Radiative Forcing of the Stratosphere by SO₂ Gas, Silicate Ash, and H₂SO₄ Aerosols Shortly after the 1982 Eruptions of El Chichón

The 1982 eruptions of the El Chichón volcano injected large quantities of sulfur dioxide gas and silicate ash into the stratosphere. Several studies have shown that the long-lived sulfuric acid aerosols derived from these volcanic effluents produced measurable changes in the radiative heating rates and the global circulation. The radiative and dynamical perturbations associated with the short-lived but more strongly absorbing sulfur dioxide and ash clouds have received much 1ess attention. The authors therefore used an atmospheric radiative transfer model and observations collected by satellites, aircraft, and ground-based observers to estimate the amplitudes of the stratospheric radiative heating rate perturbations produced by each of these components during the first few weeks after the El Chichón eruption. One week after the 4 April 1982 eruption, net radiative heating rate perturbations exceeding 20 K per day were found at altitudes near 26 km. The absorption of sunlight by the silicate ash accounts for the majority of this heating. The sulfur dioxide gas and sulfuric acid aerosols each produced net heating perturbations that never exceeded 3 K per day. In spite of the intense heating by the ash, observations indicate that stratospheric temperatures never increased by more than a few degrees Kelvin. The authors therefore concluded that this radiative heating was largely balanced by upwelling and adiabatic cooling. The amplitude and spatial extent of this upwelling was estimated with a diagnostic, two-dimensional dynamical model. The ash heating rates may have been balanced by a global enhancement in the stratospheric meridional circulation, with zonally averaged upward velocities of about 1 cm sec⁻¹ near the latitude of the plume. This enhanced stratospheric circulation persisted only for a few weeks but it may have played a major role in the vertical and horizontal dispersal of the plume. The vertical transport needed to balance the heating by sulfur dioxide gas was only 5%–10% as large, but this perturbation may have produced a 2-km increase in the altitude of the plume. These results suggest that the radiative forcing by the ash and the sulfur dioxide gas should be included in more comprehensive models of the plume evolution. They also suggest that particle size distributions inferred from ash fallout rates could be wrong if the upwelling associated with this radiative heating is not considered.