What=ifsfor a Pkrthern Ozone Hole BY ALAN NEWMAN 0.0 ill an ozone hole, like the one over Antarctica, form over the Arctic? “Who knows?” said J. D. Mahlman of the National Oceanic and Atmospheric Administration’s (NOAA) Geophysical Fluid Dynamics Laboratory in Princeton, NJ. However, Mahlman, who delivered the Jule Charney Award lecture at the recent American Geophysical Union meeting in Baltimore, raised several intriguing possibiliti to further signifi ozone losses over northern latitudes. “One would expect that there is more total chlorine in the northern hemisphere than in the south,” explained Mahlman. That expectation is supported by measurements *, taken from aircraft that showed higher chlorine monoxide (C10) concentrations over the Arctic than over Antarctica last winter. The C10 is formed from C1 atoms reacting with 0,. If, when sunlight returns in the spring, the C10 remains in the stratosphere (which didn’t occur over the Arctic last year), these molecules enter a photocatalytic cycle that regenerates ozone-destro ’ C1 atoms.
Ozone hole chemistry In the simplest analysis, C10 persists into spring over the southern PrMs articles are reports of meetings of unusual significance, international or national developments of environmental importance, significant public policy developments, and related items. 1488 Envr--- Sa. Technol.. Vol 27, No. 8, 1993
high latitudes because of reactions catalyzed by what is known as type I1 polar stratospheric clouds (PSCs). These clouds consist of ice crystals formed in the stratosphere. Because little water makes it to the stratosphere, PSCs form only under very cold conditions ( T < 187 K).Moreover, the air in this region is trapped in a massive polar vortex that, like a giant beaker, keeps the appropriate molecules and conditions swirling above Antarctica. Without the PSCs, C10 would simply react with NO, to form ClONO,-a relatively inert form of the halogen and a termination step in the catalytic cycle. Moreover, without the polar vortex, northsouth air gradients would allow 0, formed in lower latitudes to come flooding in and replace that which had been lost in reactions with C1. Experiments and measurements have identified the chemistry that takes place on type I1 PSCs as absorbed HC1 (formed i n another chain termination and ClONO, react and ”0,. The di re-enters the gas spring, it can be photolyzed to chlorine atoms. Alternatively, ClONO, can react with water in type I1 PSCs to form HOC1, another species that can be photodissociated to C1 atoms. Fortunately, type I1 PSCs e warmer Arctic there heterogeneous surfaces other than these that can catalyze similar ozone-depletion chemistry?
The role of catalytic surfaces Also speaking at the meeting, Margaret Tolbert of the University of Colorado at Boulder described laboratory experiments performed by her group and by others, with
models of two other potential catalytic surfaces: type I PSCs and sulfuric acid droplets. Type I PSCs are also confined to cold regions, forming in the stratosphere at temperatures < 193 K. Laboratory experiments generally model these clouds as nitric acid trihydrate (“0, . 3H,O), based on vapor pressure measurements in the laboratory. However, some studies suggest that the actual composition might be nitric acid dihydrate or a mixture of the two forms. The laboratory experiments indicate that HC1 can stick to type I PSCs and react just as quickly with ClONO, as it does in type I1 PSCs. “There is evidence [in the stratosphere] of HC1 loss when type I PSCs are around,” points out Tolbert, suggesting that these clouds are also contributing to ozone-loss chemistry. PSCs are confined by temperature to polar regions of the stratosphere but sulfuric acid droplets can form at all stratospheric temperatures. As a result, “tons of evidence” support the observation that sulfuric acid droplets are globally distributed in the stratosphere. Interest in sulfuric acid aerosols has been heightened since the June 1991 eruption of Mt. Pinatubo in the Philippines, which ejected into the stratos here more than 10 megatons (> 10P, g) of SO,. Significantly, the 1992 Antarctic autumn was too warm for appreciable formation of types I or I1 PSCs, yet investigators detected large amounts of active chlorine in the stratosphere. Researchers from NOAA’s Aeronomy Laboratory (Boulder, CO) have recently suggested that sulfate aerosols from the Philippine eruption might have been responsible for producing the active chlorine by the reaction of
0013-~~~~~9309227-1488$04.00/0 e 1993 American Chemical Society
These figures show instantaneous fields ot simulated nitrous oxide (N,O) on a pressure surface (48 mb) near an altitude of 20 km lrom the high resolution ( l o latitude by 1.2" longitude grid spacing) "SKYHI" model from NOAA's O , over the Southern Hemisphere on model date Geophysical Fluid Dynamics Laboratory. The top figure shows N , over the Northern Hemisphere on model date Jan. 10. Sept. 30; the bottom figure shows NO N,O. a biologically produced trace gas, is an excellent tracer of atmospheric motions. Both figures show very low , (cool colors) in the polar regions-remarkably so in the Arctic. These NO , fields essentially define the values of NO winter polar vortex structure that is a precursor to the chemistry that creates the ozone hole. The top figure shows a nearly circular vortex structure with relatively modest filaments of polar vortex air being shredded off the Antarctic vortex edge as defined by the strong NO , gradients. The bonom figure shows a much more disturbed and elongated Arctic vortex with lower NO , values (indicative of greater sinking and compression heating) , air are being pulled than in the Antarctic. The edge, or NzO "wall," is very steep, and multiple "streamers" of low NO away from the vortex edge to produce the pronounced spiral arm effect seen in the figure. Source: Figures provided courtesy of Susan Strahan and Jerry Mahlman.
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ClONO, with H,O on the aerosols. They also suspect that heterogeneous chemistry on the sulfate aerosols may have contributed to the unusually severe Antarctic ozone hole that winter. In the stratosphere, gaseous SO, is rapidly converted to sulfuric acid droplets, Observations indicate that these droplets are generally 6080% acid by weight, a key factor. “HC1 and ClONO, have very little solubility on dry sulfuric acid, but as the water concentration increases, solubility and reactivity increase,” says Tolbert. However, sulfuric acid droplets appear to be promoting a different reaction from that which occurs on PSCs. Laboratory studies have found that these droplets can effectively remove N,O, (formed from NO, and NO, in the atmosphere). The N,O, reacts with water in the droplet to form nitric acid. As a result, the nitrogen oxides are taken out of action and the droplets indirectly increase or maintain C10 concentrations. Investigations conducted from high-flying planes support this mechanism. Measurements of nitrogen oxides find that NO, concentrations are low relative to NO,,, which fits models invoking a heterogeneous reaction of N,O, on sulfuric acid droplets. In the lower latitudes the N,O, reaction is offset by the photodissociation of the nitric acid product back to nitrogen oxides. Over the southern polar regions, however, the sulfuric acid droplet chemistry competes with the much faster PSC chemistry. Measurements from the recently launched Upper Atmosphere Research Satellite indicate that sulfuric acid chemistry may be important around the edges of the Antarctic vortex (as well as when PSCs fail to form). Nevertheless, teasing out how much these droplets contribute to ozone loss is complicated. Sulfuric acid droplets can play another role. “The key is low temperatures,” says Tolbert. “How dilute do they become before they freeze?” To a first order, the N,O, reaction shows little dependence on temperature or droplet composition, but it halts when the droplet freezes. To make things even more complicated, new results by researchers at NOAA’s Aeronomy Laboratory find that the ClONO, + HC1 reaction occurs readily on frozen sulfuric acid. Unfortunately, the frozen drop-
lets are also suspected of seeding the formation of PSCs. “NO one knows the exact mechanism by which PSCs grow,” explains Tolbert. If the frozen droplets do seed PSCs, then observation of their formation could help to predict PSC growth and subsequent ozone depletion. “The majority of field experiments suggest that most sulfuric acid droplets are liquid,” says Tolbert, “although there is some evidence of frozen droplets.”
Right chemistry, wrong dynamics Even if the chemistry is right for a northern ozone hole, the dynamics are wrong. The northern polar vortex is less stable than that in the south. “They are different creatures,” says Mahlman. The northern vortex is very elongated with an asymmetric wind circulation that leads to “shredding” of the whirling mass, higher polar temperatures, and less vortex stability in the key spring months (see Figure 1). However, what happens if the predicted greenhouse warming becomes a reality? According to Mahlman several things might changeall for the worse. As CO, levels increase, heat is trapped in the lower atmosphere, which in turn leads to a cooler stratosphere. As a result, more PSCs can form. (The cooling effect can be quite pronounced; following the ClausiusClapeyron equation, a four-degree drop in temperature means a twofold lowering of the saturation vapor pressure for water.) At the same time, the greenhouse effect could increase the amount of water in the stratosphere. Most of the water in the atmosphere is effectively confined to the lower atmosphere by the tropopause-the boundary layer betweep the troposphere and the stratosphere. Here temperatures plummet to as low as 190 K, and water moving upward is caught in an efficient cold trap. Above the tropopause, water vapor levels drop by a factor of about 10,000. Greenhouse warming is predicted to create a warmer tropical tropopause that could increase the amount of water reaching the stratosphere to generate more PSCs. Furthermore, continued increases of another greenhouse gas, methane, might also foster PSC formation by providing the stratosphere with a second source of water, Unlike water, methane has a free ride through the tropopause to the stratosphere. Once methane is in the upper atmosphere, solar UV
light and OH radicals can convert this hydrocarbon to water. Thus water vapor concentration is again increased and the temperature of PSC formation rises. Finally, through a complex sequence of processes, the predicted increasing greenhouse effect could indirectly lead to a colder wintertime Arctic stratosphere, and again more PSC formation. In the current Northern Hemisphere, large-scale weather disturbances leak a portion of their energy to the less dense stratosphere. The dissipation of these disturbances into the winter stratosphere indirectly leads to the descent of air over the Arctic region. This descent produces compression heating of the polar cap. Thus, the more energetic the disturbances, the warmer the winter Arctic region. This process explains why the Arctic currently lacks a pronounced ozone hole and why one forms over Antarctica. The higher Northern Hemisphere mountain ranges act to produce larger disturbances than in the Southern Hemisphere, thereby keeping the Arctic winter stratosphere warm enough to evade the cold temperatures that would produce widespread PSCs and the associated significant ozone destruction. “The bizarre twist,” says Mahlman, “is that dynamics, or rather the lack of them, give the south permission to form the ozone hole.” In a greenhouse-warmed Earth, mathematical climate models predict a warmer, wetter lower atmosphere with a reduced north-south temperature gradient. This combination of features acts, in the model at least, to produce weaker tropospheric large-scale disturbances and consequently less energy leakage to the stratosphere. The reduction of this effect acts indirectly to make colder winter temperatures in the Arctic stratosphere. If realized, this scenario would create more PSCs and further accelerate ozone destruction. The result of these potential greenhouse-produced changes is an increase in the likelihood of a northern ozone hole. With chlorofluorocarbons now rapidly being phased out and carbon dioxide on the increase, Mahlman asks, “Who wins the race?”
Alan Newman is an associate editor on the Washington staff of ES&T. Environ. Sci. Technol., Vol. 27, No. 8 , 1993 1491
