Ozone Depletion, Chlorine, and Volcanic Eruptions
2-D Chemical Transport Model
Depletion of the ozone layer by chlorine radicals resulting from atmospheric degradation of anthropogenic chlorine-based compounds such as CFCs (chlorofluorocarbons) is a well-recognized and serious phenomenon. The AER 2-dimensional chemical transport model (CTM), developed by Dr. Nien Dak Sze, Dr. Malcolm Ko, and Debra Weisenstein, was instrumental in confirming the relationship between CFCs and global ozone depletion and helped shape policies reducing and banning the use of CFCs.
The AER 2-D CTM is a model that predicts how the ozone layer will behave through time under a variety of conditions and assumptions. It calculates concentrations of 80 chemical species in the atmosphere at 969 grid points, covering the globe from the surface to 60 km altitude. It also calculates the distribution of sulfuric acid aerosols in 40 size bins resulting from surface emissions of sulfur-bearing gases and explosive volcanic eruptions that penetrate the stratospehre. Different scenarios can be explored through model calculations, such as future emission rates of chlorine- and bromine-compounds, future concentrations of methane, changes in stratospheric temperature, or hypothesized emission of pollutants from aircraft or rockets. Results of such studies have been published in global ozone assessment reports published by the World Meteorological Organization and the Intergovernmental Panel on Climate Change.
This plot represents a calculation with the AER 2-D chemistry-transport model (CTM) illustrating the effect of explosive volcanic eruptions and anthropogenic chlorine emissions on ozone. The x-axis represents a 9 year period following a massive volcanic eruption which penetrated the middle stratosphere, such as Mt. Pinatubo which erupted in the Phillipines in June of 1991. The blue line shows the aerosol surface area density in square microns per cubic centimeter at 47N and 25 km (right-hand y-axis) resulting from conversion of the emitted sulfur dioxide to sulfuric acid aerosol particles in the stratosphere. The red line shows the evolution of total overhead ozone at 47N in Dobson units (left-hand y-axis) for the 1991-1999 period when total stratospheric chlorine abundance was at 3.2 ppbv. The green line show the evolution of total ozone for the same aerosol loading but in a future atmosphere with lower chlorine abundance of 2.5 ppbv. Volcanic eruptions that penetrate the stratosphere, a rare (once or twice a decade) though natural phenomenon, lead to stratospheric ozone depletion through heterogeneous reactions (ie. reactions between gas phase molecules and solid/liquid surfaces) on aerosol surfaces which change the balance among nitrogen, chlorine, and hydrogen radicals. The most important reaction in these high-aerosol conditions converts dinitrogen pentoxide (N2O5), a short-lived reactive compound, to nitric acid (HNO3), a longer-lived less reactive compound. With a greater fraction of nitrogen atoms as HNO3, there are fewer nitrogen atoms to react with chlorine atoms, and therefore more free reactive chlorine available to destroy ozone. The impact of volcanoes on ozone depends on the concentration of anthropogenic chlorine in the stratosphere. In the case with lower chlorine shown here, the resulting ozone depletion is much less. If there were no man-made chlorine in the stratosphere, explosive volcanic eruptions would lead to increases in ozone.
To learn more about the Ozone Layer
http://www.nas.nasa.gov/About/Education/Ozone/