Cavity ring-down spectroscopy (CRDS) is a technique that is commonly used to measure the extinction of light by aerosol particles in situ. This extinction, when normalized to particle concentration, yields the extinction cross section, a measure of a single particle's ability to scatter and absorb light. The complex index of refraction can then be retrieved by comparison of the extinction cross sections at several particle diameters with those predicted by Mie theory. This approach requires accurate determination of particle diameter and concentration as well as the length of the extinction region in the cavity, but it is often difficult to quantify the systematic errors in the measurements of these quantities. Here, we introduce a calibration technique using particles of a reference compound to account for these systematic errors. The two calibration parameters are: Cf, which scales the measured extinction cross sections, and Ad, which shifts the particle diameters. It is found that C_f correlates strongly with the condensation particle counter (CPC) used to measure particle concentration and that Δd is associated with the differential mobility analyzer (DMA) used to select particle diameters. Calibration is shown to reduce errors of subsequently-measured extinction cross sections of a test aerosol from 11% to <2% with a concomitant improvement in the accuracy of the retrieved complex index of refraction and corresponding atmospheric radiative forcing estimates.
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