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Abstract:
We present variations of methyl chloride (CH3Cl) and nitrous oxide (N2O)
in the lowermost stratosphere (LMS) obtained from air samples collected
by the In-service Aircraft for a Global Observing System-Civil Aircraft
for the Regular Investigation of the atmosphere Based on an Instrument
Container (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, Hawaii (Delta
N2O).Delta N2O 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 Delta N2O. Significant
correlations are found between CH3Cl and Delta N2O in the LMS from
winter to early summer, both being affected by mixing between
stratospheric air and upper tropospheric air. This correlation, however,
disappears in late summer to autumn. The slope of the CH3Cl-Delta N2O
correlation observed in the LMS allows us to determine the stratospheric
lifetime of CH3Cl to be 35 +/- 7 years. Finally, we examine the
partitioning of stratospheric air and tropical/extratropical
tropospheric air in the LMS based on a mass balance approach using Delta
N2O 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.
