If you set mp_zero_out = 1, it checks all the microphysics species, except for vapor. If the value is set to 2, then it checks everything, including vapor. You will also need to set mp_zero_out_thresh for the value it checks against, but we really recommend using the 'positive-definite' option (pd_moist = .true. for code prior to V3.1, and moist_adv_opt = 1 for V3.1 and later). This option should keep these variables positive-definite.