External mixture

In the general case, individual particles have differing compositions and refractive indices and to take this into account in detail is not possible from a practical point of view. To allow for a variation of refractive index, a convenient model is that of a mixture of aerosols from the several sources, each with its own extinction cross-section. The particles are assumed not to coagulate so that the aerosol is not mixed on the individual particle basis. Such an aerosol is known as an external mixture. This model would also be applicable, approximately, to an aerosol mixture whose particles are growing in size by gas-to-particle conversion. 

For an external mixture of pure, single component aerosol particles, only the first condition, (a), is necessary. 

Cass and co-workers (e.g., Rogge et al., 1996 Gray and Cass, 1998 Kleeman and Cass, 1998) have used this method to assess the contribution of various sources of particles in the Los Angeles area. If the particle emissions from each source are known and assumed to remain in air as external mixtures, they can be tracked separately in the model as they are transported and react. Hence their contribution to the concentrations at a particular location can be calculated. 

Clearly, such real-time, single-particle techniques have great promise in terms of elucidating the chemical nature of different species in tropospheric particles and determining whether they exist as internal or external mixtures. 

Kleeman, M. J., G. R. Cass, and A. Eldering, Modeling the Airborne Particle Complex as a Source-Oriented External Mixture, J. Geophys. Res., 102, 21355-21372 (1997). 

Particle-Particle Interactions. Loss of strong acid content of aerosol particles can also occur because of reactions between co-collected acidic and basic particles impacted together on the collection surface. This phenomenon most frequently occurs as the result of interaction of coarse (>2.5 xm diameter), alkaline, soil-derived particles with fine (<2.5 xm diameter) acidic sulfate particles (66). Particle-particle interactions with net neutralization can be reduced in many cases by sampling with a virtual impactor or a cyclone to remove coarse particles, although this procedure does not prevent the effect if external mixtures of fine particles of different acid contents are sampled. In situ methods with shorter sampling times can be used such that these topochemical reactions are less likely to occur. 

Figure 2.2 Development of body concentrations for 2 chemicals having different toxicokinetics, leading to an internal mixture composition that varies with time and differs from the external mixture.

Second value the most easily ignited external mixture Different values given in the literature... 

CAMS Sulfate, nitrate, ammonium, BC, OC, sea-salt, dust, water Modal (4) predicted dust and sea-salt, prescribed other aerosols single size distribution External Predicted/Predicted or predicted/Diagnosed from mass hydrophobic and hydrophilic BC/ OC with a fixed conversion rate Prescribed RI and optical properties for each aero, type, size, and wavelength, for external mixtures... 

CAMS For external mixtures only, equilibrium with RH, no hysteresis Empirical, prescribed activated mass fraction bulk CCN only Prescribed bulk activation, autoconversion, precip. rate independent of aerosols Prescribed bulk scav. efficiency, no-size dependence Bulk cloud/ice sedimentation... 

In this notation, n is the outward directed normal unit vector to the external mixture surface closing the mixture volume lAi(f), D/Dt is the... 

More generally, an infinite number of intermediate cases are possible between the internal and external mixture models. To take into account variations in chemical composition from particle to particle, the particle size distribution function must be generalized, and for that purpose the size-composition probability den.sity fimetion has been introduced (Friedlander, 1970). Let r//V be the number of particles per unit volume of gas containing molar quuiititics... 

By contrast, an external mixture contains particles, each of which is composed of a pure substance. 

The radiative effects of a population of particles depend on the composition of the particles, through their refractive indices. As we know, the atmospheric aerosol seldom consists exclusively of a single component it is generally a mixture of species from a number of sources. How all the components are distributed among the particles constitutes the mixing state of the aerosol. Two extremes of mixing state are depicted in Figure 24.9. One extreme is termed an external mixture, where, in the aerosol population, each particle... 

FIGURE 24.9 Mixing states of an aerosol—internal and external mixtures of (NILhSO, and soot. 

FIGURE 24.10 Total extinction, scattering, and absorption coefficients for internal versus external mixtures of soot and (NH4)2S04. 

FIGURE 22.23 Mixing states of an aerosol-internal and external mixtures of (NH4)2S04 and soot. 

---