Vertical distributions of N<sub>2</sub>O isotopocules in the equatorial stratosphere
Vertical profiles of nitrous oxide (N<sub>2</sub>O) and its isotopocules, isotopically substituted molecules, were obtained over the Equator at altitudes of 16–30 km. Whole air samples were collected using newly developed balloon-borne compact cryogenic samplers over the eastern equa...
| Main Authors: | , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/833/2018/acp-18-833-2018.pdf |
| Summary: | Vertical profiles of nitrous oxide (N<sub>2</sub>O) and its
isotopocules, isotopically substituted molecules, were obtained over the
Equator at altitudes of 16–30 km. Whole air samples were collected using
newly developed balloon-borne compact cryogenic samplers over the eastern
equatorial Pacific in 2012 and Biak Island, Indonesia, in 2015. They were
examined in the laboratory using gas chromatography and mass spectrometry.
The mixing ratio and isotopocule ratios of N<sub>2</sub>O in the equatorial
stratosphere showed a weaker vertical gradient than the previously reported
profiles in the subtropical and mid-latitude and high-latitude stratosphere.
From the relation between the mixing ratio and isotopocule ratios, further
distinct characteristics were found over the Equator: (1) observed
isotopocule fractionations (<i>ε</i> values) in the middle stratosphere
(25–30 km or [N<sub>2</sub>O] < ca. 260 nmol mol<sup>−1</sup>) are almost equal to
<i>ε</i> values reported from broadband photolysis experiments
conducted in the laboratory; (2) <i>ε</i> values in the lower
stratosphere (< ca. 25 km or [N<sub>2</sub>O] > ca. 260 nmol mol<sup>−1</sup>)
are about half of the experimentally obtained values, being slightly larger
than those observed in the mid-latitude and high-latitude lower stratosphere
([N<sub>2</sub>O] > ca. 170 nmol mol<sup>−1</sup>). These results from the deep
tropics suggest the following. (i) The timescale for quasi-horizontal mixing
between tropical and mid-latitude air in the tropical middle stratosphere is
sufficiently slow relative to the tropical upwelling rate that isotope
fractionation approaches the Rayleigh limit for N<sub>2</sub>O photolysis. (ii) The
air in the tropical lower stratosphere is exchanged with extratropical air on
a timescale that is shorter than that of photochemical decomposition of
N<sub>2</sub>O. Previously observed <i>ε</i> values, which are invariably
smaller than those of photolysis, can be explained qualitatively using a
three-dimensional chemical transport model and using a simple model that
assumes mixing of <q>aged</q> tropical air and extratropical air during residual
circulation. Results show that isotopocule ratios are useful to examine the
stratospheric transport scheme deduced from tracer–tracer relations. |
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| ISSN: | 1680-7316 1680-7324 |