Carbon isotopic composition of atmospheric methane: A comparison of surface level and upper tropospheric air

Tyler, S.C., H.O. Ajie, M.L. Gupta, R.J. Cicerone, D.R. Blake, and E.J. Dlugokencky (1999), Carbon isotopic composition of atmospheric methane: A comparison of surface level and upper tropospheric air, J. Geophys. Res., 104, 13895-13910.