6533b7d6fe1ef96bd1266ec4

RESEARCH PRODUCT

Retrieval of absolute SO<sub>2</sub> column amounts from scattered-light spectra – Implications for the evaluation of data from automated DOAS Networks

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Abstract. Scanning Differential Optical Absorption Spectroscopy (DOAS) networks using scattered solar radiation have become an increasingly important tool for monitoring volcanic sulphur dioxide (SO2) emissions. In order to get 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. At volcanoes, this requirement can be formulated in a weaker form, if there is a plume free viewing direction within the spectra of a scan through the complete sky. In this case, it is possible to use a specific viewing direction (e.g. zenith) as FRS and correcting for possible plume contamination in the FRS 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) is of great advantage. We followed this approach and investigated an SO2 retrieval algorithm using a modelled FRS. In this manuscript, we present results from two volcanoes that are monitored by NOVAC stations and which also often show a large volcanic plume. Results from a DOAS SO2 evaluation using a modelled FRS are presented for data from 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/cm2]. Compared to a regular evaluation (using a FRS recorded directly before the scan), 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, since the method exploits the information available in the measurements.