On the use of different spectral windows in DOAS evaluations: Effects on the estimation of SO2 emission rate and mixing ratios during strong emission of Popocatépetl volcano

- Choice of spectral window for retrieval of SO2 in volcanic plumes is discussed.
- Accuracy of the retrieved SO2 column densities is found to be strongly influenced.
- Calculation of SO2 emission rate is found to be strongly influenced.
- A simple algorithm to select the most appropriate retrieval window is proposed.
- The algorithm is suitable for the evaluation of large datasets.

Reported SO2 emission values at strongly degassing volcanoes might be underestimated. Scanning spectroscopic instruments are widely used at volcanoes to quantify SO2 emission rates and, occasionally, molar ratios between SO2 and other gases such as BrO. Frequently, Differential Optical Absorption Spectroscopy is applied to evaluate spectra in the ultraviolet range. If radiative transfer effects are not considered in the evaluations, the retrieved column densities may vary depending on which spectral window is used. We compare the use of different windows for data collected at Popocatépetl volcano during strong degassing periods, when high column densities lead to especially high dependence on the applied evaluation window. We propose the use of three different windows (310-322 nm, 314.7-326.7 nm and 322-334 nm) and a simple algorithm to choose the most accurate column density from them. The SO2 evaluation using proper spectral windows would allow a more realistic estimation of the SO2 emission rates.