Abstract. Fourier transform infrared (FTIR) spectroscopy is increasingly used as a tool forspectra of certified SO2 mixtures in order to assess the sensitivity of such instruments and the retrieval procedures used for field spectra. We investigate the effects on retrievals of the SO2 v•+v3 band (centered at 2499.87 cm ) of varying the retrieval spectral window and background fit, and of errors in the instrument line shape (ILS), temperature, and pressure. Largest deviations in retrieved amounts result from errors in the ILS (1.5-2.1%) and temperature (2.9-3.0% for a 10 K change). The total error estimate associated with these factors is similar to the uncertainty in line parameter data. Overall, the retrieval accuracy was better than 5%, except for the lowest concentration spectra. Errors calculated in the retrieval algorithm were conservative enough to cover these accuracy li2mits We estimate that in field spectra, SO2 concentrations above 2.5 x 10 •7 molecules cm-(1 (30 ppm rn at room temperature and pressure) should be measurable with high accuracy. These results are encouraging in the context of deployment of open-path FTIR spectrometers for surveillance of active volcanoes, and the findings are equally applicable to other open-path measurements of SO2, for example from anthropogenic sources.

No abstract

Abstract. An intercomparison of four Fourier transform infrared (FTIR) spectrometers, operated side by side by Jet Propulsion Laboratory (JPL), National Center for Atmospheric Research, and National Physical Laboratory groups, using two different spectral fitting algorithms, was conducted at JPL's Table Mountain Facility (TMF) during November 1996. A "blind" comparison of retrieved vertical column amounts, of preselected trace gases in preselected microwindows (mw), and subsequent reanalysis of the results are described. The species analyzed are N2 (3 mw), HF (1 mw), HC1 (1 mw), CH4 (1 mw), 03 (2 mw), N20 (2 mw), HNO3 (2 mw), and CO2 (1 mw). The column agreements from the "blind" phase were within 0.5-2%, except that for HNO3, HF, and 03 the disagreement of the results was up to 10%, 5%, and 4%, respectively. It was found that several systematic effects were neglected in the "blind" phase analysis. Taking these into account in the postanalysis reduced the disagreements to 0.5-1.0% for most cases, and to less than 4%, 3%, and 1% for HNO3, HF, and 03 respectively. It was concluded that zero offsets caused by detector nonlinearity were the main cause of the large errors in HNO3 and other gases (i.e., CO2) retrieved from the HgCdTe spectra. At shorter wavelengths (i.e., HF) we conclude that incomplete modeling of the instrument line shapes (ILS) was the main cause of column differences larger than 1%.

of the observed column ratio (C1ONO2 + HC1)/HF as an indicator for chlorine activation. Current measurement uncertainties limit the degree of activation which can be unambiguously detected using this observed quantity, but we can determine that chlorine-activated air was observed above Aberdeen (58øN) on 6 days in late

A formal intercomparison of atmospheric total column measurements of N 2 O, N 2 , CH 4 , O 3 , HCl, HNO 3 , and HF by two ground-based solar Fourier transform infrared (FTIR) spectrometers conducted as part of the Network for the Detection of Stratospheric Change (NDSC) instrument certification procedure at Lauder, New Zealand, is presented. The two instruments were nominally very similar, collocated, and collected data at the same times. Collected spectra were analyzed independently by the individual operators in a blind-phase intercomparison, then reanalyzed by a single operator using identical analysis methods to eliminate any potential bias from the spectral analysis. From the consistent reanalysis, gases with predominantly tropospheric distributions and pressure-broadened spectral lines, such as N 2 O and CH 4 , showed differences between retrieved columns of typically less than 1%. For predominantly stratospheric gases, such as HCl and O 3 , differences were less than 3%. In most cases, the differences were greater than the scatter in the individual measurements and were significant at the 95% confidence level. The worst case observed was for HF, which showed a 7% systematic bias between instruments. The differences are consistent in magnitude with those expected for known types of imperfection in spectrometer alignment and operation, but attempts to quantify these effects through instrument line shape analysis, phase error, zero offsets, and channel spectra did not remove the apparent differences.