This paper describes calibration strategies in laboratory conditions that can be applied to ensure accurate measurements of the isotopic composition of the CO in ultradry air, expressed as δC and δO on the VPDB scale, with either FT-IR (in this case a Vertex 70 V (Bruker)) or an isotope ratio infrared spectrometer (IRIS) (in this case a Delta Ray (Thermo Fisher Scientific)). In the case of FT-IR a novel methodology using only two standards of CO in air with different mole fractions but identical isotopic composition was demonstrated to be highly accurate for measurements of δC and δO with standard uncertainties of 0.09‰ and 1.03‰, respectively, at a nominal CO mole fraction of 400 μmol mol in air. In the case of the IRIS system, we demonstrate that the use of two standards of CO in air of known but differing δC and δO isotopic composition allows standard uncertainties of 0.18‰ and 0.48‰ to be achieved for δC and δO measurements, respectively. The calibration strategies were validated using a set of five traceable primary reference gas mixtures. These standards, produced with whole air or synthetic air covered the mole fraction range of (378-420) μmol mol and were prepared and/or value assigned either by the National Institute of Standards and Technology (NIST) or the National Physical Laboratory (NPL). The standards were prepared using pure CO obtained from different sources, namely, combustion; Northern Continental and Southern Oceanic Air and a gas well source, with δC values ranging between -35‰ and -1‰. The isotopic composition of all standards was value assigned at the Max Planck Institute for Biogeochemistry Jena (MPI-Jena).
Abstract. Ozone plays a crucial role in tropospheric chemistry, is the third largest contributor to greenhouse radiative forcing after carbon dioxide and methane and also a toxic air pollutant affecting human health and agriculture. Long-term measurements of tropospheric ozone have been performed globally for more than 30 years with UV photometers, all relying on the absorption of ozone at the 253.65 nm line of mercury. We have re-determined this cross-section and report a value of 11.27 × 10 −18 cm 2 molecule −1 with an expanded relative uncertainty of 0.86 % (coverage factor k = 2). This is lower than the conventional value currently in use and measured by Hearn (1961) with a relative difference of 1.8 %, with the consequence that historically reported ozone concentrations should be increased by 1.8 %. In order to perform the new measurements of cross-sections with reduced uncertainties, a system was set up to generate pure ozone in the gas phase together with an optical system based on a UV laser with lines in the Hartley band, including accurate path length measurement of the absorption cell and a careful evaluation of possible impurities in the ozone sample by mass spectrometry and Fourier transform infrared spectroscopy. This resulted in new measurements of absolute values of ozone absorption cross-sections of 9.48×10 −18 , 10.44×10 −18 and 11.07 × 10 −18 cm 2 molecule −1 , with relative expanded uncertainties better than 0.7 %, for the wavelengths (in vacuum) of 244.06, 248.32, and 257.34 nm respectively. The cross-section at the 253.65 nm line of mercury was determined by comparisons using a Standard Reference Photometer equipped with a mercury lamp as the light source. The newly reported value should be used in the future to obtain the most accurate measurements of ozone concentration, which are in closer agreement with non-UV-photometry based methods such as the gas phase titration of ozone with nitrogen monoxide.
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The use of isotope dilution mass spectrometry (IDMS) to make measurements of amount of substance that are traceable to the International System of Units (SI) depends on an analysis of possible sources of uncertainty. A comprehensive application is presented of the law of propagation of uncertainty to the two-way IDMS method, which allows sensitivity coefficients to be calculated and optimum regimes identified for carrying out IDMS measurements. This has allowed the effect of deviations from "exact matching" on the uncertainty of the measurement to be evaluated for the first time. Furthermore, the relationship between traceability and uncertainty has been investigated for the two-way IDMS method, which is shown to have the potential to be a primary ratio method and to provide traceability to the SI. Our analysis shows that proper consideration of correlation in the uncertainty calculation and appropriate experimental design can reduce the requirement for highly characterized spike reference materials.
A redetermination of the argon mole fraction in air has been undertaken in two samples of dried natural air using mass spectrometric analysis with reference to a suite of gravimetrically prepared synthetic dry air mixtures. The resulting measurement of the argon mole fraction was 9.332 mmol mol −1 with a combined standard uncertainty of 3 µmol mol −1 . This is significantly different from the value, 9.17 mmol mol −1 , conventionally employed in the CIPM formula for the determination of the density of moist air during mass standard comparisons. Using the presently reported argon mole fraction value in the CIPM formula rather than the conventional value removes the recently identified discrepancy between the two methods of determining the density of moist air during mass standard comparisons: the CIPM formula method and the air buoyancy artefacts method. Nitrogen, oxygen and carbon dioxide mole fractions in the dry air samples were obtained simultaneously.
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