Volcanic eruptions stratospheric ozone

There are numerous natural contributors of chlorine to the stratosphere, for example, volcanic eruptions. The main concern regarding ozone destruction in recent years is associated with human activities that have increased chlorine and other synthetic chemical input into the stratosphere. At the top of the list of such chemicals are chlo-rofluorocarbons, or CFCs. CFCs are compounds that contain carbon, chlorine, and fluorine they were first developed in 1928. Common CFCs are called Freons, a trade name coined by the DuPont chemical company. CFC compounds are nonreactive, nontoxic, inflammable gases. Because of their... 

There are several reasons for the dramatic ozone destruction (see Fig. 2.17) low temperatures may have prolonged the presence of polar stratospheric clouds, which play a key role in the ozone destruction, the polar vortex was very stable, there were increased sulfate aerosols from the 1991 Mount Pinatubo volcanic eruption, which also contribute to heterogeneous chemistry, and chlorine levels had continued to increase. These issues are treated in more detail shortly. 

In short, the heterogeneous chemistry that drives the Antarctic ozone hole can occur not only on solid surfaces but also in and on liquid solutions containing combinations of HN03, H2S04, and HzO. As discussed in the following section, it is believed that this is why volcanic eruptions have such marked effects on stratospheric ozone on a global basis. 

The finding that the heterogeneous chemistry that occurs on polar stratospheric clouds also occurs in and on liquid solutions in the form of liquid aerosol particles and droplets in the atmosphere provided a key link in understanding the effects of volcanic eruptions on stratospheric ozone in both the polar regions and midlatitudes. As discussed herein, the liquid particles formed from volcanic emissions are typically 60-80 wt% H2S04-H20, and hence the chemistry discussed in the previous section can also occur in these particles (Hofmann and Solomon, 1989). We discuss briefly in this section the contribution of volcanic emissions to the chemistry of the stratosphere and to ozone depletion on a global scale. For a brief review of this area, see McCormick et al. (1995). 

Tie, X., and G. Brasseur, The Response of Stratospheric Ozone to Volcanic Eruptions Sensitivity to Atmospheric Chlorine Loading, Geophys. Res. Lett., 22, 3035-3038 (1995). 

It is important to understand the sources and loss mechanisms of stratospheric sulfate aerosols. These aerosols are linked to the decrease in ozone at mid-latitudes because they hydrolyse N2O5, reducing the amount of NOx that would otherwise limit the efficiency of chlorine-catalysed ozone depletion. In addition these aerosols scatter light, cooling the planet [127]. Their concentration increases dramatically following major volcanic eruptions however they are always present at background levels. The source of these background aerosols is a matter of debate. In 1976 Paul Crutzen presented the idea that sulfate aerosols result from the photolysis of carbonyl sulphide [128] ... 

The satellite-borne Microwave Sounding Unit (MSU) records lower stratospheric temperatures. Global mean increases of up to 1.4 K were apparent following both the Pinatubo and El Chichon eruptions due to the local heating of the volcanic aerosol (Parker et al., 1996). Interestingly, for the Pinatubo case, the temperature anomaly decreased as aerosol sedimented back into the troposphere, and, by early 1993, below average, lower stratospheric temperatures were observed. This could be due to cooling coincident with the destruction of stratospheric ozone (Section 3.04.6.2.2). 

Tabazadeh A. and Turco R. P. (1993) Stratospheric chlorine injection by volcanic eruptions HCl scavenging and implications for ozone. Science 260, 1082-1086. 

The observed trends in the NAM towards higher indices may have resulted from human-induced changes in the temperature structure of the lower stratosphere in response to greenhouse gas emissions and ozone depletion (Shindell et al., 2001). The radiative effects of solar activity and of volcanic eruptions may also have produced NAM- like signatures detectable at the Earth s surface. The response of the Earth system to climate forcing may therefore involve changes in particular dynamical modes, and hence the human influence on climate at the Earth surface may occur in part by way of the stratosphere. 

In many cases, the NOx family is formed as the sum of NO and N02, and accounts for the most reactive nitrogen species. The NOx/ NOy concentration ratio, which is often reported from field observations, is an indicator of the reactivity of odd nitrogen and its ability to destroy stratospheric ozone (or to affect other chemical families including chlorine and bromine compounds). The value of this ratio increases with altitude above 30 km to reach a value of nearly one in the upper stratosphere and mesosphere. It decreases substantially when the stratospheric aerosol load is enhanced, for example, after large volcanic eruptions (Fahey et al, 1993), and substantial amounts of nitrogen oxides are converted to nitric acid by heterogeneous reaction (5.152). It is also low in the polar regions, especially in air masses processed by polar stratospheric clouds. 

It should be noted that the reaction sequences which convert S02 to sulfate do not modify the HOx budget (McKeen et al., 1984). The intrusion of large amounts of S02 in the stratosphere during volcanic eruptions could enhance the short-term amount of ozone in the middle atmosphere by direct chemical effects (Crutzen and Schmailzl, 1983 Bekki et al., 1993) through the following mechanism ... 

Jaeger, H., and K. Wege, Stratospheric ozone depletion at northern midlatitudes after major volcanic eruptions. J Atmos Chem 10, 273, 1990. 

Tie, X.X., and G. Brasseur, The response of stratospheric ozone to volcanic eruptions Sensitivity to atmospheric chlorine loading. Geophys Res Lett 22, 3035, 1995. 

Estimates have been made of the annual emissions of HCl, HF, and SO2 from volcanic eruptions to the tropo- and strato-spheres. The results indicate that man-made chlorofluorocarbons are potentially more important in stratospheric chemistry than halides of volcanic origin. Lovelock has listed the concentration and concentration profiles of halocarbons in the troposphere and the lower stratosphere over the U.K. and the mid-Atlantic he has also estimated the total quantity of chloro-species transferred to the stratosphere by halocarbon sources. The interest in such materials, and especially in chlorofluorocarbons, e.g. CFCI3 and CF2CI2, used as propellants and refrigerants, is that they are photolysed to give chlorine atoms in the stratosphere the Cl then destroys ozone by reactions (part of the CIO cycle) such as (1). 

Volcanic injection of large quantities of sulfate aerosol into the stratosphere offers the opportunity to examine the sensitivity of ozone depletion and species concentrations to a major perturbation in aerosol surface area (Hofmann and Solomon 1989 Johnston et al. 1992 Prather 1992 Mills et al. 1993). The increase in stratospheric aerosol surface area resulting from a major volcanic eruption can lead to profound effects on C10 -induced ozone depletion chemistry. Because the heterogeneous reaction of N205 and water on the surface of stratospheric aerosols effectively removes N02 from the active reaction system, less N02 is available to react with CIO to form the reservoir species C10N02. As a result, more CIO is present in active CIO cycles. Therefore an increase in stratospheric aerosol surface area, as from a volcanic eruption, can serve to make the chlorine present more effective at ozone depletion, even if no increases in chlorine are occurring. 

The tremendous force of a volcanic eruption carries a sizable amount of gas into the stratosphere. There SO2 is oxidized to SO3, which is eventually converted to sulfuric acid aerosols in a series of complex mechanisms. In addition to destroying ozone in the stratosphere (see p. 779), these aerosols can also affect climate. Because the... 

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