Hi Cris,
I have implemented what you recommended in point 1) using PAR_EXTINCTION in a update file to introduce differents values of this coefficient :
1 data sets
0 seconds between data
0           0            1                             
TIME        TIME         PAR_EXTINCTION
2022001.000  2022031.958   4.5                  
2022032.000  2022059.958   5.0
 But, the model say me "PAR_EXTINCTION is not a valid bc type". My AEM3D version is 1.1.2
Could you help me?
Thank you very much for your time