BrO/SO₂ molar ratios from scanning DOAS measurements in the NOVAC network

Abstract. The molar ratio of BrO to SO₂ is, like other halogen/sulfur ratios, a possible precursor for dynamic changes in the shallow part of a volcanic system. While the predictive significance of the BrO/SO₂ ratio has not been well constrained yet, it has the major advantage that this ratio can be readily measured using the remote-sensing technique differential optical absorption spectroscopy (DOAS) in the UV. While BrO/SO₂ ratios have been measured during several short-term field campaigns, this article presents an algorithm that can be used to obtain long-term time series of BrO/SO₂ ratios from the scanning DOAS instruments of the Network for Observation of Volcanic and Atmospheric Change (NOVAC) or comparable networks. Parameters of the DOAS retrieval of both trace gases are given. The influence of co-adding spectra on the retrieval error and influences of radiative transfer will be investigated. Difficulties in the evaluation of spectroscopic data from monitoring instruments in volcanic environments and possible solutions are discussed. The new algorithm is demonstrated by evaluating data from the NOVAC scanning DOAS systems at Nevado del Ruiz, Colombia, encompassing almost 4 years of measurements between November 2009 and end of June 2013. This data set shows variations of the BrO/SO₂ ratio several weeks prior to the eruption on 30 June 2012.