid-phase extraction method was first used to separate the size-resolved HULIS fraction, which was then quantified by a total organic carbon analyzer. The results show that regardless of particle cut sizes, the contributions of size-resolved HULIS (=1.94 ×HULIS-C) to PM size fractions (PM0.32, PM0.55, PM1.0, and PM1.8) were similar, accounting for 25.2-27.6, 15.2-22.4 and 28.2-28.7% for rice straw, pine needle, and sesame stem smoke samples, respectively. The PM1.8 fraction revealed WSOC/OC and HULIS-C/WSOC ratios of 0.51 and 0.60, 0.44 and 0.40, and 0.50 and 0.60 for the rice straw, pine needle, and sesame stem burning emissions, respectively. Strong absorption with decreasing wavelength was found by the water extracts from size-resolved biomass burning aerosols. The absorption Ångström exponent values of the size-resolved water extracts fitted between 300 and 400 nm wavelengths for particle sizes of 0.32-1.0 μm were 6.6-7.7 for the rice straw burning samples, and 7.5-8.0 for the sesame stem burning samples. The average mass absorption efficiencies of size-resolved WSOC and HULIS-C at 365 nm were 1.09 (range: 0.89-1.61) and 1.82 (range: 1.33-2.06) m2/g·C for rice straw smoke aerosols, and 1.13 (range: 0.85-1.52) and 1.83 (range: 1.44-2.05) m2/g·C for sesame stem smoke aerosols, respectively. The light absorption of size-resolved water extracts measured at 365 nm showed strong correlations with WSOC and HULIS-C concentrations (R2=0.89-0.93), indicating significant contribution of HULIS component from biomass burning emissions to the light absorption of ambient aerosols.

Keywords

• size-segregated biomass burning aerosols
• wsoc
• hulis
• absorption ångström exponent
• mass absorption efficiency