anchor.diff:  !   Default is Lueker et al. 2000) for K1 & K2
       opK1K2 = 'l'
     ENDIF
  +  IF (PRESENT(optGAS)) THEN
  +    opGAS = optGAS
  +  ELSE
  +    opGAS = 'Pinsitu'
  +  ENDIF
   
     depth0 = fco2 * 0.0
     lat0   = depth0
   ! Compute derived variables from input (DIC, ALK, ...)
     CALL vars(ph, pco2, fco2, co2, hco3, co3, OmegaA, OmegaC, BetaD, rhoSW, p, tempis,  &
  -            temp, sal, alk, dic, sil, phos, Patm, depth0, lat0, N,                        &
  -            opCON, opT, opP, opB, opK1K2, opKf                                        )
  +            temp, sal, alk, dic, sil, phos, Patm, depth0, lat0, N,                    &
  +            opCON, opT, opP, opB, opK1K2, opKf, opGAS                                 )
   
   ! Compute pCO2atm [uatm] from xCO2 [ppm], atmospheric pressure [atm], & vapor pressure of seawater
   ! pCO2atm = (Patm - pH20(i)) * xCO2,   where pH20 is the vapor pressure of seawater [atm]
  
positive.code:   !> for K1,K2 choose either \b 'l' (Lueker et al., 2000) or \b 'm10' (Millero, 2010) 
  !f2py character*3 optional, intent(in) :: optK1K2='l'
    CHARACTER(3), OPTIONAL, INTENT(in) :: optK1K2
  
  ! Output variables:
    !> air-to-sea CO2 flux <b>[mol/(m^2 * s)]</b>
  
negative.code:   !       -> 'pf' means use Perez & Fraga (1987) formulation for Kf (recommended by Dickson et al., 2007)
    !               **** BUT Valid for  9 < T < 33°C and 10 < S < 40.
    !       -> 'dg' means use Dickson & Riley (1979) formulation for Kf (recommended by Dickson & Goyet, 1994)
    !     -----------
    !     optGAS: choose in situ vs. potential fCO2 and pCO2
    !     ---------
    !       PRESSURE corrections for K0 and the fugacity coefficient (Cf) 
    !       -> 'Pzero'   = 'zero order' fCO2 and pCO2 (typical approach, which is flawed)
    !                      considers in situ T & only atm pressure (hydrostatic=0)
    !       -> 'Ppot'    = 'potential' fCO2 and pCO2 (water parcel brought adiabatically to the surface)
    !                      considers potential T & only atm pressure (hydrostatic press = 0)
    !       -> 'Pinsitu' = 'in situ' fCO2 and pCO2 (accounts for huge effects of pressure)
    !                      considers in situ T & total pressure (atm + hydrostatic)
    !     ---------
  
    !     OUTPUT variables:
    !     =================
  