Final report on international comparison CCQM-K74: Nitrogen dioxide, 10 µmol/mol

Abstract

There is a high international priority attached to activities which reduce NOx in the atmosphere. The current level of permitted emissions is typically between 50 µmol/mol and 100 µmol/mol, but lower values are expected in the future. Currently, ambient air quality monitoring regulations also require the measurement of NOx mole fractions as low as 0.2 µmol/mol. The production of accurate standards at these levels of mole fractions requires either dilution of a stable higher concentration gas standard or production by a dynamic technique, for example one based on permeation tubes.The CCQM-K74 key comparison was designed to evaluate the level of comparability of National Metrology Institutes' measurement capabilities and standards for nitrogen dioxide (NO2) at a nominal mole fraction of 10 µmol/mol.The measurements of this key comparison took place from June 2009 to May 2010.Seventeen laboratories took part in this comparison coordinated by the BIPM and VSL. The key comparison reference value was based on BIPM measurement results, and the standard measurement uncertainty of the reference value was 0.042 µmol/mol.This key comparison demonstrated that the results of the majority of the participants agreed within limits of ±3% relative to the reference value. The results of only one laboratory lay significantly outside these limits. Likewise this comparison made clear that a full interpretation of the results of the comparison needed to take into account the presence of nitric acid (in the range 100 nmol/mol to 350 nmol/mol) in the cylinders circulated as part of the comparison, as well as the possible presence of nitric acid in the primary standards used by participating laboratories.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Ozone Cross-Section Measurement by Gas Phase Titration

“…When anchoring the ozone titration to NO 2 standards, a difficulty arises from the stability and purity of the mixture when NO 2 is at the micromole per mole level. As observed during the international comparison CCQM-K74, HNO 3 at nanomole per mole levels is very commonly observed as an impurity in such mixtures, which would result from a reaction between NO 2 and traces of water inside cylinders . All three standards were therefore analyzed by FTIR as explained below.…”

“…As observed during the international comparison CCQM-K74, HNO 3 at nanomole per mole levels is very commonly observed as an impurity in such mixtures, which would result from a reaction between NO 2 and traces of water inside cylinders. 21 All three standards were therefore analyzed by FTIR as explained below.…”

Accurate Fourier Transform Infrared (FT-IR) Spectroscopy Measurements of Nitrogen Dioxide (NO2) and Nitric Acid (HNO3) Calibrated with Synthetic Spectra

“…In order to verify the consistency of the synthetic calibration method results against SI traceable mole fraction values, four NO 2 mixtures produced by the BIPM-NO 2 primary gas facility with measured values that were SI traceable were quantified by MALT+CLS+HITRAN 2004 . The BIPM facility was able to produce NO 2 gas mixtures in the range 5 to 15 μmol mol −1 with a relative standard uncertainty of 0.4% and has been fully characterized by Flores et al 1,22 The consistency of values was evaluated in terms of a difference ( Diff ) defined as…”

“…The second uncertainty source, instrument noise at a constant HNO 3 mole fraction, was estimated to result in an uncertainty contribution of only 3 nmol mol −1 and therefore negligible. Due to limitations in gas consumption imposed by the measuring protocols 22 the gas mixtures were measured only for 200 min. As described above, this period was not sufficient to achieve the necessary wall treatment process, and therefore the uncertainty arising from insufficient passivation was applied, u stab_HNO3 = u pass ( x HNO3 ) = 20 nmol mol −1 .…”

“…This work in contrast includes a sensitivity study in which the influence of all input parameters on the resulting mole fraction as calculated by MALT is considered, as well as the uncertainty in the values of line strengths included in HITRAN. This approach was validated in the determination of the mole fraction of NO 2 in N 2 at nominally 10 μmol mol −1 in reference mixtures generated at the BIPM facility, 22 and then adapted for the quantification of HNO 3 mole fractions.…”