We have heard rumors that the European Union is planning to stop funding and effectively end the TROPOMI measurements onboard the Sentinel-5P satellite, now that Sentinel-5 is in orbit. Through this petition, we would like to express our deep concern about this. Before getting into the specific reasons, we first want to highlight the importance of the TROPOMI data set.

Since its launch in 2017, TROPOMI data have been used by thousands of users in academia, governments and the private sector. TROPOMI data were used to visualize the impacts from the COVID shutdowns, to quantify trends in air pollutant emissions, to discover major methane leaks, to track volcanic plumes, and to quantify the oxidizing capacity of the atmosphere. A search for papers in the Web of Science that have "TROPOMI" or "Sentinel 5P" in their abstracts currently yields over 1,700 papers, which as a whole have been cited by more than 33,000 other papers. These numbers are still growing exponentially (see above figure).

Here is why we are deeply concerned about ending the TROPOMI measurements:

1. Ending a hugely successful mission that is not anywhere near its end-of-life is a waste of the investments made and the countless hours that went into planning and executing the mission.
2. The Sentinel 5 data are as yet unproven. Having the two missions overlap for several years is imperative to even consider ending the TROPOMI measurements.
3. TROPOMI data are being used by thousands of users, if not more, in academia, government and the private sector. Ending the TROPOMI mission will be very disruptive to their work.
4. Given the importance of the data, the redundancy of having multiple instruments in space is not a luxury. The unexpected loss of contact with MethaneSat is a good reminder that space-based instruments can and will develop problems.
5. Sentinel 5 is in a morning orbit, while Sentinel 5P, which carries TROPOMI, is in an afternoon orbit. This is important because some of the gases measured by TROPOMI and Sentinel 5 are reactive and are not the same in the morning and afternoon. Combining the data yields unique insights into chemical processes in the atmosphere.

We urge the European Union to continue the TROPOMI measurements in the foreseeable future. Please join us in signing this petition.