Photochemical box modelling of volcanic SO<sub>2</sub> oxidation: isotopic constraints

Abstract

Abstract. The photochemical box model CiTTyCAT is used to analyse the absence of oxygen mass-independent anomalies (O-MIF) in volcanic sulfates produced in the troposphere. An aqueous sulfur oxidation module is implemented in the model and coupled to an oxygen isotopic scheme describing the transfer of O-MIF during the oxidation of SO2 by OH in the gas-phase, and by H2O2, O3 and O2 catalysed by TMI in the liquid phase. Multiple model simulations are performed in order to explore the relative importance of the various oxidation pathways for a range of plausible conditions in volcanic plumes. Note that the chemical conditions prevailing in dense volcanic plumes are radically different from those prevailing in the surrounding background air. The first salient finding is that, according to model calculations, OH is expected to carry a very significant O-MIF in sulfur-rich volcanic plumes and, hence, that the volcanic sulfate produced in the gas phase would have a very significant positive isotopic enrichment. The second finding is that, although H2O2 is a major oxidant of SO2 throughout the troposphere, it is very rapidly consumed in sulfur-rich volcanic plumes. As a result, H2O2 is found to be a minor oxidant for volcanic SO2. According to the simulations, oxidation of SO2 by O3 is negligible because volcanic aqueous phases are too acidic. The model predictions of minor or negligible sulfur oxidation by H2O2 and O3, two oxidants carrying large O-MIF, are consistent with the absence of O-MIF seen in most isotopic measurements of volcanic tropospheric sulfate. The third finding is that oxidation by O2∕TMI in volcanic plumes could be very substantial and, in some cases, dominant, notably because the rates of SO2 oxidation by OH, H2O2 and O3 are vastly reduced in a volcanic plume compared to the background air. Only cases where sulfur oxidation by O2∕TMI is very dominant can explain the isotopic composition of volcanic tropospheric sulfate.

VolKilau: Volcano Rapid Response Balloon Campaign during the 2018 Kilauea Eruption

“…Coincident measurements of SO2 and aerosols near the vent of volcano likely reveal the interaction between water vapor, SO2 and aerosols (Roberts et al, 2018). In-cloud SO2 decreased rapidly probably because of different aqueous pathways leading to the formation of sulfate either from the oxidation of H2O2 (Carn et al, 2011), O3 and/or the TMI-O2 pathway (Galeazzo et al, 2018) even if we cannot rule out potential measurement artifacts of SO2 in high RH conditions. The vog was observed downwind from Fissure 8 across the Pacific Ocean.…”

“…Roberts et al (2019) recently showed through kinetic-based simulations that high temperature chemistry quickly forms oxidants such as OH, HO2 and H2O2 that can lead to the production of sulfate within a few second after the emission of SO2 starts. The aqueous pathways for sulfate formation have been studied with photochemical model including isotope markers showing that TMI (Transition Metal Ion)-O2 could indeed dominate the formation of sulfate (Galeazzo et al, 2018) relative to other pathways through H2O2 and O3, which can be rapidly titrated at low pH. In addition, the production of sulfate through new particle formation has been shown to be more frequent under the influence of volcanic emissions compared to background level conditions (Rose et al, 2019, Boulon et al, 2011, Sahyoun et al, 2019.…”

“…Finally, halogen chemistry leading to ozone depletion has been observed in volcanic plumes (Surl et al, 2015) that could affect oxidation processes. Therefore, measurements in fresh and aged volcanic plumes are extremely important to understand the different pathways of SO2 oxidation from OH in gas-phase and/or H2O2, O3 and O2 in liquid phase (Galeazzo et al, 2018).…”

Spatial and Temporal Variations in SO2 and PM2.5 Levels Around Kīlauea Volcano, Hawai'i During 2007–2018

“…The lifetime of SO 2 in the lower troposphere is generally considered to be on the order of 1-3 days to a week (Allen et al, 2002;Rotstayn and Lohmann, 2002;Pfeffer et al, 2006a;Pattantyus et al, 2018), the rate of conversion depending on relative humidity and temperature, the availability of oxidants, and interaction with cloud or fog (Saxena and Seigneur, 1987;Oppenheimer et al, 1998). However, the SO 2 oxidation pathways in a volcanic cloud are not necessarily the same as under background conditions (Galeazzo et al, 2018). Through a variety of reaction pathways (including oxidation with the hydroxyl radical, OH, and with hydrogen peroxide, H 2 O 2 , and O 3 ), SO 2 in volcanic clouds is gradually converted to sulfate aerosol (Stockwell and Calvert, 1983;Allen et al, 2002), which is a dominant component of volcanic PM 2.5 (Tam et al, 2016;Pattantyus et al, 2018).…”

Large-scale particulate air pollution and chemical fingerprint of volcanic sulfate aerosols from the 2014–2015 Holuhraun flood lava eruption of Bárðarbunga volcano (Iceland)

“…However, sulfate aerosols reduce the nucleation rate of ice crystals, affecting the properties of the ubiquitous upper troposphere cirrus clouds that play a critical role in climate (Kuebbeler et al, 2012). The properties of low-altitude meteorological clouds, their formation, lifetime and precipitation can be also substantially affected by the presence of volcanic sulfate aerosols in the lower troposphere, that are issued from persistent passive degassing activity (Gassó, 2008;Schmidt et al, 2012;Ebmeier et al, 2014) or from effusive eruptions (Yuan et al, 2011;McCoy and Hartmann, 2015;Malavelle et al, 2017).…”