Triple Oxygen and Clumped Isotope Compositions of CO 2 in the Middle Troposphere

摘要

We report the oxygen isotopic anomaly (Δ 17 O) and clumped isotopic composition (Δ 47 ) of CO 2 sampled from the middle troposphere (~10 km above ground) during two CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) flights in summer and winter seasons: one from Düsseldorf to Isla Margarita on 11 August 2001 and the other from Puerto Plata to Munich on 21 February 2002. The mid‐tropospheric Δ 17 O values were higher than the near‐surface values by 0.051?±?0.010 ‰, indicating enhanced stratospheric influence. The Δ 47 values were also higher by 0.160?±?0.015 ‰ compared to that from the lower troposphere. Stratospheric influence is supported by the observed correlations of the Δ 17 O values with the mixing ratios of N 2 O, O 3 , and 14 CO, which carry stratospheric signatures. The increase in the anomaly in the free troposphere represents a balance between the stratosphere‐troposphere exchange fluxes and biogeochemical/hydrospheric isotopic resetting rates at the Earth surface. Using a simple two box model with the free tropospheric Δ 17 O values, an average surface exchange rate of CO 2 between 327 and 772 PgC/year is inferred, giving a CO 2 turnover time of 1.4 to 2.8 years in the atmosphere. The present estimates are based on a rather small set of free tropospheric data and have relatively large uncertainties, but they agree with the existing model and proxy‐based estimates.