Particle formation nucleation burst

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... 

Clarke AD, Davis D, Kapustin VN, Eisele F, Chen G, Paluch I, Lenschow D, Bandy AR, Thornton D, Moore K, Mauldin L, Tanner D, Litchy M, Carroll MA, Collins J, Albercook G (1998) Particle nucleation in the tropical boundaiy layer and its coupling to marine sulfur sources. Science 282 89-92 Clarke AD, Kapustin, Eisele FL, Weber RJ, McMuny PH (1999) Particle production near marine clouds Sulfuric acid and predictions from classical binary nucleation. Geophys Res Lett 26 2425-2428 Clegg SL, Brimblecombe P, Wexler AS (1998) Thermodynamic model of the system H -NH/-Na -S04 -NO3 -CI -H2O at 298.15 K. JPhys Chem. A 102 2155-2171 Clement CF, Piijola L, dal Maso M, Ma kela JM, Kulmala M (2001) Analysis of particle formation bursts observed in Finland. J Aerosol Sci 32 217-236... 

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... 

Aggregation methods usually lead to the formation of polydispersed sols, mainly because the formation of new nuclei and the growth of established nuclei occur simultaneously, and so the particles finally formed are grown from nuclei formed at different times. In experiments designed to test the validity of theories, however, there are obvious advantages attached to the use of monodispersed systems. The preparation of such systems requires conditions in which nucleation is restricted to a relatively short period at the start of the sol formation. This situation can sometimes be achieved either by seeding a supersaturated solution with very small particles or under conditions which lead to a short burst of homogeneous nucleation. 

The most interesting result is the formation of a transparent colloidal solution of ceria with 2 nm particles. Cerium metal tips with the superficial layers of oxide are allowed to react in 2-methoxyethanol at 250 to 300°C, and removal of coarse ceria particles originating from the superficial layers yields the colloidal solution. Addition of water to the solution does not cause any change except dilution of the color of the solution, but addition of a drop of a solution of any kind of salt immediately causes precipitation of ceria particles. - The reaction mechanism is as follows The solvent slowly dissolves the superficial layers, and when the solvent reaches the metal, rapid reaction occurs, yielding an alkoxide solution. The concentration of the ceria precursor becomes so high that a burst of nucleation occurs, yielding the colloidal solution. The reaction of cerium acetylacetonate in the same solvent yields ceria particles but does not give a colloidal solution. 

Insofar as the solution of the GPBE is concerned, it is sufficient to define the functional form of the rate of formation of new elements of the disperse phase For example, the formation of particles in solution is typically classified as homogeneous or heterogeneous. For the former, particles are formed in the bulk solution without the support of any surface, whereas for the latter an external surface (provided, for example, by particles that have already formed or by foreign particles) is involved. According to the classical theory (Myerson, 2002), the nucleation rate prescribes a burst of monodisperse nuclei with size dc. The resulting nucleation rate is... 

The nuclei formed then grow by diffusion. An important condition needed for the formation of monodispersed particles is single act of nucleation, i.e. that no secondary burst of nuclei occurs. For this condition to be fulfilled, the newly formed molecular species of sulfur should be forming at such a rate that their diffusional transport to the already fonned solid particles, rather than secondary acts of nucleation take place. 

It is clear that if we wish to produce particles with a fairly uniform size, then one short burst of nucleation should occur in a short time interval, ti — ti. One way of achieving this is through the use of a fairly low reactant concentration. Furthermore, uniform growth of the particles requires that the solute be released slowly to allow diffusion to the particles without buildup of the solute concentration and further bursts of nucleation. This mechanism of nucleation followed by diffusion-controlled growth does not apply to the formation of particles that are aggregates of finer primary particles. Instead, it may apply only to the primary particles. 

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