Open‐path Fourier transform infrared spectroscopy of SO2: An empirical error budget analysis, with implications for volcano monitoring

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Fourier transform infrared (FTIR) spectroscopy is increasingly used as a tool for volcano monitoring, allowing measurement of a range of gases including SO2, HCl, and HF. Retrievals are complicated, since the open‐path spectra are typically pressure broadened and contaminated hy atmospheric H2O, CO2, etc., and the field instruments employed have low spectral resolution (∼0.5 cm−1). We present a detailed analysis of 0.5 cm−1 resolution infrared spectra 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 V1+v3 band (centered at 2499.87 cm−1) 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 limits. We estimate that in field spectra, SO2 concentrations above 2.5 × 1017 molecules cm−2 (100 ppm m 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.