Light absorption properties of laboratory-generated tar ball particles
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Abstract. Tar balls (TBs) are a specific particle type that is abundant in the global troposphere, in particular in biomass smoke plumes. These particles belong to the family of atmospheric brown carbon (BrC), which can absorb light in the visible range of the solar spectrum. Albeit TBs are typically present as individual particles in biomass smoke plumes, their absorption properties have been only indirectly inferred from field observations or calculations based on their electron energy-loss spectra. This is because in biomass smoke TBs coexist with various other particle types (e.g., organic particles with inorganic inclusions and soot, the latter emitted mainly during flaming conditions) from which they cannot be physically separated; thus, a direct experimental determination of their absorption properties is not feasible. Very recently we have demonstrated that TBs can be generated in the laboratory from droplets of wood tar that resemble atmospheric TBs in all of their observed properties. As a follow-up study, we have installed on-line instruments to our laboratory set-up, which generate pure TB particles to measure the absorption and scattering, as well as the size distribution of the particles. In addition, samples were collected for transmission electron microscopy (TEM) and total carbon (TC) analysis. The effects of experimental parameters were also studied. The mass absorption coefficients of the laboratory-generated TBs were found to be in the range of 0.8–3.0 m2 g−1 at 550 nm, with absorption Ångström exponents (AAE) between 2.7 and 3.4 (average 2.9) in the wavelength range 467–652 nm. The refractive index of TBs as derived from Mie calculations was about 1.84 − 0.21i at 550 nm. In the brown carbon continuum, these values fall closer to those of soot than to other light-absorbing species such as humic-like substances (HULIS). Considering the abundance of TBs in biomass smoke and the global magnitude of biomass burning emissions, these findings may have substantial influence on the understanding of global radiative energy fluxes.Keywords:
Particle (ecology)
Reactivity
Carbon fibers
Dimethyl carbonate
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The formation of particulate matter (PM or soot) emissions in diesel engines is of great interest, and the development of a predictive mechanism is an important part of understanding the production of these pollutants. In this paper, a soot-oxidization model is presented on the basis of previous soot-mechanism research, referred to as average-reaction-rate (ARR) model. The ARR soot-oxidization model was combined with the Hiroyasu soot-formation model to compose the H-ARR soot model. With the H-ARR soot model, the process of soot formation and oxidization was numerically modeled according to different conditions of fuel postinjection. Meanwhile, the impact factors and effect mechanism of fuel postinjection on soot oxidization were analyzed in combination with experimental data. Finally, it was demonstrated that the resulting H-ARR soot model could describe the real process of soot formation and oxidization in diesel engine, as well as the influence rule of fuel postinjection on soot oxidization. Especially, it was observed that the brake specific soot emission can be reduced to 30% at the best experimental point of fuel postinjection.
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