Fractal Dimension Analysis of Single, In-Situ, Restructured Carbonaceous Aggregates

Abstract

The Fractal dimension ( D ) of progressively restructured carbonaceous aggregates was investigated, whereby a modified Millikan cell allowed the measurement of D for a single, in situ aggregate in three-dimensional space. The technique relies on simultaneous measurement of the volume equivalent diameter ( d , , ) and the aerodynamic diameter (d,, ), hence allowing the number of primary spherules in the aggregate, the dynamic shape Factor x and D to be determined. Several stages of aggregate restrnctnring to a more spherical morphology, induced by reversal of the DC electric field, were observed and at each stage D was measured. Such electrostatic restructuring resulted in D exhibiting a limiting value, as high as 2.2 in some cases, beyond which additional increase by this method was not possible.

Light Absorption by Carbonaceous Particles: An Investigative Review

“…While the fractal dimensions of fresh particles are fairly well known, the same particles have not been as thoroughly characterized after residence in the atmosphere. The particles collapse from lacy structures to more compact ones when they are wetted (Hallett et al 1989;Colbeck et al 1990;Ramachandran and Reist 1995;Nyeki and Colbeck 1995) or after aging , a change that corresponds to an increase in fractal dimension. Collapse affects both absorption and scattering, and is one explanation for a decrease in absorption with atmospheric lifetime (Liousse et al 1993).…”

“…For freshly generated soot, D f is usually around 1.8 for both measured (Köylü and Faeth 1994;Nyeki and Colbeck 1995;Sorensen and Feke 1996;Lee et al 2002a) and simulated (Mountain and Mulholland 1988) agglomerates. This value of D f also corresponds to many aggregate particles in the urban atmosphere (Katrinak et al 1993).…”

Fractal Geometry of Aerosol Particles

“…Various laboratory experiments have demonstrated the existence of a number of different mechanisms by which aerosols can be restructured. Some mechanisms investigated are: (1) temperature gradient and mechanical shear in colloids [114]; (2) the tempering of silver aerosols (D f ≈ 2.18) at 80 to 270 • C into coalesced spheres for which D f ≈ 3 [50]; (3) electrostatic-induced restructuring of single in situ butane smoke aggregates in discrete steps to values as high as D f ≈ 2.2 [38]; and (4) compaction/internal rearrangement [52]. Another mechanism that may be of importance for larger aggregates in the supermicrometre range is the effect of wind shear.…”

“…Various other in-situ methods have been reported, again measuring the density fractal. These methods include techniques based on the aerodynamic and mobility properties [34,35], kinematic coagulation [36], a modified Millikan cell [37,38] and a combination of differential mobility analysis with inductively coupled plasma optical emission spectrometry [39]. Where not explicitly stated, the fractal dimensions quoted in the quoted publications refer to the mass (density) fractal dimension.…”

Fractal aggregates in the atmosphere

“…The chemical methods were applied by Eltekova et al [1993] for determining of soot FA. The value of fractal prefactor was discussed by Nyeki and Colbeck [1995a] who showed that is close to 1.…”