CMAQ-DDM and CMAQ-adjoint different

I have a question. I’d like to understand the difference between CMAQ-DDM and CMAQ-adjoint because both seem capable of calculating sensitivity between emission sources and pollutants. So, I’m a bit unclear about their distinctions. I’ve been researching inverse modeling of emission sources and noticed many articles using the Mass Balance method. However, I’d like to try 4D-Var for inversion. Should I use DDM or adjoint? The version of CMAQ-adjoint seems a bit old. I’m unsure if I can solely use CMAQ-DDM for emission inversion.

A lot of good literature exists that would answer your question - including application of both adjoint and DDM-3D for emission inversion for CMAQ and other models. Briefly, adjoint calculates “reverse” sensitivity or sensitivity of some output metric (cost function) to all input parameters. DDM-3D calculates “forward” sensitivity or sensitivity of all output fields to some defined input parameter. \