Long-Term Atmospheric Monitoring of Background CH₄ and CO₂ Using the LI-7810 and LI-7815 Trace Gas Analyzers

Introduction

Continuous atmospheric monitoring of background CH₄ and CO₂ requires instruments that meet strict requirements for accuracy, precision, and stability. In addition, they must be robust, requiring minimal maintenance, as they will be required to operate 24 hours a day, 365 days a year. The LI-7810 CH₄/CO₂/H₂O and LI-7815 CO₂/H₂O Trace Gas Analyzers were designed with just such applications in mind.

Long-term background measurements of a wide range of greenhouse gases, including CH₄ and CO₂, are overseen at the regional and global scale, with measurement networks deployed at carefully selected sites. The required performance of instruments deployed at measurement network sites is specified by the organization that oversees the collection and reporting of data. At a global scale the change in background concentrations of key greenhouse gases is monitored by the World Meteorological Organization Global Atmosphere Watch (WMO GAW), while at a European level key facilities for monitoring the health of the ecosystem, including the atmosphere, are operated by the Integrated Carbon Observation System (ICOS).

Both the LI-7810, for CH₄, and LI-7815, for CO₂, have been independently tested to measure stability, drift and precision. Both instruments were evaluated by ICOS Atmospheric Thematic Centre (ATC), Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Paris, to appraise precision, drift, pressure stability, water correction and several other key specifications. In addition, atmospheric researchers at Scripps Institution of Oceanography, University of California, USA, completed a series of experiments to determine the performance of both instruments with respect to the demands of atmospheric measurements², with reference to WMO GAW inter-laboratory measurement compatibility goals, as detailed in the 20th WMO/IAEA Meeting on Carbon Dioxide, Other Greenhouse Gases and Related Measurement Techniques (GGMT-2019) report³. The Scripps researchers concluded that ‘these experiments confirm the suitability of the LI-7810 and LI-7815 instruments for long term measurements atmospheric methane and carbon dioxide respectively’.

‘These experiments confirm the suitability of the LI-7810 and LI-7815 instruments for long term measurements atmospheric methane and carbon dioxide respectively.’

Building on these successful independent tests, the LI-7810 and LI-7815 were deployed for six months, continuous operation at the Atmospheric Research Station at Mace Head, Carna, County Galway, located on the far west coast of Ireland. Here, the two instruments were operated in parallel with a CH₄/CO₂/CO/H₂O analyzer, permanently installed at the facility, which reports data to both ICOS and WMO GAW networks. This summary describes the deployment of the LI-7810 and LI-7815 trace gas analyzers at the Mace Head facility, covering installation, data handling, calibration strategy, and measurement results, and presents in its conclusion that both instruments, in combination with an appropriate calibration strategy, meet instrument performance requirements for collection and submission of data to ICOS and WMO GAW networks.

Installation

The Atmospheric Research Station at Mace Head has an air inlet located at the top of the tall tower sampling atmospheric air, which is delivered to several gas analyzers housed in the buildings at the base of the tower (Figure 1A). The LI-7810 and LI-7815 sample atmospheric air continuously, with samples of calibration gases, stored in high-pressure cylinders, sampled periodically. Gas sample selection is provided via a VICI Valco multi-port rotary valve (Figure 1C). Instrument data and valve port selection is controlled using GCWerks, which includes routines designed specifically for the LI-COR trace gas analyzer as standard.

Results

Linearity corrected ambient CH₄ data over a six-month period, from July 2020 to January 2021, shows comparable performance of methane measurements (Figure 2). During this period there were short periods of down-time due to site maintenance and other issues, not related to instrument operation/performance. These periods are evident in the data presented.

Similarly strong performance is observed for the LI-7815 when considering CO₂ measurement data. For CH₄, with a water removal regime in place (an ICOS station requirement), a difference between measurements of ~1 ppb is observed (~0.05% of full-scale, typical background CH₄ concentration of ~2000 ppb). Without water removal, variability in water concentration may lead to this difference increasing to ~4 ppb (~0.2% of full-scale, typical background CH₄ concentration of ~2000 ppb). For CO₂ measurements made by the LI-7815 and G2401, differences were typically in the region of 0.1 ppm (~0.025% of full-scale, typical background CO₂ concentration of ~400 ppm). Differences of ~0.1 ppm were observed, irrespective of the water removal regime deployed, including total absence.

Conclusions

Previous independent testing by ICOS ATC and Scripps Institution of Oceanography represented the first steps to showing that the LI-7810 and LI-7815 are suitable for long-term background measurements of atmospheric CH₄ and CO₂, respectively. Both studies were based on relatively short-term tests, up to a maximum of approximately six weeks. The longer-term test at Mace Head shows additional confidence in terms of data stability and inter comparability, as well as reliability and convenience for continuous monitoring applications. Results from this study corroborate the conclusions from these earlier tests and show that the instruments are suitable for longer-term deployments.

From a practical perspective, this deployment demonstrated how the LI-7810 and LI-7815 instruments can be installed into existing sampling and measurement network infrastructure, with minimal impact to gas sampling or data handling hardware, and GCWerks compatibility for both data collection and gas sample selection.

In this application, the LI-7810 CH₄/CO₂/H₂O and LI-7815 CO₂/H₂O Trace Gas Analyzers show their compatibility with WMO GAW and ICOS instrument specifications and their ability to be used with varying calibration standards and protocols.

Damien Martin, who oversaw the deployment at Mace Head, stated that ‘Concerning measurement data, both instruments demonstrated that over the campaign, that with suitable calibration standards, protocols and a standard water removal regime, WMO GAW network inter-laboratory measurement compatibility goals can be achieved for ambient air sampling.’

‘Concerning measurement data, both instruments demonstrated that over the campaign, that with suitable calibration standards, protocols and a standard water removal regime, WMO GAW network inter-laboratory measurement compatibility goals can be achieved for ambient air sampling.’

Acknowledgements

• Damien Martin, National University of Ireland, Galway, Ireland
• Gerard Spain, Mace Head Research Station, Galway, Ireland
• Dickon Young, University of Bristol, UK
• Michael Geever, National University of Ireland, Galway, Ireland
• Peter Salameh, Scripps Institution of Oceanography, La Jolla, CA, USA
• Simon O’Doherty, University of Bristol, UK