Scanning spectrometer networks using scattered solar radiation in the ultraviolet spectral region have become an increasingly important tool for monitoring volcanic sulfur dioxide (SO₂) 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 SO₂ offset (e.g. the lowest SO₂ 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 SO₂ 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 SO₂ 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 SO₂ retrieval performed extraordinarily well with an SO₂ DOAS retrieval error of 1 − 2 × 10¹⁶ [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 SO₂ within one scan. The major advantage of our new retrieval is that it yields absolute SO₂ 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 SO₂ CD offset is subtracted from all retrieved dSCD in the scan to correct for possible SO₂ contamination of the FRS. The investigation showed that 21.4 % of the scans (containing significant amounts of SO₂) at Nevado del Ruiz and 7 % of the scans at Tungurahua showed much larger SO₂ CDs when evaluated using modelled FRS (more than a factor of 2). For standard evaluations the overall distribution of the SO₂ CDs in a scan can in some cases indicate whether the plume affects all viewing directions and thus these scans need to be discarded for NOVAC emission rate evaluation. However, there are other cases where this is not possible and thus the reported SO₂ emission rates would be underestimated. The new method can be used to identify these cases and thus it can considerably improve SO₂ emission budgets.