Carbon isotopic composition of atmospheric methane: A comparison of surface...

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.