Aerosol nucleation mode

Number concentrations are dominated by submicron particles, whereas the mass concentrations are strongly influenced by particle concentrations in 0.1-10 pm diameter range [13]. Similarly, the variability of the number-based measurements is strongly dominated by variability in smaller diameter ranges, whereas the variability of mass-based properties, such as PM10, are dominated by variability in the accumulation mode (usually around 500 nm of mass mean diameter) and in the coarse mode. This means the variabilities of these properties are not necessarily similar in shorter timescales, due to sensitivity of variance from very different air masses and thus aerosol types. This is demonstrated in Fig. lb, where the variance of the each size class of particle number concentrations between 3 and 1,000 nm is shown for SMEAR II station in Hyytiala, Finland. The variance has similarities to the particle number size distribution (Fig. la), but there are also significant differences, especially on smaller particles sizes. Even though in the median particle number size distribution the nucleation mode is visible only weakly, it is a major contributor to submicron particle number concentration variability. 

The accumulation mode (0.1 < d ic < 1pm) particles included in this mode originate from coagulation of particles in the nucleation mode and from condensation of vapors onto existing particles. These particles usually accounts for a substantial part of the aerosol mass and for most of the aerosol surface area (Seinfeld and Pandis, 1998). 

The approximate change in the size distribution that will have taken place after a 12-h intermission of new particle production is shown in Fig. 7-5 by the dashed curve. It results in a bimodal size distribution, which in addition to the accumulation peak now contains a transient peak caused by the incomplete coagulation of Aitken particles. We have previously designated this transient the nucleation mode. The ensuing size distribution gives a better representation of the natural aerosol, even though its resemblance to the size spectra in Fig. 7-1 is still marginal. 

A different picture of the ambient aerosol distribution is obtained if one focuses on the number of particles instead of their mass (Figure 8.10, upper panel). The particles with diameters larger than 0.1 pm, which contribute practically all the aerosol mass, are negligible in number compared to the particles smaller than 0.1 pm. Two modes usually dominate the aerosol number distribution in urban and rural areas the nucleation mode (particles smaller than 10 nm or so) and the Aitken nuclei (particles with diameters between 10 and 100 nm or so). The nucleation mode particles are usually fresh aerosols created in situ from the gas phase by nucleation. The nucleation mode may or may not be present depending on the atmospheric conditions. Most of the Aitken nuclei start their atmospheric life as primary particles, and secondary material condenses on them as they... 

The mass concentrations of the accumulation and coarse particle modes are comparable for most urban areas. The Aitken and nucleation modes, with the exception of areas close to combustion sources, contain negligible volume (Figures 8.11 and 8.13). Most of the aerosol surface area is in particles of diameters 0.1-0.5 pm in the accumulation mode (Figure 8.11). Because of this availability of area, transfer of material from the gas phase during gas-to-particle conversion occurs preferentially on them. 

During the winter and early spring (February to April) the Arctic aerosol has been found to be influenced significantly by anthropogenic sources, and the phenomenon is commonly referred to as Arctic haze (Barrie 1986). During this period the aerosol number concentration increases to over 200 cm-3. The nucleation mode mean diameter is at 0.05 pm and the accumulation mode at 0.2 pm (Covert and Heintzenberg 1993)... 

Atmospherie aerosol particles are principally divided into fine and coarse partieles. The fine-particle-size range covers geometric particle diameters (Dp) 1 >Dp > 1000 nm. Particles with Dp > 1 pm are called coarse partiele. Fine particles are also defined as Dap > 2.5 pm (e.g., by inhalation toxieologists for which Dap is defined as the aerodynamic particle diameter). The entire number-size distribution can be principally described by four different aerosol particle modes (Table 2). Fine particles belong to the nucleation (ultrafine), the Aitken, or the accumulation mode (1). The fourth mode is the coarse particle mode. 

Fig. 9.4.1 Schematic diagram of a particle growth in the aerosol technique. Nucleation proceeds in between two substrates with high and low temperature, the difference of which is several thousands of kelvins, High-temperature substrate is a heating element and low-temperature substrate is a kind of coolant such as gas, liquid, or solid substrate, depending on the operation mode. (From Ref. 1,)...

The most common way in aerosol science to represent PND data is in terms of various modes. Generally, these modes are nucleation (typically in the 1-30 nm range), Aitken (typically in the 20-100 nm range), accumulation (typically in the 30-300 nm range) and coarse (typically over 300 nm size range). Each mode contains different sources, size range, formation mechanisms, and chemical compositions [30],... 

Although the fly ash particle size distribution in the submicron regime is explained qualitatively by a vaporization/homogeneous nucleation mechanism, almost all of the available data indicate particles fewer in number and larger in size than predicted theoretically. Also, data on elemental size distributions in the submicron size mode are not consistent with the vapor-ization/condensation model. More nonvolatile refractory matrix elements such as A1 and Si are found in the submicron ash mode than predicted from a homogeneous nucleation mechanism. Additional research is needed to elucidate coal combustion aerosol formation mechanisms. 

The fine fraction of the aerosol include.s the accumulation mode (0.1 < J, < 2.5 ttn) and the ultrafine particles (dp < 0.1 /tni). The chemical characteristics of the fine fraction which arc central to the health effects and to the optical and nucleating characteristics of the aerosol can be divided into the following categories ... 

The phenomena that influence particle sizes are shown in an idealized schematic in Figure 2.7, which depicts the typical distribution of surface area of an atmospheric aerosol. Particles can often be divided roughly into modes. The nucleation (or nuclei) mode comprises particles with diameters up to about 10 nm. The Aitken mode spans the size range from about lOnm to lOOnm (0.1 pm) diameter. These two modes account for the... 

After formation of ultrafine particles (ufp) by nucleation, subsequent growth of supercritical clusters occurs by condensation and coagulation. Coagulation is discussed in this section dealing with evolution of aerosols of ufp. The discussion is limited to Brownian coagulation which is the principal mode of coagulation for ufp. 

Smoke is a mixture of black carbon (soot) and aerosol [26,27]. It has been suggested that soot nucleation and growth occur near the highly ionized regions of the flames in combustion processes, and that some of the charges are transferred to smoke particles. Multimodal distributions show that the soot particle radii belong to three modes [26] ... 

The twin mechanisms of coagulation and heterogeneous nucleation (condensation of one materiai to another) tend to accumulate submicrometre aerosol particle mass in this mode (Whitby and Cantrell, 1976 Willeke and Whitby, 1975). Because of the sharp decrease in particles larger than 0.3 pm in diameter, little mass is transferred from the accumulation mode to the coarse particle size range. Sedimentation and impaction tend to increase the relative concentration of the smallest mechanically produced particles, and then accumulate in this mode. Salt from sea spray is typically present as particles in the 1-5 pm size range, outside the normal accumulation mode. 

E). The data rather indicate that the activity median aerodynamic diameter, AMAD, of Be-aerosols increases with increasing latitude (latitudinal effect). As cosmic radiation increases with latitude, the numbers of Be atoms and ions formed also increase with latitude and so there are more Be atoms and ions available either to form small aerosol particles in the nucleation (Aitken nuclei) mode and then growing or to be attached directly to the existing large particles in the accumulation and in the coarse particle modes thereby increasing the AMAD of the Be-aerosols. 

Parameters of the activity size distribution of the aerosol-attached short-Uved radon decay products in air at different locations. Activity median aerodynamic diameter, AMAD, noted as AMD geometric standard deviation, Og fraction of the mode, fpi. The indices i = n, a and c represent the nucleation (Aitken nuclei), accumulation and coarse particles modes, Z = aerosol particle concentrations... 

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