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• Optical properties and radiative forcing of tar ball aggregates, individual tar balls, and volume equivalent spheres are calculated from 350 to 1150 nm using T-Matrix and Lorenz-Mie simulations. • A sensitivity analysis is performed for different refractive indices, and different monomer sizes and numbers. • Single scattering albedo and hemispherical upscatter fraction of tar ball aggregates differ from individual tar balls by up to 77% and more than 100%, respectively. • Over dark surfaces, the top of the atmosphere simple forcing efficiency of tar ball aggregates differs from individual tar balls by up to 53%. Image, graphical abstract Tar balls are frequently found in slightly aged biomass burning plumes. They are spherical in shape, have diameters between ∼100 and 300 nm, are amorphous and composed mostly of oxygen and carbon. Tar balls are light absorbing and considered to be a component of brown carbon. Tar balls have been typically reported and analyzed as individual spheres; however, in a recent study, we reported the presence of significant fractions of fractal-like aggregates made of several tar balls in fire plumes from different geographical locations. Aggregation affects the optical properties of particles; therefore, we use T-Matrix and Lorenz-Mie simulations to explore the effects of aggregation on the tar balls' optical properties in the 350 – 1150 nm wavelength range. We also evaluate the effects of different refractive indices available from the literature, different monomer numbers, and monomer sizes, as these are key factors determining the aggregates optical properties. Furthermore, we estimate the simple forcing efficiency for low and high surface albedos. Aggregates have a single scattering albedo (SSA) higher than that of individual tar balls (ΔSSA 550 nm up to 0.22). The hemispherical upscatter fraction of individual tar balls is more than 100% larger than for tar ball aggregates in many cases. The top of the atmosphere simple forcing efficiency over dark surfaces shows large variabilities with an increase up to ∼53% for tar ball aggregates compared to individual tar balls. These results demonstrate that aggregation of tar balls can have a significant impact on their optical properties and radiative forcing. [ABSTRACT FROM AUTHOR]