diff --git a/source/mod_analytic.f90 b/source/mod_analytic.f90
index b625ba19676a28af7bdbcbfa5bdd39ce8bef10f6..e79d9f7ef926175a065a91f77d8ea24479639185 100644
--- a/source/mod_analytic.f90
+++ b/source/mod_analytic.f90
@@ -587,10 +587,12 @@ module mod_analytic
     cost_o = 0.
     do i = 1, nobs
       if ( trim(config%spec).eq.'co2' ) then
-        obs%delta(i) = obs%model(i) + obs%nee(i) + obs%fff(i) + obs%ocn(i) - &
+        obs%delta(i) = obs%cpri(i) - obs%cakpri(i) + &
+                        obs%model(i) + obs%nee(i) + obs%fff(i) + obs%ocn(i) - &
                         obs%conc(i) + obs%bkg(i) + obs%cinipos(i)
       else
-        obs%delta(i) = obs%model(i) + obs%ghg(i) - obs%conc(i) + obs%bkg(i) + obs%cinipos(i)
+        obs%delta(i) = obs%cpri(i) - obs%cakpri(i) + &
+                        obs%model(i) + obs%ghg(i) - obs%conc(i) + obs%bkg(i) + obs%cinipos(i)
       endif
       cost_o = cost_o + obs%delta(i)**2/obs%err(i)**2
     end do