Abstract
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.
Highlights
Atmospheric aerosols are defined as particles suspended in the air, with particle diameters in the range of 10−9 to 10−4 m (0.001 to 100 μm) and various shapes, chem-Published by Copernicus Publications on behalf of the European Geosciences Union.S
We evaluate and analyze aerosol extinction (AE) profiles, aerosol optical depth (AOD), and near-surface AE retrieved from UV–visible spectral measurements collected with two MAX-DOAS instruments in Vienna, Austria, located in the northwest and south of the city center
The performance of BOREAS in this study is evaluated by considering AE profiles, AOD, and near-surface AE retrievals in the UV and visible channels that fulfill the following criteria: (1) the absolute and relative differences between measured and simulated O4 differential slant column densities (DSCDs) at all individual elevation angles are less than 1000 × 1040 molec2 cm−5 and less than 10 %, respectively, (2) the maximum AOD is less than 1.0, and (3) no more than 50 iterations were needed in the retrieval
Summary
Atmospheric aerosols are defined as particles (liquid or solid) suspended in the air, with particle diameters in the range of 10−9 to 10−4 m (0.001 to 100 μm) and various shapes, chem-. Schreier et al.: Evaluation of UV–visible MAX-DOAS aerosol profiling products ical compositions, and hygroscopic and optical properties (Seinfeld and Pandis, 2006). Aerosols are an important component of the Earth’s atmosphere and play a crucial role in atmospheric chemistry, cloud formation and lifetime, Earth’s radiation budget, and climate (IPCC, 2013). It has been widely documented that enhanced atmospheric aerosol loading has adverse effects on human health (Cohen et al, 2005; Liu et al, 2009; Russell and Brunekreef, 2009; Fann et al, 2012; Lelieveld et al, 2015)
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