Lidar system

The lidar system used in the present study consisted of a Nd:YAG laser at a wavelength of 532 nm. The receiver consists of a 20 cm Schmidt-Cassegrain telescope. Signals are measured by photomultiplier tubes (PMT) and analyzed by multi-channel analyzers (MCA) with a vertical resolution jNd: YAG]

Fig. 1. Block diagram of lidar system at Chung-Li, Taiwan. (PMT: photomultiplier tube, Dis: discriminator, MCA: multi-channel analyzer, PC: personal computer).

of 24 m and a temporal resolution of 33 s. The schematic diagram of lidar system is shown in Fig. 1. Details about our lidar studies/system can be found in the studies of Nee et al.5 and Chen et al.6 The lidar measures height profile of backscattered signal from aerosols, which are converted into backscattering ratio. The aerosols backscattering ratio, BR, is defined as follows:

where f3a and f3m are the backscatter coefficients of aerosols and molecules, respectively. The backscattering coefficient of molecule is derived from the molecular concentration measured by radiosonde multiplied by total Rayleigh scattering cross section. Calculation of aerosol backscattering ratio involved Fernald solution.7 For this study, the initial condition for the upper height is set at 5 km, and the reference value of backscattering ratio is 1.05, based on the earlier study.8 The extinction-to-backscatter ratio or lidar ratio for the aerosol and cloud cases was derived by Chiang et al.9

The polarization measurements were carried out by recording the lidar returns for perpendicular and parallel polarizations. The aerosol depolarization ratio (Sa) is calculated as a- ( \ * BRjz) - 5m da{z) = BR(z) - 1 ' (2)

where ô is the total (aerosol + molecule) depolarization ratio which is obtained by taking the ratio of the perpendicular and the parallel intensity relative to the outgoing laser beam. ôm is the molecular depolarization ratio (ôm ~ 1.4%).10 Equation (2) also indicates how correlation of the above two traces namely, BR and ôa, along a vertical interval Az will provide the information of solid or liquid particles' dominance. The depolarization ratio is 0 for spherical particles and deviates from 0 for nonspherical particles such as mineral dust.

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