Soot Aging Research Articles

Soot aging time scales in polluted regions during day and night

Abstract. The aging of soot is one of the key uncertainties in the estimation of both the direct and indirect climate effect. While freshly emitted soot is initially hydrophobic and externally mixed, it can be transferred into an internal mixture by coagulation, condensation or photochemical processes. These aging processes affect the hygroscopic qualities and hence the growth behaviour, the optical properties and eventually the lifetime of the soot particles. However, due to computational limits the aging of soot in global climate models is often only parameterised by an estimated turnover rate resulting in a lifetime of soot of several days. Hence, the aging process of soot is one of the key uncertainties governing the burden and effect of black carbon. In this study, we discuss the time scale on which diesel soot is transferred from an external to an internal mixture based on the results of our simulations with a comprehensive mesoscale model. For daytime conditions during summer condensation of sulphuric acid is dominant and the aging process occurs on a time scale of τ =8h close to the sources and τ =2h above the source region. During winter comparable time scales are found but ammonium nitrate becomes more important. During night time condensation is not effective. Then coagulation is the most important aging process and our results show time scales between 10h and 40h.

Hydration of black carbon

Hydration studies of n‐hexane soot particles in the relative H2O pressure range 0.33–0.52 have revealed the nature of the processes between water molecules in the vapor phase and primary reaction sites at the particle surface. Initially, about 40% of the carbon‐oxygen functionalities (most likely the carboxylics) on freshly prepared soot reacts irreversibly with water vapor, while at P/Po ≥ 0.48, the adsorption follows the Dubinin‐Radushkevich equation. Determination of these parameters enables calculation of surface coverage at limiting adsorption and at the chemisorption limit for all materials studied. Limiting surface coverage of fresh soot is consistent with a 50% oxygen coverage determined earlier; it increases to about 100% for nitrated soot, which is the most extensively hydrated. Hydration has been shown to increase with soot aging, a phenomenon which has revealed an important role of physisorbed O2 in hydration. Trace‐metal incorporation at the 30–110 ppm level significantly increases particle hydration and demonstrates a role of metal centers in the process.

Soot concentration and absorption coefficient in a low-pressure flame

Soot concentrations at different positions through the carbon formation zone of a premixed acetylene/oxygen flat flame at 20-mm Hg were evaluated by measuring the attenuation of a laser beam passed through the flame. The required soot absorption coefficients were measured in the same optical system by replacing the flame with standard soot samples deposited on glass slides in a molecular beam sampling instrument. The average spectral absorption coefficient of the young soot studied here (5160 cm −1) is substantially smaller than the value (76.000 cm −1) often used for full-grown soot clusters, which agrees qualitatively with the expected effect of graphitization and decreasing H C ratio in the aging of soot. In comparison with soot-particle concentrations measured previously under the same flame conditions using molecular beam sampling and electron microscope analysis of beam deposits, the concentrations observed here are substantially larger in the regions of particle nucleation and rapid surface growth, but in good agreement farther downstream in the region of predominant particle coagulation. The early difference is attributed to absorption by large hydrocarbon species whose sizes range up to that of the smallest soot particles observed under the electron microscope (about 15-Å diam or 2130 atomic mass units). The lumped concentration profile of the large hydrocarbons, estimated by assuming the absorption coefficient of these species is the same as that of the young soot, exhibits a strong peak immediately after the onset of soot formation in the region of rapidly increasing surface growth rate. Decomposition of the large hydrocarbons is probably catalyzed by soot-particle surfaces, and some of these species may serve as soot nuclei or intermediates.