Retrieval of absolute SO<sub>2</sub> column amounts from scattered-light spectra: implications for the evaluation of data from automated DOAS networks

Abstract

Abstract. Scanning spectrometer networks using scattered solar radiation in the ultraviolet spectral region have become an increasingly important tool for monitoring volcanic sulfur dioxide (SO 2 ) emissions. Often measured spectra are evaluated using the differential optical absorption spectroscopy (DOAS) technique. In order to obtain absolute column densities (CDs), the DOAS evaluation requires a Fraunhofer reference spectrum (FRS) that is free of absorption structures of the trace gas of interest. For measurements at volcanoes such a FRS can be readily obtained if the scan (i.e. series of measurements at different elevation angles) includes viewing directions where the plume is not seen. In this case, it is possible to use these viewing directions (e.g. zenith) as FRS. Possible contaminations of the FRS by the plume can then be corrected by calculating and subtracting an SO 2 offset (e.g. the lowest SO 2 CD) from all viewing directions of the respective scan. This procedure is followed in the standard evaluations of data from the Network for Observation of Volcanic and Atmospheric Change (NOVAC). While this procedure is very efficient in removing Fraunhofer structures and instrumental effects it has the disadvantage that one can never be sure that there is no SO 2 from the plume in the FRS. Therefore, using a modelled FRS (based on a high-resolution solar atlas) has a great advantage. We followed this approach and investigated an SO 2 retrieval algorithm using a modelled FRS. In this paper, we present results from two volcanoes that are monitored by NOVAC stations and which frequently emit large volcanic plumes: Nevado del Ruiz (Colombia) recorded between January 2010 and June 2012 and from Tungurahua (Ecuador) recorded between January 2009 and December 2011. Instrumental effects were identified with help of a principal component analysis (PCA) of the residual structures of the DOAS evaluation. The SO 2 retrieval performed extraordinarily well with an SO 2 DOAS retrieval error of 1−2×10 16 [molecules cm −2 ]. Compared to a standard evaluation, we found systematic differences of the differential slant column density (dSCD) of only up to ≈ 15 % when looking at the variation of the SO 2 within one scan. The major advantage of our new retrieval is that it yields absolute SO 2 CDs and that it does not require complicated instrumental calibration in the field (e.g. by employing calibration cells or broadband light sources), since the method exploits the information available in the measurements.We compared our method to an evaluation that is similar to the NOVAC approach, where a spectrum that is recorded directly before the scan is used as an FRS and an SO 2 CD offset is subtracted from all retrieved dSCD in the scan to correct for possible SO 2 contamination of the FRS. The investigation showed that 21.4 % of the scans (containing significant amounts of SO 2 ) at Nevado del Ruiz and 7 % of the Published by Copernicus Publications on behalf of the European Geosciences Union. 5678 P. Lübcke et al.: DOAS evaluation ...

SO<sub>2</sub> and BrO emissions of Masaya volcano from 2014–2020

“…Our retrieval of the SO 2 emissions fluxes is based on the standard NOVAC approach described by Johansson et al (2009) but (1) extended by a set of data filters which aim to reduce the amount of potentially problematic measurement conditions, (2) information on the plume height and the wind conditions has been assessed partially via a triangulation of NOVAC observations, and (3) the spectroscopic retrieval has been adapted to the high SO 2 emission fluxes at Masaya. One of these filters uses the absolute SO 2 background calibration method described by Lübcke et al (2016).…”

“…It has been observed, however, that the assumption of a non-contaminated background direction is not always justified (e.g. Lübcke et al, 2016). Another approach which does not rely on that assumption is the direct retrieval of SCD SO2,ref via a SO 2 DOAS fit of the zenith spectrum against a solar-atlas spectrum (see Salerno et al, 2009;Lübcke et al, 2016;Chance and Kurucz, 2010).…”

Ground-Based Remote Sensing and Imaging of Volcanic Gases and Quantitative Determination of Multi-Species Emission Fluxes

“…As illustrated by Figure 5, the plume spectra I(λ) in Equation 6are the measurements for which the lines-of-sight cross the volcanic plume. The background spectra I 0 (λ) are obtained by adjusting the observing geometry such that lines-of-sight avoid the volcanic plume or, if such an adjustment is not possible, by resorting to external sources such as solar spectral atlases [29]. In practice, this simple approach faces various complications which need to be overcome.…”

MAX-DOAS measurements of NO<sub>2</sub>, SO<sub>2</sub>, HCHO and BrO at the Mt. Waliguan WMO/GAW global baseline station in the Tibetan Plateau

“…in Venezuela (Schreier et al, 2016). The MAX-DOAS technique has been applied to monitor the absolute column densities and plumes of SO 2 from large volcano eruptions (e.g., Lübcke et al, 2016;Tulet et al, 2017), but measuring SO 2 in the background free troposphere still remains challenging.…”