What Has Been Learned About The Physical And Chemical Processes That Lead To Ozone Depletion?

New Studies Of The Catalytic Destruction Of Ozone In The Lower Stratosphere Suggest A Reordering Of The Catalytic Cycles

Recent in-situ observations (from ER-2 flights) suggest that nitrogen radicals may not be as dominant in the catalytic destruction of ozone in the lower stratosphere as previously thought. In addition, observations from the AASE II and SPADE airborne field experiments have shown that sulfate aerosols have quantitative effects on NOx chemistry in the lower stratosphere. These findings suggests a reordering of the catalytic cycles with primary importance on the reaction of ozone with HO2, and increased importance of halogen-radicals (ClO and BrO) over that of NOx in ozone depletion. In order to assess the environmental impacts on the ozone layer of anthropogenic emissions, such as nitrogen oxide effluent from supersonic transports, further research will be necessary to extend this quantitative ordering of ozone destruction cycles to a larger range of altitudes, latitudes and seasons and to understand how the system behaves in more homogeneous chemistry of the middle stratosphere.
Reference: Removal of Stratospheric O3 by Radicals: In Situ Measurements of OH, HO2, NO, NO2, ClO, and BrO, Wennberg, P.O., et al., Science, Vol. 266, pp. 398-404, 1994.

Laboratory Studies Confirm That Hydrofluorocarbon Substitutes For CFCs Pose A Negligible Threat As A Catalyst For Ozone Loss In The Stratosphere

As chlorofluorocarbons (CFCs) are phased out in compliance with the Montreal Protocol and its amendments, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have been proposed as substitutes. Many HFCs contain the functional group CF3, which was considered to have a possible catalytic role in ozone depletion. Laboratory investigations have shown that the reaction cycles involving the CF3 fragment are unimportant in the stratosphere and hence HFCs containing that functional group contain negligibly small ozone depletion potentials (ODP) (less than 0.0001). The results confirm that these compounds satisfy the requirements of the Clean Air Act Amendments that halocarbon substitutes have an ODP less than 0.2.

A combination of laboratory and computational studies has also verified that the Ozone Depletion Potential (ODP) of HFC-134a is very small, demonstrating that emissions of this substance destroy far less ozone than the CFCs it replaces. This puts to rest a recent hypothesis to the contrary, thus avoiding what would have been an unfounded halt in the use of this HFC in automobile air conditioners.

Reference: Do Hydrofluorocarbons Destroy Stratospheric Ozone?, Ravishankara, A.R., et al., Science, Vol. 263, pp. 71-74, 1994.

New Substitute For Ozone-Depleting Substances

Since the interim solutions, HCFCs (hydrochlorofluorocarbons), will be phased out in the future, attention needs to be paid to the environmental and economic consequences of any proposed substitutes. Computer modeling, laboratory chemical and physical property evaluations, and performance evaluations of substitutes for HCFCs will be continued through future studies. These substitutes include pure chemicals, azeotropes, zeotropes, and non-chemical alternatives (also called not-in-kind substitutes). A major concern is the potential effects of substitutes on climate-related parameters.
References: (1) Predictions of Azeotropes from Fluorinated Ethers, and Propanes, Gage, C. L. and Kazachi, G. S., Proceedings of the 1992 International Refrigeration Conference, Purdue University, 1993; (2) Investigation of HFC-236fa as CFC-114 Replacements in High Temperature Heat Pumps, Kazachki, G. S., Gage, C. L., Hendriks, R. V. and Rhodes. W. J., 1994. Proceedings of the International Conference CFC the Day After, Padova, Italy, 21-23, September 1994; (3) Evaluation of HFC-245fa as a potential Alternative for CFC-11 in Low Pressure Chillers, Smith, N.D. et al., Proceedings of the 1994 International CFC and Halon Alternatives Conference, Washington, D.C., 24-26, October, 1994.

Stratospheric Nitric Acid Explains Interhemispheric Differences In Polar Ozone Depletion

The importance of denitrification in the catalytic destruction of ozone has become evident through a long sequence of ground-based, balloon-borne, and aircraft-borne studies. The presence of nitric acid (HNO3) in the polar lower stratosphere affects the cumulative amount of chlorine-catalyzed ozone destruction in two major ways. First, in the low temperatures of polar winter, HNO3 condenses to form polar stratospheric clouds (PSC) which trigger conversion of stratospheric chlorine to chemically-reactive forms that destroy ozone. Second, photolysis of HNO3 releases NO2 which quenches ClO and reduces ozone destruction. Removal of stratospheric HNO3 either temporarily through condensation or permanently through the sedimentation of PSC particles, reduces the availability of NO2 and allows chlorine to remain activated longer.

The Microwave Limb Sounder (MLS) aboard the Upper Atmospheric Research Satellite (UARS) has provided the first-time ever hemispheric observations of HNO3 spanning the full winter season. Previously, nitric acid concentrations were either inferred from other in situ data or measured in snapshots from airborne instruments. MLS measurements of HNO3, along with ClO and O3 give a global set of simultaneous commonly calibrated data directly addressing the relationship between nitric acid and ozone depletion by chlorine chemistry. A sizable decrease in HNO3 is observed over Antarctica by early June and remains through November, during which time the Antarctic ozone hole forms. Similar losses of HNO3 in the Arctic are less intense, more transient and localized. However, future cooling of the stratosphere due to increasing concentrations of greenhouse gases could lead to increased denitrification over the Arctic, leading to greater ozone depletion there.

Reference: Interhemispheric Differences in Polar Stratospheric HNO3, H2O, ClO, and O3, Santee, M.L., et al., Science, Vol. 267, pp. 849-852, 1995.

Climatology Of Polar Stratospheric Clouds Determined From Satellite Data

A climatology of polar stratospheric cloud (PSC) occurrence was determined using data from the Stratospheric Aerosol Measurement II (SAM II) instrument on the Nimbus 7 spacecraft over the period 1978 to 1989. PSCs are important because they provide the catalytic surfaces mainly responsible for high latitude ozone-depleting chemistry. The SAM II measures atmospheric extinction by using the solar occultation technique, and obtains data at high latitudes in both the Arctic and Antarctic. This climatology utilizes data from all years in this time period except times strongly perturbed by the El Chichon eruption in 1982 (Arctic in 1982, 1983; Antarctic in 1983). The PSC season in the Antarctic was shown to run from mid-May to early November, with a maximum zonal average PSC probability of 0.6 occurring in August at 18-20 km. In the Arctic, the PSC season is appreciably shorter, running from late November to early March, with a maximum probability of occurrence of 0.1 in early February at 20-22 km. There is considerable year-to-year variability in PSC occurrence in the Arctic, while in the Antarctic the main year-to-year variability is seen in the number of late season clouds. In both hemispheres the maximum frequency of PSCs occurs along the Greenwich meridian (90°E - 90°W). Temperatures associated with PSC formation remain constant over the Arctic winter, while they decline in the Antarctic in the 15-20 km region, consistent with dehydration and denitrification of the stratosphere.
Reference: Polar Stratospheric Cloud Climatology Based on Stratospheric Aerosol Measurement II (SAM II) Observations from 1978 to 1989, Poole, L. R. and M. C. Pitts, Journal of Geophysical Research, Vol. 99, pp. 13,083-13,089, 1994.

Wintertime Polar Vortices Calculated To Be Well Isolated From Mid-Latitudes

A significant uncertainty in studies of stratospheric chemistry is how much air is exchanged between the wintertime polar vortices and mid-latitudes. Since air inside the polar vortexes typically has very different chemical composition than air outside, significant transport and exchange of air could have important implications for mid-latitude ozone amounts. It is therefore critical that this exchange be quantified. Calculations by several investigators have shown exceedingly strong evidence, corroborated by several lines of observational data, that air inside the vortex is well isolated from air outside through most of the stratosphere (above approximately the 400° Kelvins potential temperature surface). In the Northern Hemisphere, the isolation of the vortex is much less complete in December and March (corresponding to vortex spin-up and breakdown, respectively) than it is in January and February, however.
References: (1) The Permeability of the Antarctic Vortex Edge, Chen, P., Journal of Geophysical Research, Vol. 99, pp. 20,563-20,571, 1994; (2) Climatology of Large-Scale Isentropic Mixing in the Arctic Winter Stratosphere from Analyzed Winds, Dahlberg, S. P. and K. P. Bowman, Journal of Geophysical Research, Vol. 99, pp. 20,585-20,599, 1994.

Calculations Show Evidence Of Strong Descent In Arctic And Antarctic Polar Vortices

Observations have shown that there is appreciable descent inside winter polar vortices in both the Arctic and Antarctic, but it is difficult to infer quantitative rates and amounts of descent, as well as to demonstrate year-to-year ranges and comparisons between hemispheres. This is important because the amount of ozone in the stratosphere depends strongly on altitude. A set of calculations carried out using two techniques quantitatively analyzed this descent in the Arctic for the winters of 1988-89 and 1991-92 and in the Antarctic for fall-winter periods in 1987 and 1992. These calculations used temperature data from the National Meteorological Center along with radiative and trajectory codes. In the Northern Hemisphere, air parcels starting on November 1 inside the vortex at altitudes of 18, 25, and 50 km, by March 21 would have descended by 6, 9, and 27 km, respectively. In the Antarctic, air parcels starting on March 1 at those same altitudes would have descended by 3, 5-7, and 26-29 km by the end of October, respectively. The results for recent years are consistent with those from the Upper Atmosphere Research Satellite (UARS).
Reference: Computations of Diabetic Descent in the Stratospheric Polar Vortices, Rosenfield, J. E. et al., Journal of Geophysical Research, Vol. 99, pp. 16,677-16,689, 1994.

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