Fix duplicate heterogeneous chemistry reactions in UKCA StratTrop and CRI-Strat2

CONTRIBUTORS.md

@@ -7,3 +7,4 @@
 | jennyhickson | Jenny Hickson | Met Office | 2026-03-02 |
 | Pierre-siddall | Pierre Siddall | Met Office | 2026-03-13 |
 | yg460-cam | Yao Ge | University of Cambridge | 2026-04-17 |
+| theabro | Nathan Luke Abraham | NCAS & University of Cambridge | 2026-03-19 |

src/control/core/diagnostics/asad_flux_dat.F90

@@ -8766,6 +8766,20 @@ MODULE asad_flux_dat
 ['          ','          ','          ','          '])                         &
 ]
 
+! Add extra het chem fluxes for both troposphere and stratosphere
+! Reactions are used in both StratTrop and CRI-Strat2
+! Note that B85 is a biomolecular reaction but is aping a heterogeneous one
+TYPE(asad_flux_defn), PARAMETER, PUBLIC ::                                     &
+                         het_chem_n2o5_h2o(2) = [                              &
+! B85 N2O5+H2O
+asad_flux_defn('RXN',50993,'B',.FALSE.,0,4,                                    &
+['N2O5      ','H2O       '],                                                   &
+['HONO2     ','HONO2     ','          ','          ']),                        &
+! PSC N2O5-H2O H04
+asad_flux_defn('RXN',50994,'H',.FALSE.,0,4,                                    &
+['N2O5      ','H2O       '],                                                   &
+['HONO2     ','HONO2     ','          ','          '])                         &
+]
 
 
 PUBLIC :: asad_load_default_fluxes
@@ -8853,6 +8867,7 @@ SUBROUTINE asad_load_default_fluxes
 TYPE(asad_flux_defn), ALLOCATABLE, SAVE :: aa_ch4_budget_loss(:)
 TYPE(asad_flux_defn), ALLOCATABLE, SAVE :: aa_ch4_drydep(:)
 TYPE(asad_flux_defn), ALLOCATABLE, SAVE :: aa_ch4_ste(:)
+TYPE(asad_flux_defn), ALLOCATABLE, SAVE :: aa_het_chem_n2o5_h2o(:)
 
 INTEGER :: p1                   ! start position in asad_chemical_fluxes array
 INTEGER :: p2                   ! end position in asad_chemical_fluxes array
@@ -9025,6 +9040,9 @@ SUBROUTINE asad_load_default_fluxes
     ALLOCATE(aa_ch4_ste(SIZE(asad_ch4_ste)))
     aa_ch4_ste = asad_ch4_ste
   END IF
+  ! aa_het_chem_n2o5_h2o
+  ALLOCATE(aa_het_chem_n2o5_h2o(SIZE(het_chem_n2o5_h2o)))
+  aa_het_chem_n2o5_h2o = het_chem_n2o5_h2o
 
 ELSE IF (ukca_config%l_ukca_cristrat) THEN
   ! Select the asad diagnostics appropriate for CRI-Strat chemistry and
@@ -9233,6 +9251,9 @@ SUBROUTINE asad_load_default_fluxes
     ALLOCATE(aa_oxidN_wetdep(SIZE(cri_oxidN_wetdep)))
     aa_oxidN_wetdep = cri_oxidN_wetdep
   END IF
+  ! aa_het_chem_n2o5_h2o
+  ALLOCATE(aa_het_chem_n2o5_h2o(SIZE(het_chem_n2o5_h2o)))
+  aa_het_chem_n2o5_h2o = het_chem_n2o5_h2o
 
 
 ELSE IF (ukca_config%l_ukca_offline .OR. ukca_config%l_ukca_offline_be) THEN
@@ -9306,6 +9327,8 @@ SUBROUTINE asad_load_default_fluxes
    n_chemical_fluxes = n_chemical_fluxes + SIZE(aa_ch4_drydep)
 IF (ALLOCATED(aa_ch4_ste))                                                     &
    n_chemical_fluxes = n_chemical_fluxes + SIZE(aa_ch4_ste)
+IF (ALLOCATED(aa_het_chem_n2o5_h2o))                                           &
+   n_chemical_fluxes = n_chemical_fluxes + SIZE(aa_het_chem_n2o5_h2o)
 
 ALLOCATE(asad_chemical_fluxes(n_chemical_fluxes))
 
@@ -9449,6 +9472,11 @@ SUBROUTINE asad_load_default_fluxes
   asad_chemical_fluxes(p1:p2) = aa_ch4_ste(:)
   p1 = p2 + 1
 END IF
+IF (ALLOCATED(aa_het_chem_n2o5_h2o)) THEN
+  p2 = p1 + SIZE(aa_het_chem_n2o5_h2o) - 1
+  asad_chemical_fluxes(p1:p2) = aa_het_chem_n2o5_h2o(:)
+  p1 = p2 + 1
+END IF
 
 IF (p2 /= n_chemical_fluxes) THEN
   cmessage = ' n_chemical_fluxes and p2 are different'
@@ -9502,6 +9530,7 @@ SUBROUTINE asad_load_default_fluxes
 END IF
 
 ! Deallocate the generic arrays
+IF (ALLOCATED(aa_het_chem_n2o5_h2o))     DEALLOCATE(aa_het_chem_n2o5_h2o)
 IF (ALLOCATED(aa_ch4_ste))               DEALLOCATE(aa_ch4_ste)
 IF (ALLOCATED(aa_ch4_drydep))            DEALLOCATE(aa_ch4_drydep)
 IF (ALLOCATED(aa_ch4_budget_loss))       DEALLOCATE(aa_ch4_budget_loss)

src/control/core/interface/ukca_config_specification_mod.F90

@@ -360,6 +360,7 @@ MODULE ukca_config_specification_mod
                                        ! B-E Offline Oxidants scheme
   LOGICAL :: l_fix_ukca_h2so4_ystore   ! True to fix storage of H2SO4 in ASAD
                                        ! N-R schemes for updating in GLOMAP
+  LOGICAL :: l_fix_ukca_n2o5_h2o       ! True to filter N2O5+H2O to strat and trop only
 
   ! Settings for managing photolysis environmental driver
   ! requirements on behalf of external UKCA Photolysis code
@@ -949,6 +950,7 @@ SUBROUTINE init_ukca_configuration()
 ukca_config%l_fix_drydep_so2_water = .FALSE.
 ukca_config%l_fix_ukca_offox_h2o_fac = .FALSE.
 ukca_config%l_fix_ukca_h2so4_ystore = .FALSE.
+ukca_config%l_fix_ukca_n2o5_h2o = .FALSE.
 
 ! -- Settings for managing Photolysis driver requirements
 ukca_config%i_photol_scheme = imdi
@@ -1179,6 +1181,7 @@ SUBROUTINE ukca_get_config(                                                    &
    l_fix_drydep_so2_water,                                                     &
    l_fix_ukca_offox_h2o_fac,                                                   &
    l_fix_ukca_h2so4_ystore,                                                    &
+   l_fix_ukca_n2o5_h2o,                                                        &
    l_ukca_chem, l_ukca_trop, l_ukca_aerchem, l_ukca_raq, l_ukca_raqaero,       &
    l_ukca_offline_be, l_ukca_tropisop, l_ukca_strattrop, l_ukca_strat,         &
    l_ukca_offline, l_ukca_cristrat, l_ukca_stratcfc, l_ukca_achem,             &
@@ -1395,6 +1398,7 @@ SUBROUTINE ukca_get_config(                                                    &
 LOGICAL, OPTIONAL, INTENT(OUT) :: l_fix_drydep_so2_water
 LOGICAL, OPTIONAL, INTENT(OUT) :: l_fix_ukca_offox_h2o_fac
 LOGICAL, OPTIONAL, INTENT(OUT) :: l_fix_ukca_h2so4_ystore
+LOGICAL, OPTIONAL, INTENT(OUT) :: l_fix_ukca_n2o5_h2o
 LOGICAL, OPTIONAL, INTENT(OUT) :: l_ukca_chem
 LOGICAL, OPTIONAL, INTENT(OUT) :: l_ukca_trop
 LOGICAL, OPTIONAL, INTENT(OUT) :: l_ukca_aerchem
@@ -1685,6 +1689,8 @@ SUBROUTINE ukca_get_config(                                                    &
   l_fix_ukca_offox_h2o_fac = ukca_config%l_fix_ukca_offox_h2o_fac
 IF (PRESENT(l_fix_ukca_h2so4_ystore))                                          &
   l_fix_ukca_h2so4_ystore = ukca_config%l_fix_ukca_h2so4_ystore
+IF (PRESENT(l_fix_ukca_n2o5_h2o))                                              &
+  l_fix_ukca_n2o5_h2o = ukca_config%l_fix_ukca_n2o5_h2o
 
 ! -- UKCA internal configuration variables
 IF (PRESENT(l_ukca_chem)) l_ukca_chem = ukca_config%l_ukca_chem

src/control/core/top_level/ukca_setup_mod.F90

@@ -207,6 +207,7 @@ SUBROUTINE ukca_setup(error_code,                                              &
                       l_fix_ukca_h2dd_x,                                       &
                       l_fix_ukca_offox_h2o_fac,                                &
                       l_fix_ukca_h2so4_ystore,                                 &
+                      l_fix_ukca_n2o5_h2o,                                     &
                       l_mode_bhn_on,                                           &
                       l_mode_bln_on,                                           &
                       l_ddepaer,                                               &
@@ -540,6 +541,7 @@ SUBROUTINE ukca_setup(error_code,                                              &
 LOGICAL, OPTIONAL, INTENT(IN) :: l_fix_ukca_h2dd_x
 LOGICAL, OPTIONAL, INTENT(IN) :: l_fix_ukca_offox_h2o_fac
 LOGICAL, OPTIONAL, INTENT(IN) :: l_fix_ukca_h2so4_ystore
+LOGICAL, OPTIONAL, INTENT(IN) :: l_fix_ukca_n2o5_h2o
 LOGICAL, OPTIONAL, INTENT(IN) :: l_mode_bhn_on
 LOGICAL, OPTIONAL, INTENT(IN) :: l_mode_bln_on
 LOGICAL, OPTIONAL, INTENT(IN) :: l_ddepaer
@@ -1232,6 +1234,13 @@ SUBROUTINE ukca_setup(error_code,                                              &
     ukca_config%l_fix_ukca_h2so4_ystore = l_fix_ukca_h2so4_ystore
 END IF
 
+IF (ukca_config%i_ukca_chem == i_ukca_chem_strattrop .OR.                      &
+    ukca_config%i_ukca_chem == i_ukca_chem_cristrat) THEN
+  ukca_config%l_fix_ukca_n2o5_h2o = .TRUE.
+  IF (PRESENT(l_fix_ukca_n2o5_h2o))                                            &
+    ukca_config%l_fix_ukca_n2o5_h2o = l_fix_ukca_n2o5_h2o
+END IF
+
 ! Settings for managing photolysis environmental driver
 ! requirements on behalf of external UKCA Photolysis code
 

src/science/core/chemistry/asad/asad_bimol.F90

@@ -75,7 +75,7 @@ MODULE asad_bimol_mod
 
 CONTAINS
 
-SUBROUTINE asad_bimol( n_points )
+SUBROUTINE asad_bimol( n_points, stratflag_opt )
 
 USE asad_mod,        ONLY: t, t300, specf, spb, ab, rk, tnd, p,                &
                            wp, f, peps, nbrkx, jpspb, jpcspf, jpbk
@@ -91,6 +91,8 @@ SUBROUTINE asad_bimol( n_points )
 IMPLICIT NONE
 
 INTEGER, INTENT(IN) :: n_points
+! optional argument used for masking off the stratosphere
+LOGICAL, INTENT(IN), OPTIONAL :: stratflag_opt(n_points)
 
 ! Local variables
 
@@ -112,6 +114,7 @@ SUBROUTINE asad_bimol( n_points )
 INTEGER, SAVE :: ics2oh = 0
 INTEGER, SAVE :: ih2o = 0
 INTEGER, SAVE :: iho2no = 0
+INTEGER, SAVE :: in2o5_h2o = 0
 INTEGER :: jtr
 INTEGER :: j
 INTEGER :: jr
@@ -145,6 +148,10 @@ SUBROUTINE asad_bimol( n_points )
 REAL :: tmp1(1:n_points)
 REAL :: inv_t(1:n_points)
 
+! logical array for masking-off the stratosphere
+! default to False unless changed by stratflag_opt
+LOGICAL :: stratflag(n_points)
+
 ! ErrorStatus
 INTEGER                             :: errcode=0   ! Error flag (0 = OK)
 CHARACTER(LEN=errormessagelength)   :: cmessage    ! Error return message
@@ -167,6 +174,12 @@ SUBROUTINE asad_bimol( n_points )
 ratioa2b(:) = 0.0
 ratiob2total(:) = 0.0
 
+! set stratflag from optional argument - only True in the stratosphere
+stratflag(:) = .FALSE.
+IF (PRESENT(stratflag_opt)) THEN
+  stratflag = stratflag_opt
+END IF
+
 !       1. Calculate bimolecular rate coefficients
 !          --------- ----------- ---- ------------
 
@@ -255,6 +268,14 @@ SUBROUTINE asad_bimol( n_points )
     iho2no    = asad_findreaction( r1, r2, prods, 1, spb, nbrkx,               &
                                    jpbk+1, jpspb )
 
+    ! Find B85: N2O5 + H2O -> HONO2 + HONO2.
+    r1 = 'N2O5      '
+    r2 = 'H2O       '
+    prods(1) = 'HONO2     '
+    prods(2) = 'HONO2     '
+    in2o5_h2o = asad_findreaction( r1, r2, prods, 2, spb, nbrkx,               &
+                                   jpbk+1, jpspb )
+
 
     IF (ukca_config%l_ukca_chem_aero) THEN
       r1 = 'DMS       '
@@ -408,6 +429,13 @@ SUBROUTINE asad_bimol( n_points )
     8.53e-4*(1e-2*p(1:n_points))-1.73)/100.0
 END IF
 
+! Keep B85 (N2O5 + H2O) in the troposphere only when fix is enabled.
+IF (ukca_config%l_fix_ukca_n2o5_h2o .AND. in2o5_h2o > 0) THEN
+  WHERE (stratflag(1:n_points))
+    rk(1:n_points,in2o5_h2o) = 0.0
+  END WHERE
+END IF
+
 ! SO3 + H2O: 2nd H2O molecule dealt with here by multiplying rate by [H2O]
 IF ( ih2o /= 0 .AND. iso3h2o /= 0 ) THEN
   !       water is an advected tracer

src/science/core/chemistry/asad/asad_cdrive.F90

@@ -225,7 +225,7 @@ SUBROUTINE asad_cdrive(ftr, pp, pt, pq, co2_1d, cld_f, cld_l,                  &
 !       4.  Calculate reaction rate coefficients
 !           --------- -------- ---- ------------
 
-CALL asad_bimol (n_points)
+CALL asad_bimol (n_points, stratflag)
 CALL asad_trimol(n_points)
 
 ! Calculate aqueous-phase SO2 oxdn. and tropospheric heterogeneous rates

src/science/core/chemistry/ukca_hetero_mod.F90

@@ -490,9 +490,18 @@ SUBROUTINE ukca_hetero(n_points, have_nat, stratflag)
     rk(:,n_hocl_hcl) = hk(:,3)
   END IF
 
+  ! Optionally filter N2O5+H2O by stratflag to prevent double-counting.
   IF (n_n2o5_h2o > 0) THEN
     ! 4. N2O5 + H2O -> 2 HNO3
-    rk(:,n_n2o5_h2o) = hk(:,4)
+    IF (ukca_config%l_fix_ukca_n2o5_h2o) THEN
+      WHERE (stratflag)
+        rk(:,n_n2o5_h2o) = hk(:,4)
+      ELSE WHERE
+        rk(:,n_n2o5_h2o) = 0.0
+      END WHERE
+    ELSE
+      rk(:,n_n2o5_h2o) = hk(:,4)
+    END IF
   END IF
 
   IF (n_n2o5_hcl > 0) THEN
@@ -659,14 +668,27 @@ SUBROUTINE ukca_hetero(n_points, have_nat, stratflag)
 
   WHERE ( zh2o > peps )
     rk(:,n_clono2_h2o) = kpsc(:,2) / zh2o
-    rk(:,n_n2o5_h2o) = kpsc(:,4) / zh2o
     rk(:,n_brono2_h2o) = kpsc(:,8) / zh2o
   ELSE WHERE
     rk(:,n_clono2_h2o) = 0.0
-    rk(:,n_n2o5_h2o) = 0.0
     rk(:,n_brono2_h2o) = 0.0
   END WHERE
 
+  ! Optionally filter N2O5+H2O by stratflag to prevent double-counting.
+  IF (ukca_config%l_fix_ukca_n2o5_h2o) THEN
+    WHERE ( zh2o > peps .AND. stratflag )
+      rk(:,n_n2o5_h2o) = kpsc(:,4) / zh2o
+    ELSE WHERE
+      rk(:,n_n2o5_h2o) = 0.0
+    END WHERE
+  ELSE
+    WHERE ( zh2o > peps )
+      rk(:,n_n2o5_h2o) = kpsc(:,4) / zh2o
+    ELSE WHERE
+      rk(:,n_n2o5_h2o) = 0.0
+    END WHERE
+  END IF
+
   WHERE ( zhbr > peps )
     rk(:,n_hobr_hbr) = kpsc(:,9) / zhbr
     rk(:,n_hocl_hbr) = kpsc(:,10) / zhbr