From the paper; I am not sure if the twin system was run for a very long time. You could contact the lead for the group in Stuttgart (https://www.ifk.uni-stuttgart.de/en/institute/team/Vogt-00002/.)
I recently posted here our preliminary results from our three sensors and, unfortunately for us, the variation between the sensors is quite large (looks like about 40%) this should be sorted out with a calibration factor. However, we do not have a reference system so I am assuming an average of the three (below 70% humidity) gives the most accurate value of actual PM2.5.
Clearly, like you we are just starting out with the heated system but we would be happy to share our data with you if it would be helpful. Perhaps check out my last post first as our information might not be suitable for your purposes.