A method for resolving changes in atmospheric He ∕ N₂ as an indicator of fossil fuel extraction and stratospheric circulation

• Main Authors: B. Birner, W. Paplawsky, J. Severinghaus, R. F. Keeling
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-03-01
• Series: Atmospheric Measurement Techniques
• Online Access: https://amt.copernicus.org/articles/14/2515/2021/amt-14-2515-2021.pdf
• ISSN: 1867-1381; 1867-8548

<p>The atmospheric <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">He</mi><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="35pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="d38d2e975d71d5b9423ca4fdbbd12db9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2515-2021-ie00004.svg" width="35pt" height="14pt" src="amt-14-2515-2021-ie00004.png"/></svg:svg></span></span> ratio is expected to increase due to the emission of He associated with fossil fuels and is expected to also vary in both space and time due to gravitational separation in the stratosphere. These signals may be useful indicators of fossil fuel exploitation and variability in stratospheric circulation, but direct measurements of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">He</mi><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="35pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="7606266f310c20a7e405bef9d50dc7b3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2515-2021-ie00005.svg" width="35pt" height="14pt" src="amt-14-2515-2021-ie00005.png"/></svg:svg></span></span> ratio are lacking on all timescales. Here we present a high-precision custom inlet system for mass spectrometers that continuously stabilizes the flow of gas during sample–standard comparison and removes all non-noble gases from the gas stream. This enables unprecedented accuracy in measurement of relative changes in the helium mole fraction, which can be directly related to the <span class="inline-formula">⁴</span><span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">He</mi><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="35pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="805c4d9784ac0525fee662c2c569736f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2515-2021-ie00006.svg" width="35pt" height="14pt" src="amt-14-2515-2021-ie00006.png"/></svg:svg></span></span> ratio using supplementary measurements of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="4429455449d1e061d22bb9828b35172e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2515-2021-ie00007.svg" width="34pt" height="14pt" src="amt-14-2515-2021-ie00007.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Ar</mi><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="8ae0a88765137c85d96a7f62a8f988cf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2515-2021-ie00008.svg" width="32pt" height="14pt" src="amt-14-2515-2021-ie00008.png"/></svg:svg></span></span> and CO<span class="inline-formula">₂</span>. Repeat measurements of the same combination of high-pressure tanks using our inlet system achieves a <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">He</mi><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="35pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="887dcc6587835d80ce5e1d90c66606dc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2515-2021-ie00009.svg" width="35pt" height="14pt" src="amt-14-2515-2021-ie00009.png"/></svg:svg></span></span> reproducibility of <span class="inline-formula">∼</span> 10 per meg (i.e., 0.001 %) in 6–8 h analyses. This compares to interannual changes of gravitational enrichment at <span class="inline-formula">∼</span> 35 km in the midlatitude stratosphere of order 300–400 per meg and an annual tropospheric increase from human fossil fuel activity of less than <span class="inline-formula">∼</span> 30 per meg yr<span class="inline-formula">⁻¹</span> (bounded by previous work on helium isotopes). The gettering and flow-stabilizing inlet may also be used for the analysis of other noble-gas isotopes and could resolve previously unobserved seasonal cycles in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Kr</mi><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0a381b4505ab18ad51705371a2c23729"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2515-2021-ie00010.svg" width="32pt" height="14pt" src="amt-14-2515-2021-ie00010.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Xe</mi><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="2d21453408ba83e9e6574b999635ca4d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2515-2021-ie00011.svg" width="34pt" height="14pt" src="amt-14-2515-2021-ie00011.png"/></svg:svg></span></span>.</p>