A team led by environmental physicist Stefan Schreier at the University of Natural Resources and Applied Life Sciences in Vienna is attempting to build up a better picture of nitrogen dioxide pollution in the city. The work includes the first attempts to describe how this pollution is distributed in the vertical axis, from street-level upwards, as well as the use of data from satellite measurements.
Stefan Schreier stands 130m above the ground in front of a sheer drop. He adjusts a device in a stainless steel housing with a lens pointing up into the sky, mounted on the highest platform of the Arsenal radio tower in Vienna. A fibre optic cable runs from the device to a small maintenance room equipped with a spectrometer and a laptop. Since 2018, this device has been measuring pollutants in the air above Vienna, especially NO2, which has attracted heightened public attention as a combustion by-product from diesel engines. In combination with two identical devices installed on other buildings on the campus, the units monitor Vienna’s city centre and measure air quality.
Vertical axis view
Air quality measurements are more typically carried out close to the ground. The City of Vienna has a network of 17 measuring stations distributed throughout the city. These stations carry out point measurements, but this is not necessarily a satisfactory approach to describing the distribution of pollutants over a wide area.
Schreier and his team say they want to close this gap in a project funded by the Austrian Science Fund FWF. They use a measuring instrument called “MAX-DOAS”, short for “Multi AXis Differential Optical Absorption Spectroscopy”. The units measure characteristic deviations in the colour composition of light.
A particular achievement of the project so far, in Schreier’s view: “For the first time, we can now directly measure the vertical distribution of NO2 over the Vienna city area.”
Depending on the weather conditions and the amount of aerosols in the air, the three instruments can look several kilometres into the distance. Since their measuring ranges overlap, special opportunities arise to combine these measurements, said Schreier. “We try to derive the spatial distribution of nitrogen dioxide from the many horizontal measurements of the three MAX-DOAS devices.” The method in some ways works along similar principles to computer tomography. Building up a three-dimensional picture of the distribution of nitrogen dioxide over Vienna is the project’s main objective, and the team suggest this has yielded important insights so far. For instance, measurements from 2019 seemingly showed how, on a day with an easterly wind, the polluted air from the busy streets and industrial areas in the southeast of the city is transferred to the west.
Complementing satellite measurements
Information on nitrogen dioxide distribution in the atmosphere is also provided by a satellite operated by ESA that measures nitrogen dioxide in the Earth’s atmosphere from space. The first instrument of this kind was launched in 1995, and had a spatial resolution of more than 100×100 kilometres per pixel, said Schreier. The Tropomi spectrometer on board the European satellite Sentinel-5p has been conducting measurements since 2017. Its data provides a global picture of nitrogen dioxide distribution with a horizontal resolution of seven by three kilometres, which corresponds to a few pixels spanning the city.
Satellite measurements provide important information about air pollutants, but it is not known exactly how much these measurements tell us about the air quality just above the ground – i.e. where people are located. The MAX-DOAS instruments installed by Schreier and his team are thus designed to help validate the ESA data. “Satellite measuring devices cannot be repaired if their reliability deteriorates,” explains Schreier. “Therefore, it is important to be able to compare the results with terrestrial measurements so that they can be corrected if necessary.” Also, satellite measurements do not indicate the exact elevation at which the nitrogen dioxide is found.
Monitoring of other pollutants
There are practical reasons why Schreier’s team is currently focusing on nitrogen dioxide, as the researcher explains: “Nitrogen dioxide is the trace gas that can best be detected by means of the DOAS method.” In a next step, the Vienna MAX-DOAS measurements are however designed to look into other pollutants, including particulate matter. “We also measure gases that are responsible for the formation of ground-level ozone, which is very harmful to human health,” said Schreier. The latter is work in progress. The five-year project will run until 2021.
What is MAX-DOAS?
Multi-axis differential optical absorption spectroscopy (MAX-DOAS) is a widely used measurement technique for the detection of a atmospheric trace gases. The observation of trace gas column densities along different lines of sight allows the retrieval of aerosol and trace gas vertical profiles in the atmospheric boundary layer using appropriate retrieval algorithms.
The approach relies upon the spectral analysis of scattered sunlight and supports the simultaneous detection of numerous trace gases. Measurements along different lines of sight, with elevation angles ranging from near the horizon to the zenith, allow for the reconstruction of vertical profiles of the measured trace gases.
Using suitable inverse models, trace gas and aerosol profiles can be retrieved in the lowermost 2 km with a vertical resolution of about 50–100 m at the surface and a lower resolution above. Up to four independent pieces of information can be retrieved.