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It is suggested that aerosol particles forming the detached and main haze layers of Titan's atmosphere do not originate in the same atmospheric levels. Particles present above approximately 350 km could be formed of polyacetylenes synthetized in the 500-800 km altitude range through successive insertion reactions involving the C2H radical under the action of solar ultraviolet photons (Yung et al., Astrophys. J. Suppl. 55, 465, 1984). They might contain C-N oligomers in comparable amounts, as well as C-H-N oligomers synthetized at high altitude (900-1000 km) by the action of suprathermal Saturn plasma electrons. Physically, they are expected to consist of fluffy aggregates of density approximately 0.01-0.1 g cm-3. Their mass production rate is small (10(-15)-10(-14) kg m-2 s-1), that is typically 10% or less of the main haze production rate. Due to their low fall velocity, they are very sensitive to large scale horizontal motions and one substantial part of them may be swept away by meridional circulation at the detached haze level. The altitude range where these aerosols are created is well above the range proposed by Cabane et al. (Planet. Space Sci. 41, 257, 1993) for aerosols of the main haze layer, on the basis of a new fractal microphysical modeling of Titan's aggregates, that is approximately 350-400 km. A natural outcome of this apparent discrepancy is to suppose that there is a second formation region, below approximately 400 km altitude, giving rise to the main haze layer. The aim of the present paper is to review the different possible formation mechanisms of this main haze layer and assess their ability to account for the observed characteristics of the haze. Several conditions are established. The first one, called "condition A", concerns the formation altitude range imposed by fractal modeling. Possible chemical and energy sources are examined. Two additional constraints, relative to the minimum gas mass ("condition B") and input energy ("condition C") required for efficient conversion of gas into aerosols, are defined. By comparing the production rates of the haze, as derived from microphysical models, and of gaseous chemical species, as derived from photochemical models, five possible source constituents are identified: N2, CH4, C2H2, C2H6 and HCN. Polymerization of C2H2 into (C2H2)n through action of solar ultraviolet photons is shown to be rather improbable (condition A is hardly satisfied). From both our current knowledge of the gaseous phase photochemistry, through modeling and laboratory experiments, and existing models of the interaction between Saturn magnetosphere and Titan atmosphere, the formation of C-H-N polymers through action of Saturn magnetospheric energetic particles (E approximately 100 keV), is proposed as the basic polymerization mechanism in the lower formation region (conditions A,B and C are jointly satisfied).