The effect of ash, water vapor, and heterogeneous chemistry on the evolution of a Pinatubo-size volcanic cloud

[摘要] We employ the ECHAM5/MESSy2 atmospheric chemistry general circulation model (EMAC) that incorporates calculations of gas-phase and heterogeneous chemistry coupledwith the ozone cycle and aerosol formation, transport, and microphysics to calculate the 1991 Pinatubo volcanic cloud. We considered simultaneousinjections of SO 2 , volcanic ash, and water vapor. We conducted multiple ensemble simulations with different injection configurations to testthe evolution of SO 2 , SO 4 2 - , ash masses, stratospheric aerosol optical depth, surface area density (SAD), and the stratospherictemperature response against available observations. We found that the volcanic cloud evolution is sensitive to the altitude where volcanic debrisis initially injected and the initial concentrations of the eruption products that affect radiative heating and lofting of the volcanic cloud. Thenumerical experiments with the injection of 12  Mt SO 2 , 75  Mt of volcanic ash, and 150  Mt of water vapor at20  km show the best agreement with the observation aerosol optical depth and stratospheric temperature response. Volcanic water injectedby eruptive jet and/or intruding through the tropopause accelerates SO 2 oxidation. But the mass of volcanic water retained in thestratosphere is controlled by the stratospheric temperature at the injection level. For example, if volcanic materials are released in the cold point abovethe tropical tropopause, most of the injected water freezes and sediments as ice crystals. The water vapor directly injected into the volcanic cloudincreases the SO 4 2 - mass and stratospheric aerosol optical depth by about 5 %. The coarse ash comprises 98 % of the ash injectedmass. It sediments within a few days, but aged submicron ash could stay in the stratosphere for a few months providing SAD for heterogeneouschemistry. The presence of ash accelerates the SO 2 oxidation by 10 %–20 % due to heterogeneous chemistry, radiative heating,lofting, and faster dispersion of volcanic debris. Ash aging affects its lifetime and optical properties, almost doubling the ash radiativeheating. The 2.5-year simulations show that the stratospheric temperature anomalies forced by radiative heating of volcanic debris in ourexperiments with the 20  km  injection height agree well with observations and reanalysis data. This indicates that the model captures thelong-term evolution and climate effect of the Pinatubo volcanic cloud. The volcanic cloud's initial lofting, facilitated by ash particles' radiativeheating, controls the oxidation rate of SO 2 . Ash accelerates the formation of the sulfate layer in the first 2 months after theeruption. We also found that the interactive calculations of OH and heterogeneous chemistry increase the volcanic cloud sensitivity to watervapor and ash injections. All those factors must be accounted for in modeling the impact of large-scale volcanic injections on climate andstratospheric chemistry.

[学科分类] 医学（综合）