Correction of water vapor absorption for aerosol remote sensing with ceilometers

[摘要] In recent years attention was increasingly paid to backscatter profiles ofceilometers as a new source of aerosol information. Several case studies haveshown that – although originally intended for cloud detection only – ceilometerscan provide the planetary boundary layer height and evenquantitative information such as the aerosol backscatter coefficientβ_p, provided that the signals have been calibrated. It isexpected that the retrieval of aerosol parameters will become widespread asthe number of ceilometers is steadily increasing, and continuous andunattended operation is provided. In this context however one should be awareof the fact that the majority of ceilometers provides signals that areinfluenced by atmospheric water vapor. As a consequence, profiles of aerosolparameters can only be retrieved if water vapor absorption is taken intoaccount. In this paper we describe the influence of water vapor absorption onceilometer signals at wavelengths aroundλ = 910 nm. Spectrally high-resolved absorption coefficients arecalculated from HITRAN on the basis of realistic emission spectra ofceilometers. These results are used as a reference to develop a methodology("WAPL") for routine and near-real time corrections of the water vaporinfluence. Comparison of WAPL with the reference demonstrates its very highaccuracy. Extensive studies with simulations based on measurements revealthat the error when water vapor absorption is ignored in theβ_p-retrieval can be in the order of 20 % for mid-latitudesand more than 50 % for the tropics. It is concluded that the emissionspectrum of the laser source should be provided by the manufacturer toincrease the accuracy of WAPL, and that 910 nm is better suited than905 nm. With WAPL systematic errors can be avoided, that would exceed theinherent errors of the Klett solutions by far.

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