Evaluating the performance of a Picarro G2207- i analyser for high-precision atmospheric O 2 measurements

[摘要] Fluxes of oxygen (O 2 ) and carbon dioxide (CO 2 ) in and out of theatmosphere are strongly coupled for terrestrial biospheric exchangeprocesses and fossil fuel combustion but are uncoupled for oceanic air–seagas exchange. High-precision measurements of both species can thereforeprovide constraints on the carbon cycle and can be used to quantify fossilfuel CO 2 (ffCO 2 ) emission estimates. In the case of O 2 ,however, due to its large atmospheric mole fraction ( ∼20.9  %) it is very challenging to measure small variations to the degree ofprecision and accuracy required for these applications. We have tested anatmospheric O 2 analyser based on the principle of cavity ring-downspectroscopy (Picarro Inc., model G2207- i ), both in the laboratory and at theWeybourne Atmospheric Observatory (WAO) field station in the UK, incomparison to well-established, pre-existing atmospheric O 2 andCO 2 measurement systems. In laboratory tests analysing dry air in high-pressure cylinders, we foundthat the best precision was achieved with 30 min averaging and was ±0.5  ppm ( ∼ ± 2.4  per meg). Also from continuousmeasurements from a cylinder of dry air, we found the 24 h peak-to-peakrange of hourly averaged values to be 1.2 ppm ( ∼5.8  per meg).These results are close to atmospheric O 2 compatibility goals as set bythe UN World Meteorological Organization. However, from measurements of ambientair conducted at WAO we found that the built-in water correction of theG2207- i does not sufficiently correct for the influence of water vapour onthe O 2 mole fraction. When sample air was dried and a 5-hourly baselinecorrection with a reference gas cylinder was employed, the G2207- i 's resultsshowed an average difference from the established O 2 analyser of 13.6±7.5  per meg (over 2 weeks of continuous measurements). Over thesame period, based on measurements of a so-called “target tank”, analysedfor 12 min every 7 h, we calculated a repeatability of ± 5.7 ± 5.6  per meg and a compatibility of ± 10.0 ± 6.7  per megfor the G2207- i . To further examine the G2207- i 's performance in real-worldapplications we used ambient air measurements of O 2 together withconcurrent CO 2 measurements to calculate ffCO 2 . Due to theimprecision of the G2207- i , the ffCO 2 calculated showed largedifferences from that calculated from the established measurement systemand had a large uncertainty of ±13.0  ppm, which was roughly doublethat from the established system ( ±5.8  ppm).

[学科分类] 内科医学