The sky is currently empty, and the roads are hardly frequented. The measures taken by the Swiss Federal Council not only have an impact on our everyday lives, but also on air quality. The 16 measuring stations of the National Air Pollution Monitoring Network (NABEL) monitor and analyze numerous pollutants in the atmosphere. These data show the state of air quality in Switzerland since the lockdown mid March. The researchers at Empa's "Air Pollution/Environmental Technology" lab are particularly busy these days and are constantly updating their data.
The weather as an important factor
However, the analysis of this data is extremely complex. For the extent to which the air is polluted with pollutants does not only depend on their emission; the weather - especially wind and temperature conditions - plays a major role. For example, calm inversion weather conditions, i.e. when the higher air layers are warmer than the lower ones, tend to lead to polluted air. Simple comparisons of air pollutants before and after the lockdown are therefore not sufficient.
For example, the mild temperatures in recent months have led to a good mixing of the air layer close to the ground and thus to good air quality. In addition, in the second half of March, a pronounced breeze caused high dilution and low levels of air pollutants. For this reason, a reduction in pollutants was observed at many sites compared with the previous year, but this cannot be directly attributed to the measures taken by the Federal Council in connection with Covid-19. Computer models are helpful, however, because they allow air quality to be predicted on the basis of long series of measurements and information such as weather, time of year and time of day - provided that no exceptional events occur.
The measures taken since mid-March are precisely such an exceptional event. A comparison of forecasts and observations therefore allows us to say something about the influence of Covid-19 on air quality. Nitrogen oxides, for example, have been significantly reduced as a result of the measures taken at locations close to roads, while their concentration in rural areas is almost exclusively determined by weather conditions.