Volcanic aerosols

Two components of volcanic emissions are of most significance for aerosols: primary particles and gaseous sulfur. Most of the particles ejected from volcanoes (dust and ash) are water insoluble mineral particles, silicates, and metallic oxides such as SiO₂, Al₂O₃ and Fe₂O₃, which remain mostly in the troposphere.

The estimated dust flux reported by the IPCC (1994) for the 1980's ranges from 4 to 10,000 Tg/yr, with a estimated average of 33 Tg/yr. This estimate represents continuous eruptive activity, and is about two orders of magnitude smaller than soil dust emission. The upper value, on the other hand, is the order of magnitude of volcanic dust mass emitted during large explosive eruptions. Volcanic sources may be important to the sulfate aerosol burden in the upper troposphere, where they might act as condensation nuclei for ice particles and thus represent a potential for a large indirect radiative forcing. Support for this contention lies in evidence of cirrus cloud formation from volcanic aerosols and some data that links the interannual variability of high level clouds with explosive volcanoes. Volcanic eruptions can, however, have a large impact on stratospheric aerosol loads. Volcanic emissions sufficiently cataclysmic to penetrate the stratosphere are rare. The stratospheric lifetime of coarse particles (dust and ash) is only about 1-2 months due to the efficient removal by settling. Nevertheless, the associated transient climatic effects are large and trends in the frequency of volcanic eruptions could lead to important trends in average surface temperature. Sulfur emissions from volcanoes have a longer lived effect on stratospheric aerosol loads. Sulfur emissions from volcanoes occur mainly in the form of SO₂ , even though other sulfur species may be present in the volcanic plume, predominantly SO₄^2- aerosols and H₂S. It has been estimated that the amount of SO₄^2- and H₂S is commonly less than 1% of the total, although it may in some cases reach 10%-20%. Nevertheless, H₂S oxidizes to SO₂ in about 2 days in the troposphere or 10 days in the stratosphere. Estimates of the emission of sulfur containing species from quiescent degassing and eruptions range from 7 T to 14 Tg-S/yr. These estimates are highly uncertain because only very few of the potential sources ever have been measured and the variability between sources and between different stages of activity of the sources is considerable. The observed sulfate load in the stratosphere is about 0.14 Tg-S during volcanically quiet periods. The historical record of SO₂ emissions by erupting volcanoes shows that over 100 Tg of SO₂ can be can be emitted in a single event, such as the Tambora volcano eruption of 1815. Calculations with a global climate models suggest that the radiative effect of volcanic sulfate is only slightly smaller than that of anthropogenic sulfate, even though the anthropogenic SO₂ source strength is about five times larger. The main reason is that SO₂ is released from volcanoes at higher altitudes has a longer residence time than anthropogenic sulfate.

A case study: The eruption of Mount Pinatubo

On June 15, 1991, Mount Pinatubo in the Philippines erupted with a tremendous force, ejecting vast amounts of ash and gas high into the atmosphere; so high that the volcano's plume penetrated into the stratosphere. Pinatubo injected about 15 million tons of sulfur dioxide into the stratosphere, where it reacted with water to form sulfuric acid droplets.

Over the course of the next two years strong stratospheric winds spread these aerosol particles around the globe. The Pinatubo eruption increased aerosol optical depth in the stratosphere by a factor of 10 to 100 times normal levels measured prior to the eruption. Three months after the 1991 Mt. Pinatubo eruption in the Philippines, scientists found that the stratospheric region at latitudes near Mt. Pinatubo had warmed 2.5-3.0°C due to the increased concentrations of aerosols. Over the next 15 months, scientists measured a drop in the average global temperature of about 10°F. The image to the left shows the 1.02 µm stratospheric optical depth observed by SAGE II just after the Pinatubo eruption (June-July 1991).  

To the right, a 20 year time series of the vertical profile of aerosol extinction at 1.0 µm obtained by SAGE I and II shows the dramatic increase in upper tropospheric and stratospheric aerosols between 60° to 40° after the eruption of Mount Pinatubo in the summer of 1991.