Soot and polycyclic aromatic hydrocarbons (PAH) are main pollutants as well as NOx in combustion process. In this paper, recent advances in the understanding of the formation mechanisms of PAH and the subsequent growth by chemical reactions with radical and stable species are reviewed. In the growth processes of PAH, hydrogen abstraction-C2H2 addition (HACA) mechanism and recently-proposed phenyl addition-cyclization (PAC) mechanism are discussed. A combination of PAC mechanism with HACA mechanism is important in the production and growth to large PAH. Particle inception, surface growth, coagulation and aggregation processes of soot are also briefly reviewed.
Soot particles, which are emitted from combustion of carbonaceous materials, have complicated shapes and chemical and physical properties, and they are important factors for the global climate as well as human health problems. They consist of several hundreds of carbon nanospheres with ~40 nm in diameter. Fundamental physical properties of these carbon nanospheres and their fractal structures need to be understood in improving our knowledge on global climate and atmospheric aerosol pollution. In this report, nano- to micron-sized properties of soot particles analyzed by using transmission electron microscopy are reviewed. The results include high-resolution images of the carbon nanospheres and standard graphite samples as well as individual soot particles with various fractal structures.
Black carbon (BC) emitted from incomplete combustions of fossil fuels and biomass is likely the second strongest positive climate forcer next to the carbon dioxide. Uncertainties in calculations of global and local radiative forcing of BC are still considerably large compared to the greenhouse gases, partly because of incomplete observational understandings and unpredictability of microphysical properties of aerosols containing BC and their efficiency of the wet removal during the cloud-precipitation processes.This paper reviews the principle and applications of the single-particle laser-induced incandescence technique, a recently developed method for fast real-time measurements of the detailed properties of individual particles containing BC. Most importantly, single-particle BC mass observable by this technique can be regarded as a tracer of BC-containing particles in an air parcel, providing a novel method to explore the possible difference in wet removal efficiency among particles with different microphysical properties. Finally, future perspectives of black carbon studies are briefly given.
The role of black carbon particles in the climate system has been recognized to be particularly important because of the heating effects associated with their efficient absorption of solar radiation. An important point in evaluating the effects of black carbon particles on climate is that regional-to-global-scale radiative forcing and the spatial distribution of black carbon particles depend strongly on their mixing state, namely, the degree to which black carbon particles are coated with other aerosol components. This article reviews modeling studies of black carbon particles using from aerosol box models that explicitly represent aerosol microphysical properties to regional- and global-scale aerosol models. In particular, this article focuses on the mixing state and removal processes of black carbon particles, because their treatment causes large uncertainties in current aerosol models.
We hypothesized that PM2.5 mass concentration could be estimated by the number concentration of aerosol when the aerosol in the accumulation mode is formed by the aggregation of primary particles.In order to verify this hypothesis, long term parallel observations of PM2.5 were carried out in the urban area atmosphere of Kobe city, Japan for 211 days by using aerosol monitors for the aerosol number concentration and PM2.5 mass concentration. The product-moment correlation coefficient of PM2.5 and number concentrations is 0.88, suggesting that the hypothesis was consistent with the observations.Therefore PM2.5 mass concentration can be evaluated by the number concentration of aerosols. Parallel measurements with PM2.5 monitor and the sensor for aerosol number concentration are necessary to determine the conversion coefficient. Once the conversion coefficient is established, we may readily construct PM2.5 monitoring networks by using aerosol sensors for number concentrations.
The electrodynamic balance (EDB) has been widely employed as an experimental tool to study the physics and chemistry of a particle and a droplet with a diameter between 5 μm and 200 μm. In this technique, a single charged particle or droplet is levitated by the fields created by the DC and AC electrodes and the shape and mass of individual particles can be characterized. This review has touched on the applications of EDB in chemical thermodynamic properties of droplet and the phase changes of deliquescent substances.