Energetic radiation belt electron precipitation: a natural depletion mechanism for stratospheric ozone

During geomagnetically disturbed periods the precipitational loss of energetic electrons from the outer radiation belt of the earth can readily provide the major ionization source for the mesosphere and upper stratosphere. One particularly intense manifestation of this interaction between the radiation belts and the lower atmosphere is the relativistic electron precipitation (REP) event which occurs at subauroral latitudes during magnetospheric substorm activity. At relativistic energies the precipitating electrons produce copious fluxes of energetic bremsstrahlung x-rays, the major portion of which penetrate deep into the stratosphere before undergoing excitation and ionization collisions with the neutral atmosphere. If such REP events occur more than a few percent of the time, they can, on an annual basis, provide a local source of upper stratospheric nitric oxide molecules (via the dissociation of molecular nitrogen) comparable to that from either galactic cosmic rays or energetic solar proton events. Since nitric oxide plays a major role in the removal of stratospheric ozone, it appears that the influence of REP events must also be considered in future photochemical modeling of the terrestrial ozone layer.     

Stable isotope enrichment in stratospheric nitrous oxide

Nitrous oxide is a greenhouse gas that also plays a role in the cycling of stratospheric ozone. Air samples from the lower stratosphere exhibit 15N/14N and 18O/16O enrichment in nitrous oxide, which can be accounted for with a simple model describing an irreversible destruction process. The observed enrichments are quite large and incompatible with those determined for the main stratospheric nitrous oxide loss processes of photolysis and reaction with excited atomic oxygen. Thus, although no stratospheric source needs to be invoked, the data indicate that present understanding of stratospheric nitrous oxide chemistry is incomplete.     

Arctic ozone loss due to denitrification

Measurements from the winter of 1994-95 indicating removal of total reactive nitrogen from the Arctic stratosphere by particle sedimentation were used to constrain a microphysical model. The model suggests that denitrification is caused predominantly by nitric acid trihydrate particles in small number densities. The denitrification is shown to increase Arctic ozone loss substantially. Sensitivity studies indicate that the Arctic stratosphere is currently at a threshold of denitrification. This implies that future stratospheric cooling, induced by an increase in the anthropogenic carbon dioxide burden, is likely to enhance denitrification and to delay until late in the next century the return of Arctic stratospheric ozone to preindustrial values.     

Tunguska meteor fall of 1908: effects on stratospheric ozone

In 1908, when the giant Tunguska meteor disintegrated in the earth's atmosphere over Siberia, it may have generated as much as 30 million metric tons of nitric oxide (NO) in the stratosphere and mesosphere. The photochemical aftereffects of the event have been simulated using a comprehensive model of atmospheric trace composition. Calculations indicate that up to 45 percent of the ozone in the Northern Hemisphere may have been depleted by Tunguska's nitric oxide cloud early in 1909 and large ozone reductions may have persisted until 1912. Measurements of atmospheric transparentiy by the Smithsonian Astrophysical Observatory for the years 1909 to 1911 show evidence of a steady ozone recovery from unusually low levels in early 1909, implying a total ozone deficit of 30 +/- 15 percent. The coincidence in time between the observed ozone recovery and the Tunguska meteor fall indicates that the event may provide a test of current ozone depletion theories.     

Physical Chemistry of the H2SO4/HNO3/H2O System: Implications for Polar Stratospheric Clouds

Polar stratospheric clouds (PSCs) play a key role in stratospheric ozone depletion. Surface-catalyzed reactions on PSC particles 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 HNO(3) hydrates; however, their formation mechanism is unclear. Results of laboratory experiments are presented which indicate that the background stratospheric H(2)SO(4)/H(2)O aerosols provide an essential link in this mechanism: These liquid aerosols absorb significant amounts of HNO(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(3) and H(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(2)SO(4) solutions and on solid H(2)SO(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.     

Nitric acid trihydrate (NAT) in polar stratospheric clouds

A comprehensive investigation of polar stratospheric clouds was performed on 25 January 2000 with instruments onboard a balloon gondola flown from Kiruna, Sweden. Cloud layers were repeatedly encountered at altitudes between 20 and 24 kilometers over a wide range of atmospheric temperatures (185 to 197 kelvin). Particle composition analysis showed that a large fraction of the cloud layers was composed of nitric acid trihydrate (NAT) particles, containing water and nitric acid at a molar ratio of 3:1; this confirmed that these long-sought solid crystals exist well above ice formation temperatures. The presence of NAT particles enhances the potential for chlorine activation with subsequent ozone destruction in polar regions, particularly in early and late winter.     

Is there any chlorine monoxide in the stratosphere?

A ground-based search for stratospheric chlorine monoxide was carried out during May and October 1981 with an infrared heterodyne spectrometer in the solar absorption mode. Lines due to stratospheric nitric acid and tropospheric carbonyl sulfide were detected at about 0.2 percent absorptance levels, but the expected 0.1 percent lines of chlorine monoxide in this same region were not seen. Stratospheric chlorine monoxide is less abundant by at least a factor of 7 than is indicated by in situ measurements, and the upper limit for the integrated vertical column density of chlorine monoxide is 2.3 x 10(13) molecules per square centimeter at the 95 percent confidence level. These results imply that the release of chlorofluorocarbons may be significantly less important for the destruction of stratospheric ozone than is currently thought.     

Chlorine oxide in the stratospheric ozone layer: ground-based detection and measurement

Stratospheric chlorine oxide, a significant intermediate product in the catalytic destruction of ozone by atomic chlorine, has been detected and measured by a ground-based 204-gigahertz, millimeter-wave receiver. Data taken at latitude 42 degrees N on 17 days between 10 January and 18 February 1980 yield an average chlorine oxide column density of approximately 1.05 x 10(14) per square centimeter or approximately 2/3 that of the average of eight in situ balloon flight measurements (excluding the anomalously high data of 14 July 1977) made over the past 4 years at 32 degrees N. We find less chlorine oxide below 35 kilometers and a larger vertical gradient than predicted by theoretical models of the stratospheric ozone layer.     

Nitrous oxide: emission from soils during nitrification of fertilizer nitrogen

Nitrous oxide is released from soils to the atmosphere during nitrification of ammonium and ammonium-producing fertilizers under aerobic conditions as well as by denitrification of nitrate under anaerobic conditions. Emissions of nitrous oxide during nitrification of fertilizer nitrogen may be significant in regard to the potential threat of fertilizer-derived nitrous oxide to the stratospheric ozone layer. Such emissions can be greatly reduced through the use of nitrapyrin, which inhibits nitrification of ammonium by soil microorganisms.     

Interhemispheric Differences in Polar Stratospheric HNO3, H2O, CIO, and O3

Simultaneous global measurements of nitric acid (HNO(3)), water (H(2)O), chlorine monoxide (CIO), and ozone (O(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 HNO(3) was evident in the lower stratospheric vortex over Antarctica by early June 1992, followed by a significant reduction in gas-phase H(2)O after mid-July. By mid-August, near the time of peak CIO, abundances of gas-phase HNO(3) and H(2)O were extremely low. The concentrations of HNO(3) and H(2)O 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 HNO(3) or H(2)O were observed in the 1992-1993 Arctic winter vortex. Although CIO was enhanced over the Arctic as it was over the Antarctic, Arctic O(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.     

Vapor pressures of solid hydrates of nitric Acid: implications for polar stratospheric clouds

Thermodynamic data are presented for hydrates of nitric acid: HNO(3).H(2)O, HNO(3).2H(2)O, HNO(3).3H(2)O, and a higher hydrate. Laboratory data indicate that nucleation and persistence of metastable HNO(3).2H(2)O may be favored in polar stratospheric clouds over the slightly more stable HNO(3).3H(2)O. Atmospheric observations indicate that some polar stratospheric clouds may be composed of HNO(3).2H(2)O and HNO(3).3H(2)O. Vapor transfer from HNO(3).2H(2)O to HNO(3).3H(2)O could be a key step in the sedimentation of HNO(3), which plays an important role in the depletion of polar ozone.     

Atomic Chlorine and the Chlorine Monoxide Radical in the Stratosphere: Three in situ Observations

Three simultaneous observations of atomic chlorine (Cl) and the chlorine monoxide radical (ClO) are reported which encompass the altitude interval between 25 and 45 kilometers. Together, Cl and ClO form a gas-phase catalytic cycle potentially capable of depleting stratospheric ozone. Observed Cl and C1O densities, although variable, imply that chlorine compounds constitute an important part of the stratospheric ozone budget. The results are compared with recent models of stratospheric photochemistry which have been used as a basis for predicting ozone depletion resulting from fluorocarbon release.     

Stratospheric response to trace gas perturbations: changes in ozone and temperature distributions

The stratospheric concentration of trace gases released in the atmosphere as a result of human activities is increasing at a rate of 5 to 8 percent per year in the case of the chlorofluorocarbons (CFCs), 1 percent per year in the case of methane (CH(4)), and 0.25 percent per year in the case of nitrous oxide (N(2)O). The amount of carbon dioxide (CO(2)) is expected to double before the end of the 21st century. Even if the production of the CFCs remains limited according to the protocol for the protection of the ozone layer signed in September 1987 in Montreal, the abundance of active chlorine (2 parts per billion by volume in the early 1980s) is expected to reach 6 to7 parts per billion by volume by 2050. The impact of these increases on stratospheric temperature and ozone was investigated with a two-dimensional numerical model. The model includes interactive radiation, wave and mean flow dynamics, and 40 trace species. An increase in CFCs caused ozone depletion in the model, with the largest losses near the stratopause and, in the vertical mean, at high latitudes. Increased CO(2) caused ozone amounts to increase through cooling, with the largest increases again near 45 kilometers and at high latitudes. This CO(2)-induced poleward increase reduced the CFC-induced poleward decrease. Poleward and downward ozone transport played a major role in determining the latitudinal variation in column ozone changes.     

Increase in the stratospheric background sulfuric Acid aerosol mass in the past 10 years

Data obtained from measurements of the stratospheric aerosol at Laramie, Wyoming (41 degrees N), indicate that the background or nonvolcanic stratospheric sulfuric acid aerosol mass at northern mid-latitudes has increased by about 5 +/- 2 percent per year during the past 10 years. Whether this increase is natural or anthropogenic could not be determined at this time because of inadequate information on sulfur sources, in particular, carbonyl sulfide, which is thought to be the dominant nonvolcanic source of stratospheric sulfuric acid vapor. An increase in stratospheric sulfate levels has important climatic implications as well as heterogeneous chemical effects that may alter the concentration of stratospheric ozone.     

The dynamics of the stratospheric polar vortex and its relation to springtime ozone depletions

Dramatic springtime depletions of ozone in polar regions require that polar stratospheric air has a high degree of dynamical isolation and extremely cold temperatures necessary for the formation of polar stratospheric clouds. Both of these conditions are produced within the stratospheric winter polar vortex. Recent aircraft missions have provided new information about the structure of polar vortices during winter and their relation to polar ozone depletions. The aircraft data show that gradients of potential vorticity and the concentration of conservative trace species are large at the transition from mid-latitude to polar air. The presence of such sharp gradients at the boundary of polar air implies that the inward mixing of heat and constituents is strongly inhibited and that the perturbed polar stratospheric chemistry associated with the ozone hole is isolated from the rest of the stratosphere until the vortex breaks up in late spring. The overall size of the polar vortex thus limits the maximum areal coverage of the annual polar ozone depletions. Because it appears that this limit has not been reached for the Antarctic depletions, the possibility of future increases in the size of the Antarctic ozone hole is left open. In the Northern Hemisphere, the smaller vortex and the more restricted region of cold temperatures suggest that this region has a smaller theoretical maximum for column ozone depletion, about 40 percent of the currently observed change in the Antarctic ozone column in spring.     

Chlorine monoxide radical, ozone, and hydrogen peroxide: stratospheric measurements by microwave limb sounding

Profiles of stratospheric ozone and chlorine monoxide radical (C1O) have been obtained from balloon measurements of atmospheric limb thermal emission at millimeter wavelengths. The C1O measurements, important for assessing the predicted depletion of stratospheric ozone by chlorine from industrial sources, are in close agreement with present theory, The predicted decrease of C1O at sunset was measured. A tentative value for the stratospheric abundance of hydrogen peroxide was also determined.     

The potential for ozone depletion in the arctic polar stratosphere

The nature of the Arctic polar stratosphere is observed to be similar in many respects to that of the Antarctic polar stratosphere, where an ozone hole has been identified. Most of the available chlorine (HCl and ClONO(2)) was converted by reactions on polar stratospheric clouds to reactive ClO and Cl(2)O(2) throughout the Arctic polar vortex before midwinter. Reactive nitrogen was converted to HNO(3), and some, with spatial inhomogeneity, fell out of the stratosphere. These chemical changes ensured characteristic ozone losses of 10 to 15% at altitudes inside the polar vortex where polar stratospheric clouds had occurred. These local losses can translate into 5 to 8% losses in the vertical column abundance of ozone. As the amount of stratospheric chlorine inevitably increases by 50% over the next two decades, ozone losses recognizable as an ozone hole may well appear.     

Stratospheric ozone effects on temperature

Calculated surface temperature changes, DeltaT(8), due to stratospheric ozone depletion (at 35 degrees N latitude in April) are less than previously estimated and range between -0.6 and +0.9 degrees K. The sign of DeltaT(8), is determined by the surface albedo and the presence or absence of a low-lying particulate layer (heating with particles, cooling without particles). The calculations indicate that a 90 percent stratospheric ozone depletion does not cause the temperature inversion at the tropopause to vanish, although it is weakened substantially.     

The impact of stratospheric ozone recovery on the Southern Hemisphere westerly jet

In the past several decades, the tropospheric westerly winds in the Southern Hemisphere have been observed to accelerate on the poleward side of the surface wind maximum. This has been attributed to the combined anthropogenic effects of increasing greenhouse gases and decreasing stratospheric ozone and is predicted to continue by the Intergovernmental Panel on Climate Change/Fourth Assessment Report (IPCC/AR4) models. In this paper, the predictions of the Chemistry-Climate Model Validation (CCMVal) models are examined: Unlike the AR4 models, the CCMVal models have a fully interactive stratospheric chemistry. Owing to the expected disappearance of the ozone hole in the first half of the 21st century, the CCMVal models predict that the tropospheric westerlies in Southern Hemisphere summer will be decelerated, on the poleward side, in contrast with the prediction of most IPCC/AR4 models.     

Stratospheric ozone destruction by man-made chlorofluoromethanes

Calculations indicate that chlorofluoromethanes produced by man can greatly affect the concentrations of stratospheric ozone in future decades. This effect follows the release of chlorine from these compounds in the stratosphere. Present usage levels of chlorofluoromethanes can lead to chlorine-catalyzed ozone destruction rates that will exceed natural sinks of ozone by 1985 or 1990.