Understanding the interaction between volcanic gases and ash is important to derive gas compositions from ash leachates and to constrain the environmental impact of eruptions. Volcanic HCl could potentially damage the ozone layer, but it is unclear what fraction of HCl actually reaches the stratosphere. The adsorption of HCl on volcanic ash was therefore studied from-76 to + 150 ˚C to simulate the behavior of HCl in the dilute parts of a volcanic plume. Finely ground synthetic glasses of andesitic, dacitic, and rhyolitic composition as well as a natural obsidian from Vulcano (Italy) served as proxies for fresh natural ash. HCl adsorption is an irreversible process and appears to increase with the total alkali content of the glass. Adsorption kinetics follow a first order law with rate constants of 2.13. 10-6 s-1 to 1.80. 10-4 s-1 in the temperature range investigated. For dacitic composition, the temperature and pressure dependence of adsorption can be described by the equation ln c = 1.26 + 0.27 ln p-715.3/T, where c is the surface concentration of adsorbed HCl in mg/m 2 , T is temperature in Kelvin, and p is the partial pressure of HCl in mbar. A comparison of this model with a large data set for the composition of volcanic ash suggests that adsorption of HCl from the gas phase at relatively low temperatures can quantitatively account for the majority of the observed Cl concentrations. The model implies that adsorption of HCl on ash increases with temperature, probably because of 2 the increasing number of accessible adsorption sites. This temperature dependence is opposite to that observed for SO 2 , so that HCl and SO 2 are fractionated by the adsorption process and the fractionation factor changes by four orders of magnitude over a temperature range of 250 K. The assumption of equal adsorption of different species is therefore not appropriate for deriving volcanic gas compositions from analyses of adsorbates on ash. However, with the experimental data provided here, the gas compositions in equilibrium with the ash surfaces can be calculated. In particular, for dacitic composition, the molar ratio of S/Cl adsorbed to the ash surface is related to the molar S/Cl ratio in the gas phase according to the equation ln (S/Cl) adsorbed = 2855 T-1 + 0.28 ln (S/Cl) gas-11.14. Our data also show that adsorption on ash will significantly reduce the fraction of HCl reaching the stratosphere, only if the initial HCl content in the volcanic gas is low (< 1 mole %). For higher initial HCl concentrations, adsorption on ash has only a minor effect. While HCl scavenging by hydrometeors may remove a considerable fraction of HCl from the eruption column, recent models suggest that this process is much less efficient than previously thought. Our experimental data therefore support the idea that the HCl loading from major explosive eruptions may indeed cause severe depletions of stratospheric ozone.

Volcanic degassing provides important information for the assessment of volcanic hazards. Santa Ana and San Miguel are open vent volcanoes along the Central American Volcanic Arc–CAVA, where the magmatism, basaltic to dacitic, is related to the near-orthogonal convergence of the Caribbean Plate and the subducting Cocos Plate. Both volcanoes are the most active ones in El Salvador with recent eruptive events in October 2005 (Santa Ana) and December 2013 (San Miguel), but still not much data on gas composition and emission are available today. At each volcano, SO2 emissions are regularly monitored using ground-based scanning Differential Optical Absorption Spectrometer (Scan-DOAS) instruments that are part of the global “Network for Observation of Volcanic and Atmospheric Change” (NOVAC). We used the data series from these NOVAC stations in order to retrieve SO2 and minimum bromine emissions, which can be retrieved from the same spectral data for the period 2006–2020 at Santa Ana and 2008–2019 at San Miguel. However, BrO was not detected above the detection limit. SO2 emission ranged from 10 to 7,760 t/d, and from 10 to 5,870 t/d for Santa Ana and San Miguel, respectively. In addition, the SO2 emissions are complemented with in situ plume data collected during regular monitoring surveys (2018–2020) and two field campaigns in El Salvador (2019 and 2020). MultiGAS instruments recorded CO2, SO2, H2S and H2 concentrations. We determined an average CO2/SO2 ratio of 2.9 ± 0.6 when peak SO2 concentration exceeded 15 ppmv at Santa Ana, while at San Miguel the CO2/SO2 ratio was 7.4 ± 1.8, but SO2 levels reached only up to 6.1 ppmv. Taking into account these ratios and the SO2 emissions determined in this study, the resulting CO2 emissions are about one order of magnitude higher than those determined so far for the two volcanoes. During the two field campaigns Raschig tubes (active alkaline trap) were used to collect plume samples which were analyzed with IC and ICP-MS to identify and quantify CO2, SO2, HCl, HF, and HBr. Additionally, also 1,3,5-trimethoxybenzene (TMB)-coated denuders were applied and subsequently analyzed by GC-MS to determine the sum of the reactive halogen species (RHS: including Cl2, Br2, interhalogens, hypohalous acids). The RHS to sulfur ratios at Santa Ana and San Miguel lie in the range of 10−5. Although no new insights could be gained regarding changes with volcanic activity, we present the most comprehensive gas geochemical data set of Santa Ana and San Miguel volcanoes, leading to a solid data baseline for future monitoring purposes at both volcanoes and their improved estimate of CO2, SO2 and halogens emissions. Determining the reactive fraction of halogens is a first step towards a better understanding of their effects on the atmosphere.

&lt;p&gt;Volcanic degassing plays an important role in a volcano&amp;#8217;s behavior. Going from large emissions at craters and fumaroles, to invisible degassing at vents and soil; a volcano releases H2O, CO2, SO2, HCl, HF, H2S, CO, H2, HBr, HI, Hg and noble gases.&lt;/p&gt;&lt;p&gt;SO2 emissions are considered a basic monitoring tool, mainly measured by remote-sensing techniques. The Differential Optical Absorption Spectroscopy (DOAS) is a well-established method currently used to regularly measure volcanic SO2 emission rates with about 80 scanning DOAS operating in 37 volcanoes within the framework of the global &quot;Network for Observation of Volcanic and Atmospheric Change&quot; (NOVAC) (Galle et al., 2010). Typically, SO2&lt;sub&gt;&lt;/sub&gt;fluxes are often combined with in-situ gas measurements of SO2 and other volatiles (CO2, H2S), to evaluate the degassing regime. In-situ sampling can be made by collecting the gases directly in evacuated flasks or solution-filled bottles (alkaline traps), or by sampling with a multi-sensor instrument (MultiGAS) that enables real-time measurements of several gases at once (Aiuppa et al. 2005b; Shinohara 2005, Roberts et al. 2017).&lt;/p&gt;&lt;p&gt;Santa Ana and San Miguel are the most active volcanoes in El Salvador, with an average SO2 emission rate of 220 and 326 t/d, respectively during 2018. Both volcanoes arise along the Central American Volcanic Arc &amp;#8211; CAVA, where the magmatism, fundamentally basaltic, is related to the convergence of the Caribbean Plate and the subducting Cocos Plate (Leeman and Carr 1995). Also, Santa Ana and San Miguel are part of the NOVAC group since 2008 with just a few published gas data (Rodriguez et al. 2004, Cartagena et al. 2004, Olmos et al. 2007, Colvin et al. 2013, Laiolo et al. 2017). The most recent studies were performed by Granieri et al. 2015 and Hasselle et al. 2019. The first, reported CO2/SO2, HCl/SO2 and HF/SO2 mass ratios (0.95, 0.13 and 0.016, respectively) measured at San Miguel volcano in early 2014; while the second, presented CO2/SO2, H2S/SO2 and H2O/SO2 ratios (&lt;3-37.9, 0.03-0.1 and 32-205, respectively), measured in 2017-2018 at Santa Ana&amp;#8217;s crater lake and rim.&lt;/p&gt;&lt;p&gt;In this study, we present an SO2 long-time data series (2008-2018) for San Miguel and Santa Ana obtained from the DOAS stations of each volcano, and complement with data collected during regular monitoring (2018-2020) and field campaigns in El Salvador (2019 and 2020) by means of MultiGAS devices. The aim of the study is to extend the characterization of these two volcanoes in El Salvador and the establishment of SO2 and CO2 baselines and inventories for them.&lt;/p&gt;