SUBROUTINE METHOX(KIDIA, KFDIA, KLON, KLEV, PQ, PTENQ, PAP)

  !**** *METHOX*   - Calculate humidity tendencies from methane
  !                  oxidation and photolysis

  !**   INTERFACE.
  !     ----------
  !        CALL *METHOX* FROM *CALLPAR*
  !              ------        -------

  !        EXPLICIT ARGUMENTS :
  !        --------------------
  !     PARAMETER     DESCRIPTION                                   UNITS
  !     ---------     -----------                                   -----
  !     INPUT PARAMETERS (INTEGER):

  !    *KIDIA*        START POINT
  !    *KFDIA*        END POINT
  !    *KLON*         NUMBER OF GRID POINTS PER PACKET
  !    *KLEV*         NUMBER OF LEVELS

  !     INPUT PARAMETERS (REAL):

  !    *PAP*          PRESSURE                                      PA
  !    *PQ*           SPECIFIC HUMIDITY                             KG/KG

  !     UPDATED PARAMETERS (REAL):

  !    *PTENQ*        TENDENCY OF SPECIFIC HUMIDITY                 KG/(KG*S)

  !        NONE

  !        IMPLICIT ARGUMENTS :
  !        --------------------
  !        MODULE YOEMETH
  !        MODULE YOMCST

  !     METHOD.
  !     -------
  !        SEE RD-MEMO R60.1/AJS/31

  !     EXTERNALS.
  !     ----------
  !        NONE

  !     REFERENCE.
  !     ----------
  !        SEE RD-MEMO R60.1/AJS/31

  !     AUTHOR.
  !     -------
  !        C.JAKOB   *ECMWF*

  !     MODIFICATIONS.
  !     --------------
  !        ORIGINAL : 98-04-07
  !        M.Hamrud      01-Oct-2003 CY28 Cleaning
  !        D. Cugnet     24-Feb-2012 Adapted for LMDZ
  !     ------------------------------------------------------------------

  USE YOEMETH, ONLY: RALPHA1, RALPHA2, RQLIM, &
          RPBOTOX, RPBOTPH, RPTOPOX, RPTOPPH, &
          RALPHA3, RLOGPPH
  USE lmdz_yomcst

  IMPLICIT NONE

  INTEGER, INTENT(IN) :: KLON
  INTEGER, INTENT(IN) :: KLEV
  INTEGER, INTENT(IN) :: KIDIA
  INTEGER, INTENT(IN) :: KFDIA
  REAL, INTENT(IN) :: PQ(KLON, KLEV)
  REAL, INTENT(INOUT) :: PTENQ(KLON, KLEV)
  REAL, INTENT(IN) :: PAP(KLON, KLEV)
  LOGICAL :: LLOXID, LLPHOTO

  INTEGER :: JK, JL

  REAL :: ZARG, ZPRATIO, ZTAU1, ZTAU2, ZTDAYS

  DO JK = 1, KLEV
    DO JL = KIDIA, KFDIA

      LLOXID = PAP(JL, JK) < RPBOTOX.AND.PQ(JL, JK) < RQLIM
      LLPHOTO = PAP(JL, JK) < RPBOTPH

      !     METHANE OXIDATION

      IF(LLOXID) THEN
        IF(PAP(JL, JK) <= RPTOPOX) THEN
          ZTDAYS = 100.
        ELSE
          ZPRATIO = (LOG(PAP(JL, JK) / RPTOPOX))**4. / LOG(RPBOTOX / PAP(JL, JK))
          ZTDAYS = 100. * (1 + RALPHA1 * ZPRATIO)
        ENDIF
        ZTAU1 = 86400. * ZTDAYS
        PTENQ(JL, JK) = PTENQ(JL, JK) + (RQLIM - PQ(JL, JK)) / ZTAU1
      ENDIF

      !     PHOTOLYSIS

      IF(LLPHOTO) THEN
        IF(PAP(JL, JK) <= RPTOPPH) THEN
          ZTDAYS = 3.
        ELSE
          ZARG = RALPHA2 - RALPHA3 * (1 + COS((RPI * LOG(PAP(JL, JK) / RPBOTPH)) / RLOGPPH))
          ZTDAYS = 1.0 / (EXP(ZARG) - 0.01)
        ENDIF
        ZTAU2 = 86400. * ZTDAYS
        PTENQ(JL, JK) = PTENQ(JL, JK) - PQ(JL, JK) / ZTAU2
      ENDIF
    ENDDO
  ENDDO

END SUBROUTINE METHOX