Gas composition of Popocatépetl Volcano between 2007 and 2008: FTIR spectroscopic measurements of an explosive event and during quiescent degassing W. Stremme a, ⁎, I. Ortega a , C. Siebe b , M. Grutter a a Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico b Departamento de Vulcanología, Instituto de Geofísica, Universidad Nacional Autónoma de México, Mexico City, Mexico abstract article info Article history: Received 30 August 2010 Received in revised form 15 November 2010 Accepted 18 November 2010 Available online 13 December 2010 Editor: R.W. Carlson Keywords: volcanic gas Vulcanian explosion cooling remote sensing FTIR spectroscopy solar absorption SiF4 HF SO2 HCl Popocatépetl volcano On December 1, 2007, the solar absorption infrared spectra of the Popocatépetl volcanic plume was recorded during an eruptive event and complementarily on November 17, 2008, the passive quiescent degassing was measured from the same site. A portable FTIR spectrometer with a scanning mirror for fast tracking of the sun provided the flexibility, quality, and simplicity needed for field deployment. Slant columns of the gases SO 2 , HCl, HF, and SiF 4 were retrieved and strong differences could be observed when comparing gas ratios in both time periods. During the explosive eruption, the SO 2 /HCl ratio was three times greater and the HF/HCl ratio was slightly smaller than during passive degassing. While the ratios among SO 2 , HCl, HF, and SiF 4 describe the chemical composition of the volcanic gas mixture, the SiF 4 /HF ratio provides information about the equilibrium temperatures of the stored gases which in this study were calculated at 150° and 185 °C for the explosive and quiescent degassing episodes, respectively. We conclude that cooling of lava domes in the crater precedes Vulcanian explosions as suggested by Schaaf et al (2005). Based on SO 2 flux (Grutter et al., 2008) and measurements and data from the November 2008 event, the average fluxes for HCl, HF, SiF 4 , and F through quiescent degassing are estimated to be 204, 22.7, 9.8, and 31.7 tons/day, respectively. These values are similar to those reported by Love et al. (1998) more than 10 yrs ago. © 2010 Elsevier B.V. All rights reserved. 1. Introduction 1.1. Popocatépetl Popocatépetl (smoking mountain in Nahuatl language) is a stratovolcano (19.02°N, 98.62°W, 5465 m a.s.l.) located 60 km southeast of Mexico City and is known to be one of the world's largest SO 2 sources (Grutter et al., 2008). During the last 15,000 yrs several high-magnitude Plinian erup- tions have occurred at Popocatépetl and ejected ash and pumice has even reached the area today occupied by Mexico City (Arana-Salinas et al., 2010; Siebe et al., 1996, 1999). After almost one century of dormancy the volcano started to erupt again since 1994 (Goff et al., 1998). This activity has been characterized by short periods of dome growth accompanied by explosive activity (Macías and Siebe, 2005) that has produced columns of ash separated by much longer and almost permanent periods of strong quiescent passive degassing. The passive degassing has an accumulated SO 2 emission over time that compares to the amount of SO 2 produced during the Pinatubo Plinian eruption of 1991 (Delgado-Granados et al., 2001), which had an impact on global climate. The ash emitted by Popocatépetl in small, short-lived explosions with moderate frequency has been of great concern, especially for the aviation industry. The behavior of the volcano is continuously monitored by various parameters measured by the National Center for Disaster Prevention (CENAPRED) and the Geophysics Institute of UNAM. Its SO 2 flux is estimated periodically by COSPEC measurements and more continu- ously by DOAS measurements as well as during special projects such as the MILAGRO international campaign, in which various methods (scanning DOAS, airborne DOAS, and imaging infrared spectrometry) were combined to estimate its flux (Grutter et al., 2008). The plume composition described as HCl, HF, and SiF 4 to SO 2 molecule ratios was reported from FTIR measurements during the initial eruptive period between 1996 and 1997 more then one decade ago (Goff et al., 2001; Love et al., 1998). Earth and Planetary Science Letters 301 (2011) 502–510 ⁎ Corresponding author. E-mail address: stremme@atmosfera.unam.mx (W. Stremme). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.11.032 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl