The adsorption of HCl on volcanic ash

International audienceUnderstanding the interaction between volcanic gases and ash is important 8 to derive gas compositions from ash leachates and to constrain the environmental 9 impact of eruptions. Volcanic HCl could potentially damage the ozone layer, but it is 10 unclear what fraction of HCl actually reaches the stratosphere. The adsorption of HCl 11 on volcanic ash was therefore studied from-76 to + 150 ˚C to simulate the behavior 12 of HCl in the dilute parts of a volcanic plume. Finely ground synthetic glasses of 13 andesitic, dacitic, and rhyolitic composition as well as a natural obsidian from 14 Vulcano (Italy) served as proxies for fresh natural ash. HCl adsorption is an 15 irreversible process and appears to increase with the total alkali content of the glass. 16 Adsorption kinetics follow a first order law with rate constants of 2.13. 10-6 s-1 to 17 1.80. 10-4 s-1 in the temperature range investigated. For dacitic composition, the 18 temperature and pressure dependence of adsorption can be described by the equation 19 ln c = 1.26 + 0.27 ln p-715.3/T, where c is the surface concentration of adsorbed HCl 20 in mg/m 2 , T is temperature in Kelvin, and p is the partial pressure of HCl in mbar. A 21 comparison of this model with a large data set for the composition of volcanic ash 22 suggests that adsorption of HCl from the gas phase at relatively low temperatures can 23 quantitatively account for the majority of the observed Cl concentrations. The model 24 implies that adsorption of HCl on ash increases with temperature, probably because of 25 2 the increasing number of accessible adsorption sites. This temperature dependence is 26 opposite to that observed for SO 2 , so that HCl and SO 2 are fractionated by the 27 adsorption process and the fractionation factor changes by four orders of magnitude 28 over a temperature range of 250 K. The assumption of equal adsorption of different 29 species is therefore not appropriate for deriving volcanic gas compositions from 30 analyses of adsorbates on ash. However, with the experimental data provided here, the 31 gas compositions in equilibrium with the ash surfaces can be calculated. In particular, 32 for dacitic composition, the molar ratio of S/Cl adsorbed to the ash surface is related 33 to the molar S/Cl ratio in the gas phase according to the equation ln (S/Cl) adsorbed = 34 2855 T-1 + 0.28 ln (S/Cl) gas-11.14. Our data also show that adsorption on ash will 35 significantly reduce the fraction of HCl reaching the stratosphere, only if the initial 36 HCl content in the volcanic gas is low (< 1 mole %). For higher initial HCl 37 concentrations, adsorption on ash has only a minor effect. While HCl scavenging by 38 hydrometeors may remove a considerable fraction of HCl from the eruption column, 39 recent models suggest that this process is much less efficient than previously thought. 40 Our experimental data therefore support the idea that the HCl loading from major 41 explosive eruptions may indeed cause severe depletions of stratospheric ozone. 42 4