Measurements of the carbon and hydrogen isotopes of atmospheric methane at Izaña, Tenerife: Seasonal cycles and synoptic‐scale variations

Abstract

Atmospheric CH4 and its stable isotope ratios 13C/12C and D/H have been investigated at the Global Atmospheric Watch station Izaña, Tenerife (28°N, 16°W, 2370 m above sea level), since late 1996. Every fortnight both spot samples and integral samples were taken, the latter collected continuously over 2‐week periods. While spot samples show considerable synoptic‐scale variability, the continuous samples clearly define seasonal cycles of CH4, δ13C, and δD, with peak‐to‐peak amplitudes of 30 ppb, 0.2‰ and 3.5‰, respectively. The measurement of the δD seasonality is the first ever reported for atmospheric background CH4 and has been made possible by the development of a tunable diode laser based optical Methane Isotopomer Spectrometer (MISOS). The δD is well in phase with CH4 mixing ratios, and the compact correlation between them allows to derive the average kinetic isotope effect (KIE) of the tropospheric sinks to be 1.23±0.04 (1σ), consistent with recent laboratory measurements of the kinetic isotope effect in the reaction of CH4±OH. In contrast to δD, δ13C is out of phase to CH4 mixing ratios, clearly indicating that δ13C is not only effected by the KIE of the CH4±OH reaction, but also by seasonally varying source mixtures. Considerable short‐term variations are observed in the isotopic composition and mixing ratios of the spot samples. This can be attributed to different origin of air masses arriving at Izaña. Significant CH4 enhancement is observed for air masses originating from the North American continent or Europe, of 12.7±11.7 and 13.4±0.3 ppb, respectively, while air from the African continent or the North Atlantic is depleted (−11.2±2.8 and −7.0 ± 8.2 ppb). Deviations from the mean seasonal CH4 cycle are correlated with significant deviations of δ13C and δD, allowing to estimate the δ13C and δD signatures of major source regions. Furthermore, deviations in CH4 mixing ratio are also clearly correlated with deviations of CO and SF6 mixing ratios. A three‐dimensional inverse model is employed in order to assist with the interpretation of observational data. In general, the model shows excellent agreement with the observed mean seasonal cycles of CH4, δ13C, and δD, including the observed phase behavior.