Complex Refractive Indices of Aerosols Retrieved by Continuous Wave-Cavity Ring Down Aerosol Spectrometer N. Lang-Yona, † Y. Rudich, † E. Segre, ‡ E. Dinar, † and A. Abo-Riziq* ,† Department of Environmental Sciences and Physics Services, Weizmann Institute, Rehovot 76100, Israel The major uncertainties associated with the direct impact of aerosols on climate call for fast and accurate characterization of their optical properties. Cavity ring down (CRD) spectroscopy provides highly sensitive measurement of aerosols’ extinction coefficients from which the complex refractive index (RI) of the aerosol may be retrieved accurately for spherical particles of known size and number density, thus it is possible to calculate the single scattering albedo and other atmo- spherically relevant optical parameters. We present a CRD system employing continuous wave (CW) single mode laser. The single mode laser and the high repeti- tion rate obtained significantly improve the sensitivity and reliability of the system, compared to a pulsed laser CRD setup. The detection limit of the CW-CRD system is between 6.67 × 10 -10 cm -1 for an empty cavity and 3.63 × 10 -9 cm -1 for 1000 particles per cm 3 inside the cavity, at a 400 Hz sampling and averaging of 2000 shots for one sample measurement taken in 5 s. For typical pulsed-CRD, the detection limit for an empty cavity is less than 3.8 × 10 -9 cm -1 for 1000 shots averaged over 100 s at 10 Hz. The system was tested for stability, accuracy, and RI retrievals for scattering and absorbing laboratory- generated aerosols. Specifically, the retrieved extinc- tion remains very stable for long measurement times (1 h) with an order of magnitude change in aerosol number concentration. In addition, the optical cross section (σ ext ) of a 400 nm polystyrene latex sphere (PSL) was determined within 2% error compared to the calculated value based on Mie theory. The complex RI of PSL, nigrosin, and ammonium sulfate (AS) aerosols were determined by measuring the extinction efficiency (Q ext ) as a function of the size parameter ((πD)/λ) and found to be in very good agreement with literature values. A mismatch in the retrieved RI of Suwannee River fulvic acid (SRFA) compared to a previous study was observed and is attributed to variation in the sample composition. The small system presented delivers high ability for fast measurements and accurate analysis, making it a good candidate for field aerosol optical properties studies. The impact of aerosols on the Earth’s climate and air quality is a major uncertainty in climate change models as was empha- sized in the latest Intergovernmental Panel on Climate Change (IPCC) report, 1 pointing to an urgent need for further studies on aerosol properties and processes. A major component of the aerosol’s climatic effect is their scattering and absorption of solar radiation, which is governed by their optical and physical properties. 2-7 Common scattering aerosols in the atmosphere include inorganic salts (sea spray, sulfate aerosols). 8,9 These aerosols have mainly a “cooling effect” on the climate due to a decrease in the solar radiation that reaches the Earth’s surface. Mineral dust, 10-12 soot aerosols, 13-18 and organic aerosols * To whom correspondence should be addressed. E-mail: Ali.Abo-riziq@ weizmann.ac.il. † Department of Environmental Sciences. ‡ Physics Services. (1) Solomon, S.; Qin, D.; Manning, M.; Alley, R. B.; Berntsen, T.; Bindoff, N. 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Science 2002, 297, 2250– 2253. Anal. Chem. 2009, 81, 1762–1769 10.1021/ac8017789 CCC: $40.75  2009 American Chemical Society 1762 Analytical Chemistry, Vol. 81, No. 5, March 1, 2009 Published on Web 02/06/2009