Nucleation theory aerosol particles

The role of ions in the production of aerosols is among the least understood, but potentially is an important, process in the Earth s atmosphere. Atmospheric and experimental observations have shown that the nucleation of aerosol particles can occur under conditions that cannot be explained by classical nucleation theory" [29]. 

In the past half-decade, extensive studies have focused on aerosol nucleation in aircraft exhaust plumes [79]. This interest has brought attention to the formation of volatile aerosols that might eventually evolve into cloud condensation nuclei [80], Measurements of ultrafine particles reveal remarkably high abundances in jet wakes at very early times (within 1 second of emission) (e.g., [81]). As in the background atmosphere, the classical homogeneous nucleation theory has been applied to explain the number and size distribution of these volatile microscopic particles [82,83], However, while achieving some initial success, the theory has not been able to explain more recent, detailed observations. 

The dependence of / on S is known from homogeneous nucleation theory discussed earlier in this chapter. Thus, there are four differential equations in five unknowns. A, M, S,N, and /, with a relationship between / and S. The set of equations for the dynamics of the stable aerosol is remarkable because the calculation of the important moments, A and N, does not require the detemiination of the size distribution of the stable particles. 

In determining their ability to nucleate clouds, the chemical composition of aerosol particles is much less important than their size, a result that will clarify aerosol effects on climate (Rosenfeld 2006). This is what is expected from theory because the radius-to-volume ratio determines the molecular transfer from the gas phase, whereas the hygroscopicity (surface characteristics of CCN) determines the uptake coefficient (Chapter 4.3.7.4). The other very important parameter for cloud formation is the CCN number, determining the cloud droplet number (cf Fig. 2.20). As shown originally by Twomey (1991), and recently reviewed by Lohmann and Feich-ter (2005), the sensitivity of climate to CCN number density is nonlinear, with the effect being much stronger at low particle numbers. 

Turbidimetry measurements, using monochromatic light, yields data that can be used to determine particle size distributions. It requires simple optical technology, but complex computational software to handle the Mie theory conversion. The sensitive diameter range for latex-water suspensions was found to be 0.1 to 10 pm. Different types of sensors have been conceived and applied to various experimental situations. The method is particularly useful in crystallization experiments. Other applications include agglomeration, attrition and nucleation studies. Applications of the equipment and software to studies of emulsions, fumes and aerosols are also envisaged [18]. 

The subject of ultrafine particles (ufp) is perhaps currently the most challenging and interesting area in aerosol science. This subject entails the difficult area of nucleation processes to explain the origin of most ufp. Analysis of the evolution in size, composition, space and time of ufp involves one with current problems in statistical mechanics, kinetic theory, probability theory, quantum chemistry, etc. One also encounters very difficult measurement problems for ufp, although these will not be discussed here. 

Numerous results concerning analyte transport/ losses were presented in the literature and these very often lead to different conclusions on where and how much is lost during transport of the analyte vapors to the ICPMS. However, there is a dear general conclusion that can be drawn from aU these studies the transport of the ETV-formed aerosol is never quantitative for any ETV device. Following Kantor s theory, the self-nucleation process can hardly take place within the aerosol itself (i.e., without any external help) and the analyte particles easily deposit onto the colder parts of the ETV interface (e.g., the walls of the transport tubing or the switch valve) that act as nucleation/condensation sites. However, this tendency can be greatly influenced by a number of external factors. 

Both aerosol modeling and more fundamental atomistic and molecular level models have been applied to this problem. Aerosol dynamics modeling has lead to a better understanding of the individual steps that comprise the formation of particles, all the way from nucleation to subsequent growth. Both molecualar orbital and reaction rate theory was used as sources of fundamental data for input to the aerosol dynamics model. A simplistic molecular dynamics computation has been used to explain the particle morphology observed. 

Above the CMC, the theory fits well with the experimental results. For example, with increasing [peroxodisulfate] from 3.0 x 10 to 24.0 x 10 mol dm decreased N from 4.7 to 4.3 x 10 particles/dm (2 g of Aerosol DBM/dm, 50 C). On the contrary, below CMC the model deviates from the observed data. This was ascribed to increased effect of homogeneous nucleation of particles. 

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