The hypothesis of volcanogenic soot: a critical analysis and possible constructive additions

It is known that after powerful volcanic eruptions, long-term temperature and ozone anomalies are observed in the stratosphere. An explanation of these effects is possible if we assume that the stratospheric aerosol contains highly dispersed particles of soot (black carbon) formed in the eruptive column (the so-called hypothesis of volcanogenic soot by Zuev et al., 2014). A prerequisite for confirming this hypothesis should be reliable evidence of the detection and accumulation of soot particles in the stratosphere after powerful (Plinian) eruptions. However, a thorough analysis of high-altitude aircraft and balloon measurements does not confirm any noticeable post-volcanic enrichment of the stratosphere with soot particles constantly present in very small concentrations. Thus, a contradiction arises between the obvious possibility of the formation of synthesized black carbon particles in explosive-type eruptions (according to the hypothesis) and the absence of convincing traces of them in stratospheric aerosols. The proposed report presents the authors' point of view on the possible resolution of this contradiction. It is shown that explosive volcanic eruptions are necessarily characterized by a two-phase flow in the eruption column above the fragmentation level (volcanic gases plus pyroclasts), and not a single-phase flow of volcanic gases only (as assumed in the hypothesis). On highly heated pyroclastic fragments, the catalytic decomposition reactions of volcanic methane can occur with the formation of various forms of restored black carbon. Known data on the detection of various forms of black carbon in the ashes and tephra of Kamchatka and Kuril Island volcanoes correspond to the conclusions of the two-phase volcanic flow model. Thus, black carbon can be formed in explosive volcanic eruptions, but it is realized not in the form of highly dispersed particles thrown into the stratosphere but in the form of various black carbon forms strongly associated with deposited pyroclastic material (ashes and tephra).