Methyl chloride as a tracer of tropical tropospheric air in the lowermost stratosphere inferred from IAGOS-CARIBIC passenger aircraft measurements

We present variations of methyl chloride (CH3Cl) and nitrous oxide (N2O) in the lowermost stratosphere (LMS) obtained from air samples collected by the IAGOS-CARIBIC passenger aircraft observatory for the period 2008–2012. To correct for the temporal increase of atmospheric N2O, the CARIBIC N2O data are expressed as deviations from the long-term trend at the northern hemispheric baseline station Mauna Loa (MLO) Hawaii (ΔN2O). ΔN2O undergoes a pronounced seasonal variation in the LMS with a minimum in spring. The amplitude increases going deeper in the LMS (up to potential temperature of 40 K above the thermal tropopause), as a result of the seasonally varying subsidence of air from the stratospheric overworld. Seasonal variation of CH3Cl above the tropopause is similar in phase to that of ΔN2O. Significant correlations are found between CH3Cl and ΔN2O in the LMS from winter to early summer, both being affected by mixing between stratospheric air and upper tropospheric (UT) air. This correlation however disappears in late summer to autumn. The slope of the CH3Cl-ΔN2O correlation observed in the LMS allows us to determine the stratospheric lifetime of CH3Cl to be 35±7 yr. Finally, we examine the partitioning of stratospheric air and tropical/extra-tropical tropospheric air in the LMS based on a mass balance approach using ΔN2O and CH3Cl. This analysis clearly indicates efficient inflow of tropical tropospheric air into the LMS in summer and demonstrates the usefulness of CH3Cl as a tracer of tropical tropospheric air.