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

Abstract

The photochemical box-model CiTTyCAT is used to analyse the absence of oxygen mass-independent anomalies (O-MIF) in volcanic sulphates produced in the troposphere. An aqueous sulphur oxidation module is implemented in the model and coupled to an oxygen isotopic scheme describing the transfer of O-MIF during the oxidation of SO 2 by OH in the gas-phase, and by H 2 O 2 , O 3 and O 2 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 5 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 sulphur-rich volcanic plumes and, hence, that the volcanic sulphate produced in the gas phase would have a very significant positive isotopic enrichment. The second finding is that, although H 2 O 2 is a major oxidant of SO 2 throughout the troposphere, it is very rapidly consumed in sulphur-rich volcanic plumes. As a result, H 2 O 2 is found to be a 10 minor oxidant for volcanic SO 2 . According to the simulations, oxidation of SO 2 by O 3 is negligible because volcanic aqueous phases are too acidic. The model predictions of minor or negligible sulphur oxidation by H 2 O 2 and O 3 , two oxidants carrying large O-MIF, are consistent with the absence of O-MIF seen in most isotopic measurements of volcanic tropospheric sulphate.The third finding is that oxidation by O 2 /TMI in volcanic plumes could be very substantial and, in some cases, dominant, notably because the rates of SO 2 oxidation by OH, H 2 O 2 , and O 3 are vastly reduced in a volcanic plume compared to the 15 background air. Only cases where sulphur oxidation by O 2 /TMI is very dominant can explain the isotopic composition of volcanic tropospheric sulphate.

Reaction Rates Control High-Temperature Chemistry of Volcanic Gases in Air

“…The sulfate:SO 2 ratios typically range from 0.00002 up to around 0.01 mol/mol, Table 1 and references therein. Many of the ratios are too high to be explained by atmospheric oxidation of SO 2 at low-temperatures on the plume transport timescale of seconds to minutes (e.g., Galeazzo et al, 2018). Instead, the observations indicate a sulfate-rich aerosol formed very close to source.…”

“…Unusually, at Erebus volcano a rapid loss of SO 2 has been identified in the very young plume possibly aided by cloud processing (Oppenheimer et al, 2010). However, oxidation of SO 2 in volcanic plumes is typically slow (e.g., Galeazzo et al, 2018). The measurements of CO/SO 2 in the downwind Eyjafjallajökull plume do not show any clear dependency on plume age.…”

“…These measurements have eluded any explanation given the lack of photochemistry, a cooled plume, and anticipated fast HO x destruction by reaction with volcanic SO 2 and halogens (both in excess abundances). In general, the reported elevated abundances of HO xy in volcanic plumes compared to the background atmosphere are surprising because low-temperature volcanic plume chemistry is expected to deplete HO xy (Galeazzo et al, 2018). Volcanic HO xy may originate from high-temperature chemistry of the near-source plume.…”

Advances in Bromine Speciation in Volcanic Plumes

“…Assuming slow in-plume oxidation, SO 2 will act as a plume tracer on the relatively short observation timescales (typically minutes to hours) of measurements on the volcano flank. Indeed, volcanic plume SO 2 can often be considered to be conserved over the short distance between "source" and "downwind" observation points on the volcano flank because the acidity of the volcanic aerosol in the plume, prevents the SO 2 uptake, and because SO 2 concentrations are in excess of most oxidants (McGonigle, 2004;von Glasow et al, 2009;Galeazzo et al, 2018). Variations in bromine/sulfur observed at just one fixed measurement point might be a result from changes in emission composition.…”

“…Oxidation can occur by gas-phase reaction with OH, and aqueous-phase reaction with ozone, H 2 O 2 , and O 2 with transition metal ions. In volcanic plumes, catalysis by metallic ions might significantly enhance oxidation of SO 2 with O 2 (Galeazzo et al, 2018) . H 2 SO 4 is highly hygroscopic and can create aqueous H 2 SO 4(aq) droplets via homogeneous nucleation or condense onto existing accumulation mode particles (Seinfeld et al, 1998;Mather et al, 2006;Martin et al, 2012b;Galeazzo et al, 2018).…”