Volcanos produce a complicated mixture of rocks, ash, small particles, and gases. Rocks and large ash particles fall out of the volcanic plume rapidly near the location of the volcano. Smaller particles are removed more slowly but are still insignificant as material for injection into the stratosphere. However, the gases released may have an impact on stratospheric ozone.
Volcanic gases are formed during the melting of rock within the magma chamber. Most of the measurements of the composition of these gases are made in noneruptive vents called fumaroles. This is because they can be sampled most easily. The few measurements on volcanic eruptions tend to be on the non-explosive type such as Kilauea crater in Hawaii, a photo of which is shown in Figure 10.12. These measurements show that about 98% of the gas emitted is water vapor. Of the remaining gases, the most significant element present is sulfur, emitted as hydrogen sulfide (H2S) and sulfur dioxide (SO2) in about equal amounts. Any chlorine and fluorine emitted tends to be in the reduced forms of hydrochloric acid (HCl) and hydrofluoric acid (HF). These generally make up on the order of one or two tenths of a percent of the total gas emitted but their amounts can be highly variable.
Measurements are much more difficult on explosive volcanos. Massive eruption columns can reach well into the stratosphere, such as occurred in the 1991 eruption of Mt. Pinatubo in the Philippines, shown spectacularly in the photo of Figure 10.13. These eruption columns contain a complex mixture of gases, ash, and other debris. By the time the column reaches the stratosphere, most of the large debris and ash particles have fallen out of the cloud. What is left is a mixture of smaller particles, which then fall out in a few weeks, and gases which can have residence times on the order of 1-2 years.
Because measurements of the gases emitted in explosive volcanos are difficult, if not impossible, our knowledge of them is extrapolated from experience measuring nonexplosive volcanos, enhanced by measurements of the gases in the stratosphere soon after the eruption. Measurements in the stratosphere after the eruption of El Chichon in Mexico in 1982 showed an enhancement in HCl of about 20-30% at midlatitudes in the lower stratosphere. Measurements after the Mt. Pinatubo eruption, which was about three times larger than El Chichon, showed no enhancement at all.
The likely explanation for the absence of significant chlorine from the Mt. Pinatubo eruption lies in the solubility of HCl in water. The eruption cloud generates rapid updraft and storms with rain, which may remove many soluble gases and small particles. In addition, the Mt. Pinatubo eruption nearly coincided with the passage of a typhoon over the Philippines. It is still not completely understood why the El Chichon eruption caused a measurable increase in stratospheric chlorine while the Mt. Pinatubo eruption did not.
Sulfur dioxide is the most important of the gases emitted from explosive volcanos as far as the stratosphere is concerned. Sulfur can account for about 1% of the gases emitted, and sulfur dioxide about half of that. It is estimated that El Chichon deposited about 7 million tons of SO2 into the stratosphere, while Mt. Pinatubo deposited about 20 million tons.
SO2 is oxidized in the stratosphere to sulfuric acid (H2SO4) which coalesces into small particles or aerosols. This takes about 30 days after the injection of the SO2 into the stratosphere. These particles are generally much smaller than those originally emitted by the volcano. They act more like gases in that their fall velocity (rate of descent) is small and turbulent motions are sufficient to keep them in the same air mass with any remaining gas from the volcano. These aerosol particles can enhance the formation of polar stratospheric clouds, key players in the formation of the Antarctic ozone hole.
By far the largest continuous sources of volcanic gases to the atmosphere are the numerous vents and fumaroles. Because chlorine is emitted primarily in the soluble form HCl, these do not represent a significant source of chlorine for the stratosphere.
Kilauea crater and its much larger counterpart, Mauna Loa, represent a type of nonexplosive volcano that erupts frequently with large lava flows. They sit on top of the ocean vent which is responsible for the formation of the Hawaiian Island chain. Kilauea is one of the locations from which a significant number of the measurements on the gaseous composition of volcanic emissions are made (see Figure 10.12).
Mt. Erebus in Antarctica is another nonexplosive volcano that is frequently in eruption. Because of its high altitude (~4km) and location in Antarctica, Erebus has been frequently cited as a possible source for the chlorine that is active in formation of the Antarctic ozone hole. However, its eruption plumes are generally confined to altitudes of about a half a kilometer above the volcano and are thus entirely in the troposphere. The general downward motion of air over the polar vortex results in little of the chlorine emitted from the volcano ever reaching the stratosphere.
Explosive volcanic eruptions can inject material directly into the stratosphere to altitudes greater than 30 km. Any chlorine that is included in the volcanic mixture will be deposited in the stratosphere at these high altitudes. Although it is soluble, the HCl will not be rained out quickly because the stratosphere is very dry and tropospheric storm clouds do not reach these altitudes. The chlorine content of eruptions is extremely variable. Little is known of the specifics of the chlorine content of historical eruptions. As was seen above, the El Chichon eruption in 1982 caused a measurable but small increase in stratospheric chlorine that lasted for a year or two, while the Mt.. Pinatubo eruption in 1991 apparently had no effect at all on stratospheric chlorine.
5.5.1 Historical Frequencies and Eruption Magnitudes -- The historical record of volcanic eruptions has to be pieced together from diverse sources of anecdotal data. This has been done by H.H. Lamb in his book "Climate: Present, past, and future". He lists the known explosive eruptions from 1680 through 1970 and makes an estimate of their effect on the stratosphere. He uses a measure called the "Dust Veil Index" or DVI. The DVI is a good measure for comparing volcanic eruptions in the past because it is an estimate of the most obvious stratospheric effect of the injection of volcanic material (with subsequent aerosol production).
The presence of dust in the stratosphere leads to long lasting spectacular sunsets that are often purple. These will last for months to a year or two after the eruption. Perhaps the most dramatic example in recorded history was the 1815 eruption of the volcano Tambora in Indonesia. The following year, 1816, was known as "the year without a summer" around the globe because of the dramatic cooling induced by sulfate aerosols in the atmosphere. Another significant eruption was Krakatau, near Java, in 1883.
Lamb's DVI for the period 1680 to 1970 is shown in Figure 10.14. The period from about 1750 to the late 1800s was one of intense explosive volcanic activity. The period from about 1910 to 1960 was one of little or no explosive volcanic activity. The Agung eruption in Bali in 1963 ended this quiescent period for explosive volcanic activity.
5.5.2 Recent Large Eruptions -- Subsequent to the time period shown by Lamb's DVI, the largest explosive volcanic eruptions have been El Chichon in Mexico in 1982 and Mt. Pinatubo in the Philippines in 1991. El Chichon was of about the same magnitude as Agung in 1963, while Mt. Pinatubo has been estimated at three times larger. For these eruptions, we have much more detail on the formation and spread of aerosols. The Stratospheric Aerosol and Gas Experiment (SAGE) instruments on satellites have been measuring stratospheric aerosol amounts globally since 1979. The surface area of aerosols at a specific height in the stratosphere as a function of latitude and time is shown in Figure 10.15.