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

Abstract

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.

Measurements of volcanic SO2 and CO2 fluxes by combined DOAS, Multi-GAS and FTIR observations: a case study from Turrialba and Telica volcanoes

“…Independent studies quantifying the error budget of the gas concentration retrievals have estimated that for a SO 2 concentration of 250 ppm, the choice of the background polynomial and inaccuracies in the instrumental line shape account for an uncertainty of 1.2 and 1.5 %, respectively (Horrocks et al 2001). Similarly, Smith et al (2011) estimated that for a CO 2 concentration of 280 ppm, the choice of the polynomial and FOV variations of the instrument account for an uncertainty of 1.7 and 5.2 %, respectively.…”

Retrieval of sulfur dioxide from a ground-based thermal infrared imaging camera

“…In these applications single field-ofview (FoV), medium-spectral-resolution (6-0.5 cm −1 ) interferometers are used to gather quantitative information on multiple gas species simultaneously. Fourier-transform interferometers (FT-IRs) have become a very valuable device for volcanic gas studies (Love et al, 1998;Oppenheimer et al, 1998;Burton et al, 2000;Horrocks, 2001), including measurements of gas ratios reported by Oppenheimer et al (2002). Systems using ultraviolet light as a source have recently been developed for volcanic SO 2 measurements (McGonigle, 2005;Horton et al, 2006), for volcanic BrO measurements (Bobrowski et al, 2003), and also for CO 2 slantpath columns (Goff et al, 2001).…”

Line-averaging measurement methods to estimate the gap in the CO<sub>2</sub> balance closure – possibilities, challenges, and uncertainties

“…This value for total uncertainty seems to be high compared to active OP-FTIR investigations (Horrocks et al, 2001;Smith et al, 2011). The uncertainty in the temperature difference between background and considered atmospheric gas compound could be identified as the main error source for the passive measurements, and a threshold of 2 K for data filtering was defined.…”

“…These doubts in the true ILS of the applied spectrometer can lead to uncertainties in smoothing of spectral information and later on in concentration determination errors. Horrocks et al (2001) estimated a concentration retrieval error of about 2 % due to an ILS uncertainty by measuring defined gas concentrations under fixed conditions. However, recent investigations concerning the sensitivity of OP-FTIR retrievals by Smith et al (2011) point out that using a broader spectral feature for concentration retrieval is suitable for the minimization of the effect of ILS on individual absorption lines.…”

“…Infrared spectral data are mainly controlled by the environmental conditions such as pressure and temperature variations. Horrocks et al (2001) demonstrated that especially temperature has a significant impact on retrieval error and is an important parameter under consideration for subsequent data processing. The challenge in determining gas concentration using passive OP-FTIR under conditions with changing temperatures was described by Cieszczyk (2014).…”