An uncertainty budget for ground‐based Fourier transform infrared column measurements of HCl, HF, N2O, and HNO3 deduced from results of side‐by‐side instrument intercomparisons

The results of side‐by‐side instrument intercomparisons of Fourier transform infrared (FTIR) spectrometers at Ny‐Ålesund, Spitzbergen (79°N, 12°E), during May/June 1995 and at Harestua, Norway (60°N, 10°E), during September/October 1994 are reported. The spectrometers were operated simultaneously recording atmospheric spectra in the midinfrared using the Sun as a source. The differences in vertical columns of HCl, HF, N2O, and HNO3 measured simultaneously by different instruments were as large as 5.8, 7.7, 2.8, and 4.3%, respectively, having mean absolute values 0.5, 1.6, 1.0, and 1.6%, respectively. These results were used to derive 1σ overall uncertainties of 9.7% for HCl, 7.7% for HF, 6.4% for N2O, and 14.3% for HNO3. Separate uncertainties are quoted for the precision of daily column measurements which exclude the contribution due to uncertainty in the line parameters. These are 7.7% for HCl, 5.7% for HF, 4.9% for N2O, and 6.8% for HNO3. These numbers are estimated for the local conditions at the intercomparison sites but will differ from site to site depending on the knowledge of local atmospheric conditions at the time of measurement.