Accurate Fourier Transform Infrared (FT-IR) Spectroscopy Measurements of Nitrogen Dioxide (NO 2 ) and Nitric Acid (HNO 3 ) Calibrated with Synthetic Spectra

A novel method for determining the accuracy of laboratory-based measurements of nitrogen dioxide (NO₂) and nitric acid (HNO₃) mole fractions using Fourier transform infrared (FT-IR) spectroscopy 1 cm⁻¹ resolution instruments calibrated with synthetic spectra has been developed. The traceability of these measurement results is to the reference line strength data contained within the high-resolution transmission molecular absorption (HITRAN) database. Incorporating a proper estimate of the uncertainty of this data into the measurement results will ensure that the SI traceable values are encompassed within the uncertainty of the measurement results. The major contributors to the uncertainties of the results are, in descending order of importance, the uncertainty in the line strength values (HITRAN 2004), the uncertainty attributed to the generation of reference spectra (including knowledge of the optical path length of the FT-IR gas cell), and temperature measurements of the gas. The stability of the FT-IR instrument itself is only a minor contributor to the overall uncertainty of the measurements. FT-IR measurements of NO₂ mole fractions at nominal values of 10 μmol mol⁻¹ calibrated with synthetic spectra lead to standard uncertainties of 0.34 μmol mol⁻¹ (3.4% relative). In contrast, calibration of the FT-IR instrument with SI traceable gas standards generated by a dynamic weighing system resulted in measurements results with standard uncertainties of 0.04 μmol mol⁻¹ (0.4% relative). When comparing the consistency of measurement results based on the synthetic calibration method against those obtained by calibrations with SI traceable gas standards, the existence of a potential bias of ∼5% was observed, although this was within the stated uncertainties of the results. The FT-IR measurements of HNO₃ mole fractions at nominal values of 200 nmol mol⁻¹ calibrated with synthetic spectra resulted in values with standard uncertainties of 23 nmol mol⁻¹ (11% relative) with the dominating uncertainty in this case arising from the stabilization of the mole fraction value within the FT-IR gas cell.