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Scanning spectrometer networks using scattered solar radiation in theultraviolet spectral region have become an increasingly important tool formonitoring volcanic sulfur dioxide (SO2) emissions. Often measuredspectra are evaluated using the differential optical absorption spectroscopy(DOAS) technique. In order to obtain absolute column densities (CDs), theDOAS evaluation requires a Fraunhofer reference spectrum (FRS) that is freeof absorption structures of the trace gas of interest. For measurements atvolcanoes such a FRS can be readily obtained if the scan (i.e. series ofmeasurements at different elevation angles) includes viewing directions wherethe plume is not seen. In this case, it is possible to use these viewingdirections (e.g. zenith) as FRS. Possible contaminations of the FRS by theplume can then be corrected by calculating and subtracting an SO2offset (e.g. the lowest SO2 CD) from all viewing directions of therespective scan. This procedure is followed in the standard evaluations ofdata from the Network for Observation of Volcanic and Atmospheric Change(NOVAC). While this procedure is very efficient in removing Fraunhoferstructures and instrumental effects it has the disadvantage that one cannever 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 SO2retrieval algorithm using a modelled FRS. In this paper, we presentresults from two volcanoes that are monitored by NOVAC stations and whichfrequently emit large volcanic plumes: Nevado del Ruiz (Colombia) recordedbetween January 2010 and June 2012 and from Tungurahua (Ecuador) recordedbetween January 2009 and December 2011. Instrumental effects were identifiedwith help of a principal component analysis (PCA) of the residual structuresof the DOAS evaluation. The SO2 retrieval performed extraordinarilywell with an SO2 DOAS retrieval error of1 − 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 theSO2 within one scan. The major advantage of our new retrieval is thatit yields absolute SO2 CDs and that it does not require complicatedinstrumental calibration in the field (e.g. by employing calibration cellsor broadband light sources), since the method exploits the informationavailable in the measurements. We compared our method to an evaluation that is similar to the NOVACapproach, where a spectrum that is recorded directly before the scan is usedas an FRS and an SO2 CD offset is subtracted from all retrieved dSCDin the scan to correct for possible SO2 contamination of the FRS. Theinvestigation showed that 21.4 % of the scans (containing significant amountsof SO2) at Nevado del Ruiz and 7 % of the scans at Tungurahua showedmuch larger SO2 CDs when evaluated using modelled FRS (more than afactor of 2). For standard evaluations the overall distribution of theSO2 CDs in a scan can in some cases indicate whether the plumeaffects all viewing directions and thus these scans need to be discarded forNOVAC emission rate evaluation. However, there are other cases where this isnot possible and thus the reported SO2 emission rates would beunderestimated. The new method can be used to identify these cases and thusit can considerably improve SO2 emission budgets

atmospheric measurement techniques

Retrieval of absolute SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; column amounts from scattered-light spectra: implications for the evaluation of data from automated DOAS networks