Merge pull request #1481 from jimmielin/hplin/fix_gc_so4_presc

cam6_4_151: GEOS-Chem within CESM: fix MAM4 mapping for SO4 in default configuration

Author: jimmielin

doc/ChangeLog

@@ -1,3 +1,59 @@
+===============================================================
+
+Tag name: cam6_4_151
+Originator(s): Haipeng Lin
+Date: Feb 12, 2026
+One-line Summary: GEOS-Chem within CESM: fix MAM4 mapping for SO4 in default configuration
+Github PR URL: https://github.com/ESCOMP/CAM/pull/1481
+
+Purpose of changes (include the issue number and title text for each relevant GitHub issue):
+- Fixes #1480 - incorrect sulfate mapping from MAM4 to GEOS-Chem chemistry
+
+Describe any changes made to build system: N/A
+
+Describe any changes made to the namelist: N/A
+
+List any changes to the defaults for the boundary datasets: N/A
+
+Describe any substantial timing or memory changes: N/A
+
+Code reviewed by: lizziel, cacraig
+
+List all files eliminated: N/A
+
+List all files added and what they do: N/A
+
+List all existing files that have been modified, and describe the changes:
+M       src/chemistry/geoschem/chemistry.F90
+  - fixes #1480 (incorrect sulfate mapping from MAM4 to GEOS-Chem chemistry)
+
+If there were any failures reported from running test_driver.sh on any test
+platform, and checkin with these failures has been OK'd by the gatekeeper,
+then copy the lines from the td.*.status files for the failed tests to the
+appropriate machine below.  All failed tests must be justified.
+
+derecho/intel/aux_cam:
+  ERI_D_Ln18.ne16pg3_ne16pg3_mt232.FHIST_C4.derecho_intel.cam-outfrq3s_eri (Overall: FAIL) details:
+     - pre-existing failure
+     - ERI test discovered a bug in CICE:  See: https://github.com/ESCOMP/CESM_CICE/issues/34
+
+  SMS_Ld1.f09_f09_mg17.FCHIST_GC.derecho_intel.cam-outfrq1d (Overall: DIFF) details:
+     - expected baseline change due to change in GEOS-Chem chemistry interface for bug-fix.
+
+derecho/nvhpc/aux_cam: All PASS
+
+izumi/nag/aux_cam: All PASS
+
+izumi/gnu/aux_cam: All PASS
+
+CAM tag used for the baseline comparison tests if different than previous
+tag:
+
+Summarize any changes to answers:
+  Answer differences in GEOS-Chem compset only due to bug fix.
+
+If bitwise differences were observed, how did you show they were no worse
+than roundoff?
 
 ===============================================================
 

src/chemistry/geoschem/chemistry.F90

@@ -2341,10 +2341,12 @@ subroutine chem_timestep_tend( state, ptend, cam_in, cam_out, dT, pbuf, fh2o )
           P = map2MAM4(SM,M) ! Constituent index for GEOS-Chem
           IF ( P > 0 ) K = map2GC(P) ! Index in State_Chm
 
-          ! do not zero out sulfate aerosol here since aerosol distribution for sulfate
-          ! will be prescribed (hplin, 5/9/23)
-          call rad_cnst_get_info(0,M,SM,spec_name=aerName)
-          IF ( to_upper(aerName(:3)) == "SO4" ) CYCLE
+          if (usePrescribedAerDistribution) then
+             ! do not zero out sulfate aerosol here since aerosol distribution for sulfate
+             ! will be prescribed (hplin, 5/9/23)
+             call rad_cnst_get_info(0,M,SM,spec_name=aerName)
+             IF ( to_upper(aerName(:3)) == "SO4" ) CYCLE
+          end if
 
           IF ( K > 0 ) State_Chm(LCHNK)%Species(K)%Conc(1,:nY,:nZ) = 0.0e+00_fp
        ENDDO
@@ -2361,10 +2363,12 @@ subroutine chem_timestep_tend( state, ptend, cam_in, cam_out, dT, pbuf, fh2o )
           ! species (with cnst index P, which corresponds to index K in
           ! State_Chm)
 
-          ! do not zero out sulfate aerosol here since aerosol distribution for sulfate
-          ! will be prescribed (hplin, 5/9/23)
-          call rad_cnst_get_info(0,M,SM,spec_name=aerName)
-          IF ( to_upper(aerName(:3)) == "SO4" ) CYCLE
+          if (usePrescribedAerDistribution) then
+             ! do not zero out sulfate aerosol here since aerosol distribution for sulfate
+             ! will be prescribed (hplin, 5/9/23)
+             call rad_cnst_get_info(0,M,SM,spec_name=aerName)
+             IF ( to_upper(aerName(:3)) == "SO4" ) CYCLE
+          end if
 
           ! Multiple MAM4 bins are mapped to same GEOS-Chem species
           State_Chm(LCHNK)%Species(K)%Conc(1,:nY,:nZ) = State_Chm(LCHNK)%Species(K)%Conc(1,:nY,:nZ) &
@@ -2573,9 +2577,9 @@ subroutine chem_timestep_tend( state, ptend, cam_in, cam_out, dT, pbuf, fh2o )
                                              + REAL(state%q(:nY,nZ:1:-1,N),fp) *         &
                                                 adv_mass(l_SO4) / adv_mass(mapCnst(N))
        ! SO4_gasRatio is in mol/mol
-       SO4_gasRatio(:nY,:nZ) = state%q(:nY,:nZ,N)                      &
-                             * adv_mass(l_SO4) / adv_mass(mapCnst(N))  &
-                             / State_Chm(LCHNK)%Species(K)%Conc(1,:nY,nZ:1:-1)
+       SO4_gasRatio(:nY,:nZ) = state%q(:nY,:nZ,N)                      &       ! kg(H2SO4) kg-1 air
+                             * adv_mass(l_SO4) / adv_mass(mapCnst(N))  &       ! g(SO4) mol(SO4)-1 / g(H2SO4) mol(H2SO4)-1
+                             / State_Chm(LCHNK)%Species(K)%Conc(1,:nY,nZ:1:-1) ! kg(SO4) kg-1 air
        MMR_Beg(:nY,:nZ,K)    = State_Chm(LCHNK)%Species(K)%Conc(1,:nY,:nZ)
     ENDIF
 #endif