Human Effects on Ozone

Human activities have had a devastating effect on the concentration and distribution of stratospheric ozone over the last few decades. Measurements of ozone by satellite and ground-based measurements over the last several decades indicate that stratospheric ozone levels have decreased. Atmospheric ozone has decreased globally by more than 5% since 1970 (see Figure 5). Atmospheric measurements have also shown that the depleted levels of ozone have indeed increased the amount of UV at the surface. The significant global decrease in stratospheric ozone since the 1970s is well correlated with increasing amounts of chlorine and bromine in the stratosphere. The sources of this chlorine and bromine are chlorofluorocar-bons (CFCs) and other halocarbons produced industrially for a variety of uses such as refrigerants in refrigerators, air conditioners, and large chillers, as propellants for aerosol cans, as blowing agents for making plastic foams, and as solvents for dry-cleaning and for degreasing of materials. Atmospheric measurements have clearly corroborated theoretical studies showing that the chlorine and bromine released from the destruction of these halocarbons in the stratosphere is reacting to destroy ozone. Figure 6 shows the excellent comparison between the observed trend in ozone based on measurements from the solar backscatter ultraviolet (SBUV) satellite instrument averaged over latitudes from 50° N to 50° S and results from the University of Illinois zonally averaged model that is based on the changes in atmospheric gases and particles. In addition to the human related emissions, natural forcings on ozone

-Ground-based data TOMS zonal means SBUV-SBUV/2 Merged satellite data NIWA assimilated dataset

I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1965 1970 1975 1980 1985 1990 1995 2000

Figure 5 Deviations in total ozone with time relative to January 1979 from various ground-based and satellite measurements (TOMS and SBUV). The data are area weighted over 90° S-90° N. Graph provided by Vitali Fioletov as an update to earlier analyses presented in Fioletov VE, BodekerGE, Miller AJ, McPeters RD, and Stolarski R (2002) Global and zonal total ozone variations estimated from ground-based and satellite measurements: 1964-2000. Journal of Geophysical Research 107: 4647 (doi: 10.1029/2001JD001350).

-Ground-based data TOMS zonal means SBUV-SBUV/2 Merged satellite data NIWA assimilated dataset

I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1965 1970 1975 1980 1985 1990 1995 2000

Figure 5 Deviations in total ozone with time relative to January 1979 from various ground-based and satellite measurements (TOMS and SBUV). The data are area weighted over 90° S-90° N. Graph provided by Vitali Fioletov as an update to earlier analyses presented in Fioletov VE, BodekerGE, Miller AJ, McPeters RD, and Stolarski R (2002) Global and zonal total ozone variations estimated from ground-based and satellite measurements: 1964-2000. Journal of Geophysical Research 107: 4647 (doi: 10.1029/2001JD001350).

Figure 6 Comparison of trend in annually averaged, observed total ozone (red dots) from 50° N-50° S based on measurements from the solar backscatter ultraviolet (SBUV) satellite instrument and the derived trend from the University of Illinois zonally averaged chemistry-transport model of the global atmosphere. The effects of the natural cycle due to the quasi-biennial oscillation have been removed from the satellite data. While the overall decline in ozone results from increases in concentrations of stratospheric chlorine and bromine from human-related emissions of various halocarbons, the cyclic variations in the observations and model results are due to the effects of the 11-year solar sunspot cycle, while the extra deep minimum in ozone in the early 1990s is due to the effects of emissions from the Mount Pinatubo volcanic eruption in 1991. The model results also show the projected recovery of ozone if the Montreal Protocol reduces halocarbon emissions and atmospheric concentrations as expected.

from the effects of the 1991 Mount Pinatubo volcanic eruption (due to sulfur emissions) and from the effects of solar flux variations during the 11-year sunspot cycle also contribute to the observed trends.

Beginning in the late 1970s, a special phenomenon began to occur in the springtime over Antarctica, referred to as the Antarctic ozone 'hole'. A large decrease in total ozone, now over 60% relative to pre-hole levels, has been observed in the springtime (September to November) over Antarctica. Dr. Joseph Farman and colleagues first documented this rapid springtime decrease in Antarctic ozone over their British Antarctic Survey (BAS) station at Halley Bay, Antarctica. These analyses attracted the attention of the scientific community, who soon found that decreases in the total ozone column were greater than 50% compared with historical values observed by both ground-based and satellite techniques. At the time, there was no expectation that such a phenomenon would be discovered. As a result of the Farman paper, a number of hypotheses arose attempting to explain the large ozone destruction in the springtime over Antarctica. It was initially proposed that the chlorine catalytic cycle might explain the observed ozone decrease, but this did not match the expected ozone decrease possible from the reactive chlorine available at the high latitudes. A special measurement campaign in 1987, as well as later measurements, proved that chlorine and bromine chemistry indeed was indeed responsible for the ozone 'hole' but because of heterogeneous reactions occurring on polar stratospheric clouds in the lower stratosphere.

The air over the Antarctic becomes extremely cold during the winter as a result of the lack of sunlight over the polar region and because of greatly reduced mixing of the lower stratospheric air over this region with air outside this region. During the winter, a circumpolar vortex, also called the polar winter vortex, forms which isolates the air in the polar region from that outside ofthe region as a result of a stratospheric jet of wind circulating between approximately between 50° S and 65° S. The extremely cold temperatures inside the vortex lead to the formation of clouds in the lower stratosphere (from roughly 12 to 22 km), called 'polar stratospheric clouds'. Heterogeneous reactions occur on these particles that convert less reactive forms of chlorine to much more reactive ones with ozone. When daylight starts to occur over Antarctica in the early spring, this chlorine is available to react with and destroy ozone. Bromine compounds and nitrogen oxides can also react heterogeneously on the particles of these clouds. The ozone destruction continues until the polar vortex breaks up, usually in November.

In the late 1980s, it was generally thought that the Arctic lower stratosphere did not get cold enough to lead to decreases in ozone during the winter and springtime like those found in the Antarctic. The polar vortex is not generally as strong in the Northern Hemisphere, and, although polar stratospheric clouds would form, they would not likely last long enough for extensive decreases in ozone. However, since 1990, ozone decreases of as much as 30% have been found in the Arctic in those years when lower-stratospheric temperatures in the Arctic vortex have been sufficiently low to lead to ozone destruction processes similar to those found in the Antarctic ozone 'hole'. As with Antarctica, large increases in concentrations in reactive chlorine have been measured in the regions where the large ozone destruction is occurring.

The recognition of the harmful effect of chlorine and bromine on ozone spawned international action to restrict the production and use of CFCs and halons and protect stratospheric ozone. These included the 1987 Montreal Protocol on substances that deplete the ozone layer, the subsequent 1990 London Amendment, the 1992 Copenhagen Amendment, and the 1997 Montreal Amendment. In the Montreal Protocol and its Amendments, there is a distinction between the control measures in developed and developing countries. These agreements initially called for reduction ofCFC consumption in developed countries. A November 1992 meeting of the United Nations Environment Program held in Copenhagen resulted in substantial modifications to the protocol because of the large observed decrease in ozone, and called for the phase-out of CFCs, carbon tetrachloride (CCl4), and methyl chloroform (CH3CCl3) by 1996 in developed countries. As part of this, the United States, through the Clean Air Act, has eliminated production and import of these chemicals. Production of these compounds is to be totally phased out in developing countries by 2006, while production of halons in developed countries was stopped in 1994. Human-related production and emissions of methyl bromide were not to increase after 1994 in developed countries, and should slowly decline with total elimination by 2005. Hydrochlorofluorocarbons (HCFCs), many of which have been used as replacements for the CFCs, still contain chlorine that can destroy ozone, are to be phased out in the developed countries by 2030.

Worldwide compliance with the international agreements to protect ozone is resulting in significant reductions in the emissions of the CFCs, halons, and other halocarbons having the largest effects on ozone; as a result, levels of stratospheric ozone should slowly begin to recover over the coming decades as the reactive chlorine and bromine in the stratosphere declines. This recovery will be gradual, primarily because of the long time it takes for CFCs and halons to be removed from the atmosphere. Atmospheric model results, such as that shown in Figure 6, suggest that the effects of halocarbons on ozone should return to 1980 ozone levels by 2040-50.

While ozone slowly recovers, scientists and policymakers will need to work together to ensure that new problems do not develop as a result of the introduction of new chemicals into the marketplace. They will also need to interact with industry and governments to ensure that the potential effects of increasing concentrations of other gases changing as a result of human activities, such as methane and nitrous oxide, do not produce their own significant impacts on ozone. Further understanding is also needed towards evaluating the potential effects on ozone from various natural events, such as volcanic eruptions and solar events, and other possible human activities, including nuclear explosions.

See also: Carbon Cycle; Climate Change 2: Long-Term Dynamics; Matter and Matter Flows in the Biosphere; Ozone Layer; Radionuclides: Their Biogeochemical Cycles and the Impacts on the Biosphere; The Significance of O2 for Biology.

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