Aitken particles

Bigg, E. K., Gras, J. L., and Evans, C. (1984). Origin of Aitken particles in remote regions of the Southern Hemisphere. /. Atmos. Chem. 1, 203-214. 

The particle number size distributions and concentration levels measured at the Central European stations were remarkably similar. The median particle number size distributions did not change significantly from season to season, and the differences between the stations were not very large (N50 from 2,500 to 3,100 cm-3). The variability of nucleation and small Aitken particles was elevated... 

The tropospheric air always contains a considerable amount of small submicron particles, the so-called Aitken particles (AP), which are widely spread. Experimental data show that, on the average, not less than 50-75% of the total mass concentration of AP fall on sulfates [43]. The average percentage of the mass concentration of organic and nitrate components in the aerosol of the sub-micron range is estimated at 20-25% [2, 22]. Undoubtedly, nuclear explosions (with a yield of about 10 NO molecules per 1 Mt of trinitrotoluene equivalent [12] should generate great amounts of nitrate and sulfate aerosols, but there are no estimates so far. 

The in situ produced aerosol has a characteristic bimodal distribution dS/dlgr. The first mode is determined by Aitken particles themselves (r < 0.1 pm). The second, coagulative mode (0.1 pm < r < 1 pm) is constantly supplied with heterogeneously coagulating Aitken particles and, for these particles, it is the last form of existence before the aerosol particles in question leave the atmosphere. The parameters of the coagulative mode are dg (mean geometrical diameter) = 0.37 pm, o = 2.00... 

Winkler P., Chemical analysis of Aitken particles over the Atlantic ocean. Geophys. Res. Lett. , 2, 45-48 (1975). 

Figure 4. Residence time of particles in seconds (left axis) and days (right axis) as a function of particle radius. The shaded areas represent estimates of the lifetimes made as follows 1, molecular or ionic clusters C, coagulation of particles P, removal by precipitation F, gravitational settling A, derived from spatial distribution of Aitken particles R, derived from the distribution of small radioactive particles. From Kreidenweis et al. (1999) in Atmospheric Chemistry and Global Change by Brassem et al. 1999 by Oxford University Press, Inc. Used by permission.

This division is very convenient from the point of view of particle characterization and measurement. Thus, in the range of Aitken particles diffusion effects are significant and particle coagulation is rapid. However, in case of giant particles these phenomena can be neglected and the behaviour of aerosol particles is mostly determined by their sedimentation due to gravitation. The large particles constitute... 

Because of their small radius, the size and size distribution (see Subsection 4.3.2) of Aitken particles may be determined with a diffusion battery. This device is composed by an ensemble of capillary tubes, through which the air is drawn at low velocity. As a result of their Brownian diffusion, the smaller aerosol particles are deposited on the walls of the tubes during the aspiration. This particle loss is a function of the diffusion coefficient and consequently of the size of the particles (see equation [4.4]). 

The smaller aerosol particles can be captured from the air for subsequent counting and size measurement by means of so-called thermal precipitators. In these instruments, metal wires are heated to produce a temperature gradient. Aerosol particles move away from the wire in the direction of a cold surface, since the impact of more energetic gas molecules from the heated side gives them a net motion in that direction. The particles captured are studied with an electron microscope. Another possible way to measure Aitken particles is by charging them electrically under well-defined conditions. The charged particles are passed through an electric field and are captured as a result of their electrical mobility (see equation [4.6]). Since size and electrical mobility are related, the size distribution of particles can be deduced. These devices are called electrical mobility analyzers. 

Thus, a large set of data obtained by McWilliams (1969) in clean air (W. Ireland) by means of expansion chambers showed that the concentration of Aitken particles is lower in the winter than in the summertime. Furthermore, McWilliams observations also demonstrated that more aerosol particles can be detected during daylight than at night. This finding was confirmed by the investigations of Vohra et... 

Average concentration of Aitken particles under various conditions according to Landsberg (see Junge. 1963)... 

Vertical profile of Aitken particles according to Junge (1963) and Weickmann (1957). (/) under ambient conditions (2) at standard temperature and pressure. (By courtesy of Academic Press and Junge)... 

To estimate the significance of impaction by trees we refer to the results of Neuburger and his co-workers (see Hidy, 1973), based on particle concentration measurements carried out inside and outside a forest area. Neuburger s data show that more than 80 % of pollen (coarse particles) is removed by trees while the corresponding value for Aitken particles is 34 %. Further research is needed, however, to confirm these results. 

Finally, it is to be noted that in the foregoing discussion of dry removal we considered the total ensemble of particles. If size ranges are taken into account separately, additional sinks have to be mentioned. Thus, thermal coagulation of particles with very small size, as well as the condensation (below a relative humidity of 100 %) of vapours with low saturation vapour pressure provide effective removal for Aitken particles. It is believed (Hidy, 1973) that these processes are dominant in the removal of aerosol particles in the size range below 0.1 /im radius. 

Fig. 7-5. Coagulation behavior of particles produced by nucleation. [Adapted from Walter (1973).] A continuous generation of embryos with 1.2x 10 3 pm radius is assumed. Left Variation of the distribution function with time in the absence of preexisting particles production rate q = 106 cm" s"1. Right Steady-state distributions for different concentrations of preexisting large particles (r > 0.1 pm) production rate q = 102 cm 3 s-1 number density of preexisting large particles N0 = 270/3. The dashed curve (for /3 = 1) is obtained from the steady-state distribution 12 h after terminating the source of embryos. The apparent lifetime of Aitken particles for coagulation is here greater than that indicated in Fig. 7-4 because of the smaller number density of particles.

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. 

Aitken particles are removed mainly by coagulation with other particles. The corresponding time constant is obtained from the decline of... 

Me Carmack t Hilliard (1970), U02 particles Adamt Semonin (1970).subtilis spores Beilke (1970). ambient Aitken particles... 

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