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

Abstract. 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 the
northwest (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 optical
depth (AOD), and near-surface AE are retrieved from MAX-DOAS measurements
recorded on cloud-free days applying the Bremen Optimal estimation REtrieval
for Aerosols and trace gaseS (BOREAS) algorithm. Measurements of atmospheric
profiles of pressure and temperature obtained from routinely performed sonde
ascents are used to calculate box-air-mass factors and weighting functions for different seasons. The performance of BOREAS was evaluated against
co-located ceilometer, sun photometer, and in situ instrument observations
covering all four seasons. The results show that the vertical AE profiles
retrieved from the BOKU UV–visible MAX-DOAS observations are in very good agreement with data from the co-located ceilometer, reaching correlation
coefficients (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>0.865 and linear regression
slopes 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 and
R=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 Vienna
is assessed by analyzing the retrieved and evaluated BOREAS aerosol
profiling products in terms of different azimuth angles of the two MAX-DOAS
instruments and for different seasons. We found that the relative
differences of averaged AOD between different azimuth angles are 7–13 %,
depending on the season. Larger relative differences of up to 32 % are
found for near-surface AE in the different azimuthal directions. This study
revealed the strong capability of BOREAS to retrieve AE profiles, AOD, and
near-surface AE over urban environments and demonstrated its use for
identifying the spatial variability of aerosols in addition to the temporal variation.
