Uncertainty estimation of aerosol properties from a Vaisala CT25k ceilometer based on in situ aerosol measurements

Abstract. In recent years, the use of automatic lidars and ceilometers (ALC) for atmospheric research has increased. Originally, these instruments were developed to measure cloud base height automatically, utilising the LIDAR principle. However, multiple studies have shown their usability for aerosol remote sensing and planetary boundary layer height detection. It is not only possible to calibrate a ceilometer and derive the attenuated backscatter signal, but also to retrieve aerosol extinction coefficients and aerosol mass concentrations by means of estimated extinction to mass coefficients (EMC). The ACTRIS national facility JOYCE (Jülich Observatory for Cloud Evolution) offers a multiyear dataset of cloud remote sensing measurements and ceilometer observations. So far, a method for measuring aerosol properties has been missing to use this dataset to quantify aerosol-cloud interaction. The goal of this study is to evaluate the applicability of a ceilometer aerosol retrieval to prove the value of this dataset for aerosol remote sensing. We present the workflow, starting with the raw ceilometer data, followed by a calibration of the backscatter coefficient profiles and a retrieval of aerosol properties. To evaluate the result of this workflow for the JOYCE ceilometer (Vaisala CT25k), in situ aerosol measurements at the Jülich meteorological tower were performed, where an optical particle sizer (OPS) was installed at 120 m above ground. Aerosol extinction coefficients σ_a were retrieved from the ceilometer attenuated backscatter signal with σ_a correlated with the in situ total aerosol mass concentration with R = 0.73. For our measurement set-up, aerosol mass concentrations can be derived from the retrieved σ_a with a mean absolute percentage error of 39 %. However, the extinction to mass conversion factor EMC = (2.2 ± 0.9) m²g^-1 derived from the measurements for a wavelength of 906 nm was found to be greater by a factor of about 1.8 compared to literature and to EMC calculated from Mie simulations based on the in situ aerosol size distributions. The mismatch is tentatively attributed to the limited aerosol size range of the OPS.