Abstract
Abstract. Biomass burning is one of the major sources of carbonaceous aerosols, which affects air quality, the 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 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, organic carbon (OC) and elemental carbon (EC) were 9.3±3.4, 4.6±1.9 and 0.21±0.07 g kg−1 for corn burning and 8.7±5.0, 3.9±2.8 and 0.22±0.05 g kg−1 for wheat burning, generally lower than wood or forest burning emissions. Though the mass contribution of WISOC to OC (32 %–43 %) was lower than WSOC, the light absorption contribution of WISOC (57 %–84 % at 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 the water-soluble fractions were considered. However, the light absorption of WSOC in the near-UV range, occupying 39 %–43 % of the total extracted OC absorption at 300 nm, cannot be negligible due to the sharper increase of absorption towards shorter wavelengths compared with WISOC. HULIS were the major light absorption contributors to 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 study. 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 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 ratios of K+/OC and Cl-/OC increased under higher MCE conditions due to the enhancement of potassium and chlorine released under higher fire temperatures during flaming combustion. This indicates that the potassium ion, as a commonly used biomass burning tracer, may lead to estimation uncertainty if the burning conditions are not considered.
Highlights
Biomass burning emissions, as a major primary source of carbonaceous aerosols, have significant effects on air quality, human health and the regional or global radiation budget (Bond, 2004; Chen et al, 2017a; Reid et al, 2005; Saleh et al, 2015)
Smoldering-dominated conditions, with an expected modified combustion efficiency (MCE) < 0.9 or even lower, have been widely observed during the combustion of agricultural residues in the field (IARI, 2012; Wang et al, 2017); the results in this study are applicable to field or related model studies
Though the mass contribution of water-insoluble organic carbon (WISOC) was lower than water-soluble organic carbon (WSOC) (Fig. 2), the light absorption of WISOC surpassed WSOC due to the higher light absorption capability of water-insoluble biomass burning organic aerosols (BBOA), indicated by the higher mass absorption efficiency (MAE) of WISOC (Fig. 6)
Summary
As a major primary source of carbonaceous aerosols, have significant effects on air quality, human health and the regional or global radiation budget (Bond, 2004; Chen et al, 2017a; Reid et al, 2005; Saleh et al, 2015). Biomass burning could contribute onethird of the black carbon (BC) budget and two-thirds of the primary organic aerosol budget on the global scale (Bond, 2004; Bond et al, 2013). Biomass burning organic aerosols (BBOA) attracted much attention due to their substantial contribution to light-absorbing organic aerosols, known as brown carbon (BrC) (Andreae and Gelencsér, 2006; Laskin et al, 2015; Lin et al, 2016; Saleh et al, 2014; Washenfelder et al, 2015; Yan et al, 2018). Regional radiative forcing effects of BrC could be comparable to those of BC over major areas dominated by biomass burning and biofuel combustion, such as South and East Asia (Feng et al, 2013)
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