Abstract
Abstract. The emission characteristics of refractory black carbon (rBC) from biomass burning are essential information for numerical simulations of regional pollution and climate effects. We conducted combustion experiments in the laboratory to investigate the emission ratio and mixing state of rBC from the burning of wheat straw and rapeseed plants, which are the main crops cultivated in the Yangtze River Delta region of China. A single particle soot photometer (SP2) was used to measure rBC-containing particles at high temporal resolution and with high accuracy. The combustion state of each burning case was indicated by the modified combustion efficiency (MCE), which is calculated using the integrated enhancement of carbon dioxide and carbon monoxide concentrations relative to their background values. The mass size distribution of the rBC particles showed a lognormal shape with a mode mass equivalent diameter (MED) of 189 nm (ranging from 152 to 215 nm), assuming an rBC density of 1.8 g cm−3. rBC particles less than 80 nm in size (the lower detection limit of the SP2) accounted for ∼ 5 % of the total rBC mass, on average. The emission ratios, which are expressed as ΔrBC ∕ ΔCO (Δ indicates the difference between the observed and background values), displayed a significant positive correlation with the MCE values and varied between 1.8 and 34 ng m−3 ppbv−1. Multi-peak fitting analysis of the delay time (Δt, or the time of occurrence of the scattering peak minus that of the incandescence peak) distribution showed that rBC-containing particles with rBC MED = 200 ± 10 nm displayed two peaks at Δt = 1.7 µs and Δt = 3.2 µs, which could be attributed to the contributions from both flaming and smoldering combustion in each burning case. Both the Δt values and the shell / core ratios of the rBC-containing particles clearly increased as the MCE decreased from 0.98 (smoldering-dominant combustion) to 0.86 (flaming-dominant combustion), implying the great importance of the rapid condensation of semi-volatile organics. This laboratory study found that the mixing state of rBC particles from biomass burning strongly depends on its combustion processes, and overall MCE should be taken carefully into consideration while the climate effect of rBC particles from open biomass burning is simulated.
Highlights
Black carbon aerosols in the atmosphere play a vital role in climate change by absorbing solar radiation and altering the formation, lifespan and albedo of clouds (Novakov et al, 2005; Ramanathan and Carmichael, 2008; Bond et al, 2013)
Biomass burning experiments were conducted in the laboratory using wheat straw and rapeseed plants, two major agriculture crop residues, which were obtained from the Yangtze River Delta region (YRDR) during the Rudong field campaign
The combustion state of each biomass burning experiment was assessed using the modified combustion efficiency (MCE) that was calculated on the basis of fire-integrated excess carbon monoxide (CO) and CO2 mixing ratios, relative to their background values
Summary
Black carbon aerosols in the atmosphere play a vital role in climate change by absorbing solar radiation and altering the formation, lifespan and albedo of clouds (Novakov et al, 2005; Ramanathan and Carmichael, 2008; Bond et al, 2013). X. Pan et al.: Emission characteristics of refractory black carbon aerosols from fresh biomass burning activities and natural causes, abbreviated as OBB) is one of the important sources of rBC, and it contributes ∼ 42 % of atmospheric loadings in the global emissions budget (Bond et al, 2004). The rBC particles sometimes were reported to be attached on the surface of non-rBC matter (Moteki et al, 2014) Either of these circumstances renders the core–shell model invalid or introduces biases into the results. OBB is an important source of brown carbon (BrC), which has distinct light absorbing features with different wavelength dependence, and their coexistence of rBC and BrC influences the overall absorption enhancement of rBC-containing particles OBB is an important source of brown carbon (BrC), which has distinct light absorbing features with different wavelength dependence, and their coexistence of rBC and BrC influences the overall absorption enhancement of rBC-containing particles (Lack et al, 2012; Saleh et al, 2014; D. Liu et al, 2015; S. Liu et al, 2015; Saleh et al, 2015)
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