Articles | Volume 8, issue 9
Research article
30 Sep 2015
Research article |  | 30 Sep 2015

Correction of water vapor absorption for aerosol remote sensing with ceilometers

M. Wiegner and J. Gasteiger

Abstract. In recent years attention was increasingly paid to backscatter profiles of ceilometers as a new source of aerosol information. Several case studies have shown that – although originally intended for cloud detection only – ceilometers can provide the planetary boundary layer height and even quantitative information such as the aerosol backscatter coefficient βp, provided that the signals have been calibrated. It is expected that the retrieval of aerosol parameters will become widespread as the number of ceilometers is steadily increasing, and continuous and unattended operation is provided. In this context however one should be aware of the fact that the majority of ceilometers provides signals that are influenced by atmospheric water vapor. As a consequence, profiles of aerosol parameters can only be retrieved if water vapor absorption is taken into account. In this paper we describe the influence of water vapor absorption on ceilometer signals at wavelengths around λ = 910 nm. Spectrally high-resolved absorption coefficients are calculated from HITRAN on the basis of realistic emission spectra of ceilometers. These results are used as a reference to develop a methodology ("WAPL") for routine and near-real time corrections of the water vapor influence. Comparison of WAPL with the reference demonstrates its very high accuracy. Extensive studies with simulations based on measurements reveal that the error when water vapor absorption is ignored in the βp-retrieval can be in the order of 20 % for mid-latitudes and more than 50 % for the tropics. It is concluded that the emission spectrum of the laser source should be provided by the manufacturer to increase the accuracy of WAPL, and that 910 nm is better suited than 905 nm. With WAPL systematic errors can be avoided, that would exceed the inherent errors of the Klett solutions by far.

Short summary
For the past few years a large number of autonomous continuously operating single-wavelength backscatter lidars, so called ceilometers, have been installed. Currently the assessment of their potential for aerosol remote sensing is a major research topic. This paper focusses on the need to consider water vapor absorption if ceilometers emitting at wavelengths in the 905 to 910 nm range are used and proposes a correction scheme to improve the retrieval of the aerosol backscatter coefficient.