UK greenhouse gas measurements at two new tall towers for aiding emissions verification

• Main Authors: A. R. Stavert, S. O'Doherty, K. Stanley, D. Young, A. J. Manning, M. F. Lunt, C. Rennick, T. Arnold
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-08-01
• Series: Atmospheric Measurement Techniques
• Online Access: https://www.atmos-meas-tech.net/12/4495/2019/amt-12-4495-2019.pdf

<p>Under the UK-focused Greenhouse gAs and Uk and Global Emissions (GAUGE) project, two new tall tower greenhouse gas (GHG) observation sites were established in the 2013/2014 Northern Hemispheric winter. These sites, located at existing telecommunications towers, utilized a combination of cavity ring-down spectroscopy (CRDS) and gas chromatography (GC) to measure key GHGs (<span class="inline-formula">CO₂</span>, <span class="inline-formula">CH₄</span>, CO, <span class="inline-formula">N₂O</span> and SF<span class="inline-formula">₆</span>). Measurements were made at multiple intake heights on each tower. <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> dry mole fractions were calculated from either CRDS measurements of wet air, which were post-corrected with an instrument-specific empirical correction, or samples dried to between 0.05&thinsp;% <span class="inline-formula">H₂O</span> and 0.3&thinsp;% <span class="inline-formula">H₂O</span> using a Nafion^® dryer, with a smaller correction applied for the residual <span class="inline-formula">H₂O</span>. The impact of these two drying strategies was examined. Drying with a Nafion^® dryer was not found to have a significant effect on the observed <span class="inline-formula">CH₄</span> mole fraction; however, Nafion^® drying did cause a 0.02&thinsp;<span class="inline-formula">µ</span>mol&thinsp;mol<span class="inline-formula">⁻¹</span> <span class="inline-formula">CO₂</span> bias. This bias was stable for sample <span class="inline-formula">CO₂</span> mole fractions between 373 and 514&thinsp;<span class="inline-formula">µ</span>mol&thinsp;mol<span class="inline-formula">⁻¹</span> and for sample <span class="inline-formula">H₂O</span> up to 3.5&thinsp;%. As the calibration and standard gases are treated in the same manner, the 0.02&thinsp;<span class="inline-formula">µ</span>mol&thinsp;mol<span class="inline-formula">⁻¹</span> <span class="inline-formula">CO₂</span> bias is mostly calibrated out with the residual error (<span class="inline-formula">≪</span>0.01&thinsp;<span class="inline-formula">µ</span>mol&thinsp;mol<span class="inline-formula">⁻¹</span> <span class="inline-formula">CO₂</span>) well below the World Meteorological Organization (WMO) reproducibility requirements. Of more concern was the error associated with the empirical instrument-specific water correction algorithms. These corrections are relatively stable and reproducible for samples with <span class="inline-formula">H₂O</span> between 0.2&thinsp;% and 2.5&thinsp;%, <span class="inline-formula">CO₂</span> between 345 and 449&thinsp;<span class="inline-formula">µ</span>mol&thinsp;mol<span class="inline-formula">⁻¹</span>, and <span class="inline-formula">CH₄</span> between 1743 and 2145&thinsp;nmol&thinsp;mol<span class="inline-formula">⁻¹</span>. However, the residual errors in these corrections increase to &gt;&thinsp;0.05&thinsp;<span class="inline-formula">µ</span>mol&thinsp;mol<span class="inline-formula">⁻¹</span> for <span class="inline-formula">CO₂</span> and &gt;&thinsp;1&thinsp;nmol&thinsp;mol<span class="inline-formula">⁻¹</span> for <span class="inline-formula">CH₄</span> (greater than the WMO internal reproducibility guidelines) at higher humidities and for samples with very low (&lt;&thinsp;0.5&thinsp;%) water content. These errors also scale with the absolute magnitude of the <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> mole fractions. As such, water corrections calculated in this manner are not suitable for samples with low (&lt;&thinsp;0.5&thinsp;%) or high (&gt;&thinsp;2.5&thinsp;%) water contents and either alternative correction methods should be used or partial drying or humidification considered prior to sample analysis.</p>