THEORYElectromagnetic coupling among primary particles in an aggregate yields new optical properties. Often the optical response of a N-particle aggregate is even bigger than the optical response of N isolated particles. In this contribution the concept of extinction and scattering by aggregates of spherical particles with arbitrary topology (Fig. 1) according to GERARDY and AUSLOOS (1982) is used to study the optical response of soot agglomerates.For N spherical particles with arbitrary sizes 2a i lumped into an aggregate of arbitrary topography in which they are separated with center-to-center distances d i j ≥ a i + a j , scattering and absorption of electromagnetic waves is modified due to the near-field coupling among the particles, resulting in modified expansion coefficients α n m (i) and β n m (i) for the scattered wave of each particle i.In turn, the extinction and scattering efficiencies for an aggregate with N spherical particles are: RESULTSIntroducing an enhancement factor , comparison to the single particle extinction is enabled. Fig. 2 shows broadband spectra in the wavelength range from 300 nm to 1000 nm for five aggregates of carbonaceous particles with N = 5 and various topology. The single particle size is 2a = 20 nm. Compared to the single particle spectrum (solid line) the spectra of the aggregates are rather similar. However most aggregates exhibit a larger extinction efficiency than the single particle. Fig. 3 depicts the corresponding spectra for the enhancement factor ENH. It is obvious that the enhancement lies in the range between 5% and 15%. Only for the trapezoid the aggregate scatters and absorbs less than the single particle.