Stable carbon isotopic composition of atmospheric methane: A comparison of surface level and free tropospheric air

We report CH4 mixing ratios and delta(13)C of CH4 values for remote air at two groundbased atmospheric sampling sites for the period December 1994 to August 1998 and similar data from aircraft sampling of air masses from near sea level to near tropopause in September and October of 1996 during the Global Tropospheric Experiment Pacific Exploratory Mission (PEM)Tropics A. Surface values of delta(13)C-CH4 ranged from -47.02 to -47.52 parts per thousand at Niwot Ridge, Colorado (40 degrees N, 105 degrees W), and from -46.81 to -47.64 parts per thousand at Montana de Ore, California (35 degrees N, 121 degrees W). Samples for isotopic analysis were taken from 2 degrees to 27 degrees S latitude and 81 degrees to 158 degrees W longitude and from sea level to 11.3 km in altitude during the PEM-Tropics A mission. They represent the first study of (CH4)-C-13 in the tropical free troposphere. At similar to 11 km, delta(13)C-CH4 was similar to 1 parts per thousand greater than surface level values. Methane was generally enriched in C-13 as altitude increased and as latitude increased (toward the South Pole). Using criteria to filter out stratospheric subsidence and convective events on the basis of other trace gases present in the samples, we find evidence of a vertical gradient in delta(13)C-CH4 in the tropical troposphere. The magnitude of the isotopic shifts in atmospheric CH4 with altitude are examined with a two-dimensional tropospheric photochemical model and experimentally determined values for carbon kinetic isotope effects in chemical loss processes of CH4 Model-calculated values for delta(13)C-CH4 in both the troposphere and lower stratosphere significantly underpredict the enrichment in (CH4)-C-13 with altitude observed in our measurement data and data of other research groups.