Accurate measurement of optical extinction of atmospheric aerosols is important for quantifying the direct climate effects of aerosols. A portable cavity ringdown spectrometer utilizing a modulated multimode blue diode laser (linewidth ~0.2 nm) is developed to measure the aerosol optical extinction. Laboratory generated ammonia sulfate particles (<1 μm in diameter) are characterized, with good agreements between the experimental measurements and Mie theory calculations. An optical extinction detection sensitivity of 0.24 Mm −1 (1σ) is achieved. Measurements of ambient aerosols are also carried out. This study demonstrates the feasibility of a compact, multimode diode laser cavity ringdown spectrometer for sensitive measurements of the optical extinction of atmospheric aerosols. aerosol, optical extinction, spectroscopy Citation:Liu Y D, Zhang J S. Measurement of aerosol optical extinction using diode laser cavity ringdown spectroscopy.