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

We report CH 4 mixing ratios and δ 13 C of CH 4 values for remote air at two ground‐based 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 δ 13 C‐CH 4 ranged from −47.02 to −47.52‰ at Niwot Ridge, Colorado (40°N, 105°W), and from −46.81 to −47.64‰ at Montaña de Oro, California (35°N, 121°W). Samples for isotopic analysis were taken from 2° to 27°S latitude and 81° to 158°W longitude and from sea level to 11.3 km in altitude during the PEM‐Tropics A mission. They represent the first study of 13 CH 4 in the tropical free troposphere. At ∼11 km, δ 13 C‐CH 4 was ∼1‰ greater than surface level values. Methane was generally enriched in 13 C 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 δ 13 C‐CH 4 in the tropical troposphere. The magnitude of the isotopic shifts in atmospheric CH 4 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 CH 4 Model‐calculated values for δ 13 C‐CH 4 in both the troposphere and lower stratosphere significantly underpredict the enrichment in 13 CH 4 with altitude observed in our measurement data and data of other research groups.