First measurements of spatial NO2 variability in Bremen with a newly developed tram DOAS instrument

Mobile DOAS measurements are a good option to determine the spatial distribution of NO₂ or other trace gases such as HCHO and SO₂ within a city. In previous works, such measurements have been performed using cars and have provided interesting insights into spatial inhomogeneities of the NO₂ distribution and valuable contributions to satellite validation. A disadvantage of car DOAS measurements is the time required for driving the car.

Therefore, a new DOAS instrument was developed and installed on a tram in Bremen in November 2021. The instrument is mostly operating in zenith-sky mode, but also takes measurements at 20° elevation every 7 minutes. The advantage of the tram DOAS instrument is that it performs measurements during the normal operation of the tram. As a result, regular measurements along the route network can be carried out without additional time spent driving.

Up to now, the instrument has gathered one year of data all over the Bremen tram network. It can be clearly seen that the areas of increased tropospheric NO₂ VCDs change throughout the day, but certain areas show reproducibly higher NO₂ pollution. We found elevated NO₂ levels in the industrially dominated northwest and lower levels in the more rural northeast of Bremen. This is confirmation for a very similar distribution visible in the TROPOMI tropospheric NO₂ VCDs data. In addition, the tram DOAS measurements are compared to the measurements from a stationary MAX-DOAS instrument installed close to one of the tram lines.