Abstract
Vertical profiles of the linear particle depolarization ratio of pure dust clouds were measured during the Saharan Mineral Dust Experiment (SAMUM) at Ouarzazate, Morocco (30.9◦N, –6.9◦E), close to source regions in May–June 2006, with four lidar systems at four wavelengths (355, 532, 710 and 1064 nm). The intercomparison of the lidar systems is accompanied by a discussion of the different calibration methods, including a new, advanced method, and a detailed error analysis. Over the whole SAMUM periode pure dust layers show a mean linear particle depolarization ratio at 532 nm of 0.31, in the range between 0.27 and 0.35, with a mean Ångström exponent (AE, 440–870 nm) of 0.18 (range 0.04–0.34) and still high mean linear particle depolarization ratio between 0.21 and 0.25 during periods with aerosol optical thickness less than 0.1, with a mean AE of 0.76 (range 0.65–1.00), which represents a negative correlation of the linear particle depolarization ratio with the AE. A slight decrease of the linear particle depolarization ratio with wavelength was found between 532 and 1064 nm from 0.31 ± 0.03 to 0.27 ± 0.04.
Highlights
Shape, size distribution and composition of aerosol particles influence their scattering characteristics and the radiative impact
We report measurements of the linear particle depolarization ratios δp of pure Saharan dust with four lidar systems at four wavelengths in Quarzazate, Morocco, during Saharan Mineral Dust Experiment (SAMUM) in May–June 2006
We evaluate the errors and their sources of the calibration methods used for the different lidars, and achieve trustworthy error estimations for the linear depolarization ratios
Summary
Size distribution and composition of aerosol particles influence their scattering characteristics and the radiative impact. Observations of the linear depolarization ratio at several wavelengths may be used in retrieval schemes (Dubovik et al, 2006) to improve the estimation of the microphysical properties of dust from optical measurements First dual-wavelength aerosol polarization lidar measurements were presented by Sugimoto et al (2002). The linear depolarization ratio δ is defined as the ratio of the cross–polarized lidar return signal to the parallel-polarized backscatter signal. Technique appears to be rather simple, and robust and reliable, several sources for systematic errors can have a significant impact on the result and can introduce large errors.
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