Publication Date:
2015-11-05
Description:
We present variations of methyl chloride (CH 3 Cl) and nitrous oxide (N 2 O) 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 N 2 O, the CARIBIC N 2 O data are expressed as deviations from the long-term trend at the northern hemispheric baseline station Mauna Loa (MLO) Hawaii (ΔN 2 O). ΔN 2 O 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 CH 3 Cl above the tropopause is similar in phase to that of ΔN 2 O. Significant correlations are found between CH 3 Cl and ΔN 2 O 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 CH 3 Cl-ΔN 2 O correlation observed in the LMS allows us to determine the stratospheric lifetime of CH 3 Cl 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 ΔN 2 O and CH 3 Cl. This analysis clearly indicates efficient inflow of tropical tropospheric air into the LMS in summer and demonstrates the usefulness of CH 3 Cl as a tracer of tropical tropospheric air.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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