I. Kilbane-Dawe

Arctic Ozone Loss in Threshold Conditions: Match Observations in 1997/1998 and 1998/1999

Chemical ozone loss rates inside the Arctic polar vortex were determined in early 1998 and early 1999 by using the Match technique based on coordinated ozonesonde measurements. These two winters provide the only opportunities in recent years to investigate chemical ozone loss in a warm Arctic vortex under threshold conditions, i.e., where the preconditions for chlorine activation, and hence ozone destruction, only occurred occasionally. In 1998, results were obtained in January and February between 410 and 520 K. The overall ozone loss was observed to be largely insignificant, with the exception of late February, when those air parcels exposed to temperatures below 195 K were affected by chemical ozone loss. In 1999, results are confined to the 475 K isentropic level, where no significant ozone loss was observed. Average temperatures were some 8°–10° higher than those in 1995, 1996, and 1997, when substantial chemical ozone loss occurred. The results underline the strong dependence of the chemical ozone loss on the stratospheric temperatures. This study shows that enhanced chlorine alone does not provide a sufficient condition for ozone loss. The evolution of stratospheric temperatures over the next decade will be the determining factor for the amount of wintertime chemical ozone loss in the Arctic stratosphere.

Three-dimensional studies of the 1991/1992 northern hemisphere winter using domain-filling trajectories with chemistry

We describe a new and computationally efficient technique for global three-dimensional modeling of stratospheric chemistry. This technique involves integrating a photochemical package along a large number of independent trajectories to produce a Lagrangian view of the atmosphere. Although Lagrangian chemical modeling with trajectories is an established procedure, this extension of integrating chemistry along a large number of domain-filling trajectories is a novel technique. This technique is complementary to three-dimensional Eulerian chemical transport modeling and avoids spurious mixing caused by low resolutions or diffusive transport schemes in these models. We illustrate the technique by studying the chlorine activation in the Arctic winter lower stratosphere. A photochemical model was integrated along large ensembles of calculated trajectories between 20 and 100 mbar for the 1991/1992 winter in order to produce a three-dimensional chemical picture. Large amounts of chlorine was activated at low altitudes (80 to 100 mbar) as well as altitudes near 50 mbar. This activated air was well contained at all levels, with little indication of mixing into lower latitudes. Model results for early January 1992 were compared to daily Microwave Limb Sounder (MLS) ClO observations at 465 K. The structure and evolution of the activated chlorine was well reproduced, giving faith in the technique, although absolute modeled ClO amounts were smaller than the MLS data. A larger number of domain-filling isentropic trajectories were also run at 475 K to produce a higher-resolution picture of vortex evolution in late January 1992. The model successfully reproduced the wave breaking events which characterized this period causing transport of activated air to lower latitudes.

Early modelling results from the SESAME and ASHOE campaigns

Data from the second European stratospheric arctic and middle latitude experiment (SESAME) and the airborne southern hemisphere ozone experiment (ASHOE)(recent campaigns in both hemispheres), and from the upper atmosphere research satellite (UARS) provide information about the movement, and possible mixing, of stratospheric air from the vortex and vortex edge into middle latitudes. Model studies reported here reproduce the observed features and may provide insight into their larger-scale structure. It is now clear that filaments of activated air, removed from the vortex edge, contribute to the observed decline of ozone in middle latitudes.

Trajectory model studies of ClOx activation during the 1991/92 northern hemispheric winter

The authors present the result of chemical box model calculations carried out along air mass trajectories between November 1991, and January 1992. The calculations show that reactive chlorine compounds, such as chlorine oxide, built up in these air masses over time. The calculated values are compared with measurements from satellite platforms.

A Comparison of Match Ozonesonde-Derived and 3D Model Ozone Loss Rates in the Arctic Polar Vortex during the Winters of 1994/95 and 1995/96

Ozone loss rates from ozonesonde data reported in the Match experiments of winters 1994/95 and 1995/96 inside the Arctic polar vortex are compared with simulations of the same winters performed using the SLIMCAT 3D chemistry and transport model. For 1994/95 SLIMCAT reproduces the location and timing of the diagnosed ozone destruction, reaching 10 ppbv/sunlit hour in late January as observed. SLIMCAT underestimates the loss rates observed in February and March by 1–3 ppbv/sunlit hour. By the end of March, SLIMCAT ozone exceeds the observations by 25–35%. In January 1995 the ozonesonde-derived loss rates at levels above 525 K are not chemical in origin but due to poor conservation of air parcels. Correcting temperature biases in the model forcing data significantly improved the agreement between the model and observed ozone at the end of winter 1994/95, increasing ozone destruction in SLIMCAT in February and March. The SLIMCAT simulation of winter 1995/96 does not reproduce the maximum ozone loss rates diagnosed by Match of 13 ppbv/sunlit hour. Comparing the data for the two winters reveals that the SLIMCAT photochemistry is least able to reproduce observed losses at low temperatures or when low temperatures coincide with high solar zenith angles (SZA). When cold (T = 192 K), high SZA (≥90°)matches are excluded from the 1995/96 analysis, agreement between the diagnoses and SLIMCAT is better with ozone loss rates of up to 6 ppbv/sunlit hour. For the rest of the winter SLIMCAT consistently underestimates the Match rates of ozone loss by 1–3 ppbv/sunlit hour. In March 1996 the monthly mean SLIMCAT ozone is 50% greater than observations at 430–540 K. In both winters, ozone destruction rates peaked more rapidly and declined more slowly in the Match observations than in the SLIMCAT simulations. The differences between the observed and modelled cumulative ozone losses demonstrate that the total ozone destruction by the end of the winter is sensitive to errors in the instantaneous ozone loss rates of 1–3 ppbv/sunlit hour.