Evolution of the stratospheric aerosol in the northern hemisphere following the June 1991 volcanic eruption of Mount Pinatubo: Role of tropospheric‐stratospheric exchange and transport

Abstract

Since the eruption of Mount Pinatubo in June, 1991, measurements of particle size and concentration have intermittently been carried out from an ER‐2 aircraft at altitudes of up to 21 km at midlatitudes and high latitudes in the northern hemisphere. They show the evolution and purge of the volcanic aerosol to be due to an interaction of aerosol mechanics with tropospheric‐stratospheric exchange processes, transport, and mixing. During the first 5 months after the eruption the volcanic plume spread to higher latitudes in laminae and filaments, producing steep spatial gradients in the properties of the stratospheric aerosol. At the same time the concentration of newly formed particles in the plume rapidly decreased toward background values as a result of coagulation while particle size and aerosol surface area continued to increase. By December 1991, the particle number mixing ratios and aerosol surface area mixing ratios had become spatially uniform over a wide range of latitudes above 18 km. The surface area mixing ratios peaked in this region of the stratosphere at ∼35 times their background values in the winter of 1992. The corresponding condensed mass mixing ratio enhancement was by a factor of ∼200. After the winter of 1992, a gradual removal of the volcanic mass began and initially was dominated by sedimentation above 18 km. The aerosol surface area mixing ratio thus decreased by an order of magnitude over 2.5 years, and the aerosol volume, or condensed mass, mixing ratio decayed by an order of magnitude over approximately 1.7 years. Below 18 km, the purging of the Pinatubo aerosol at mid‐latitudes appeared sporadic and disorderly and was strongly influenced by episodal rapid quasi‐isentropic transport and dilution by tropical air of tropospheric origin having high condensation nuclei mixing ratios but low mixing ratios of aerosol surface area or condensed mass compared to the volcanic aerosol.