From ddeafbbef7c961aa2a4ace81e338313d1da7ba22 Mon Sep 17 00:00:00 2001
From: Haipeng Lin <hplin+github@ucar.edu>
Date: Mon, 20 Jul 2026 13:06:08 -0400
Subject: [PATCH 1/2] Populate diagnostic_volume_mixing_ratio_of_co2_to_coupler
 instead of co2diag_tbd to land model.

Provenance: FIX-31 octopus
Assisted-by: claude-fable:5
---
 schemes/utilities/co2_diagnostic_export.F90  | 31 ++++++++++++++++++++
 schemes/utilities/co2_diagnostic_export.meta | 31 ++++++++++++++++++++
 suites/suite_cam5.xml                        |  2 ++
 3 files changed, 64 insertions(+)
 create mode 100644 schemes/utilities/co2_diagnostic_export.F90
 create mode 100644 schemes/utilities/co2_diagnostic_export.meta

diff --git a/schemes/utilities/co2_diagnostic_export.F90 b/schemes/utilities/co2_diagnostic_export.F90
new file mode 100644
index 00000000..75103500
--- /dev/null
+++ b/schemes/utilities/co2_diagnostic_export.F90
@@ -0,0 +1,31 @@
+! Export the diagnostic CO2 concentration to the surface coupler.
+module co2_diagnostic_export
+
+  implicit none
+  private
+
+  ! Public interfaces
+  public :: co2_diagnostic_export_run
+contains
+!> \section arg_table_co2_diagnostic_export_run Argument Table
+!! \htmlinclude co2_diagnostic_export_run.html
+subroutine co2_diagnostic_export_run(co2_vmr, co2diag, errmsg, errcode)
+   use ccpp_kinds,       only: kind_phys
+
+   ! Input arguments
+   real(kind_phys),  intent(in)  :: co2_vmr      ! Prescribed CO2 volume mixing ratio [mol mol-1]
+
+   ! Output arguments
+   real(kind_phys),  intent(out) :: co2diag(:)   ! Diagnostic CO2 concentration for coupler [ppmv]
+   character(len=*), intent(out) :: errmsg
+   integer,          intent(out) :: errcode
+
+   errmsg = ''
+   errcode = 0
+
+   ! mol/mol -> ppmv expected by the coupler. (chem_surfvals sets CO2VMR * 1e6)
+   co2diag(:) = co2_vmr * 1.0e6_kind_phys
+
+end subroutine co2_diagnostic_export_run
+
+end module co2_diagnostic_export
diff --git a/schemes/utilities/co2_diagnostic_export.meta b/schemes/utilities/co2_diagnostic_export.meta
new file mode 100644
index 00000000..20e577a6
--- /dev/null
+++ b/schemes/utilities/co2_diagnostic_export.meta
@@ -0,0 +1,31 @@
+[ccpp-table-properties]
+  name = co2_diagnostic_export
+  type = scheme
+
+[ccpp-arg-table]
+  name  = co2_diagnostic_export_run
+  type  = scheme
+[ co2_vmr ]
+  standard_name = prescribed_volume_mixing_ratio_of_co2
+  units = mol mol-1
+  type = real | kind = kind_phys
+  dimensions = ()
+  intent = in
+[ co2diag ]
+  standard_name = diagnostic_volume_mixing_ratio_of_co2_to_coupler
+  units = ppmv
+  type = real | kind = kind_phys
+  dimensions = (horizontal_loop_extent)
+  intent = out
+[ errmsg ]
+  standard_name = ccpp_error_message
+  units = none
+  type = character | kind = len=*
+  dimensions = ()
+  intent = out
+[ errcode ]
+  standard_name = ccpp_error_code
+  units = 1
+  type = integer
+  dimensions = ()
+  intent = out
diff --git a/suites/suite_cam5.xml b/suites/suite_cam5.xml
index d5e6b3fa..53d13fba 100644
--- a/suites/suite_cam5.xml
+++ b/suites/suite_cam5.xml
@@ -160,6 +160,8 @@
          of the "before_coupler" group. -->
     <scheme>calc_dry_air_ideal_gas_density</scheme>
     <scheme>set_surface_coupling_vars</scheme>
+    <!-- Write CO2 concentration for surface model (Sa_co2diag) diagnostic CO2 mode -->
+    <scheme>co2_diagnostic_export</scheme>
 
   </group>
 

From b7709c1855552ef55fb3a99839873f2847c22662 Mon Sep 17 00:00:00 2001
From: Haipeng Lin <hplin+github@ucar.edu>
Date: Wed, 22 Jul 2026 16:25:00 -0400
Subject: [PATCH 2/2] Wire up to cam4, cam7 suites

---
 suites/suite_cam4.xml | 1 +
 suites/suite_cam7.xml | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/suites/suite_cam4.xml b/suites/suite_cam4.xml
index 06c89ac7..e953b1b2 100644
--- a/suites/suite_cam4.xml
+++ b/suites/suite_cam4.xml
@@ -278,6 +278,7 @@
          of the "before_coupler" group. -->
     <scheme>calc_dry_air_ideal_gas_density</scheme>
     <scheme>set_surface_coupling_vars</scheme>
+    <scheme>co2_diagnostic_export</scheme>
 
   </group>
 
diff --git a/suites/suite_cam7.xml b/suites/suite_cam7.xml
index aa091ebb..5e5cfac3 100644
--- a/suites/suite_cam7.xml
+++ b/suites/suite_cam7.xml
@@ -81,7 +81,8 @@
          of the "before_coupler" group. -->
     <!-- UNCOMMENT ONCE ALL OTHER CAM7 SCHEMES ARE PRESENT -->
     <!-- <scheme>calc_dry_air_ideal_gas_density</scheme>
-    <scheme>set_surface_coupling_vars</scheme> -->
+    <scheme>set_surface_coupling_vars</scheme>
+    <scheme>co2_diagnostic_export</scheme> -->
 
   </group>