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We consider the impact of uncertainties in the refractive index and size of polystyrene beads on the retrieved optical properties of aerosol particles by aerosol cavity ring down spectroscopy (A-CRDS). Polystyrene beads are frequently used to verify and calibrate light extinction measurements by cavity ring down instruments. Any uncertainties in either the polymer particle size or the refractive index can contribute to systematic errors in properties retrieved for any subsequent measurements on other aerosol types. We demonstrate that the tolerances on bead size reported by the manufacturers can lead to a range in the real part of the refractive index of the polymer beads retrieved from A-CRDS measurements of as large as 2.9%. Further, we show that the current uncertainty in the refractive index of polystyrene beads in the visible part of the electromagnetic spectrum limits the accuracy with which the real part of the refractive index of other aerosol types can be retrieved to uncertainties of -0.5% and +0.3% at a minimum. This error should be included in any subsequent retrieval of aerosol optical properties from aerosol cavity ring down instruments that are dependent on polystyrene bead calibration. It is expected that such calibrations could lead to significantly larger uncertainties if the complex part of the refractive index is to be retrieved.