A pulsed cavity ring-down transmissometer is shown here to be capable of sensitively measuring the aerosol extinction coefficient at two wavelengths (λ = 532, 1064 nm) simultaneously. This instrument can be coupled with a nephelometer (yielding a measurement of the 530 nm scattering coefficient of the same aerosol) and a particle counter to allow the in situ measurement of extensive and intensive optical properties and particle number concentrations of aerosols. From the scattering and extinction coefficient measurements, the aerosol absorption coefficient can be calculated and the intensive properties single scattering albedo and extinction Ångström exponent can be determined. In this report, the pulsed cavity-ring down transmissometer (CRDT) is validated through a series of laboratory experiments. Agreement between the extinction coefficients from the cavity ring-down transmissometer and the scattering coefficient from the nephelometer is demonstrated for purely scattering sub-micron particles. Then agreement between measured extinction and Mie theory is demonstrated when using size-selected particles of measured number concentration. The agreement with Mie theory ranges from excellent (less than 1% deviation) to marginal (12%) depending on the particle size and composition. Similar deviations from Mie theory have been observed (Baynard et al. 2007) and we suggest that they could be due to the influence of multiply charged particles in the size-selection (DMA) process. The 95% confidence level limits of detection and quantitation for the extinction measurement by the CRD transmissometer are estimated to be bext = 4.0 Mm−1 and 13.4 Mm−1, respectively