PRO vmrtodu_int_cumoz, press_bounds, phi, plow, vmro3, total 

; this is a program to convert from Volume Mixing Ratio to DU. It returns
; total ozone between phi and plo (phi is lowest layer in atmosphere, 
; plo is highest level, ie lowest pressure level). phi and plo can
; be a vector or layers. 

; press_bounds is defined as the boundaries of levels in which vmr03 is
; defined, and should be entered from top to bottom of the atmosphere.
; press_bounds should be entered in hPa. VMR is also top of atmosphere to 
; bottom.

; define constants
g=9.806 ;gravity (m/s2)
ra=287.05 ;specific gas constant (J/kg/K)
st=273.15 ;standard temperature (K)
sp=101325. ;standard pressure(N/m2)

; compute delta pressure and convert to Pa
np=n_elements(press_bounds)-1
delpr=(press_bounds(1:np)-press_bounds(0:np-1))*100.
nlyrs=n_elements(phi)

lyroz=fltarr(np)
cumoz=fltarr(np)
for k=0, np-1 do lyroz(k)=0.1*ra*st/g/sp*vmro3(k)*delpr(k)
for k=0, np-1 do cumoz(k)=total(lyroz(0:k))

; Interpolate cumulative ozone to requested layer boundaries
boz=interpol(cumoz,alog(press_bounds(1:*)), alog(phi), /spline)
toz=interpol(cumoz,alog(press_bounds(1:*)), alog(plow), /spline)
total=boz-toz

over:
end