2-DIMENSIONAL COUPLED DYNAMICAL CHEMICAL MICROPHYSICAL SIMULATION OF GLOBAL DISTRIBUTION OF EL CHICHON VOLCANIC AEROSOLS

We have developed a coupled two-dimensional dynamical/chemical/microphysical model to study the global distribution of stratospheric sulfate aerosols. In particular, we use this model to simulate the global distribution of volcanic aerosols after the eruption of El Chichon in Mexico in April 1982. The simulated background aerosol distributions are highly dispersed, while a slight latitudinal gradient is also noticed. The calculated background aerosol surface area and mass are about 0.7 to 1.0 mum 2/cm3 and 0.3 to 0.5 parts per billion by mass, respectively, at midlatitude in the northern hemisphere, in fair agreement with available observations. After the eruption of El Chichon in April 1982, the stratospheric aerosol load rapidly increases in the tropics at an altitude of 20 to 25 km. The aerosol area in the tropics reaches a maximum 50 mum2/cm3 in the lower stratosphere, which is about 70-100 times the background aerosol area. Six months after the eruption, volcanic aerosols spread out globally but are still centered in the tropics. One year after the eruption the enhanced aerosol begins to decrease and tends to become uniformly distributed in the lower stratosphere. Two years after the eruption the global aerosol is about 5 times the background aerosol load in the lower stratosphere. The e-folding time of the aerosol load is about 10 months in the tropics during the postvolcanic period. Compared to observations (in terms of spatial, temporal, and size distributions), the model quantitatively simulates the evolution of volcanic aerosol clouds. Thus this model could be a useful tool for studying the impacts of volcanic eruptions on stratospheric ozone and climate. Moreover, we find that for a model simulation in which the gas phase SO2 is the only material ejected by the eruption, the model substantially underestimates the volcanic aerosol load. Thus we expect that the direct ejection of sulfate aerosol particles may be a very important process.