To calculate PM2.5 from individual PM components stored in CMAQv5.4 DDM or ISAM files, you need to go back to the equations in the CMAQv5.3.3 SpecDef files (e.g. this one).
If you are interested in calculating sharp cut-off PM2.5 species, you need to use the FPM25AIT, FPM25ACC, and FPM25COR variables from the CMAQv5.4 AELMO files instead of the PM25AT, PM25AC, and PM25CO variables referenced in that SpecDef file and stored in the CMAQv5.3.3 APMDIAG files.
Please see the following threads for additional information on how to calculate PM2.5 for DDM or ISAM in CMAQv5.4 (note that while these posts discuss ISAM, the same approach holds true for calculating PM2.5 from the DDM output stored in the ASENS files):
Questions about PM2.5 calculation in CMAQv5.4 cb6r5AERO7 mechanism
How to get total pm10/pm25 from cmaq v5.4_isam output
How to get PM2.5_TOT from CMAQ5.4-ISAM outputs by combine?