Stratospheric Chemistry and Arctic Ozone Decline

Part 2.3 from ACE: An Overview by Dr. Peter Bernath

The anthropogenic release of CFCs affects the stratospheric ozone layer through gas phase chemical reactions (such as the Cl/ClO cycle) as well as by heterogeneous chemistry on PSCs and aerosols [1]. CFCs, transported to the stratosphere, are broken down largely by UV light and release "active" chlorine species such as Cl and ClO. These species destroy odd oxygen (O + O3) via various gas phase catalytic cycles. However, the full extent of the destruction is ameliorated by the storage of the active chlorine in the reservoir species HCl and ClONO2, neither of which reacts efficiently with O3 or O. If only the gas phase chemical reactions are considered then the predicted effect on the total ozone column is relatively small. For example, Brasseur [7] predicts with a 2D model (latitude-elevation) that the ozone column would decrease by less than 1% over the ten-year period 1980-1990 in the absence of heterogeneous chemistry.

The discovery of the Antarctic ozone hole in 1985 by Farman et al. [6] led to the realization that heterogeneous reactions were important in determining the ozone budget in polar regions. Later work [8] indicated that heterogeneous reactions on and within sulphate aerosols also play an important role in the stratosphere. Volcanic SO2 from "random" eruptions penetrating the stratosphere is also important in maintaining the stratospheric sulphate layer budget. Each winter the vortex forms in the polar regions due to the IR cooling that occurs causing temperatures to drop well below 200 K. The Antarctic vortex is colder than the Arctic vortex. With these cold temperatures H2SO4, HNO3, H2O and various mixtures can freeze or exist as supercooled solutions. As the temperature drops below 195 K, large amounts of HNO3 can dissolve into the sulphate aerosols to form ternary solutions, amounts large enough to deplete almost entirely the gas phase HNO3 levels. The volume of these aerosol solutions can dramatically increase with this large uptake of gaseous HNO3 and H2O. It is thought that these super-cooled ternary H2SO4/HNO3/H2O liquid solutions can exist to within a few degrees of the frost point without forming type I PSC NAT (solid HNO3·3H2O) and SAT (H2SO4·4H2O). If the temperature falls below the ice frost point (~188 K), type II PSC water ice can form. As temperatures again rise, adsorption of gas phase HNO3 onto solid SAT can help to regenerate ternary solutions and melt SAT far below the temperature (210-215 K) SAT is believed to melt [9, 10]. Due to the low temperatures needed for solid PSCs to form, it is expected that processing on liquid surfaces is more important in the Arctic than the Antarctic.

Heterogeneous reactions on these condensed phases can activate chlorine and bromine while tying up odd-nitrogen as HNO3(a) [11]. The active chlorine and to a lesser extent bromine drive the reactions that form the ozone hole. For example, on the ice crystals, inactive or reservoir forms of the halogen catalysts are freed,

ClONO2 + HX (ice) --> HNO3 (ice) + XCl
(where X = Cl, Br).

Low temperatures drive these processes. As the polar lower stratospheric temperatures drop at the end of the fall season, the aerosol reactions become important. For example, the solubility of HCl is very temperature sensitive and as the temperature drops, it begins to dissolve in the ternary solution / sulphate aerosol. These and similar reactions drive the formation of the ozone hole in the polar late winter (Northern Hemisphere) and austral springtime. These reactions can occur during the nighttime and when polar sunrise occurs: species such as Cl2, BrCl and ClNO2 are readily photolysed into more labile species such as Cl. The processed air can persist for several weeks since the reformation of the reservoir species HCl and ClNO3 is a relatively slow process at polar latitudes. Gravitational sedimentation of the PSCs removes stratospheric HNO3 and H2O (denitrification and dehydration). One of the main features of ozone loss in polar regions is that it is not rate-limited by the low abundance of atomic oxygen. One of the main loss mechanisms involves self-reaction of ClO:

ClO + ClO --> Cl2O2
Cl2O2 + hv --> Cl + ClO2
ClO2 + M --> Cl + O2
2(Cl + O3 --> ClO + O2)
O3 + O3 --> 3O2 (net)

As long as temperatures are less than ~210 K, then thermal decomposition of Cl2O2
Cl2O2 + M --> ClO + ClO,
which does not lead to ozone loss, does not play a major role. Another important ozone loss mechanism is due to the synergistic reaction between BrO and ClO [13]:

BrO + ClO --> Br + Cl + O2
BrO + ClO --> BrCl + O2
BrCl + hv --> Br + Cl
Cl + O3 --> ClO + O2
Br + O3 --> BrO + O2

As before, this loss rate is not limited by the abundance of atomic oxygen.

The inclusion of heterogeneous reactions appears to be able to account for the severe ozone loss in the Antarctic spring [1]. There have been problems accounting for the ozone decline in the Arctic [15] although recent modelling work by Lefevre et al. [5] using a chemical transport model produces quite good agreement with column ozone measurements. Springtime ozone depletion differs between the Arctic and Antarctic due to differences in seasonal temperature extremes largely caused by atmospheric dynamics related to topography. The northern polar vortex is somewhat larger in extent, less well defined spatially, and more unstable than the southern vortex. More of the northern vortex is exposed to sunlight during the boreal winter, increasing the complexity of both its chemistry and physics. Another effect that may be important is processing that occurs at mesoscales (< 50 km). Most model simulations to date are run at rather low resolution. High-resolution (mesoscale) model runs by Carslaw et al. [16] suggest that temperatures sufficiently low for PSCs to form can be induced by mountain waves even when the synoptic temperatures appear to be too high for the formation of PSCs. And as noted above the air, once processed, they will remain processed for several weeks. In this manner large volumes of stratospheric air may undergo processing.

There is no dramatic "Arctic ozone hole" in the spring because downward transport of ozone-rich air masks the strong ozone depletion [3 to 5]. Additionally, Arctic stratospheric temperatures are generally warmer than those in the Antarctic because the vortex is often in sunlight. The PSCs associated with strong ozone depletion in the springtime form in the stratosphere at temperatures below about 195 K (see Figure 4). Such temperatures are common in the Antarctic winter but are rarer in the Arctic. Typical Arctic winter temperatures lie just above 195 K so that there is a strong correlation between springtime Arctic ozone loss and the formation of PSCs. Salawitch has noted the correlation between Arctic ozone column and the minimum temperature measured in March at 50 mbar (about 20 km) (see Figure 4 ). When this temperature dropped below 195 K as it did in 1994, 1995 and 1997, strong ozone loss was detected. The spring of 1998, however, was relatively warm (198 K) and the ozone loss was much reduced. These observations make a strong case for the importance of heterogeneous PSC chemistry in Arctic ozone declines. In addition to gases, ACE will thus strongly focus on the chemistry and physics of PSC particles. Note that the ozone declines at mid-latitudes cannot be attributed to the polar chemistry, although transport of ozone-depleted air might be a contributory factor.

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