Dear Kai Wu,
Thanks for contacting me about HONO chemistry in CMAQ.
Gas-phase chemical reactions are included in “mech.def”, heterogeneous chemistry on aerosols is included in “mech.def” and “AEROSOL_CHEMISTRY.F”, and heterogeneous chemistry on ground is included in “vdiffacmx.F“. HONO emissions are included in emission file (GC_cb6*_ae7_aq.nml).
Thanks,
Sarwar