Department of Environmental Sciences and Physics Services, Weizmann Institute, Rehovot 76100, Israel.
Anal Chem. 2009 Mar 1;81(5):1762-9. doi: 10.1021/ac8017789.
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 atmospherically relevant optical parameters. We present a CRD system employing continuous wave (CW) single mode laser. The single mode laser and the high repetition 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 x 10(-10) cm(-1) for an empty cavity and 3.63 x 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 x 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 extinction 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 (sigma(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 ((piD)/lambda) 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.
与气溶胶对气候的直接影响相关的主要不确定性要求快速准确地描述它们的光学性质。腔衰荡（CRD）光谱学提供了对气溶胶消光系数的高灵敏度测量，从中可以准确地为已知大小和数密度的球形颗粒检索气溶胶的复折射率（RI），从而可以计算单次散射反照率和其他与大气相关的光学参数。我们提出了一种采用连续波（CW）单模激光的 CRD 系统。与脉冲激光 CRD 装置相比，单模激光和高重复率显著提高了系统的灵敏度和可靠性。对于空腔，CW-CRD 系统的检测极限在 6.67 x 10(-10) cm(-1) 到 1000 个颗粒/cm(3) 之间，在 400 Hz 采样和 2000 次shots 平均 5 秒内取一个样品的情况下，检测极限为 3.63 x 10(-9) cm(-1)。对于典型的脉冲 CRD，对于空腔，在 10 Hz 下平均 100 s 内进行 1000 次shots 的检测极限小于 3.8 x 10(-9) cm(-1)。该系统经过稳定性、准确性和 RI 检索测试，用于散射和吸收实验室生成的气溶胶。具体来说，在气溶胶数浓度发生数量级变化的情况下，长时间（1 小时）测量时，消光的恢复非常稳定。此外，通过测量消光效率（Q(ext)）作为尺寸参数（((piD)/lambda)）的函数，确定了 400nm 聚苯乙烯乳胶球（PSL）的光学截面（sigma(ext)），与基于米氏理论计算的值相比，误差在 2%以内。通过测量消光效率（Q(ext)）作为尺寸参数（((piD)/lambda)）的函数，确定了 PSL、nigrosin 和硫酸铵（AS）气溶胶的复折射率（RI），与文献值非常吻合。与之前的研究相比，苏万尼河富里酸（SRFA）的检索 RI 不匹配，这归因于样品成分的变化。所提出的小型系统具有快速测量和准确分析的高能力，使其成为现场气溶胶光学特性研究的良好候选者。
