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. Since May 2017 and August 2018, two ground-based MAX-DOAS (multi-axis differential optical absorption spectroscopy) instruments have been continuously recording daytime spectral UV–visible measurements in thenorthwest (University of Natural Resources and Life Sciences (BOKU) site) and south (Arsenal site), respectively, of the Vienna city center (Austria). In this study, vertical aerosol extinction (AE) profiles, aerosol opticaldepth (AOD), and near-surface AE are retrieved from MAX-DOAS measurementsrecorded on cloud-free days applying the Bremen Optimal estimation REtrievalfor Aerosols and trace gaseS (BOREAS) algorithm. Measurements of atmosphericprofiles of pressure and temperature obtained from routinely performed sondeascents are used to calculate box-air-mass factors and weighting functions for different seasons. The performance of BOREAS was evaluated againstco-located ceilometer, sun photometer, and in situ instrument observationscovering all four seasons. The results show that the vertical AE profilesretrieved from the BOKU UV–visible MAX-DOAS observations are in very good agreement with data from the co-located ceilometer, reaching correlationcoefficients (R) of 0.936–0.996 (UV) and 0.918–0.999 (visible) during the fall, winter, and spring seasons. Moreover, AE extracted using the lowest part of MAX-DOAS vertical profiles (up to 100 m above ground) is highly consistent with near-surface ceilometer AE (R&gt;0.865 and linear regressionslopes of 0.815–1.21) during the fall, winter, and spring seasons. A strong correlation is also found for the BOREAS-based AODs when compared to the AERONET ones. Notably, the highest correlation coefficients (R=0.953 andR=0.939 for UV and visible, respectively) were identified for the fall season. While high correlation coefficients are generally found for the fall, winter, and spring seasons, the results are less reliable for measurements taken during summer. For the first time, the spatial variability of AOD and near-surface AE over the urban environment of Viennais assessed by analyzing the retrieved and evaluated BOREAS aerosolprofiling products in terms of different azimuth angles of the two MAX-DOASinstruments and for different seasons. We found that the relativedifferences of averaged AOD between different azimuth angles are 7–13 %,depending on the season. Larger relative differences of up to 32 % arefound for near-surface AE in the different azimuthal directions. This studyrevealed the strong capability of BOREAS to retrieve AE profiles, AOD, andnear-surface AE over urban environments and demonstrated its use foridentifying the spatial variability of aerosols in addition to the temporal variation.

atmospheric measurement techniques

Evaluation of UV–visible MAX-DOAS aerosol profiling products by comparison with ceilometer, sun photometer, and in situ observations in Vienna, Austria