diff --git a/.gitmodules b/.gitmodules
index 03cbcece4c..ebe013da74 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -71,7 +71,7 @@
 [submodule "hemco"]
 	path = src/hemco
 	url = https://github.com/ESCOMP/HEMCO_CESM.git
-        fxtag = hemco-cesm2_0_hemco3_9_0
+        fxtag = hemco-cesm2_1_0_hemco3_9_0
         fxrequired = AlwaysRequired
 	fxDONOTUSEurl = https://github.com/ESCOMP/HEMCO_CESM.git
 
diff --git a/bld/namelist_files/namelist_defaults_cam.xml b/bld/namelist_files/namelist_defaults_cam.xml
index 21c3c22add..03e5479ac6 100644
--- a/bld/namelist_files/namelist_defaults_cam.xml
+++ b/bld/namelist_files/namelist_defaults_cam.xml
@@ -3567,9 +3567,9 @@
 
 <!-- HEMCO: Harmonized Emissions Component -->
 <use_hemco>.false.</use_hemco>
-<hemco_data_root>atm/cam/geoschem/emis/ExtData/HEMCO</hemco_data_root>
-<hemco_config_file>atm/cam/geoschem/emis/HEMCO_Config.CC.CEDS_AEIC19.NEx.c230615.rc</hemco_config_file>
-<hemco_diagn_file>atm/cam/geoschem/emis/HEMCO_Diagn.3_5_0.c230307.rc</hemco_diagn_file>
+<hemco_data_root>atm/cam/geoschem/ExtData/HEMCO</hemco_data_root>
+<hemco_config_file>atm/cam/hemco/HEMCO_Config.CC.CEDS_AEIC19.NEx.c241122.rc</hemco_config_file>
+<hemco_diagn_file>atm/cam/hemco/HEMCO_Diagn.3_5_0.c230307.rc</hemco_diagn_file>
 <hemco_emission_year>-1</hemco_emission_year>
 
 <!-- Default emissions grid resolution at 2x2.5 degree: coarser may result in masking artifacts -->
diff --git a/cime_config/testdefs/testlist_cam.xml b/cime_config/testdefs/testlist_cam.xml
index bf8b42e467..4f73aadad1 100644
--- a/cime_config/testdefs/testlist_cam.xml
+++ b/cime_config/testdefs/testlist_cam.xml
@@ -1966,7 +1966,7 @@
     </machines>
     <options>
       <option name="wallclock">00:20:00</option>
-      <option name="comment">FCSD CAM-chem f19 with HEMCO exact restart</option>
+      <option name="comment">FCSD CAM-chem f09 with HEMCO exact restart</option>
     </options>
   </test>
   <test compset="FCSD_HCO" grid="f09_f09_mg17" name="ERP_Ln9" testmods="cam/outfrq9s" supported="false">
@@ -1976,7 +1976,7 @@
     </machines>
     <options>
       <option name="wallclock">00:40:00</option>
-      <option name="comment">FCSD CAM-chem f19 with HEMCO exact restart/PE tests</option>
+      <option name="comment">FCSD CAM-chem f09 with HEMCO exact restart/PE tests</option>
     </options>
   </test>
 
diff --git a/doc/ChangeLog b/doc/ChangeLog
index c2ef4c0487..3888d125d7 100644
--- a/doc/ChangeLog
+++ b/doc/ChangeLog
@@ -1,4 +1,68 @@
+===============================================================
+
+Tag name: cam6_4_082
+Originator(s): lizziel
+Date: 2 Apr 2025
+One-line Summary: Update HEMCO interface external to HEMCO_CESM 2.1.0
+Github PR URL: https://github.com/ESCOMP/CAM/pull/1221
+
+Purpose of changes (include the issue number and title text for each relevant GitHub issue):
+     - Update HEMCO_CESM to get HEMCO clock time bug fix
+     - Update the HEMCO configuration file for *_HCO cases to read 3D emissions every timestep
+     - Together these updates fix failing FCSD_HCO ERS restart tests
+     - Other FCSD_HCO tests will continue to fail due to remaining reproducibility issues that
+       require a new HEMCO version to fix (https://github.com/ESCOMP/CAM/issues/856).
+
+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: fvitt, 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       .gitmodules
+        - Update HEMCO_CESM version from hemco-cesm2_0_hemco3_9_0 to hemco-cesm2_1_0_hemco3_9_0
+
+M       bld/namelist-files/namelist_defaults_cam.xml
+	- Update default hemco_config_file to read and regrid aviation emissions every timestep
+	- Change default paths of hemco_config_file, hemco_diagn_file, and hemco_data_root
 
+M       cime_config/testdefs/testlist_cam.xml
+	- Fix typos in comments of FCSD_HCO tests
+
+M       src/hemco
+	- Committed hemco submodule (HEMCO_CESM) hash for tag hemco-cesm2_1_0_hemco3_9_0
+
+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:
+  ERP_Ln9.f09_f09_mg17.FCSD_HCO.derecho_intel.cam-outfrq9s (Overall: FAIL) details:
+  SMS_Lh12.f09_f09_mg17.FCSD_HCO.derecho_intel.cam-outfrq3h (Overall: DIFF) details:
+  SMS_Ld1.f09_f09_mg17.FCHIST_GC.derecho_intel.cam-outfrq1d (Overall: DIFF) details:
+    - pre-existing failures due to HEMCO not having reproducible results (issues #1018 and #856)
+
+  SMS_D_Ln9_P1280x1.ne0CONUSne30x8_ne0CONUSne30x8_mt12.FCHIST.derecho_intel.cam-outfrq9s (Overall: FAIL) details:
+    - pre-existing failures due to build-namelist error requiring CLM/CTSM external update
+
+derecho/nvhpc/aux_cam: all BFB
+
+izumi/nag/aux_cam: all BFB
+
+izumi/gnu/aux_cam: all BFB
+
+===============================================================
 ===============================================================
 
 Tag name: cam6_4_081
diff --git a/src/hemco b/src/hemco
index 7bd8358229..d108012f84 160000
--- a/src/hemco
+++ b/src/hemco
@@ -1 +1 @@
-Subproject commit 7bd8358229eefd2cbb910ff30c46dfc97e34fb6f
+Subproject commit d108012f8419ec13a56c0955684f5aaaf65fcacf