diff --git a/code/ysolve.f90 b/code/ysolve.f90
index aa221e5..d79727f 100644
--- a/code/ysolve.f90
+++ b/code/ysolve.f90
@@ -92,7 +92,7 @@
        IF (d_mode == 0) THEN
          CALL solve_with_diffusivity (update_dm)
        ELSE IF (d_mode == 1) THEN
-         CALL solve_with_diffusivity (update_dm)
+         CALL solve_with_diffusivity (update_dm_cd)
        ELSE
          WRITE(*,*)'ERROR, UNSUPPORTED DIFFUSIVITY ', d_mode, '. STOPPING'
          STOP
@@ -547,10 +547,23 @@
 
      SUBROUTINE update_dm_cd(r1)
       REAL, INTENT(IN),DIMENSION(:,:) :: r1
+      REAL :: conv_rxn_boundary
+      REAL :: beta, tutb
       INTEGER :: i
 
+      tutb = 1. / (bc+1.)
+
+      ! \beta = Ta/Tb * (Tb-Tu)/Tb
+      beta = ac * tutb * (1. - tutb)
+
+      conv_rxn_boundary = 1./beta
+
       DO i=1,nx
-        dm(i) = diff + d_turb
+        IF (r1(i,fctrl_species) > conv_rxn_boundary) THEN
+          dm(i) = diff + d_turb
+        ELSE
+          dm(i) = diff
+        END IF
       ENDDO
      END SUBROUTINE update_dm_cd