function sulfate,pres_in,temp_in,qh2o_in 

;-------------------------------------------------------------------
;
; Purpose:
;    This routine returns the equilibrium composition
;    of an aerosol composed of water and sulfuric acid
;    in weight percent of H2SO4,  as a function of temp
;    pressure and H2O mixing ratio.
;
;    Uses an equation published in Hanson et al., 1994 JGR,
;    which is a fit to the data of Steel and Hamill 1981
;
; Parameters:
;
;   input:
;      temp_in.....temperature (K)
;      qh2o_in.....water vapor mixing ratio (ppmv)
;      pres_in.....pressure (mb)
;
;   output:
;      result......the sulfuric acid composition (wt % h2so4)
;
; Source:  Mark Hervig 
;
;---------------------------------------------------------------------

   pp     = alog(pres_in * (qh2o_in * 1.e-6) ) ; h2o partial pressure

   top    = (-14.458 + 0.62456 * pp) * temp_in + 3565.0
   bot    = 44.777 + 1.3204 * pp - 0.19988 * temp_in

   result = top / bot

   return,result
   end