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The gamma ray spectrometer on the Mars Odyssey spacecraft measured an enhancement of atmospheric argon over southern high latitudes during autumn followed by dissipation during winter and spring. Argon does not freeze at temperatures normal for southern winter (~145 kelvin) and is left in the atmosphere, enriched relative to carbon dioxide (C[O.sub.2]), as the southern seasonal cap of C[O.sub.2] frost accumulates. Calculations of seasonal transport of argon into and out of southern high latitudes point to meridional (north-south) mixing throughout southern winter and spring., Between autumn (areocentric longitude of the Sun [L.sub.s] 0° to 90°) (1) and winter in the southern hemisphere of Mars, about 25% of the atmosphere accumulates as a thick southern [...]

Polar deserts of the High Arctic contain vast areas of minimal plant cover and low primary productivity. Significant development of polar desert plant communities is largely restricted to areas with considerable cover of a cryptogamic soil crust, which develops in sites exposed to continued surface runoff from melting snow for some of the short growing season. Thus, soil drought and plant water stress have often been assumed to be major constraints to plant community development in polar deserts. To examine this issue, water availability and plant water relations of common herbaceous perennial plants were studied over three growing seasons in a typically barren ('noncrusted') site and a site with a well-developed cryptogamic crust and vascular plant community ('crusted'). Soil water content was consistently higher in the crusted site than the noncrusted site through all growing seasons. These differences had limited biological relevance because subsurface soils at both sites remained effectively saturated (soil water potential > -0.1 MPa) through each growing season, despite low amounts of precipitation that varied nearly twofold from year to year. However, the surface soils (0.5-2.0 cm in depth), especially in noncrusted sites, dry considerably in some years. There were no significant differences in plant water potential and midday values of whole-plant transpiration and water vapor conductance for adult plants growing in crusted vs. noncrusted sites. Water stress for established plants was minor in both sites. Greater plant community development in crusted areas of this polar desert does not result from a reduction in plant water stress by a greater supply of meltwater through the growing season. Instead, surface meltwaters probably benefit vascular plants indirectly by facilitating the growth, development, and nitrogen fixation activities of cryptogamic organisms in the soil crust. The presence and activity of these cryptograms favor vascular plant success through increased nutrient availability, soil organic matter, surface temperatures, reduced soil cryoturbation, and more favorable sites for germination and seedling establishment. Key words: arctic; cryptograms; Devon Island, Canada; plant community development; polar deserts; soil crusts; soil moisture; water availability; water relations., INTRODUCTION Large expanses of the High Arctic are occupied by polar deserts (Aleksandrova 1983, Bliss and Matveyeva 1992). These landscapes are characterized by a sparse cover ( Short growing seasons [...]

Simultaneous global measurements of nitric acid (HN[O.sub.3]), water ([H.sub.2O]), chlorine monoxide (CIO), and ozone ([O.sub.3]) in the stratosphere have been obtained over complete annual cycles in both hemispheres by the Microwave Limb Sounder on the Upper Atmosphere Research Satellite. A sizeable decrease in gas-phase HN[O.sub.3] was evident in the lower stratospheric vortex over Antarctica by early June 1992, followed by a significant reduction in gas-phase [H.sub.2O] after mid-July. By mid-August, near the time of peak CIO, abundances of gas-phase HN[O.sub.3] and [H.sub.2O] were extremely low. The concentrations of HN[O.sub.3] and [H.sub.2O] over Antarctica remained depressed into November, well after temperatures in the lower stratosphere had risen above the evaporation threshold for polar stratospheric clouds, implying that denitrification and dehydration had occurred. No large decreases in either gas-phase HN[O.sub.3] or [H.sub.2O] were observed in the 1992-1993 Arctic winter vortex. Although CIO was enhanced over the Arctic as it was over the Antarctic, Arctic [O.sub.3] depletion was substantially smaller than that over Antarctica. A major factor currently limiting the formation of an Arctic ozone 'hole' is the lack of denitrification in the northern polar vortex, but future cooling of the lower stratosphere could lead to more intense denitrification and consequently larger losses of Arctic ozone., The severe depletion of stratospheric ozone over Antarctica[1] in late winter and early spring (the so-called Antarctic ozone 'hole') is now known[2] to be caused by chlorine chemistry. Recently, there [...]

Polar stratospheric clouds (PSCs) play a key role in stratospheric ozone depletion. Surface-catalyzed reactions on PSC particies generate chlorine compounds that photolyze readily to yield chlorine radicals, which in turn destroy ozone very efficiently. The most prevalent PSCs form at temperatures several degrees above the ice frost point and are believed to consist of [HN0.sub.3] hydrates; however, their formation mechanism is unclear. Results of laboratory experiments are presented which indicate that the background stratospheric [H.sub.2][SO.sub.4][H.sub.2 O] aerosols provide an essential link in this mechanism: These liquid aerosols absorb significant amounts of [HNO.sub.3] vapor, leading most likely to the crystallization of nitric acid trihydrate (NAT). The frozen particles then grow to form PSCs by condensation of additional amounts of [HNO.sub.3] and [H.sub.2 O] vapor. Furthermore, reaction probability measurements reveal that the chlorine radical precursors are formed readily at polar stratospheric temperatures not just on NAT and ice crystals, but also on liquid [H.sub.2][SO.sub.4] solutions and on solid [H.sub.2][SO.sub.4] hydrates. These results imply that the chlorine activation efficiency of the aerosol particles increases rapidly as the temperature approaches the ice frost point regardless of the phase or composition of the particles., Polar stratospheric clouds (PSCs) play a crucial role in the depletion of ozone ([O.sub.3]) in the polar stratosphere in the winter and spring months: They promote the conversion of stable [...]

The deepest waters of the ocean--North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW)--originate from polar surface waters that sink and then spread over the ocean bottom toward the [...]

Upper Atmosphere Research Satellite observations indicate that extensive denitrification without significant dehydration currently occurs only in the Antarctic during mid to late June. The fact that denitrification occurs in a relatively warm month in the Antarctic raises concern about the likelihood of its occurrence and associated effects on ozone recovery in a colder and possibly more humid future Arctic lower stratosphere. Polar stratospheric cloud lifetimes required for Arctic denitrification to occur in the future are presented and contrasted against the current Antarctic cloud lifetimes. Model calculations show that widespread severe denitrification could enhance future Arctic ozone loss by up to 30%., Polar stratospheric clouds (PSCs) play important roles in the formation of the springtime Antarctic 'ozone hole' by activating chlorine and denitrifying the stratosphere (1-7). Although similar abundances of reactive chlorine [...]

It is now well established that heterogeneous chlorine activation reactions on polar stratospheric clouds (PSCs) play a central role in polar ozone depletion. In contrast, the chemical composition and formation [...]

Before the discovery of the Antarctic ozone hole, it was generally assumed that gas-phase chemical reactions controlled the abundance of stratospheric ozone. However, the massive springtime ozone losses over Antarctica [...]