Plinian eruptions as a potential source of black carbon in the stratosphere

Abstract

Black carbon (BC) aerosol particles, when being in sufficient amount in the Earth’s atmosphere, significantly influence its radiation balance and intensively deplete ozone by heterogeneous chemical reactions. There are many sources of BC in the troposphere: bio- and fossil-fuel burning, forest and bush fires, power generation and industrial processes, engines, etc. Aircraft emissions are widely assumed to be the principal source of BC nanoparticles in the stratosphere. In this paper, we make a hypothesis that Plinian eruptions with VEI ≥ 5 are a strong occasional source of long-lived BC in the stratosphere and suggest a possible mechanism of BC formation in volcanic conduits during the eruptions. Based on this hypothesis, and using the 1991 Pinatubo eruption (VEI = 6) as an example, we roughly estimate CBC and number concentration of BC particles NBC, which are required to cause ozone depletion events in the tropical stratosphere (at altitudes where ozone depletion cannot occur due to the presence of volcanic sulfuric acid aerosols). For this purpose, we use vertical ozone profiles obtained over the tropical station Hilo (19.72° N, 155.11° W; Hawaii) in September and November 1992. The September CBC and NBC values, we calculated for BC particles of mean radius 120 nm at an altitude of ~20.5 km, are 35.4 ±30.6 ng/m3 and (2.2±1.9)×106 particles/m3, respectively. The November CBC and NBC values for BC particles of mean radius 40 nm at an altitude of ~30 km are 25.3 ±7.8 ng/m3 and (4.2±1.3)×107 particles/m3, respectively.