Vienna horizontal and vertical distribution observations of nitrogen dioxide and aerosols

The overall goal of the proposed project is to improve our current knowledge of air pollution in large agglomerations caused by mankind. The investigation of nitrogen dioxide (NO2) and aerosol amounts will be based on spectral measurements from two Multi AXis Differential Optical Absorption Spectroscopy (MAX- DOAS) instruments located at two different sites with ideal measurement conditions in Vienna, Austria. The MAX-DOAS instrument allows for measurements at different viewing directions and thus, it can be used for obtaining both the horizontal and vertical variations of trace gases and aerosols in the troposphere useful measurements for quantifying air pollution. The retrieval of tropospheric columns and vertical profiles of NO2 and aerosols will be based on the fast geometrical approximation and the more accurate optimal estimation method implemented in the Bremian advanced MAX-DOAS Retrieval Algorithm (BREAM). On the one hand, altitude-averaged tropospheric vertical trace gas columns retrieved from MAX-DOAS together with meteorological measurements (e.g. wind speed, wind direction) allow the monitoring of air pollutants (NO2 and aerosols) for urban background, high-traffic roads, and industrial point sources on the horizontal level. On the other hand, vertical profiles provide valuable information for the comparison with in-situ measurements in Vienna. The proposed long-term measurements (more than two years) will provide a valuable data set for analyzing the temporal variability of air pollutants over Vienna. Moreover, these data will be used for the comparison with MAX-DOAS observations in Athens, Greece, and Bremen, Germany to investigate similarities and differences between polluted sites at different latitudes and meteorological and photochemical regimes. Tropospheric vertical columns of NO2 will also allow the validation of satellite measurements from the OMI, GOME-2, and TROPOMI instruments as well as comparison with model simulations, for example from the COPERNICUS atmospheric monitoring service. As the measurements from the two instruments cover several azimuthal directions with partially overlapping fields of view, these data together with in-situ measurements provide a multitude of information on the spatial NO2 distribution, enabling an attempt to develop a spatially resolved image of air pollution for Vienna using a tomographic imaging approach a novel method with respect to MAX-DOAS and a large city like Vienna. The results of the project will provide information about the horizontal and vertical distributions of NO 2 and aerosols over an urban environment. In addition to improvements in trace gas retrieval methods, the results will also provide important information for the model community, as the vertical profiles of NO 2 are a useful supplement to the point measurements of in-situ instruments.

Satellite observations of tropospheric nitrogen dioxide (NO2) form an important basis for estimating the environmental impact of nitrogen oxide emissions and for assessing the impact of atmospheric pollution on human health. There is a great need to evaluate the accuracy of satellite tropospheric NO2 vertical columns by validating these data products against other measurements, for example ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations. Moreover, the conversion of tropospheric NO2 vertical columns to surface NO2 concentrations, which is of great interest for exposure studies, requires new scientific approaches to reduce existing uncertainties. One major goal of the VINDOBONA project was to improve the spatial representativeness of ground-based MAX-DOAS observations in urban environments by making use of measurements taken simultaneously at three locations in Vienna, each covering a range of azimuth directions. By comparing MAX-DOAS integrated NO2 concentrations along horizontal columns with each other as well as with in-situ NO2 data from local air quality measuring stations, interesting insights into the spatial distribution of NO2 in Vienna was gained. Even more insights and in fact, a higher spatial variability of NO2 on the scale of the city was found from case study-based DOAS horizontal measurements taken on the rotating Café of the Danube Tower. These results highlight the need to refine the colocation of ground-based MAX-DOAS with satellite pixels in future validation activities. Car DOAS NO2 measurements taken in Vienna on several days were analyzed together with surface NO2 concentrations and meteorological data to develop an empirical method to convert tropospheric NO2 vertical columns into surface NO2 concentrations along the route. This new method has potential applications in the conversion of MAX-DOAS and satellite-derived tropospheric NO2 vertical columns on the local and global scale, respectively. Spatial and temporal patterns of vertical aerosol profiles were derived from MAX-DOAS observations taken at selected elevation angles and azimuth directions applying the Bremen Optimal estimation REtrieval for Aerosols and trace gaseS (BOREAS) algorithm. It was found that substantial variations could occur over the urban environment of Vienna, both on the spatial and temporal scales. These results have possible implications for the retrieval of tropospheric NO2 vertical columns from satellite measurements, as a priori information on aerosols and the shape of vertical profiles is usually based on monthly climatologies. Finally, a completely new method to obtain the spatial distribution of NO2 over the urban environment of Vienna by making use of horizontal MAX-DOAS measurements taken at many different azimuth directions by the three instruments was developed within the framework of VINDOBONA. For the first time, a two-dimensional picture of NO2 above an urban setting based on MAX-DOAS data could be produced - another valuable tool for future satellite validation activities and human exposure studies.