diff --git a/.circleci/config.yml b/.circleci/config.yml
index 6de527ca..31a4f50c 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -1,11 +1,11 @@
 version: 2.1
 
 # Anchors in case we need to override the defaults from the orb
-#baselibs_version: &baselibs_version v7.27.0
-#bcs_version: &bcs_version v11.6.0
+#baselibs_version: &baselibs_version v8.24.0
+#bcs_version: &bcs_version v12.0.0
 
 orbs:
-  ci: geos-esm/circleci-tools@4
+  ci: geos-esm/circleci-tools@5
 
 workflows:
   build-test:
@@ -21,7 +21,10 @@ workflows:
           #baselibs_version: *baselibs_version
           repo: GEOSgcm
           checkout_fixture: true
-          mepodevelop: true
+          # V12 code uses a special branch for now.
+          fixture_branch: feature/sdrabenh/gcm_v12
+          # We comment out this as it will "undo" the fixture_branch
+          #mepodevelop: true
           persist_workspace: true # Needs to be true to run fv3/gcm experiment, costs extra
 
       # Run AMIP GCM (1 hour, no ExtData)
diff --git a/.github/workflows/workflow.yml b/.github/workflows/workflow.yml
index 41fd37c7..49f9448e 100644
--- a/.github/workflows/workflow.yml
+++ b/.github/workflows/workflow.yml
@@ -21,11 +21,13 @@ jobs:
       matrix:
         compiler: [ifort, gfortran-14, gfortran-15]
         build-type: [Debug]
+      fail-fast: false
     uses: GEOS-ESM/CI-workflows/.github/workflows/geosgcm_build_tests.yml@project/geosgcm
     with:
       compiler: ${{ matrix.compiler }}
       cmake-build-type: ${{ matrix.build-type }}
       fixture-repo: GEOS-ESM/GEOSgcm
+      fixture-ref: feature/sdrabenh/gcm_v12
 
   spack_build:
     uses: GEOS-ESM/CI-workflows/.github/workflows/spack_gcc_build.yml@project/geosgcm
@@ -34,4 +36,6 @@ jobs:
       BUILDCACHE_TOKEN: ${{ secrets.BUILDCACHE_TOKEN }}
     with:
       fixture-repo: GEOS-ESM/GEOSgcm
+      fixture-ref: feature/sdrabenh/gcm_v12
+      load-fms: true
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 39c783da..07b9ef15 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,11 +7,21 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
-### Removed
 ### Changed
+### Removed
 ### Fixed
 ### Deprecated
 
+## [2.0.0] - 2026-05-28
+
+### Changed
+
+- HEMCO Run1 now checks the `RUN_DT` alarm and skips work when the alarm is not ringing
+- Moved `DELP` resource read to before the interpolation loop in PChemGridComp relaxation
+- `OX` (ozone) relaxation in PCHEM now uses a strongly-constrained (instantaneous) relaxation
+  below the tropopause (~150 hPa) and a weak TAU-based relaxation above it, via a new
+  `OX_PCRIT` resource (default 15000 Pa / 150 hPa)
+
 ## [1.16.2] - 2026-01-22
 
 ### Added
diff --git a/GEOSpchem_GridComp/GEOS_PChemGridComp.F90 b/GEOSpchem_GridComp/GEOS_PChemGridComp.F90
index 80247da2..bc7bf450 100644
--- a/GEOSpchem_GridComp/GEOS_PChemGridComp.F90
+++ b/GEOSpchem_GridComp/GEOS_PChemGridComp.F90
@@ -1749,6 +1749,10 @@ subroutine UPDATE(NN,NAME,XX)
 
        PROD1 = PCHEM_STATE%MNCV(:,:,NN,1)*FAC + PCHEM_STATE%MNCV(:,:,NN,2)*(1.-FAC)
 
+       call MAPL_GetResource(MAPL, DELP,  LABEL=trim(NAME)//"_DELP:" , DEFAULT=5000. ,RC=STATUS)
+       VERIFY_(STATUS)
+       DELP = max(DELP, 1.e-16) ! avoid division by zero
+
        do j=1,jm
           do l=1,nlevs
              call INTERP_NO_EXTRAP( PROD(:,L), LATS(:,J), Prod1(:,L), PCHEM_STATE%LATS)
@@ -1758,11 +1762,6 @@ subroutine UPDATE(NN,NAME,XX)
           enddo
        end do
 
-       call MAPL_GetResource(MAPL, DELP,  LABEL=trim(NAME)//"_DELP:" , DEFAULT=5000. ,RC=STATUS)
-       VERIFY_(STATUS)
-
-       DELP = max(DELP, 1.e-16) ! avoid division by zero
-
        if(trim(NAME)=="H2O") then
           call MAPL_GetResource(MAPL, PCRIT, LABEL=trim(NAME)//"_PCRIT:", DEFAULT=20000. ,RC=STATUS)
           VERIFY_(STATUS)
@@ -1775,10 +1774,20 @@ subroutine UPDATE(NN,NAME,XX)
           end where
           WRK = min(WRK, PCRIT)
           do L=1,LM
+             ! only contrained above the troposphere
              LOSS_INT(:,:,L) = (1./TAU) * max( min( (WRK-PL(:,:,L))/DELP, 1.0), 0.0)
           end do
           deallocate(WRK)
+       elseif(trim(NAME)=="OX") then
+          call MAPL_GetResource(MAPL, PCRIT, LABEL=trim(NAME)//"_PCRIT:", DEFAULT=15000. ,RC=STATUS)
+          VERIFY_(STATUS)
+          do L=1,LM
+             ! strongly constrained (TAU=DT) below the 150hPa
+             LOSS_INT(:,:,L) = (1./TAU) * (      max( min( (PCRIT-PL(:,:,L))/DELP, 1.0), 0.0)) + &
+                               (1./DT ) * (1.0 - max( min( (PCRIT-PL(:,:,L))/DELP, 1.0), 0.0)) 
+          end do
        else
+          ! relaxed by TAU everywhere
           call MAPL_GetResource(MAPL, PCRIT, LABEL=trim(NAME)//"_PCRIT:", DEFAULT=1.e+16 ,RC=STATUS)
           VERIFY_(STATUS)
           LOSS_INT = (1./TAU) * max( min( (PCRIT   -PL)/DELP, 1.0), 0.0)
diff --git a/HEMCO_GridComp/HEMCO_GridCompMod.F90 b/HEMCO_GridComp/HEMCO_GridCompMod.F90
index 7332e893..05322e51 100644
--- a/HEMCO_GridComp/HEMCO_GridCompMod.F90
+++ b/HEMCO_GridComp/HEMCO_GridCompMod.F90
@@ -592,9 +592,22 @@ SUBROUTINE Run1 ( GC, Import, Export, Clock, RC )
 ! LOCAL VARIABLES:
 !  
     TYPE(Instance), POINTER  :: ThisInst => NULL()
+    type(MAPL_MetaComp), pointer :: MAPL
+    type(ESMF_Alarm)             :: ALARM
+    logical                      :: timeToDoWork
 
     __Iam__('Run1')
 
+    call MAPL_GetObjectFromGC ( GC, MAPL, _RC)
+
+!  Get RUN_DT alarn from MAPL
+!  --------------------------
+    call MAPL_Get(MAPL, RUNALARM=ALARM, _RC)
+    timeToDoWork = ESMF_AlarmIsRinging (ALARM, _RC)
+    if (.not. timeToDoWork) then
+       _RETURN(ESMF_SUCCESS)
+    end if
+
     !=======================================================================
     ! Run1 begins here!
     !=======================================================================