Evaluating the performance of a Picarro G2207-i analyser for highprecision atmospheric O2 measurements.

Fluxes of oxygen (O₂) and carbon dioxide (CO₂) in and out of the atmosphere are strongly coupled for terrestrial biospheric exchange processes and fossil fuel combustion but are uncoupled for oceanic air-sea gas exchange. High-precision measurements of both species can therefore provide constraints on the carbon cycle and can be used to quantify fossil fuel CO₂ (ffCO₂) emission estimates. In the case of O₂, however, due to its large atmospheric mole fraction of O₂ (~20.9 %) it is very challenging to measure small variations to the degree of precision and accuracy required for these applications. We have tested an atmospheric O₂ analyser based on the principle of cavity ring-down spectroscopy (Picarro Inc., model G2207- i), both in the laboratory and at the Weybourne Atmospheric Observatory (WAO) field station in the UK, in comparisons to well-established, pre-existing atmospheric O₂ and CO₂ measurement systems. In laboratory tests analysing air in high-pressure cylinders, from the Allan deviation we calculated a precision of ± 1 ppm (1σ standard deviation of 300 seconds mean), and a 24-hour peak-to-peak range of hourly averaged values of 1.2 ppm. These results are close to atmospheric O₂ compatibility goals as set by the UN World Meteorological Organization. From measurements of ambient air conducted at WAO we found that the built-in water correction of the G2207-i does not sufficiently correct for the influence of water vapour on the O₂ mole fraction. When sample air was pre-dried and employing a 5-hourly baseline correction with a reference gas cylinder, the G2207-i’s results showed an average difference from the established O₂ analyser of 13.6 ± 7.5 per meg (over two weeks of continuous measurements). Over the same period, based on measurements of a so-called “target tank” (sometimes known as a “surveillance tank”), analysed for 12 minutes every 7 hours, we calculated a repeatability of ± 5.7 ± 5.6 per meg and a compatibility of ± 10.0 ± 6.7 per meg for the G2207-i . To further examine the G2207-i’s performance in real-world applications we used ambient air measurements of O₂ together with concurrent CO₂ measurements to calculate ffCO₂. Due to the imprecision of the G2207-i, the ffCO₂ calculated showed large differences from that calculated from the established system, and had a large uncertainty of ± 13.0 ppm, which was roughly double that from the established system (± 5.8 ppm). [ABSTRACT FROM AUTHOR]