Search

ABSTRACT Measurements with two Raman-depolarization lidars of the Meteorological Institute of the Ludwig-Maximilians-Universität, München, Germany, performed during SAMUM-2, were used to characterize the planetary boundary layer (PBL) over Praia, Cape Verde. A novel approach was used to determine the volume fraction of dust υ d in the PBL. This approach primarily relies on accurate measurements of the linear depolarization ratio. Comparisons with independent in situ measurements showed the reliability of this approach. Based on our retrievals, two different phases could be distinguished within the measurement period of almost one month. The first (22-31 January 2008) was characterized by high aerosol optical depth (AOD) in the PBL and large υ d > 95%. During the second phase, the AOD in the PBL was considerably lower and υ d less than ∼40%. These findings were in very good agreement with ground based in situ measurements, when ambient volume fractions are considered that were calculated from the actual measurements of the dry volume fraction. Only in cases when dust was not the dominating aerosol component (second phase), effects due to hygroscopic growth became important. [ABSTRACT FROM AUTHOR]

In the last years a significant number of eye-safe single-wavelength backscatter lidars, oftenreferred to as ceilometers, was installed for unattended operation. Though the primaryreason for such ceilometer networks is the automation of synoptic observations, inparticular the determination of the cloud base height, the potential of quantitativeremote sensing of aerosols has been investigated for approximately 10 years. Oneof the main benefits is the very good spatial coverage that allows to track aerosollayers in real time. The optical characterization of aerosols is however limited tothe particle backscatter coefficient βp. In this context it was ignored for a longtime that most of the ceilometers work in a spectral range where water vapor isabsorbing, so that a correction is required if one aims at the quantitative retrieval ofβp. As a consequence we have investigated the relevance of water vapor absorption forceilometer signals and developed a numerical procedure to correct for it (Wiegner andGasteiger, 2015). Recently the validation of this correction scheme was attempted (Wiegneret al., 2018), based on data from the CeiLinEx2015 campaign in Lindenberg, Germany, whenseveral types of ceilometers and a reference lidar system were deployed. In this context itmust be considered that a strict validation is hardly possible as the optical properties ofparticles (backscatter and extinction) at the wavelength of the reference system (withoutwater vapor absorption, in our case at 1064 nm) and at the wavelength of the ceilometer(with absorption, around 910 nm) are different. They can only be estimated from sunphotometer data and the inversion of the reference lidar signal. Moreover, the emittedwavelength of the ceilometers is typically not exactly known due to missing technicalspecifications and possible drifts with temperature. Taking these uncertainties intoaccount it was demonstrated that the correction of CL51 (Vaisala) measurements istrustworthy. We want to emphasize that water vapor absorption does not play a significant role for theassessment of the mixing layer height (mlh) and the detection of (elevated) aerosol layers. Forthese applications the low signal-to-noise ratio and (possible) signal artefacts, but also theambiguity of most mlh-retrievals are more relevant. Nevertheless, we recommend monitoringthe emitted wavelength of the instrument to obtain new options of aerosol remotesensing. Wiegner, M. and Gasteiger, J.: Correction of water vapor absorption foraerosol remote sensing with ceilometers, Atmos. Meas. Tech., 8, 3971-3984,https://doi.org/10.5194/amt-8-3971-2015, 2015. Wiegner, M. et al.: Aerosol backscatter profiles from ceilometers: validation of watervapor correction in the framework of CeiLinEx2015, Atmos. Meas. Tech. Discuss.,https://doi.org/10.5194/amt-2018-307, in review, 2018. [ABSTRACT FROM AUTHOR]