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. We present the evaluation activity of the EuropeanAerosol Research Lidar Network (EARLINET) for the quantitative assessment ofthe Level 2 aerosol backscatter coefficient product derived by theCloud-Aerosol Transport System (CATS) aboard the International SpaceStation (ISS; Rodier et al., 2015). The study employs correlative CATS and EARLINET backscattermeasurements within a 50 km distance between the ground station and the ISSoverpass and as close in time as possible, typically with the starting time orstopping time of the EARLINET performed measurement time window within 90 min of the ISS overpass, for the period from February 2015 to September 2016. The results demonstrate the good agreement of the CATS Level 2 backscattercoefficient and EARLINET. Three ISS overpasses close to the EARLINETstations of Leipzig, Germany; Evora, Portugal; and Dushanbe, Tajikistan, areanalyzed here to demonstrate the performance of the CATS lidar system underdifferent conditions. The results show that under cloud-free, relativehomogeneous aerosol conditions, CATS is in good agreement with EARLINET,independent of daytime and nighttime conditions. CATS low negative biases areobserved, partially attributed to the deficiency of lidar systems to detecttenuous aerosol layers of backscatter signal below the minimum detectionthresholds; these are biases which may lead to systematic deviations and slightunderestimations of the total aerosol optical depth (AOD) in climatestudies. In addition, CATS misclassification of aerosol layers as clouds,and vice versa, in cases of coexistent and/or adjacent aerosol and cloudfeatures, occasionally leads to non-representative, unrealistic, and cloud-contaminated aerosol profiles. Regarding solar illumination conditions, lownegative biases in CATS backscatter coefficient profiles, of the order of6.1 %, indicate the good nighttime performance of CATS. During daytime,a reduced signal-to-noise ratio by solar background illumination preventsretrievals of weakly scattering atmospheric layers that would otherwise bedetectable during nighttime, leading to higher negative biases, of the orderof 22.3 %.

EARLINET evaluation of the CATS Level 2 aerosol backscatter coefficient product