; set desired region to be 0'd

    latS   =  0                     
    latN   = 10
    lonW   = 10   
    lonE   = 30

system("/bin/rm -f wrfbiochemi_d03_ZERO.nc")   ; remove any pre-existing file
fout = addfile ("wrfbiochemi_d03_ZERO.nc"  , "c")   

f = addfile ("wrfbiochemi_d03.nc" , "r")  

    copy_VarAtts (f, fout)                     ; copy file attributes

    xlat2d   = f->XLAT(0,:,:)
    xlon2d   = d->XLONG(0,:,:)

    ji = region_ind (xlat2d,xlon2d, latS, latN, lonW, lonE)

    jStrt = ji(0)      ; lat start
    jLast = ji(1)      ; lat last
    iStrt = ji(2)      ; lon start
    iLast = ji(3)      ; lon las

    vNames = getfilevarnames (f) ; get names of all variables on file
    nNames = dimsizes (vNames)   ; number of variables on the file
    print (vNames)               ; print all variable names on file
      
    do n=0,nNames-1              ; loop thru each variable
       v    := f->$vNames(n)$    ; read the varible to memory
       vdim := dimsizes(v)       ; dimension size of the variable
       vrank= dimsizes(vdim)     ; rank [ie: number of dimensions]
      
       if (vrank.ge.3.and.(vNames(n).ne."XLAT" .and. vNames(n).ne."XLONG"))  then
           vtype = typeof(v)
           if (vtype.eq."integer") then
               if (vrank.eq.3) then
                   v(:,jStrt:jLast,iStrt:iLast) = 0
               else
                   v(:,:,jStrt:jLast,iStrt:iLast) = 0
               end if
           elseif (vtype.eq."float") then
               if (vrank.eq.3) then
                   v(:,jStrt:jLast,iStrt:iLast) = 0.0
               else
                   v(:,:,jStrt:jLast,iStrt:iLast) = 0.0
               end if
           elseif (vtype.eq."double") then
               if (vrank.eq.3) then
                   v(:,jStrt:jLast,iStrt:iLast) = 0.0d0
               else
                   v(:,:,jStrt:jLast,iStrt:iLast) = 0.0
               end if
           end if
        end if

	fout->$vNames(n)$ = v          
          
   end do