Antarctic Ozone Depletion Research Articles

Mountain waves, polar stratospheric clouds, and the ozone depletion over Antarctica

We report observations of ozone column and clouds at typically stratospheric altitudes, obtained during the 1987 Airbone Antarctic Experiment from the TIROS Operational Vertical Sounder (TOVS)/High‐Resolution Infrared Sounder (HIRS 2) instrument on the National Oceanic and Atmospheric Administration NOAA 10 satellite. Cloud formation occurs mainly along the coast of Antarctica when strong tropospheric winds blow from the ocean to the continent. The period of September 2–9 provides a good example of the formation of localized high‐altitude clouds over the Palmer Peninsula and the Weddell Sea. The appearance of these clouds (a subset of the more prevalent polar stratospheric clouds (PSCs)) is consistent with the presence of a strong tropospheric jet over the elevated topography of the peninsula. Mountain waves and their associated high‐altitude adiabatic cooling are the driving mechanism for the cloud formation. On the basis of the cloud area coverage and wind analyses, we estimate that in the altitude range 14–18 km the air within the polar vortex has spent 5% of its time inside these clouds from early August to late September. The importance of this observation for springtime ozone depletion mechanisms is discussed. It is also concluded that the TOVS data set recorded during the past decade would be suitable to search for a possible trend in the cloud amount. Such a trend could be relevant to the rate of the Antarctic ozone depletion.

Antarctic Ozone Hole: Possible implications for ozone trends in the southern hemisphere

Satellite‐borne instruments (the Total Ozone Mapping Spectrometer and the Solar Backscattered Ultraviolet Instrument) show that, compared to 1979, total column ozone has a year‐round decrease of more than 5% in the neighborhood of 60°S. The meteorological conditions (warmer temperatures, the absence of polar stratospheric clouds) at these latitudes do not seem to favor heterogeneous chemistry as the direct cause for the observed year‐round ozone reduction. A mechanism involving the seasonal transport of ozone‐poor air from within the polar vortex to lower latitudes (the so‐called “dilution effect”) is proposed as a possible explanation for the observed year‐round ozone reduction in subpolar regions. A two‐dimensional model with an imposed springtime Antarctic ozone depletion is used to study the post‐ozone hole impact on the spatial and temporal distributions of column ozone at latitudes north of 60°S. It is found that the time constant associated with the dilution effect in the latitude region 40°–60°S is about 1 year, long enough to contribute to the observed year‐round decrease of total ozone in that region. Because of the relatively short ozone replacement time constant (∼1 month) at latitudes north of 30°S, the calculated dilution effect over these latitudes is small. Measurement strategies aimed at distinguishing various possible causes for the observed ozone reduction at middle to high latitudes in the southern hemisphere are proposed.

Visible and near‐ultraviolet spectroscopy at McMurdo Station, Antarctica: 4. Overview and daily measurements of NO2, O3, and OClO during 1987

Near‐ultraviolet absorption spectroscopy in the wavelength range from 330 to 370 nm was used to measure O3, NO2, OClO, and BrO at McMurdo Station (78°S) during 1987. Visible absorption measurements of O3, NO2, and OClO were also obtained using the wavelength range from about 403 to 453 nm. These data are described and compared to observations obtained in 1986. It is shown that comparisons of observations in the two wavelength ranges provide a sensitive measure of the altitude where the bulk of atmospheric absorption takes place. The measurements indicate that the bulk of the NO2 column abundance is located near 30 km, while those of OClO and O3 are near 20 km. The measurements of NO2 display a systematic increase during the month of September, probably reflecting the release of odd nitrogen from reservoirs formed earlier in the winter season. The measurements of OClO display a strong diurnal variation, with considerably higher values being obtained in the evening than those measured in the morning. The evening twilight OClO column abundances obtained in 1987 were notably larger than those in 1986, perhaps because stratospheric temperatures were colder, and associated heterogeneous chemistry may have been more intense. This in turn implies a faster rate of ozone destruction in 1987 than in 1986 by halogen chemistry. These observations provide important constraints on the coupled nitrogen‐halogen chemistry of Antarctic spring and its influence on the springtime Antarctic ozone depletion.

Rate constants for reactions between atmospheric reservoir species. 2. Water

The kinetics of the reactions of H{sub 2}O with ClONO{sub 2}, N{sub 2}O{sub 5}, O{sub 3}, and COCl{sub 2} have been investigated by using a large-volume static cell and a Fourier transform infrared spectrometer at 296 K. Upper limits for the homogeneous gas-phase reaction rate constants of the ClONO{sub 2} + H{sub 2}O, N{sub 2}O{sub 5} + H{sub 2}O, O{sub 3} + H{sub 2}O, and COCl{sub 2} + H{sub 2}O reactions were found to be 3.4 {times} 10{sup {minus}21}, 2.8 {times} 10{sup {minus}21}, 1.1 {times} 10{sup {minus}22}, and 1.2 {times} 10{sup {minus}23}, respectively (all in units of cm{sup 3} s{sup {minus}1}), based on the observed decay rates of ClONO{sub 2}, N{sub 2}O{sub 5}, O{sub 3}, and COCl{sub 2}. Product analyses gave 0.82 {plus minus} 0.07 for the yield of HNO{sub 3} in the ClONO{sub 2} + H{sub 2}O {yields} HOCl + HNO{sub 3} reaction and 1.1 {plus minus} 0.3 for the yield of HNO{sub 3} from the N{sub 2}O{sub 5} + H{sub 2}O {yields} 2HNO{sub 3} reaction. The quoted error represents one standard deviation of the measurement. An attempt was also made to monitor possible reaction products such as H{sub 2}O{sub 2} for the O{sub 3} + H{sub 2}O reaction,more » and CO{sub 2} or HCl for the COCl{sub 2} + H{sub 2}O reaction. These results may be important in the elucidation of the springtime Antarctic ozone depletion over the past decade. The implication for NO{sub x} chemistry in the nighttime troposphere based on their results of the N{sub 2}O{sub 5} + H{sub 2}O reaction will be discussed.« less

Evidence of the mid-latitude impact of Antarctic ozone depletion

THE 1987 Antarctic Airborne Ozone Expedition established that the springtime depletion of Antarctic ozone1,2 is due to photochemical destruction3 following a preconditioning phase involving heterogeneous reactions on polar stratospheric clouds4. Ozone destruction is concentrated in the cold Antarctic stratospheric vortex of winter and early spring, where these clouds occur. Nevertheless, ozone data reveal a secular decline into mid-latitudes5,6. Modelling studies6,7 suggest a 'dilution effect' whereby ozone-poor air is transported into mid-latitudes when the vortex breaks up in late spring (the 'final warming'). The extent to which the effect penetrates into middle latitudes is clouded by statistical uncertainty in trend analyses and by the difficulty of separating photochemical and dynamical effects on seasonal timescales. Here we analyse record low ozone values found over Australia and New Zealand during December 19875 following the record low Antarctic values of October 19875. In fact, there was a sudden decline of ozone amounts in mid-month; this rules out photochemical effects as a cause and allows us to investigate the underlying processes on a 'case study' basis. Using data from ozonesondes, radiosondes, the Nimbus-7 total ozone mapping spectrometer (TOMS), and meteorological analyses from the National Meteorological Center (Washington), we argue that these low values resulted from transport of ozone-poor air from higher latitudes. It therefore seems that the chemical destruction of ozone over Antarctica in early spring is having an impact on lower latitudes.

Diffusion and location of hydrochloric acid in ice: Implications for polar stratospheric clouds and ozone depletion

We have carried out experiments to study the incorporation and movement of HCl within the structure of ice. These involved freezing HCl solutions, and observing them in a scanning electron microscope fitted with an X‐ray microanalysis system. We are able to show that HCl is not easily incorporated into ice crystals, but is strongly partitioned towards the grain boundaries. Furthermore, the diffusion of HCl through ice crystals is slow. These results contradict the interpretation of earlier experiments. They mean that if HCl is to be available for reaction on polar stratospheric cloud particles, as required by current theories of Antarctic ozone depletion, then it must be present in some form other than a solid solution.

The 1988 Antarctic ozone depletion: Comparison with previous year depletions

The 1988 spring Antarctic ozone depletion was observed by TOMS to be substantially smaller than in recent years. The minimum polar total ozone values declined only 15% during September 1988 compared to nearly 50% during September 1987. At southern midlatitudes, exceptionally high total ozone values were recorded beginning in July 1988. The total integrated southern hemispheric ozone increased rapidly during the Austral spring, approaching 1980 levels during October. The high midlatitude total ozone values were associated with a substantial increase in eddy activity as indicated by the standard deviation in total ozone in the zonal band 30°–60°S. The standard deviation also correlates with the QBO cycling of the tropical winds. Mechanisms through which the increased midlatitude eddy activity could disrupt the formation of the Antarctic ozone hole are briefly discussed.

Ozone destruction through heterogeneous chemistry following the eruption of El Chichón

It is now well established that heterogeneous reactions provide an important mechanism for Antarctic ozone depletion. Recent laboratory studies suggest that the same reactions that occur on HNO3/H2O ice clouds in the cold Antarctic stratosphere can also take place on sulfuric acid particles (e.g., volcanic and background aerosols) typical of lower latitudes, albeit at slower rates. The reduction in stratospheric ozone observed at northern mid‐latitudes in late 1982 through 1983 following the volcanic eruption of El Chichón is investigated in terms of ozone loss through heterogeneous chemistry on the aerosol which formed in the stratosphere. The rates of the relevant heterogeneous reactions are believed to be critically dependent on (1) the aerosol surface area density and (2) the percent by weight sulfuric acid in the liquid particles. Direct measurements of both of these important quantities for El Chichón aerosol are described and used as a basis for model calculations of their possible effects on ozone and other trace species. The observed volcanic particle surface area reached a maximum at mid‐latitudes of about 50 μm2 cm−3 (above a typical background value of about 0.75) at an altitude of 18–20 km in early 1983. This enhancement of surface area is about the same as that encountered in stratospheric clouds in the Antarctic, suggesting a possible basis for ozone depletion through heterogeneous chemistry. Observations of NO2 and HNO3 also suggest that heterogeneous reactions on both background and volcanic aerosol play a significant role in partitioning reactive nitrogen species in middle and high latitudes in winter. It is shown that heterogeneous reactions similar to those occurring in Antarctica may have been responsible for at least a portion of the anomalous ozone reduction observed at mid‐latitudes in early 1983.

Direct ozone depletion in springtime Antarctic lower stratospheric clouds

It is now generally accepted that springtime Antarctic ozone depletion in the atmosphere between 17 and 22 km is related to the presence of ClO produced through heterogeneous chemistry on the surface of polar stratospheric cloud particles1,2. Below 17 km, however, where ozone depletion also occurs3, a causal link with ClO has not been established because of apparently low ClO values. Vertical motions, associated with mountain lee waves and the formation of nacreous clouds, also seemingly cannot explain observed ozone depletion4. Here we present balloon measurements of ozone and cloud particles in the lower stratosphere (10–17 km) at McMurdo Station (787° N), Antarctica, on 9 September 1988 which indicate that the cloud particles are directly involved in ozone depletion in this region. The reductions in ozone appear to be correlated with high concentrations of smaller particles, of about 0.2 micrometre radius, which dominate the surface area density in the cloud. The observations suggest a high degree of spatial inhomogeneity of free chlorine, associated with lower stratospheric clouds, and are important in understanding ozone depletion in this region.

近年のオゾンホールの発達に及ぼす海面水温の影響の可能性

A possible cause of the recent decrease of planetary wave activity and Antarctic ozone depletion in spring is proposed. The tropospheric stationary wave structure in the southern hemisphere shows substantial variation during 1979 to 1988. Before the 1980's, especially in 1979, a well-developed tropospheric ridge was situated in the South Pacific and the stationary wave propagated into the stratosphere, while after 1985 the tropospheric ridge became weaker and split into two ridges, and no vertical propagation was clearly seen. In 1988, however, the vertically propagating wave number 1 structure was recovered. This change of planetary wave structure in the South Pacific seems to be related to the sea surface temperature anomaly in the low latitude South Pacific. A possible role of the sea surface temperature variation in the recent development of the ozone hole is also discussed.

ChemInform Abstract: Antarctic Ozone Depletion Chemistry: Reactions of N2O5 with H2O and HCl on Ice Surfaces

The reactions of dinitrogen pentoxide (N(2)O(5)) with H(2)O and hydrochloric acid (HCl) were studied on ice surfaces in a Knudsen cell flow reactor. The N(2)O(5) reacted on ice at 185 K to form condensed-phase nitric acid (HNO(3)). This reaction may provide a sink for odd nitrogen (NO(x)) during the polar winter, a requirement in nearly all models of Antarctic ozone depletion. A lower limit to the sticking coefficient, gamma, for N(2)O(5) on ice is 1 x 10(-3). Moreover, N(2)O(5) reacted on HCl-ice surfaces at 185 K, with gamma greater than 3 x 10(-3). This reaction, which produced gaseous nitryl chloride (ClNO(2)) and condensed-phase HNO(3), proceeded until all of the HCl within the ice was depleted. The ClNO(2), which did not react or condense on ice at 185 K, can be readily photolyzed in the Antarctic spring to form atomic chlorine for catalytic ozone destruction cycles. The other photolysis product, gaseous nitrogen dioxide (NO(2)), may be important in the partitioning of NO(x) between gaseous and condensed phases in the Antarctic winter.

ChemInform Abstract: The Stability and Photochemistry of Dimers of the ClO Radical and Implications for Antarctic Ozone Depletion

Measurements of the ultraviolet spectrum of the Cl2O2 molecule formed at temperatures in the range 203–300 K by recombination of ClO radicals are reported. The equilibrium constant for the reaction has been determined and the products of photolysis of C12O2 at 254 nm investigated. The main photodissociation pathway for C12O2 probably produces Cl atoms and chlorine peroxy radicals, as assumed in calculation of the ozone loss by the ClO and C12O2 catalytic cycle in the Antarctic stratosphere.

Influence of polar stratospheric clouds on the depletion of Antarctic ozone

Precipitation of nitrate in polar stratospheric clouds (PSCs) can provide a significant sink for Antarctic stratospheric odd nitrogen. It is argued that the depth of the Ozone Hole is sensitive to the occurrence of temperatures below about 196 K. An increase in the prevalence of temperatures below 196 K would enhance ozone loss by increasing the spatial extent and persistence of PSCs, and by decreasing the level of HNO3 that remains following PSC evaporation. Concentrations of halogen gases in the 1960s and earlier were insufficient to support major ozone loss, even if thermal conditions were favorable.

Heterogeneous reactions of N2O5 with H2O and HCl on ice surfaces: Implications for Antarctic ozone depletion

Laboratory studies of heterogeneous reactions of N2O5 with H2O and HCl on ice surfaces have been performed in a fast flow reactor. A differentially pumped quadrupole mass spectrometer with an electron impact ionizer was used as a detector for the trace gas analysis. The reaction probability (or the heterogeneous reaction rate) of N2O5 on water ice was measured to be 0.028 (±0.011) at 195 K, while nitric acid was the sole product in the condensed phase. This reaction may effectively provide a sink for odd nitrogen species during the polar winter. In the presence of HCl with mole fractions between 0.015 and 0.04 in ice, the measured reaction probability of N2O5 was enhanced by about a factor of 2 at 195 K while nitryl chloride (ClNO2) or chlorine nitrite (ClONO) was found to be the major product in the gas phase. The quoted error bar represents the statistical error only. ClNO2 or ClONO can be readily photolyzed in the austral spring to form active chlorine which removes stratospheric ozone through several catalytic cycles. Another reaction product was nitric acid which remained in the solid phase. Since the polar stratospheric clouds are thought to consist of ice particles or possibly mixtures of HCl‐ice on the surface, the large reaction probabilities measured in this work suggest that these reactions should be a major factor in producing the observed springtime ozone depletion in the Antarctic stratosphere. Finally, a new ozone destruction mechanism based on surface recombination of ClO radicals is also proposed.

Two‐dimensional modelling of the Antarctic lower stratosphere

We have used a two‐dimensional radiative‐dynamical‐chemical model to study some aspects of Antarctic ozone depletion. While many aspects of the preconditioning of the vortex are reproduced by simply reducing the horizontal diffusion during the Antarctic winter and early spring, significant problems remain. The lack of knowledge of the meridional circulation throughout the year represents a serious limitation.When a springtime ozone depletion is imposed on the model in the Antarctic, the effect of the depletion is confined mainly to high southern latitudes. The consequent changes in radiative transfer lead to lower temperatures in the lower polar atmosphere during spring, with some smaller increases in temperature above about 20mb.

Antarctic Ozone Depletion Chemistry: Reactions of N 2 O 5 with H 2 O and HCl on Ice Surfaces

The reactions of dinitrogen pentoxide (N(2)O(5)) with H(2)O and hydrochloric acid (HCl) were studied on ice surfaces in a Knudsen cell flow reactor. The N(2)O(5) reacted on ice at 185 K to form condensed-phase nitric acid (HNO(3)). This reaction may provide a sink for odd nitrogen (NO(x)) during the polar winter, a requirement in nearly all models of Antarctic ozone depletion. A lower limit to the sticking coefficient, gamma, for N(2)O(5) on ice is 1 x 10(-3). Moreover, N(2)O(5) reacted on HCl-ice surfaces at 185 K, with gamma greater than 3 x 10(-3). This reaction, which produced gaseous nitryl chloride (ClNO(2)) and condensed-phase HNO(3), proceeded until all of the HCl within the ice was depleted. The ClNO(2), which did not react or condense on ice at 185 K, can be readily photolyzed in the Antarctic spring to form atomic chlorine for catalytic ozone destruction cycles. The other photolysis product, gaseous nitrogen dioxide (NO(2)), may be important in the partitioning of NO(x) between gaseous and condensed phases in the Antarctic winter.

Antarctic springtime ozone depletion computed from temperature observations

An observationally based, mechanistic dynamical model is used to simulate the decline of total ozone during September and October for the years 1979 through 1986. Vertical velocities derived from observed stratospheric temperature changes and computed radiative heating rates are used to advect an ozone mixing ratio profile during the Antarctic spring period. An early August 1982 Syowa balloonsonde ozone profile is used to initialize the computations. The model reasonably simulates the September and October changes in total ozone, considering the uncertainties in the observed data and the radiative heating. The simulated decline is found to be very sensitive to the choice of initial ozone profile and to small changes in the radiative heating. The results of this study suggest that the dynamical hypothesis of the Antarctic ozone depletion is both quantitatively credible and consistent with the observed temperature changes.

The stability and photochemistry of dimers of the ClO radical and implications for Antarctic ozone depletion

Measurements of the ultraviolet spectrum of the Cl2O2 molecule formed at temperatures in the range 203–300 K by recombination of ClO radicals are reported. The equilibrium constant for the reaction has been determined and the products of photolysis of C12O2 at 254 nm investigated. The main photodissociation pathway for C12O2 probably produces Cl atoms and chlorine peroxy radicals, as assumed in calculation of the ozone loss by the ClO and C12O2 catalytic cycle in the Antarctic stratosphere.

Interactions between HCl, NOx and H2O ice in the Antarctic stratosphere: Implications for ozone

Thermodynamic properties of solid phases containing H2O and HCl are examined for conditions expected over Antarctica in winter. It is shown that solid solutions with 0.02–0.035 mole fraction HCl in H2O ice will form in the stratosphere at temperatures between 193° and 190°K. Crystalline HNO3·H2O and/or HNO3·3H2O form in the same temperature range. Thus condensation of small quantities of H2O (<1 ppm) leads to nearly complete removal of both HCl and HNO3 from the gas phase. Formation of ice‐HCl solid solutions provides a favorable setting for the heterogeneous reaction of HCl with ClNO3, initiating a rapid sequence of reactions that converts solid‐phase HCl into gaseous ClNO3 or chlorine oxide radicals. Odd nitrogen species are simultaneously converted into solid‐phase nitrates. The cycle producing reactive chlorine gases is likely to be completed before particles grow large enough to fall from the stratosphere. Hence precipitation of particles from the polar stratospheric clouds is expected to remove odd nitrogen from the stratosphere efficiently, leaving chlorine gases behind. High concentrations of reactive chlorine oxide radicals are rapidly produced by heterogeneous reactions if the initial concentration of HCl exceeds a critical value, 0.5 times the concentration of NOx before onset of condensation. Production of unreactive ClNO3 and HOCl is favored if HCl levels are lower than this value. The onset of Antarctic ozone depletion in the late 1970s may in part reflect growth of HCl levels beyond this threshold.

Antarctic ozone hole may affect other areas

Now that chlorine species derived from chlorofluorocarbons have been inextricably tied to the Antarctic ozone hole, atmospheric scientists are turning their attention to the global implications of the phenomenon. Some initial attempts to evaluate the global impact of the recurring dramatic loss of ozone were aired at a symposium on Antarctic ozone depletion held in Boston earlier this month at the annual meeting of the American Association for the Advancement of Science. Each September and October since the late 1970s, just as winter has come to an end in the Southern Hemisphere, the stratospheric ozone layer over Antarctica has drastically shrunk. The ozone depletion occurs within the polar vortex, a stream of air in the stratosphere that tends to circle the continent in winter. Later in the spring, ozone concentrations return to normal as the polar vortex breaks up and air from the North enters the Antarctic stratosphere. In 1987, the ozone hole was larger ...