Aitken mode

Aerosol surface area is likely to be variable even within a remote marine air mass. Previous MBL aerosol studies describe changes in aerosol concentration and composition due to entrainment from the free troposphere (Bates et al., 1998, 2001 Covert et al., 1998). Raes et al. (1997) found an observable link between vertical transport patterns and aerosol variability in the MBL specifically in the Aitken mode (<0.2/u.m). Hence entrainment of aerosol from the free troposphere appears to occur frequently, even in remote MBL air masses. In addition, aerosols have the capacity to travel great distances in the free troposphere, before being entrained into the MBL. 

Number concentration of particles between 30 and 50 nm of dry diameter (N30-50), given in particles per cubic centimeter (cm-3). These particles represent smaller end of the Aitken mode particles and are representative of recently emitted or formed nanoparticles, which have high probability to end up in... 

At the Mediterranean station Finokalia (FKL) located in the Greek island of Crete, the particle number size distributions were bimodal for winter with an Aitken mode around 50 nm particle diameter and accumulation mode at 150 nm. The spring and summer were dominated by strong accumulation mode at around 100 nm. 

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. 

Recently, a fourth mode has been introduced into this nomenclature It appears that particles with sizes less than 0.1 pm consist of two modes, the nucleation mode, which includes particles with dae between 0.01 and 0.03 pm representing quite recently formed particles, and the Aitken mode containing particles between 0.03 and 0.1pm (Horvath, 2000). 

This section provides a conceptual framework and several examples of modeling and fieldwork on the growth of atmospheric nanoparticles. The growth of nanoparticles is an important source of Aitken mode and accumulation mode particles, including cloud condensation nuclei, especially in remote regions with few primary particle sources. For more quantitative descriptions of growth processes, as well as their parameterizations in models, see Kulmala (1993), Kulmala et al. (1993), Kerminen et al. (1997), Mattila et al. (1997), Vesala et al. (1997), Seinfeld and Pandis (1998), and Friedlander (2000). 

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... 

Their sizes range from a few nanometers to a few micrometers often with pronounced concentration modes around a few tens of nm Aitken mode), in the range 100-500 nm (accumulation mode) and at a few (im (coarse mode) see Fig. 4.17. Coarse mode particles typically originate from the dispersion of solid and liquid matter such as soil, dust and sea spray (Chapter 2.6.4). Submicron particles are usually mixtures of primary particles (almost combustion products from biomass smoke and diesel soot) and secondary produced particles via the gas-to-particle conversion (most important is sulfuric acid from SO2 oxidation and organics from oxidation of VOCs). 

The nucleation mode occurs in the atmosphere only sporadically, depending on favorable conditions for new particle formation (see under Sec. III.B) or on mixing processes (2). The Aitken mode and accumulation mode are usually always present in the boundary layer of the atmosphere. The Aitken mode... 

Oceans. Hoppel et al. (4) found, for background conditions, a double-peaked characteristic for the submicrometer particles (Aitken and accumulation mode) in the remote Atlantic. The particle number concentration varied between 200 and 300 cm . The total particle number in the two modes for background conditions is of the same order of magnitude, whereas under continental influence, the particle number in the Aitken mode is much higher than in the accumulation mode. Under continental influence, the total particle number concentration can reach several thousands in the Atlantic. 

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