function dobson,p,o3

;-----------------------------------------------------------
;
; Purpose:
;   Compute column ozone ammount in dobson units (DU), from a
;   profile of O3 mixing ratio vs. pressure.
;   1 DU = 2.69e16 molecules cm-2
;
;  Input:
;    p.......pressure, mb
;    o3......ozone mixing ratio,  ppmv
;
;  Output:
;    dobs....column o3 abundance, DU
;
;  Source: Mark Hervig
;
;-----------------------------------------------------------

;- some constants

   g        = 9.8         ; gravity, m/s2
   avagadro = 6.02252e23  ; molec/mol
   mdry     = 0.028964    ; molec. wt. of dry air, kg/mol

   const    = 0.01 * avagadro / (g * mdry)

   n        = n_elements(p)

   dobs     = 0.

;- sum o3 over height

   for j = 1,n-1,1 do begin

     term = 0.5*(o3(j) + o3(j-1)) *1.e-6 * abs(p(j-1) - p(j)) * const/2.69e16

     dobs = dobs + term 

   endfor

   return, dobs

   end