When MARBL uses a constant ALT_CO2 value, it uses the ATM_CO2 instead of ATM_ALT_CO2

[mnlevy1981]

The issue is in this block of MARBL_forcing_mod.F90:

      case (atm_co2_constant_iopt)
        do j=js,je ; do i=is,ie
          fluxes%atm_alt_co2(i,j) = G%mask2dT(i,j) * CS%atm_co2_const
        enddo ; enddo

We want to use atm_alt_co2_const:

       case (atm_co2_constant_iopt)
         do j=js,je ; do i=is,ie
-          fluxes%atm_alt_co2(i,j) = G%mask2dT(i,j) * CS%atm_co2_const
+          fluxes%atm_alt_co2(i,j) = G%mask2dT(i,j) * CS%atm_alt_co2_const
         enddo ; enddo