Preparation of primary standard mixtures for atmospheric oxygen measurements with less than 1 µmol mol<sup>−1</sup> uncertainty for oxygen molar fractions
<p>Precise monitoring of changes in atmospheric <span class="inline-formula">O<sub>2</sub></span> levels was implemented by preparing primary standard mixtures with less than 1 <span class="inline-formula">µ</span>mol&thins...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-05-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/12/2631/2019/amt-12-2631-2019.pdf |
| Summary: | <p>Precise monitoring of changes in atmospheric <span class="inline-formula">O<sub>2</sub></span> levels was
implemented by preparing primary standard mixtures with less than
1 <span class="inline-formula">µ</span>mol mol<span class="inline-formula"><sup>−1</sup></span> standard uncertainty for <span class="inline-formula">O<sub>2</sub></span> molar fractions. In this
study, these mixtures were crafted in 10 L high-pressure aluminium alloy
cylinders using a gravimetric method in which unknown uncertainty factors
were theoretically determined and subsequently reduced. Molar fractions of
the constituents (<span class="inline-formula">CO<sub>2</sub></span>, Ar, <span class="inline-formula">O<sub>2</sub></span>, and <span class="inline-formula">N<sub>2</sub></span>) in the primary
standard mixtures were mainly resolved using masses of the respective source
gases (<span class="inline-formula">CO<sub>2</sub></span>, Ar, <span class="inline-formula">O<sub>2</sub></span>, and <span class="inline-formula">N<sub>2</sub></span>) that had been filled into the
cylinders. To precisely determine the masses of the source gases, the
difference in mass of the cylinder before and after filling the respective
source gas was calculated by comparison with an almost identical reference
cylinder. Although the masses of the cylinders filled with the source gas with
respect to the reference cylinder tended to deviate in relation to
temperature differences between the source-gas-filled cylinder and
surrounding air, the degree of the deviation could be efficiently reduced by
measuring the two cylinders at the exact same temperature. The standard
uncertainty for the cylinder mass obtained in our weighing system was
determined to be 0.82 mg. The standard uncertainties for the <span class="inline-formula">O<sub>2</sub></span> molar
fractions in the primary standard mixtures ranged from 0.7
to 0.8 <span class="inline-formula">µ</span>mol mol<span class="inline-formula"><sup>−1</sup></span>. Based on the primary standard
mixtures, the annual average molar fractions of atmospheric <span class="inline-formula">O<sub>2</sub></span> and Ar
in 2015 at Hateruma island, Japan, were found to be <span class="inline-formula">209339.1±1.1</span>
and <span class="inline-formula">9334.4±0.7</span> <span class="inline-formula">µ</span>mol mol<span class="inline-formula"><sup>−1</sup></span>,
respectively. The molar fraction for atmospheric Ar was in agreement with
previous reports.</p> |
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| ISSN: | 1867-1381 1867-8548 |