Nucleation bursting

One can speculate on the nature of the material that contributed to each burst of nucleation and the growth of the initial nuclei. The early nucleation did not occur under the same conditions without SO , so it is probable that it results from the primary oxidation product of that species, namely, H2S04. The second nucleation burst is probably the same material that condensed without the initial S02, that is, the condensible hydrocarbons that result from the 1-octene photooxidation. Because the initial SO concentration was much smaller than that of the hydrocarbon, much of the growth of the early nuclei is likely due to hydrocarbon condensation, that is, condensation of species that did not nucleate until much later in the first experiment. An examination of the quantity of aerosol produced in the two experiments supports this interpretation. As shown in Figure 8, particle formation occurs before significant hydrocarbon reaction in the SO -containing experiment. Once the hydrocarbon reaction begins in earnest, the aerosol yield increases by an amount that is comparable to that in the S02-free experiment. Two... 

The introduction in 1993 of a high temperature preparation method using organic solvents constituted an important step towards the fabrication of monodisperse CdS, CdSe, and CdTe NCs.14 As demonstrated in classical studies by LaMer and Dinegar,43 the synthesis of monodisperse colloids via homogeneous nucleation requires a temporal separation of nucleation and growth of the seeds. The LaMer plot (Fig. 5.4) is very useful to illustrate the separation of nucleation and growth by means of a nucleation burst. 

After a nucleation burst occurs, the aerosol population (fresh clusters and preexisting particles) evolves through coagulation, condensation, evaporation, emis-... 

Alternatively, supersaturation and nucleation can be triggered by a slow ramping of the reaction temperature. Precursors are mixed at low temperature and slowly brought to the temperature at which precursor reaction and decomposition occur sufficiently quickly to result in supersaturation." Supersaturation is again relieved by a nucleation burst , after which temperature is controlled to avoid additional nucleation events, allowing monomer addition to existing nuclei to occur more rapidly than new nuclei formation. Thus, nucleation does not need to be instantaneous, but it should be a single, temporally discreet event to provide for the desired nucleation-controlled narrow size dispersions. ... 

Figure 14. Nucleation burst of nanoparticles (Dp = 3-10 mn) and associated mixing ratios of gases in Atlanta, Georgia, on April 1, 1999. The vertical lines represent the period of enhanced number concentrations of 3-10 mn nanoparticles. Note, however, that the number concentrations of these particles are often still appreciable outside of this period, especially at night. From Woo et al. (2001). Used by permission of Taylor Francis, Inc.

The first evidence of in situ particle formation in the atmosphere was provided by John Aitken at the end of the nineteenth century (Aitken 1897). He built the first apparatus to measure the number of dust and fog particles in the atmosphere. However, little progress was made in understanding what causes new particle formation or how widespread it might be for almost a century. In the 1990s the development of instruments capable of measuring the size distribution of particles as small as 3 nm led to the discovery that nucleation and growth of new particles is a rather common event in many areas around the world (Kulmala ct al. 2004). Areas where frequent nucleation bursts have been observed include... 

Summary of Experimental Evidence. The above mentioned facts are summarized in Table 1. From the facts obtained in our studies we can consider the mechanism of the purity drop as one that a very small amount of D-Thr, present on the surface of the seeds or inside of the seeds which happen to be exposed on breakage starts to grow in the supersaturated solution until some critical size when breeding (secondary nucleation) bursts in the solution. The time needed for D-Thr, to grow to the critical size and the number of such elementary D-Thr. determine the time of the start of purity drop. 

In (8), all quantities are constant for a given synthesis, except for the temperature T and the supersaturation S. It can be seen that the rate of nucleation increases on increasing temperature and supersaturation. Reactants deplete (usually) when nuclei are formed, and thus the reaction rate decreases. Monodisperse nanoparticles can be synthesised by choosing the reaction temperature so that it is just above the nucleation temperature at moderate supersaturation. Then, a so-called nucleation burst occurs (rapid nucleation followed by a sudden drop in nucleation rate), followed by slow growth of the nuclei. This generic mechanism is applicable to the majority of synthesis methods. 

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