Comment on amt-2022-207

Abstract

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).