Contribution of C1- and F-bearing gases to the atmosphere by volcanoes

Abstract

As halogen gases catalyse the destruction of ozone in the stratosphere^1–3, it is important to quantify the natural emissions of halogens from active volcanoes. Here we use equilibrium thermodynamics to predict the speciation of Cl and F in volcanic gases and provide new estimates of the global emission rates to the atmosphere. Our calculations show that HCl and HF are the dominant species of Cl and F in volcanic gases, at least several orders of magnitude more abundant than all other species. We estimate the annual global volcanic fluxes of HCl and HF to be 0.4–11 Tg (10¹² g) and 0.06–6 Tg, respectively. On average, <10% of these emissions come in large explosive eruptions that transmit them efficiently to the stratosphere. Although they are infrequent, large volcanic eruptions may inject significant amounts of HCl and HF into the stratosphere. Passively degassing volcanoes are also a major source of tropospheric HF, although sea salt is the main source of tropospheric HC1.