Comparison of equations used to estimate soot agglomerate absorption efficiency with the Rayleigh-Debye-Gans approximation

•Absorption efficiency equations for soot particles in Rayleigh regime were compared.•Errors were quantified for the application of the Rayleigh-Debye-Gans approximation.•Large primary particles lead to unacceptable errors at short wavelengths.•Fourth-order equation by Bohren-Huffman provided the lowest errors.•Wavelength spectrum and particle sizes should be considered to select equation.