Ah yes. This was mentioned in Implementation of new Sensirion SEN-5X sensors - #99 by fredvduin . Based on that I would suggest to ignore the Sensirion PM4 (*_P4) and PM10 (*_P1) measurements. Then if you compare the SEN55_P2 with the PMS_P2 and SDS_P2 values perhaps for just PM 2.5 it’s not that bad after all 