High-precision atmospheric oxygen measurement comparisons between a newly built CRDS analyzer and existing measurement techniques
Abstract. Carbon dioxide and oxygen are tightly coupled in land biosphere CO2 – O2 exchange processes, whereas they are not coupled in oceanic exchange.
For this reason, atmospheric oxygen measurements can be used to constrain
the global carbon cycle, especially oceanic uptake. However, accurately
quantifying small ( ∼1 –100 ppm) variations in O2 is
analytically challenging due to the very large atmospheric background which
constitutes about 20.9 % ( ∼209 500  ppm) of atmospheric air.
Here we present a detailed description of a newly
developed high-precision oxygen mixing ratio and isotopic composition
analyzer (Picarro G2207) that is based on cavity ring-down spectroscopy
(CRDS) as well as to its operating
principles; we also demonstrate comprehensive laboratory and field studies using the abovementioned instrument. From the laboratory tests, we calculated a short-term precision
(standard error of 1 min O2 mixing ratio measurements) of
< 1 ppm for this analyzer based on measurements of eight standard
gases analyzed for 2 h, respectively. In contrast to the currently
existing techniques, the instrument has an excellent long-term stability;
therefore, calibration every 12 h is sufficient to get an overall
uncertainty of < 5 ppm. Measurements of ambient air were also
conducted at the Jungfraujoch high-altitude research station and the
Beromunster tall tower in Switzerland. At both sites, we observed
opposing and diurnally varying CO2 and O2 profiles due to
different processes such as combustion, photosynthesis, and respiration.
Based on the combined measurements at Beromunster tower, we determined
height-dependent O2:CO2 oxidation ratios varying between −0.98 and
 −1.60 ; these ratios increased with the height of the tower inlet, possibly due to
different source contributions such as natural gas combustion, which has a high
oxidation ratio, and biological processes, which have oxidation ratios that are relatively lower.