subroutine methanesat(nsize,t,p,qsat,qsurf_n2)

  ! use callkeys_mod
  use dimphy, only: klon

  IMPLICIT NONE


!=======================================================================
!
!  Methane mass mixing ratio at saturation (kg/kg) for a given pressure (Pa)
!   and Temperature (K) array
!
!=======================================================================


#include "dimensions.h"


!   INPUT
      integer nsize
      real t(nsize) , p(nsize)
!   OUTPUT
      real qsat(nsize),qsurf_n2(nsize)
      INTEGER i

      do i=1,nsize

!       from Fray and schmitt fit by formulation L = 6.125e5 J/kg
!        qsat(i)=0.117*exp((16*612.5/8.314)*(1/90.7-1/t(i)))*100000
        qsat(i)=0.117*exp((1178.734664)*(0.01102536-1/t(i)))*100000
        qsat(i)=qsat(i)* 16.0 / (28.0*p(i))

!       security :
        qsat(i)=min(qsat(i),0.99)

      enddo

!     Raoult law if mixte in N2 ice : assume 0.5% CH4 in N2
      if (nsize.le.klon) then
         do i=1,nsize
           if (qsurf_n2(i).gt.0.001) qsat(i)=qsat(i)*0.005 !*(1.-dayfrac)       ! .5 /100
         end do
      endif

  return
end subroutine methanesat