Chemical composition and light absorption of carbonaceous aerosols emitted from crop residue burning: Influence of combustion efficiency

Abstract

Abstract. Biomass burning is one of the major sources of carbonaceous aerosols, which affects air quality, radiation budget and human health. Field straw residue burning is a widespread type of biomass burning in Asia, while its emissions are poorly understood compared with the wood burning emissions. In this study, lab-controlled straw (wheat and corn) burning experiments were designed to investigate the emission factors and light absorption properties of different biomass burning organic aerosol (BBOA) fractions, including water soluble organic carbon (WSOC), humic-like substances (HULIS) and water insoluble organic carbon (WISOC). The influences of biofuel moisture content and combustion efficiency on emissions are comprehensively discussed. The emission factors of PM2.5, OC and EC were 9.3 ± 3.4, 4.6 ± 1.9 and 0.21 ± 0.07 g/kg for corn burning and 8.7 ± 5.0, 3.9 ± 2.8 and 0.22 ± 0.05 g/kg for wheat burning, generally lower than wood or forest burning emissions. Though the mass contribution of WISOC among OC (32 %–43 %) was lower than WSOC, the light absorption contribution of WISOC (57 %–84 % @ 300–400 nm) surpassed WSOC due to the higher mass absorption efficiency (MAE) of WISOC. The results suggested that BBOA light absorption would be largely underestimated if only considering the water soluble fractions. However, the light absorption of WSOC among near-UV ranges, occupying 39 %–43 % of the total extracted OC absorption at 300 nm, cannot be negligible due to the sharper increase of absorption towards shorter wavelength compared with WISOC. HULIS were the major light absorption contributors among WSOC, due to the higher MAE of HULIS than other high-polarity WSOC components. The emission levels and light absorption of BBOA were largely influenced by the burning conditions, indicated by modified combustion efficiency (MCE) calculated by measured CO and CO2 in this work. The emission factors of PM2.5, OC, WSOC, HULIS and organic acids were enhanced under lower-MCE conditions or during higher-moisture straw burning experiments. Light absorption coefficients of BBOA at 365 nm were also observed higher under lower-MCE conditions, which was mainly due to the elevated mass emission factors. Our results suggested that the influence of varied combustion efficiency on particle emissions could surpass the differences caused by different types of biofuels. Thus, the burning efficiency or conditions should be taken into consideration when estimating the influence of biomass burning. In addition, we observed that the K+ / OC and Cl− / OC ratios increased under higher-MCE conditions due to the enhancement of released potassium and chlorine under higher fire temperatures during flaming combustion. This indicates that potassium ion, as a commonly used biomass burning tracer, may lead to estimation uncertainty if without considering the burning conditions.

Linking the chemical composition and optical properties of biomass burning aerosols in Amazonia

“…24 During combustion, thermal decomposition and pyrolysis of compounds found in plants, such as lignin, tannins, etc., can release BrC. Thus, BB is an important source of primary BrC, 25 as well as precursors that produce secondary organic aerosols (SOAs) that absorb light. Moreover, the composition and optical properties of BrC evolve during its atmospheric life cycle, resulting in the processes of "whitening" or "darkening", decreasing or increasing the ability to absorb UV and visible light.…”