Abstract
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 Beromünster 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 Beromünster 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.
Highlights
Atmospheric oxygen comprises about 20.9 % of the global atmosphere, and over the past decade its concentration has decreased at a rate of ∼ 20 per meg yr−1 (Keeling and Manning, 2014) which has mainly been associated with the increase in fossil fuel combustion
The short-term variability in atmospheric oxygen can be used to estimate marine biological productivity and air–sea gas exchange (Keeling et al, 1998; Nevison et al, 2012). The accuracy of these estimates is primarily linked to the accuracy and precision of atmospheric O2 measurements and the assumed oxidation ratio (OR) for the different processes that are highly variable in contrast to atmospheric CO2, which can be well measured within the precision guidelines set by the Global Atmospheric Watch (GAW; ±0.1 ppm for the Northern Hemisphere)
The pressure sensitivity is strikingly small, indicating a good cancelation of the pressure dependence of the absorption amplitude and line width. Both temperature and pressure sensitivities are small enough to have a negligible effect on the short-term precision of measurements made with the stabilized ring-down cavity, long-term drifts in the sensors are always a matter of concern
Summary
Atmospheric oxygen comprises about 20.9 % of the global atmosphere, and over the past decade its concentration has decreased at a rate of ∼ 20 per meg yr−1 (Keeling and Manning, 2014) which has mainly been associated with the increase in fossil fuel combustion. Berhanu et al.: High-precision atmospheric oxygen measurement comparisons alyzer presented here) into changes in δO2/N2 This is associated with the influence of dilution effects on the mole fractions but not necessarily on the ratios. As the variability of atmospheric oxygen is directly linked to the carbon cycle, both its shortand long-term observations can be used to better constrain the carbon cycle Since it was first suggested by Keeling and Shertz (1992), the long-term trends derived from concurrent measurements of atmospheric CO2 and O2 have been widely used to quantify the partitioning of atmospheric CO2 between the land biosphere and oceanic sinks (Battle et al, 2000; Goto et al, 2017; Manning and Keeling, 2006; Valentino et al, 2008). The accuracy of these estimates is primarily linked to the accuracy and precision of atmospheric O2 measurements and the assumed ORs for the different processes that are highly variable in contrast to atmospheric CO2, which can be well measured within the precision guidelines set by the Global Atmospheric Watch (GAW; ±0.1 ppm for the Northern Hemisphere)
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