Accurate measurement of optical properties of aerosols is crucial for quantifying the influence of aerosols on climate. This article describes laboratory testing of a cavity ring down (CRD) system for measurement of the optical extinction of aerosol. The system is tested with well-characterized laboratory generated aerosols, and the measured extinctions agree within 5% of predictions by Mie theory. Extension of the lab prototype to field measurements of atmospheric aerosol is discussed. It is shown that the precision of measurements of the optical extinction of aerosol in the field will be determined by statistical fluctuations in the number of particles in the sample volume. Averaging times on the order of minutes are required to reduce statistical fluctuations to less than 5%. Improvements to the CRD system to reduce the averaging times are proposed. (C) 2004 Elsevier Ltd. All rights reserved.