Aerosol accumulation mode

Fig. 6. Size distribution of an urban aerosol showing the three modes containing much of the aerosol mass. The fine mode contains particles produced by condensation of low volatility gases. The mid-range, or accumulation mode, results from coagulation of smaller aerosols and condensation of gases on preexisting particles. Coarse particulates, the largest aerosols, are usually generated mechanically.

Chu et al. (1987) present results of laboratory studies of the formation of an ultrafine aerosol by converting SO- to sulfuric acid using measurement methods described by Holub and Knutson (1987) and Kulju et al. (1987). It was found that the size of the resulting activity distributions is dependent on the S02 concentration. The role of humidity is still unclear and more studies are needed, but it appears that both future theoretical models and laboratory studies will be extremely fruitful in elucidating the behavior of Po-218 from shortly after its formation until its incorporation into the existing accumulation mode aerosol. 

Recoil of RaB ions from aerosol particles is taken into account in the way proposed by Mercer (6). If RaA ions or molecules are attached to aerosol particles in the accumulation mode, it is assumed that they remain on the surface of the particles for RaA... 

Table I presents the average aerodynamic distributions of Pb-212 and Pb-214, as well as the frequency with which Pb-214 or Pb-212 was the dominant isotope in each size range. The Aitken nuclei fraction (below 0.08 pm) contained a higher percentage of Pb-212 activity compared with Pb-214 in 69.6% of the measurements. The predominance of Pb-212 in this fraction is also illustrated by the distributions reported in Figure 1. In the remaining measurements, where Pb-214 was fractionally more abundant below 0.08 um, the disparity between the relative amounts of each isotope was not nearly as dramatic. Conversely, Figure 1 and Table I illustrate that Pb-214 is generally enriched in the accumulation mode aerosol, particularly between 0.11 and 0.52 ]xm, where most of the surface area and mass occurs.

Accumulation mode, aerosol particles, 328 Actinon—See Radon-219... 

FIGURE 9.7 Schematic of an atmospheric aerosol size distribution showing four modes. The original hypothesis of Whitby and co-workers is shown by the solid, trimodal curves, and the fourth, ultrafine particle mode, as well as the two peaks sometimes observed in the accumulation mode are shown by the dashed lines (adapted from Whitby and Sverdrup, 1980). 

For example, Fig. 9.23a shows the measured volume distribution of one ambient aerosol sample. When this volume distribution is multiplied by the size distribution of the scattering coefficient per unit volume in Fig. 9.22, one obtains the calculated curve for light scattering in Fig. 9.23b. It is seen that the particles in the 0.1-to 1-yu.m-diameter range, that is, in the accumulation mode, are clearly expected to predominate the light scattering. 

Hering, S., A. Eldering, and J. H. Seinfeld, Biniodal Character of Accumulation Mode Aerosol Mass Distributions in Southern California, Atmos. Environ., 31, 1-11 (1997). 

Kerminen, V.-M., and A. S. Wexler, Growth Laws for Atmospheric Aerosol Particles An Examination of the Bimodality of the Accumulation Mode, Atmos. Environ., 29, 3263-3275 (1995). 

Smith, M. H., and C. D. O Dowd, Observations of Accumulation Mode Aerosol Composition and Soot Carbon Concentrations by Means of a High-Temperature Volatility Technique, J. Geophys. Res., 101, 19583-19591 (1996). 

Although the SEMS represents a marked advance in the state of the art for measurement of aerosol size distribution, an important gap remains in current measurement technology, namely, the ability to make rapid, high-resolution measurements of the accumulation-mode aerosols on-line. The limitation of the DMA or SEMS for measurement of particles larger than 0.2 xm in diameter is the multiple charging that allows particles of two or more different sizes to contribute a given mobility fraction. Regardless of... 

THE AMBIENT ATMOSPHERIC AEROSOL consists of liquid and solid particles that can persist for significant periods of time in air. Generally, most of the mass of the atmospheric aerosol consists of particles between 0.01 and 100 xm in diameter distributed around two size modes a coarse or mechanical mode centered around 10- to 20- xm particle diameter, and an accumulation mode centered around 0.2- to 0.8- xm particle diameter (1). The former is produced by mechanical processes, often natural in origin, and includes particles such as fine soils, sea spray, pollen, and other materials. Such particles are generated easily, but they also settle out rapidly because of deposition velocities of several centimeters per second. The accumulation mode is dominated by particles generated by combustion processes, industrial processes, and secondary particles created by gases converting to par-... 

Figure 1. Example of compositionally resolved bimodal and monomodal distributions of aerosols. The ordinate gives the percent of the species found in the given size fraction of the impactor. The mode near 0.3 xm is the accumulation mode , and that above 8 xm is the coarse mode The minimum of mass between 1 and 2 xm is typical the chlorine distribution is anomalous. Chlorine is in fact a coarse-mode marine aerosol that has lost its larger particles during transport from the ocean to Davis, California, a distance of roughly 100 km. (Reproduced with permission from reference 15. Copyright 1988.)...

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. 

Figure 9 (adapted from [18]) shows some of the typical correlations between particle number concentrations between 30 and 100 nm (here referred to as Aitken mode, although a more rigorous derivation would require actual modal fitting) and concentrations between 100 and 500 nm ( accumulation mode ). The idea of this kind of plot is to show the possible correlation between the two aerosol modes, to indentify some of the main particle number size distribution types, and whether the particle number concentrations in both modes increase in the same rate. 

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). 

Most of the studies on size-resolved aerosol mass concentrations in areas with different levels of pollution show that particulate matter typically exhibit a bimodal distribution, with most of their mass being found in the submicron size range (dae < 1pm) and an additional minor mode in the coarse fraction (1 < dae < 10 pm) (Maenhaut et al., 2002 Smolfk et al., 2003 Wang et al., 2003 Gajananda et al., 2005 Samara and Voutsa, 2005). However, with instrumentation allowing more precise measurements, the aerosol mass size distribution was found to be multimodal with the preponderance of a fine mode (dae < 0.2 pm) and an accumulation mode (dae 0.5pm), with a minor coarse mode at d 3-4pm (Raes et al., 2000 Pillai and Moorthy, 2001 Berner et al., 2004). Traditionally, atmospheric researchers classify airborne particles into three size classes coarse (2.5 < c/ ie < 10pm), fine... 

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