Analysis of accurate ¹³C and ¹⁸O isotope measurements of CO₂ in CARIBIC aircraft air samples from the tropical troposphere, and the upper troposphere/lowermost stratosphere

Abstract

Abstract. The project CARIBIC (http://caribic-atmospheric.com) aims to study atmospheric chemistry and transport by regularly measuring many compounds in the free troposphere (FT) and the upper troposphere/lowermost stratosphere (UT/LMS) by using passenger aircraft. Here CO2 concentrations and highly accurate isotope results are presented in detail together with supporting trace gas data. 509 CARIBIC-2 samples (highest precision and accuracy δ13C(CO2) and δ18O(CO2) data) from June 2007 until March 2009, together with CARIBIC-1 samples (flights between November 1999 and April 2002, 350 samples in total, 270 for NH, mostly δ13C(CO2) data) give a fairly extensive, unique data set for the NH free troposphere and the UT/LMS region. To compare data from different years a de-trending is applied. In the UT/LMS region δ13C(CO2), δ18O(CO2) and CO2 are found to correlate well with stratospheric tracers, in particular N2O. These correlations are in good agreement with current understanding of stratospheric circulation. δ18O(CO2) appears to be a useful, hitherto unused, tracer of atmospheric transport in the UT/LMS region. By filtering out the LMS data (based on N2O distribution), the isotope variations for the free and upper troposphere are obtained. These show however little latitudinal gradient, if any, and are in good agreement with the data of selected NOAA stations in NH tropics. Correlations between δ13C(CO2) and CO2 are observed both within single flight(s) covering long distances and for certain seasons. The overall variability in de-trended δ13C(CO2) and CO2 for CARIBIC-1 and CARIBIC-2 are similar and basically agree with each other, which also underscores the high quality of measurement. Based on all correlations, we discuss that CO2 distribution in the NH FT and UT (at CARIBIC flight routes) is regulated by uplift and pole-wards transport of tropical air up to approximately 50° N. The main reasons for variability of signals in FT and UT (which is larger for the high spatial resolution sampling during CARIBIC-2) is mixing of different tropospheric air masses affected by CO2 sources and sinks. The effect of stratospheric flux appears to be limited. All in all it is demonstrated that CARIBIC produced new important and reliable data sets for little explored regions of the atmosphere. A logical next step will be global scale modeling of δ13C and especially δ18O, which is linked to the hydrological cycle.