Online technique for isotope and mixing ratios of CH<sub>4</sub>, N<sub>2</sub>O, Xe and mixing ratios of organic trace gases on a single ice core sample
Firn and polar ice cores enclosing trace gas species offer a unique archive to study changes in the past atmosphere and in terrestrial/marine source regions. Here we present a new online technique for ice core and air samples to measure a suite of isotope ratios and mixing ratios of trace gas specie...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-08-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | http://www.atmos-meas-tech.net/7/2645/2014/amt-7-2645-2014.pdf |
| Summary: | Firn and polar ice cores enclosing trace gas species offer a unique archive
to study changes in the past atmosphere and in terrestrial/marine source
regions. Here we present a new online technique for ice core and air samples
to measure a suite of isotope ratios and mixing ratios of trace gas species
on a single sample. Isotope ratios are determined on methane, nitrous oxide
and xenon with reproducibilities for ice core samples of
0.15‰ for δ<sup>13</sup>C–CH<sub>4</sub>,
0.22‰ for δ<sup>15</sup>N–N<sub>2</sub>O,
0.34‰ for δ<sup>18</sup>O–N<sub>2</sub>O, and
0.05‰ per mass difference for δ<sup>136</sup>Xe for
typical concentrations of glacial ice. Mixing ratios are determined on
methane, nitrous oxide, xenon, ethane, propane, methyl chloride and
dichlorodifluoromethane with reproducibilities of 7 ppb for CH<sub>4</sub>, 3 ppb
for N<sub>2</sub>O, 70 ppt for C<sub>2</sub>H<sub>6</sub>, 70 ppt for C<sub>3</sub>H<sub>8</sub>, 20 ppt
for CH<sub>3</sub>Cl, and 2 ppt for CCl<sub>2</sub>F<sub>2</sub>. However, the blank contribution for
C<sub>2</sub>H<sub>6</sub> and C<sub>3</sub>H<sub>8</sub> is large in view of the measured values
for Antarctic ice samples. The system consists of a vacuum extraction
device, a preconcentration unit and a gas chromatograph coupled to an
isotope ratio mass spectrometer. CH<sub>4</sub> is combusted to CO<sub>2</sub> prior to
detection while we bypass the oven for all other species. The highly
automated system uses only ~ 160 g of ice, equivalent to
~ 16 mL air, which is less than previous methods. The
measurement of this large suite of parameters on a single ice sample is new
and key to understanding phase relationships of parameters which are usually
not measured together. A multi-parameter data set is also key to understand
in situ production processes of organic species in the ice, a critical issue
observed in many organic trace gases. Novel is the determination of xenon
isotope ratios using doubly charged Xe ions. The attained precision for
δ<sup>136</sup>Xe is suitable to correct the isotopic ratios and mixing
ratios for gravitational firn diffusion effects, with the benefit that this
information is derived from the same sample. Lastly, anomalies in the Xe
mixing ratio, δXe/air, can be used to detect melt layers. |
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| ISSN: | 1867-1381 1867-8548 |